| 1. |
- Groenholm, T., et al.
(författare)
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Aerosol particle dry deposition to canopy and forest floor measured by two-layer eddy covariance system
- 2009
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Ingår i: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996 .- 0148-0227 .- 2156-2202. ; 114
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Tidskriftsartikel (refereegranskat)abstract
- We present, to our knowledge for the first time, results from subcanopy (forest trunk space) aerosol flux measurements with the eddy covariance (EC) technique. The measurements were performed at the SMEAR II measurement station located in a Scots pine forest in southern Finland during spring 2003 when the ground was snow covered and in situ biogenic particle formation took place almost every day. Spectral analysis showed that the EC method can be applied to estimate subcanopy aerosol fluxes and thereby ground deposition in a forest. By using the two-layer EC measurements we were able to investigate the fraction of the total aerosol deposition taking place in the canopy and on the forest floor. We found that the ratio of subcanopy to above- canopy flux is dependent on the strength of turbulence. When the friction velocity was low (u(*) < 0.25 m s(-1)), approximately 35% of the depositing particles penetrated the canopy and deposited on the ground. When u* increased, the fractional deposition on the forest floor decreased almost linearly, and in strongly turbulent conditions (u(*)> 1.0 m s(-1)) the ground deposition contributed only around 10% to the total deposition. Thus, it seems that increased turbulence enhances the importance of the canopy for aerosol deposition and the relative importance of the forest floor to the total deposition diminishes.
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| 2. |
- Amanda Collaboration, -, et al.
(författare)
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Optical Properties of Deep Glacial Ice at the South Pole
- 2006
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202 .- 2169-897X. ; 111:D13, s. D13203-
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Tidskriftsartikel (refereegranskat)abstract
- We have remotely mapped optical scattering and absorption in glacial ice at the South Pole for wavelengths between 313 and 560 nm and depths between 1100 and 2350 m. We used pulsed and continuous light sources embedded with the AMANDA neutrino telescope, an array of more than six hundred photomultiplier tubes buried deep in the ice. At depths greater than 1300 m, both the scattering coefficient and absorptivity follow vertical variations in concentration of dust impurities, which are seen in ice cores from other Antarctic sites and which track climatological changes. The scattering coefficient varies by a factor of seven, and absorptivity (for wavelengths less than ∼450 nm) varies by a factor of three in the depth range between 1300 and 2300 m, where four dust peaks due to stadials in the late Pleistocene have been identified. In our absorption data, we also identify a broad peak due to the Last Glacial Maximum around 1300 m. In the scattering data, this peak is partially masked by scattering on residual air bubbles, whose contribution dominates the scattering coefficient in shallower ice but vanishes at ∼1350 m where all bubbles have converted to nonscattering air hydrates. The wavelength dependence of scattering by dust is described by a power law with exponent −0.90 ± 0.03, independent of depth. The wavelength dependence of absorptivity in the studied wavelength range is described by the sum of two components: a power law due to absorption by dust, with exponent −1.08 ± 0.01 and a normalization proportional to dust concentration that varies with depth; and a rising exponential due to intrinsic ice absorption which dominates at wavelengths greater than ∼500 nm.
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| 3. |
- Achtert, Peggy, 1982-, et al.
(författare)
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Hygroscopic growth of tropospheric particle number size distributions over the North China Plain
- 2009
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 114, s. D00G07-
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Tidskriftsartikel (refereegranskat)abstract
- The hygroscopic growth of atmospheric submicrometer particle size distributions (diameter D-p ranging from 22 to 900 nm) was studied at a rural/suburban site in the North China Plain within the framework of the international Campaigns of Air Quality Research in Beijing and Surrounding Region 2006 (CAREBeijing-2006) research project. The goal was to characterize the regional aerosol in the polluted northeastern plain in China. Size descriptive hygroscopic growth factors (DHGFs) were determined as a function of relative humidity (RH) by relating the particle number size distribution at a dry condition ( 100 nm), the DHGF are substantially higher than in the Aitken particle mode (D-p < 100 nm) as a result of different chemical composition. The size-dependent behavior of the DHGF highlights the relevance of particulate sulfate production over the North China Plain, accomplished by secondary formation from the gas phase and, potentially, liquid phase processes in convective clouds. Furthermore, all results concerning the DHGF show a significant dependency on meteorological air masses. The hygroscopic growth of accumulation mode particles correlates significantly with the PM1-mass fraction of sulfate ions determined by chemical analysis. Finally, this investigation provides a parameterization of the hygroscopic growth of 250-nm particles, which might be useful when predicting visibility and radiative forcing and performing atmospheric aerosol model validations.
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| 4. |
- Adriani, O., et al.
(författare)
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Measurements of quasi-trapped electron and positron fluxes with PAMELA
- 2009
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 114, s. A12218-
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents precise measurements of the differential energy spectra of quasi-trapped secondary electrons and positrons and their ratio between 80 MeV and 10 GeV in the near-equatorial region (altitudes between 350 km and 600 km). Latitudinal dependences of the spectra are analyzed in detail. The results were obtained from July until November 2006 onboard the Resurs-DK satellite by the PAMELA spectrometer, a general purpose cosmic ray detector system built around a permanent magnet spectrometer and a silicon-tungsten calorimeter.
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| 5. |
- Andreeova, K., et al.
(författare)
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Shock propagation in the magnetosphere : Observations and MHD simulations compared
- 2008
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113:A9, s. A09224-
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Tidskriftsartikel (refereegranskat)abstract
- We examine the propagation of disturbances in the Earth's magnetosphere caused by fast forward shock interaction with the magnetopause. Our statistical study and event analyses show that the propagation speeds are larger in the magnetosphere than in the solar wind and are larger in the nightside magnetosphere than in the dayside magnetosphere. A case study of a double shock during 9 November 2002 is examined both observationally and using the GUMICS-4 global MHD simulation. Tracing the disturbance propagation allows us to confirm that the MHD simulation results are in good agreement with the in situ observations. The simulation results show that the propagation of the disturbance occurs in the antisunward direction at all clock angles simultaneously. However, changes in the magnetosheath are largest at high latitudes, while in the magnetotail the largest variations are seen in the plasma sheet.
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| 6. |
- Backstrand, K., et al.
(författare)
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Total hydrocarbon flux dynamics at a subarctic mire in northern Sweden
- 2008
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113, s. G03026-
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Tidskriftsartikel (refereegranskat)abstract
- This is a study of the spatial and temporal variability of total hydrocarbon (THC) emissions from vegetation and soil at a subarctic mire, northern Sweden. THCs include methane (CH4) and nonmethane volatile organic compounds (NMVOCs), both of which are atmospherically important trace gases and constitute a significant proportion of the carbon exchange between biosphere and atmosphere. Reliable characterization of the magnitude and the dynamics of the THC fluxes from high latitude peatlands are important when considering to what extent trace gas emissions from such ecosystems may change and feed back on climate regulation as a result of warmer climate and melting permafrost. High frequency measurements of THC and carbon dioxide (CO2) were conducted during four sequential growing seasons in three localities representing the trophic range of plant communities at the mire. The magnitude of the THC flux followed the moisture gradient with increasing emissions from a dry Palsa site (2.2 +/- 0.1 mgC m(-2) d(-1)), to a wet intermediate melt feature with Sphagnum spp. (28 +/- 0.3 mgC m(-2) d(-1)) and highest emissions from a wet Eriophorum spp. site (122 +/- 1.4 mgC m(-2) d(-1)) (overall mean +/- 1 SE, n = 2254, 2231 and 2137). At the Palsa site, daytime THC flux was most strongly related to air temperature while daytime THC emissions at the Sphagnum site had a stronger relation to ground temperature. THC emissions at both the wet sites were correlated to net ecosystem exchange of CO2. An overall spatial correlation indicated that areas with highly productive vegetation communities also had high THC emission potential.
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| 7. |
- Barret, B., et al.
(författare)
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Intercomparisons of trace gases profiles from the Odin/SMR and Aura/MLS limb sounders
- 2006
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 111:D21
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents the intercomparison of O(3), HNO(3), ClO, N(2)O and CO profiles measured by the two spaceborne microwave instruments MLS ( Microwave Limb Sounder) and SMR ( Submillimetre Radiometer) on board the Aura and Odin satellites, respectively. We compared version 1.5 level 2 data from MLS with level 2 data produced by the French data processor version 222 and 225 and by the Swedish data processor version 2.0 for several days in September 2004 and in March 2005. For the five gases studied, an overall good agreement is found between both instruments. Most of the observed discrepancies between SMR and MLS are consistent with results from other intercomparison studies involving MLS or SMR. O(3) profiles retrieved from the SMR 501.8 GHz band are noisier than MLS profiles but mean biases between both instruments do not exceed 10%. SMR HNO(3) profiles are biased low relative to MLS's by similar to 30% above the profile peak. In the lower stratosphere, MLS ClO profiles are biased low by up to 0.3 ppbv relative to coincident SMR profiles, except in the Southern Hemisphere polar vortex in the presence of chlorine activation. N(2)O profiles from both instruments are in very good agreement with mean biases not exceeding 15%. Finally, the intercomparison between SMR and MLS CO profiles has shown a good agreement from the middle stratosphere to the middle mesosphere in spite of strong oscillations in the MLS profiles. In the upper mesosphere, MLS CO concentrations are biased high relative to SMR while negative values in the MLS retrievals are responsible for a negative bias in the tropics around 30 hPa.
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| 8. |
- Bastviken, D., et al.
(författare)
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Fates of methane from different lake habitats : Connecting whole-lake budgets and CH4 emissions
- 2008
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113:G2
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Tidskriftsartikel (refereegranskat)abstract
- Methane (CH4) represents a major product of organic matter decomposition in lakes. Once produced in the sediments, CH4 can be either oxidized or emitted as a greenhouse gas to the atmosphere. Lakes represent an important source of atmospheric CH4, but the relative magnitudes of the internal pathways that lead to CH4 emissions are not yet clear. We quantified internal cycling and methane emissions in three lakes during summer stratification. These methane budgets included: sediment release of CH4 at different depths; water column transport patterns and methane oxidation; methane storage in the water column; and methane emissions to the atmosphere by diffusion and ebullition. The contribution of CH4 carbon, via oxidation by methanotrophic bacteria, to pelagic food webs was also estimated. Despite the very low concentration of CH4 in surface waters, shallow, epilimnetic sediments were major contributors of CH4 to the atmosphere. While 51 - 80% of the CH4 produced in deep sediments was oxidized in the water column, most of the CH4 released from shallow sediment escaped oxidation and reached the atmosphere. Epilimnetic sediments accounted for 100% of CH4 emitted during summer stratification, and 14 - 76% considering the release of CH4 stored in deep water layers during lake circulation after the stratification period; diffusive emission accounted for 26 - 48% and ebullition the remainder. These results indicate that it is important to address transport rates of CH4 from the shallow sediment along with the production-consumption processes when trying to understand methane dynamics and the regulation of lake methane emissions.
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| 9. |
- Bavassano Cattaneo, M. Bice, et al.
(författare)
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Kinetic signatures during a quasi-continuous lobe reconnection event : Cluster Ion Spectrometer (CIS) observations
- 2006
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 111:A9, s. A09212-
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Tidskriftsartikel (refereegranskat)abstract
- On 3 December 2001 the Cluster spacecraft observed a long-lasting lobe reconnection event in the southern high-latitude dusk magnetopause (MP) tailward of the cusp, during a 4 hour interval of mainly northward interplanetary magnetic field ( IMF) and of sub-Alfvenic magnetosheath flow. Almost all the MP encounters have accelerated flows ( for which the Walen test has been successfully verified by Retino et al. ( 2005)) as well as a large number of secondary populations related to reconnection, that is, ions of magnetosheath or magnetospheric origin which cross the MP either way. The detailed analysis of the distribution functions shows that the reconnection site frequently moves relative to the spacecraft, but simultaneous measurements by two spacecraft on opposite sides of the reconnection site indicate that the spacecraft's distance from the X line is small, i.e., below 3200 km. The vicinity to the X line throughout the event is probably the reason why the distribution functions characteristics agree with theoretical expectations on both sides of the reconnection site throughout this long event. Moreover, the detailed analysis of the distribution functions shows evidence, during a few time intervals, of dual reconnection, i.e., of reconnection simultaneously going on also in the northern hemisphere.
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| 10. |
- Becerra, Marley, et al.
(författare)
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Effect of the space charge layer created by corona at ground level on the inception of upward lightning leaders from tall towers
- 2007
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 112:D12, s. D12205-
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Tidskriftsartikel (refereegranskat)abstract
- Electric field measurements above ground have shown that the space charge layer created by corona at ground level shields the background electric field produced by the thundercloud. Therefore it is expected that this space charge layer can also influence the conditions required to initiate upward lightning from tall objects. For this reason, a numerical model that describes the evolution of the main electrical parameters below a thunderstorm is used to compute the space charge layer development. The time variation of the electric field measured at 600 m above ground during the 1989 rocket triggered lightning experiment at the Kennedy Space Center (Florida) is used to drive the model. The obtained space charge density profiles are used to compute the conditions required to initiate stable upward lightning positive leaders from tall towers. Corona at the tip of the tower is neglected. It is found that the space charge layer significantly affects the critical thundercloud electric fields required to initiate upward lightning leaders from tall objects. The neutral aerosol particle concentration is observed to have a significant influence on the space charge density profiles and the critical thundercloud electric fields, whereas the corona current density does not considerably affect the results for the cases considered in the analysis. It is found that a lower thundercloud electric field is required to trigger a lightning flash from a tall tower or other tall slender grounded structure in the case of sites with a high neutral aerosol particle concentration, like polluted areas or coastal regions.
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| 11. |
- Brenning, Nils, et al.
(författare)
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Radiation from an electron beam in a magnetized plasma : Whistler mode wave packets
- 2006
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Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 111:A11
-
Tidskriftsartikel (refereegranskat)abstract
- Experimental studies are reported of oscillations and radiation that is spontaneously excited by an electron beam which is shot along a diverging magnetic field into a plasma from a hot cathode. In the present study we focus on oscillations below the electron gyrofrequency, where we find that whistler mode radiation appears in the form of bursts, or wave packets, each with typically 0.1-1 mu s time duration, and which together cover typically a few percent of the full time. Wave packets are found in a broad frequency range of 7-40 MHz, while each individual wave packet is dominated by a single frequency. There is propagation along two routes: at the group velocity resonance cone angle, away from the central channel where the waves are excited, and in a channel along the magnetic field. Features of the whistler mode wave packets that are studied include (1) the statistics of amplitudes, frequencies, and time durations; (2) the propagation and decay of wave packets with different frequencies; (3) the group and phase velocities; and (4) how the wave packet production varies with the energy, and the current density, in the electron beam.
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| 12. |
- Brohede, Samuel, 1977, et al.
(författare)
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Validation of Odin/OSIRIS stratospheric NO2 profiles
- 2007
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 112:D07310
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents the validation study of stratospheric NO2 profiles retrieved from Odin/OSIRIS measurements of limb-scattered sunlight (version 2.4). The Optical Spectrograph and Infrared Imager System (OSIRIS) NO2 data set is compared to coincident solar occultation measurements by the Halogen Occultation Experiment (HALOE), Stratospheric Aerosol and Gas Experiment (SAGE) II, SAGE III, and Polar Ozone and Aerosol Measurement (POAM) III during the 2002–2004 period. Comparisons with seven Systeme d'Analyse par Observation Zenithal (SAOZ) balloon measurements are also presented. All comparisons show good agreement, with differences, both random and systematic, of less than 20% between 25 km and 35 km. Inconsistencies with SAGE III below 25 km are found to be caused primarily by diurnal effects from varying NO2 concentrations along the SAGE III line-of-sight. On the basis of the differences, the OSIRIS random uncertainty is estimated to be 16% between 15 km and 25 km, 6% between 25 km and 35 km, and 9% between 35 km and 40 km. The estimated systematic uncertainty is about 22% between 15 and 25 km, 11–21% between 25 km and 35 km, and 11–31% between 35 km and 40 km. The uncertainties for AM (sunrise) profiles are generally largest and systematic deviations are found to be larger at equatorial latitudes. The results of this validation study show that the OSIRIS NO2 profiles are well behaved, with reasonable uncertainty estimates between 15 km and 40 km. This unique NO2 data set, with more than hemispheric coverage and high vertical resolution will be of particular interest for studies of nitrogen chemistry in the middle atmosphere, which is closely linked to ozone depletion.
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| 13. |
- Buehler, Stefan A., et al.
(författare)
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An upper tropospheric humidity data set from operational satellite microwave data
- 2008
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113:14, s. D14110-
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Tidskriftsartikel (refereegranskat)abstract
- 183.31 GHz observations from the Advanced Microwave Sounding Unit B (AMSUB) instruments onboard the NOAA 15, 16, and 17 satellites were used to derive a new data set of Upper Tropospheric Humidity (UTH). The data set consist of monthly median and mean data on a 1.5 degrees latitude-longitude grid between 60 degrees S and 60 degrees N, and covers the time period of January 2000 to February 2007. The data from all three instruments are very consistent, with relative difference biases of less than 4% and relative difference standard deviations of 7%. Radiometric contributions by high ice clouds and by the Earth's surface affect the measurements in certain areas. The uncertainty due to clouds is estimated to be up to approximately 10%RH in areas with deep convection. The uncertainty associated with contamination from surface emission can exceed 10%RH in midlatitude winter, where the data therefore should be regarded with caution. Otherwise the surface influence appears negligible. The paper also discusses the UTH median climatology and seasonal cycle, which are found to be broadly consistent with UTH climatologies from other sensors. Finally, the paper presents an initial validation of the new data set against IR satellite data and radiosonde data. The observed biases of up to 9%RH (wet bias relative to HIRS) were found to be broadly consistent with expectations based on earlier studies. The observed standard deviations against all other data sets were below 6%RH. The UTH data are available to the scientific community on http://www.sat.ltu.se.
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| 14. |
- Buehler, Stefan, et al.
(författare)
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A simple method to relate microwave radiances to upper tropospheric humidity
- 2005
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 110:2, s. D02110-
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Tidskriftsartikel (refereegranskat)abstract
- A brightness temperature (BT) transformation method can be applied to microwave data to retrieve Jacobian weighted upper tropospheric relative humidity (UTH) in a broad layer centered roughly between 6 and 8 km altitude. The UTH bias is below 4% RH, and the relative UTH bias below 20%. The UTH standard deviation is between 2 and 6.5% RH in absolute numbers, or between 10 and 27% in relative numbers. The standard deviation is dominated by the regression noise, resulting from vertical structure not accounted for by the simple transformation relation. The UTH standard deviation due to radiometric noise alone has a relative standard deviation of approximately 7% for a radiometric noise level of 1 K. The retrieval performance was shown to be of almost constant quality for all viewing angles and latitudes, except for problems at high latitudes due to surface effects. A validation of AMSU UTH against radiosonde UTH shows reasonable agreement if known systematic differences between AMSU and radiosonde are taken into account. When the method is applied to supersaturation studies, regression noise and radiometric noise could lead to an apparent supersaturation even if there were no supersaturation. For a radiometer noise level of 1 K the drop-off slope of the apparent supersaturation is 0.17% RH−1, for a noise level of 2 K the slope is 0.12% RH−1. The main conclusion from this study is that the BT transformation method is very well suited for microwave data. Its particular strength is in climatological applications where the simplicity and the a priori independence are key advantages.
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| 15. |
- Buehler, Stefan, et al.
(författare)
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Radiative transfer calculations for a passive microwave satellite sensor : comparing a fast model and a line-by-line model
- 2006
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 111:20, s. 20304-
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Tidskriftsartikel (refereegranskat)abstract
- A comparison between the fast radiative transfer model Radiative Transfer for the TIROS Operational Vertical Sounder (RTTOV-7) and the physical radiative transfer model Atmospheric Radiative Transfer Simulator ( ARTS) was carried out. Radiances were simulated for the sounding channels of the Advanced Microwave Sounding Unit B (AMSU-B) for the whole globe for a single time of a single day ( 1 January 2000, 0000 UT). Temperature, pressure, and specific humidity profiles from the reanalysis data set ERA-40 of the European Centre for Medium-Range Weather Forecasts (ECMWF) were used as input for both models; geopotential height profiles were also used but only as input for ARTS. The simulations were made for two different surface emissivities, 0.60 and 0.95. The low surface emissivity case exhibits the larger radiance differences. Although the global values of the mean difference and standard deviation are small ( for example, the global mean difference for channel 18 is 0.014 K and the standard deviation is 0.232 K), the examination of the geographical distribution of the differences shows that large positive or negative values are observed over dry regions of high northern and southern latitudes and over dry elevated regions. The origin of these differences was found to be due to errors introduced by the transmittance parametrization used in RTTOV.
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| 16. |
- Buffam, Ishi, et al.
(författare)
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Landscape-scale variability of acidity and dissolved organic carbon during spring flood in a boreal stream network
- 2007
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 112
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Tidskriftsartikel (refereegranskat)abstract
- Acidity is well known to influence stream biota, but the less well-studied spatial and temporal distributions of acidity are likely to play a larger ecological role than average values. We present data on spatial variability of chemical parameters contributing to acidity during winter baseflow and spring flood periods in Krycklan, a fourth-order boreal stream network in northern Sweden. Fifteen stream sites were monitored in subcatchments spanning 3 orders of magnitude in size and representing a wide range of percent wetland. At baseflow, pH ranged from 3.9 to 6.5 at the different sites. Baseflow dissolved organic carbon (DOC) concentration varied by an order of magnitude and was positively correlated with subcatchment percent wetland, resulting in high spatial variability in dissociated organic acids (OA(-)). During spring flood, DOC and OA(-) increased in forested sites and decreased in wetland sites, resulting in reduced spatial variability in their concentrations. In contrast, base cations and strong acid anions diluted throughout the stream network, resulting in decreased acid neutralizing capacity (ANC) at all sites. The spatial variability of base cations increased slightly with high flow. As a result of the changes in OA(-) and ANC, pH dropped at all but the most acidic site, giving a slightly narrowed pH range during spring flood (4.2-6.1). The transition from winter to spring flood stream chemistry could largely be explained by: (1) a shift from mineral to upper riparian organic soil flow paths in forested catchments and (2) dilution of peat water with snowmelt in wetland catchments.
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| 17. |
- Chaufray, J. Y., et al.
(författare)
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Mars solar wind interaction : Formation of the Martian corona and atmospheric loss to space
- 2007
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 112:E9, s. E09009-
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Forskningsöversikt (refereegranskat)abstract
- A three- dimensional ( 3- D) atomic oxygen corona of Mars is computed for periods of low and high solar activities. The thermal atomic oxygen corona is derived from a collisionless Chamberlain approach, whereas the nonthermal atomic oxygen corona is derived from Monte Carlo simulations. The two main sources of hot exospheric oxygen atoms at Mars are the dissociative recombination of O-2(+) between 120 and 300 km and the sputtering of the Martian atmosphere by incident O+ pickup ions. The reimpacting and escaping fluxes of pickup ions are derived from a 3- D hybrid model describing the interaction of the solar wind with our computed Martian oxygen exosphere. In this work it is shown that the role of the sputtering crucially depends on an accurate description of the Martian corona as well as of its interaction with the solar wind. The sputtering contribution to the total oxygen escape is smaller by one order of magnitude than the contribution due to the dissociative recombination. The neutral escape is dominant at both solar activities ( 1 x 10(25) s(-1) for low solar activity and 4 x 10(25) s(-1) for high solar activity), and the ion escape flux is estimated to be equal to 2 x 10(23) s(-1) at low solar activity and to 3.4 x 10(24) s(-1) at high solar activity. This work illustrates one more time the strong dependency of these loss rates on solar conditions. It underlines the difficulty of extrapolating the present measured loss rates to the past solar conditions without a better theoretical and observational knowledge of this dependency.
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| 18. |
- Chen, Li-Jen, et al.
(författare)
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Evidence of an extended electron current sheet and its neighboring magnetic island during magnetotail reconnection
- 2008
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113:A12, s. A12213-
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Tidskriftsartikel (refereegranskat)abstract
- We have identified a spatially extended electron current sheet (ECS) and its adjacent magnetic island during a magnetotail reconnection event with no appreciable guide field. This finding is based on data from the four Cluster spacecraft and is enabled by detailed maps of electron distribution functions and DC electric fields within the diffusion region. The maps are developed using two-dimensional particle-in-cell simulations with a mass ratio m(i)/m(e) = 800. One spacecraft crossed the ECS earthward of the reconnection null and, together with the other three spacecraft, registered the following properties: (1) The ECS is colocated with a layer of bipolar electric fields normal to the ECS, pointing toward the ECS, and with a half width less than 8 electron skin depths. (2) In the inflow region up to the ECS and separatrices, electrons have a temperature anisotropy (Te-parallel to/Te-perpendicular to > 1), and the anisotropy increases toward the ECS. (3) Within about 1 ion skin depth (d(i)) above and below the ECS, the electron density decreases toward the ECS by a factor of 3-4, reaching a minimum at edges of the ECS, and has a local distinct maximum at the ECS center. (4) A di-scale magnetic island is attached to the ECS, separating it from another reconnection layer. Our simulations established that the electric field normal to the ECS is due to charge imbalance and is of the ECS scale, and ions exhibit electron-scale structures in response to this electric field.
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| 19. |
- Collier, A. B., et al.
(författare)
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Correlation between global lightning and whistlers observed at Tihany, Hungary
- 2009
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 114, s. A07210-
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Tidskriftsartikel (refereegranskat)abstract
- Although the generation and propagation mechanisms for whistlers are fairly well understood, the location and extent of the lightning source region for the whistlers observed at a given station are currently unknown. The correlation of whistler observations against global lightning data allows an estimate of the size and position of the source region. For whistlers detected at Tihany, Hungary, an area of positive correlation with radius of similar to 1000 km was found to be centered on the conjugate point. Although the maximal sample correlation coefficient was relatively low, r = 0.065, it has a high statistical significance, indicating that it is extremely improbable that the whistlers and lightning in this region are actually uncorrelated. Other smaller areas of positive correlation were found further afield in South America and the Maritime Continent. Lightning in the northern hemisphere displayed a negative correlation with whistlers at Tihany.
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| 20. |
- Cui, J., et al.
(författare)
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Diurnal variations of Titan's ionosphere
- 2009
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 114:6, s. A06310-
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Tidskriftsartikel (refereegranskat)abstract
- We present our analysis of the diurnal variations of Titan's ionosphere (between 1000 and 1300 km) based on a sample of Ion Neutral Mass Spectrometer (INMS) measurements in the Open Source Ion (OSI) mode obtained from eight close encounters of the Cassini spacecraft with Titan. Although there is an overall ion depletion well beyond the terminator, the ion content on Titan's nightside is still appreciable, with a density plateau of similar to 700 cm(-3) below similar to 1300 km. Such a plateau is a combined result of significant depletion of light ions and modest depletion of heavy ones on Titan's nightside. We propose that the distinctions between the diurnal variations of light and heavy ions are associated with their different chemical loss pathways, with the former primarily through "fast'' ion-neutral chemistry and the latter through "slow'' electron dissociative recombination. The strong correlation between the observed night-to-day ion density ratios and the associated ion lifetimes suggests a scenario in which the ions created on Titan's dayside may survive well to the nightside. The observed asymmetry between the dawn and dusk ion density profiles also supports such an interpretation. We construct a time-dependent ion chemistry model to investigate the effect of ion survival associated with solid body rotation alone as well as superrotating horizontal winds. For long-lived ions, the predicted diurnal variations have similar general characteristics to those observed. However, for short-lived ions, the model densities on the nightside are significantly lower than the observed values. This implies that electron precipitation from Saturn's magnetosphere may be an additional and important contributor to the densities of the short-lived ions observed on Titan's nightside.
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| 21. |
- Cully, C.M., et al.
(författare)
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Electrostatic structure around spacecraft in tenuous plasmas
- 2007
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 112:A9, s. A09211-
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Tidskriftsartikel (refereegranskat)abstract
- Most satellite-based in situ plasma experiments are affected in some manner by the electrostatic structure surrounding the spacecraft. In order to better understand this structure, we have developed a fully three-dimensional self-consistent model that can accept realistic spacecraft geometry, including both thin (similar to 10(-4) m) wires and long (similar to 10(2) m) booms, with open boundary conditions. The model uses an integral formulation incorporating boundary element, multigrid and fast multipole methods to overcome problems associated with the large range in scale sizes and inherently three-dimensional structure. By applying the model to the Cluster spacecraft, we show that the electric potential structure is dominated by the charge on the wire booms, with the spacecraft body contributing at small distances. Consequently, the potential near the EFW ( Electric Fields and Waves experiment) probes at the end of the wire booms is typically significantly above the true plasma potential. For the Cluster spacecraft, we show that this effect causes a 19% underestimation of the spacecraft potential and 13% underestimation of the ambient electric field. We further assess the electric field due to the sunward-oriented photoelectron cloud, showing that the cloud contributes little to the observed spurious sunward field in the EFW data.
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| 22. |
- Cumnock, Judy A.
(författare)
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High-latitude aurora during steady northward interplanetary magnetic field and changing IMF B-y
- 2005
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 110:A2, s. A02304-
-
Tidskriftsartikel (refereegranskat)abstract
- High-latitude transpolar arcs (TPAs) are often observed during northward interplanetary magnetic field (IMF); of these, theta aurora are seen when transpolar arcs move in the dawn or dusk direction across the entire polar region in response to IMF By changes. Periods of study were chosen when By changes sign during steady northward IMF in order to determine the influence of IMF B-x, B-y, the strength of the IMF, the solar wind, and Earth dipole tilt on the occurrence and motion of high-latitude TPAs forming theta aurora. For a 4.5-year period there are 55 events for which IMF B-z is northward for at least 2 hours before and at least 3 hours after a B-y sign change. Of these, 19 occurred when the Polar satellite was over the Northern Hemisphere for the duration of the event. We find that for northward IMF and a B-y sign change theta aurora are almost always formed in the Northern Hemisphere, regardless of B-x and dipole tilt. This implies that theta aurorae form simultaneously in both hemispheres. IMF B-y does not appear to influence the intensity and duration of the arc. Strongest UV emissions occur in the summer hemisphere. Evolution time has a fairly complex dependence on solar wind parameters.
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| 23. |
- De Conti, Alberto, et al.
(författare)
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A comparison of different approaches to simulate a nonlinear channel resistance in lightning return stroke models
- 2008
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113:D14, s. D14129-
-
Tidskriftsartikel (refereegranskat)abstract
- Different physical models that describe the time variation of the channel resistance are investigated in a lightning return stroke model. Such models consider one of the three following hypotheses: (1) the channel resistance decays exponentially with time, (2) the channel resistance decays with the radial expansion of the channel core, which is assumed to be described by the strong-shock approximation, or (3) the channel resistance varies with time according to three different arc resistance models (defined by Toepler, Barannik and Kushner et al.). Analyses illustrate the effect of a time-varying channel resistance on channel currents and corresponding electromagnetic fields. It is shown that the strong-shock approximation is able to predict typical features of experimentally observed lightning electromagnetic fields and return stroke speed profiles. It is also shown that results predicted by the strong-shock approximation can be qualitatively reproduced by either using simplified arc resistance equations (such as Toepler's and Barannik's ones) or considering an exponential decay of the channel resistance with attenuation constants linearly increasing with height.
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| 24. |
- Deng, X. H., et al.
(författare)
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Dynamics and waves near multiple magnetic null points in reconnection diffusion region
- 2009
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Ingår i: Journal of Geophysical Research. - : Blackwell Publishing. - 0148-0227 .- 2156-2202. ; 114:7
-
Tidskriftsartikel (refereegranskat)abstract
- Identifying the magnetic structure in the region where the magnetic field lines break and how reconnection happens is crucial to improving our understanding of three-dimensional reconnection. Here we show the in situ observation of magnetic null structures in the diffusion region, the dynamics, and the associated waves. Possible spiral null pair has been identified near the diffusion region. There is a close relation among the null points, the bipolar signature of the Z component of the magnetic field, and enhancement of the flux of energetic electrons up to 100 keV. Near the null structures, whistler-mode waves were identified by both the polarity and the power law of the spectrum of electric and magnetic fields. It is found that the angle between the fans of the nulls is quite close to the theoretically estimated maximum value of the group-velocity cone angle for the whistler wave regime of reconnection.
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| 25. |
- Divine, D. V., et al.
(författare)
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Deuterium excess record from a small Arctic ice cap
- 2008
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113:D19, s. D19104-
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we present a deuterium excess (d) record from an ice core drilled on a small ice cap in Svalbard in 1997. The core site is located at Lomonosovfonna at 1255 m asl, and the analyzed time series spans the period 1400-1990 A. D. The record shows pronounced multidecadal to centennial-scale variations coherent with sea surface temperature changes registered in the subtropical to southern middle-latitude North Atlantic during the instrumental period. We interpret the negative trend in the deuterium excess during the 1400s and 1500s as an indication of cooling in the North Atlantic associated with the onset of the Little Ice Age. Consistently positive anomalies of d after 1900, peaking at about 1950, correspond with well-documented contemporary warming. Yet the maximum values of deuterium excess during 1900-1990 are not as high as in the early part of the record (pre-1550). This suggests that the sea surface temperatures during this earlier period of time in the North Atlantic to the south of approximately 45 degrees N were at least comparable with those registered in the 20th century before the end of the 1980s. We examine the potential for a cold bias to exist in the deuterium excess record due to increased evaporation from the local colder sources of moisture having isotopically cold signature. It is argued that despite a recent oceanic warming, the contribution from this local moisture to the Lomonosovfonna precipitation budget is still insufficient to interfere with the isotopic signal from the primary moisture region in the midlatitude North Atlantic.
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| 26. |
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| 27. |
- Eastwood, J.P., et al.
(författare)
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Multi-point observations of the Hall electromagnetic field and secondary island formation during magnetic reconnection
- 2007
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 112:A6, s. A06235-
-
Tidskriftsartikel (refereegranskat)abstract
- A key feature of collisionless magnetic reconnection is the formation of Hall magnetic and electric field structure in the vicinity of the diffusion region. Here we present multi‐point Cluster observations of a reconnection event in the near‐Earth magnetotail where the diffusion region was nested by the Cluster spacecraft; we compare observations made simultaneously by different spacecraft on opposite sides of the magnetotail current sheet. This allows the spatial structure of both the electric and magnetic field to be probed. It is found that, close to the diffusion region, the magnetic field displays a symmetric quadrupole structure. The Hall electric field is symmetric, observed to be inwardly directed on both sides of the current sheet. It is large (∼40 mV m−1) on the earthward side of the diffusion region, but substantially weaker on the tailward side, suggesting a reduced reconnection rate reflected by a similar reduction in Ey. A small‐scale magnetic flux rope was observed in conjunction with these observations. This flux rope, observed very close to the reconnection site and entrained in the plasma flow, may correspond to what have been termed secondary islands in computer simulations. The core magnetic field inside the flux rope is enhanced by a factor of 3, even though the lobe guide field is negligible. Observations of the electric field inside the magnetic island show extremely strong (∼100 mV m−1) fields which may play a significant role in the particle dynamics during reconnection.
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| 28. |
- Edberg, Niklas, et al.
(författare)
-
Statistical analysis of the location of the Martian magnetic pileup boundary and bow shock and the influence of crustal magnetic fields
- 2008
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113:A8, s. A08206-
-
Tidskriftsartikel (refereegranskat)abstract
- We use the data set from the magnetometer and electron reflectometer instruments on board the Mars Global Surveyor spacecraft to show that the crustal magnetic fields of Mars affect the location of the magnetic pileup boundary (MPB) and bow shock (BS) globally. We search for crossings of the MPB and BS in the data that were observed over the first 16 months of the mission. To identify the influence of the crustal magnetic fields, all crossings are extrapolated to the terminator plane in order to remove the solar zenith angle (SZA) dependence, and to make it possible to compare crossings independently of location. The MPB crossings that were observed over regions on Mars, which contain strong crustal magnetic fields, are on average located further out than crossings observed over regions with weak crustal fields. This is shown in three separate longitude intervals. We also find that the dayside BS crossings observed over the southern hemisphere of Mars are on average located further out than the BS crossings observed over the northern hemisphere, possibly because of the influence of the crustal fields. We also study the magnetic field strength and its variation at the inside of the MPB and their dependence on the SZA and altitude. We find that the magnitude of the magnetic field in the MPB is closely linked to the altitude of the MPB, with the magnitude increasing as the MPB is observed closer to the planet.
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| 29. |
- Ejiri, M.K., et al.
(författare)
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Validation of the Improved Limb Atmospheric Spectrometer-II (ILAS-II) Version 1.4 nitrous oxide and methane profiles
- 2006
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 111:D22
-
Tidskriftsartikel (refereegranskat)abstract
- This study assesses polar stratospheric nitrous oxide (N(2)O) and methane (CH(4)) data from the Improved Limb Atmospheric Spectrometer-II (ILAS-II) on board the Advanced Earth Observing Satellite-II (ADEOS-II) retrieved by the Version 1.4 retrieval algorithm. The data were measured between January and October 2003. Vertical profiles of ILAS-II volume mixing ratio (VMR) data are compared with data from two balloon-borne instruments, the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS-B) and the MkIV instrument, as well as with two satellite sensors, the Odin Sub-Millimetre Radiometer (SMR) for N(2)O and the Halogen Occultation Experiment (HALOE) for CH(4). Relative percentage differences between the ILAS-II and balloon/satellite data and their median values are calculated in 10-ppbv-wide bins for N(2)O (from 0 to 400 ppbv) and in 0.05-ppmv-wide bins for CH(4) (from 0 to 2 ppmv) in order to assess systematic differences between the ILAS-II and balloon/satellite data. According to this study, the characteristics of the ILAS-II Version 1.4 N(2)O and CH(4) data differ between hemispheres. For ILAS-II N(2)O VMR larger than 250 ppbv, the ILAS-II N(2)O agrees with the balloon/SMR N(2)O within +/- 20% in both hemispheres. The ILAS-II N(2)O in the VMR range from 30-50 to 250 ppbv (corresponding to altitudes of similar to 17-30 km in the Northern Hemisphere (NH, mainly outside the polar vortex) and similar to 13-21 km in the Southern Hemisphere (SH, mainly inside the polar vortex) is smaller by similar to 10-30% than the balloon/SMR N(2)O. For ILAS-II N(2)O VMR smaller than 30 ppbv (>similar to 21 km) in the SH, the differences between the ILAS-II and SMR N(2)O are within +/- 10 ppbv. For ILAS-II CH(4) VMR larger than 1 ppmv (similar to 30 km) and the ILAS-II CH(4) for its VMR smaller than 1 ppmv (>similar to 25 km) only in the NH, are abnormally small compared to the balloon/satellite data.
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| 30. |
- Eliasson, Bengt, et al.
(författare)
-
Full-scale simulation study of the initial stage of ionospheric turbulence
- 2008
-
Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 113:0148-0227, s. A02305-1-A02305-9
-
Tidskriftsartikel (refereegranskat)abstract
- We present a full-scale simulation of the nonlinear interaction between an intense electromagnetic wave and the Earth's ionosphere, by means of a generalized Zakharov model. The radio wave propagates from the neutral atmosphere into the ionospheric plasma layer and reaches the turning points of the ordinary and extraordinary wave modes. At the turning point of the ordinary mode, a parametric instability takes place in which the electromagnetic wave decays into an electron plasma wave and an ion acoustic wave with a typical wavelength of one meter. This is followed by collapse and caviton formation and trapping of the intense electron plasma wave. The cavitons lead to an efficient excitation of slow X (or Z) waves that propagate further into the denser ionospheric layer at higher altitudes. We use a realistic ion (oxygen) mass, length scales, and other plasma parameters. This numerical study should be useful for understanding the nonlinear interaction between intense radio waves and the ionosphere.
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| 31. |
- Eliasson, Bengt, et al.
(författare)
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Penetration of ELF currents and electromagnetic fields into the Earth's equatorial ionosphere
- 2009
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 114, s. A10301-A10314
-
Tidskriftsartikel (refereegranskat)abstract
- The penetration of extremely low frequency (ELF) transient electromagnetic fields and associated currents in the Earth's equatorial E-region plasma is studied theoretically and numerically. In the low-frequency regime, the plasma dynamics of the E-region is characterized by helicon waves since the ions are viscously coupled to neutrals while the electrons remain mobile. For typical equatorial E-region parameters, the plasma is magnetically insulated from penetration of very long timescale magnetic fields by a thin diffusive sheath. Wave penetration driven by a vertically incident pulse localized in space and time leads to both vertical penetration and the triggering of ELF helicon/whistler waves that carry currents obliquely to the magnetic field lines. The study presented here may have relevance for ELF wave generation by lightning discharges and seismic activity and can lead to new concepts in ELF/ULF injection in the earth-ionosphere waveguide.
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| 32. |
- Eliasson, Bengt, et al.
(författare)
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Zakharov simulation study of spectral features of on-demand Langmuir turbulence in an inhomogeneous plasma
- 2008
-
Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 113:A2
-
Tidskriftsartikel (refereegranskat)abstract
- [1] We have performed a simulation study of Langmuir turbulence in the Earth's ionosphere by means of a Zakharov model with parameters relevant for the F layer. The model includes dissipative terms to model collisions and Landau damping of the electrons and ions, and a linear density profile, which models the ionospheric plasma inhomogeneity whose length scale is of the order 10-100 km. The injection of energy into the system is modeled by a constant source term in the Zakharov equation. Langmuir turbulence is excited "on-demand" in controlled ionospheric modification experiments where the energy is provided by an HF radio beam injected into the overhead ionospheric plasma. The ensuing turbulence can be studied with radars and in the form of secondary radiation recorded by ground-based receivers. We have analyzed spectral signatures of the turbulence for different sets of parameters and different altitudes relative to the turning point of the linear Langmuir mode where the Langmuir frequency equals the local plasma frequency. By a parametric analysis, we have derived a simple scaling law, which links the spectral width of the turbulent frequency spectrum to the physical parameters in the ionosphere. The scaling law provides a quantitative relation between the physical parameters ( temperatures, electron number density, ionospheric length scale, etc.) and the observed frequency spectrum. This law may be useful for interpreting experimental results.
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| 33. |
- Eriksson, S., et al.
(författare)
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Magnetic island formation between large-scale flow vortices at an undulating postnoon magnetopause for northward interplanetary magnetic field
- 2009
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 114:2, s. A00C17-
-
Tidskriftsartikel (refereegranskat)abstract
- Time History of Events and Macroscale Interactions during Substorms multispacecraft observations are presented for a similar to 2-h-long postnoon magnetopause event on 8 June 2007 that for the first time indicate that the trailing (sunward) edges of Kelvin-Helmholtz (KH) waves are commonly related to small-scale < 0.56 R-E magnetic islands or flux transfer events (FTE) during the growth phase of these surface waves. The FTEs typically show a characteristic bipolar B-N structure with enhanced total pressure at their center. Most of the small-scale FTEs are not related to any major plasma acceleration. TH-A observations of one small FTE at a transition from the low-latitude boundary layer (LLBL) into a magnetosheath plasma depletion layer were reconstructed using separate techniques that together confirm the presence of a magnetic island within the LLBL adjacent to the magnetopause. The island was associated with a small plasma vortex and both features appeared between two large-scale (similar to 1 R-E long and 2000 km wide) plasma vortices. We propose that the observed magnetic islands may have been generated from a time-varying reconnection process in a low ion plasma beta (beta(i) < 0.2) and low 8.3 degrees field shear environment at the sunward edge of the growing KH waves where the local magnetopause current sheet may be compressed by the converging flow of the large-scale plasma vortices as suggested by numerical simulations of the KH instability.
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| 34. |
- Forkman, Peter, 1959, et al.
(författare)
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Observing the Vertical Branch of the Mesospheric Circulation at lat N 60° Using Ground Based Measurements of CO and H2O
- 2005
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 110:D5, s. 1-11
-
Tidskriftsartikel (refereegranskat)abstract
- In this report daily ground-based measurements of vertical profiles of CO and H2O from 2002 are used to trace the vertical movement of air caused by the seasonally varying mean meridional circulation. In the spring (days 100-130) the estimated ascending velocity is found to vary with time and altitude from about between 250 m/day at 65 km altitude on day 100 to a maximum of similar to 450 m/day at 85-90 km altitude on day 130. In the late summer and fall the descending velocity goes from 0 (when the downwelling starts) to similar to 250 m/day at 75 km day 280. The mesospheric adiabatic temperature changes caused by the vertical movement of air are compared to mesopause temperature as estimated by the radiance of the Meinel band OH airglow.
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| 35. |
- Forsstrom, S., et al.
(författare)
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Elemental carbon distribution in Svalbard snow
- 2009
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 114
-
Tidskriftsartikel (refereegranskat)abstract
- The concentration of apparent elemental carbon (ECa, based on a thermal-optical method) in the snow was investigated in Svalbard (European Arctic) during spring 2007. The median ECa concentration of 81 samples was 4.1 mu g l(-1) and the values ranged from 0 to 80.8 mu g l(-1) of melt water. The median concentration is nearly an order of magnitude lower than the previously published data of equivalent black carbon (BCe, based on an optical method), obtained from Svalbard snow in the 1980s. A systematic regional difference was evident: ECa concentrations were higher in east Svalbard compared to west Svalbard. The observations of snow ECa cover spatial scales up to several hundred kilometers, which is comparable to the resolution of many climate models. Measurements of atmospheric carbonaceous aerosol (2002-2008) at Zeppelin station in Ny-Alesund, Svalbard, were divided to air mass sectors based on calculated back trajectories. The results show that air originating from the eastern sector contains more than two and half times higher levels of soot than air arriving from south to west. The observed east-west gradient of ECa concentrations in snow may be because of a combination of the atmospheric concentration gradient, the orographic effect of the archipelago, and the efficient scavenging of the carbonaceous particles through precipitation
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| 36. |
- Galli, André, et al.
(författare)
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The tailward flow of energetic neutral atoms observed at Mars
- 2008
-
Ingår i: Journal of Geophysical Research. - Washington : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 113
-
Tidskriftsartikel (refereegranskat)abstract
- The ASPERA-3 experiment on Mars Express provides the first measurements of energetic neutral atoms (ENAs) from Mars. These measurements are used to study the global structure of the interaction of the solar wind with the Martian atmosphere. In this study we describe the tailward ENA flow observed at the nightside of Mars. After characterizing energy spectra of hydrogen ENA signals, we present composite images of the ENA intensities and compare them to theoretical predictions (empirical and MHD models). We find that the tailward flow of hydrogen ENAs is mainly generated by shocked solar wind protons. Despite intensive search, no oxygen ENAs above the instrument threshold are detected. The results challenge existing plasma models and constrain the hydrogen exospheric densities and atmospheric hydrogen and oxygen loss rates at low solar activity.
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| 37. |
- Gao, Ge, 1972, et al.
(författare)
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Trend of estimated actual evapotranspiration over China during 1960-2002
- 2007
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 112:D11120
-
Tidskriftsartikel (refereegranskat)abstract
- In this study, the water balance methodology introduced by Thornthwaite and Mather (1955) is modified to estimate monthly actual evapotranspiration for 686 stations over China during 1960–2002. The modification is done by replacing the Thornthwaite potential evapotranspiration estimation with the Penman-Monteith method. Temporal trend and spatial distribution of the estimated annual actual evapotranspiration during the past 43 years are analyzed. The results show that (1) the actual evapotranspiration had a decreasing trend in most areas east of 100°E, and there was an increasing trend in the west and the north parts of northeast China; (2) the spatial distribution of the trend for the actual evapotranspiration is similar to that of the potential evapotranspiration in south China, while the trends are opposite in north China; (3) for most parts of China, the change in precipitation played a key role for the change of estimated actual evapotranspiration, while in southeast China, the change of potential evapotranspiration appeared to be the major factor; and (4) in general, the hydrological cycle was intensified in western China, whereas it was weakened from the Yellow River basin northward.
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| 38. |
- Garnier, P., et al.
(författare)
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The lower exosphere of Titan : Energetic neutral atoms absorption and imaging
- 2008
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113:A10, s. A10216-
-
Tidskriftsartikel (refereegranskat)abstract
- The Saturn magnetosphere interacts with the Titan atmosphere through various mechanisms. One of them leads, by charge exchange reactions between the energetic Saturnian ions and the exospheric neutrals of Titan, to the production of energetic neutral atoms (ENAs). The Ion and Neutral Camera (INCA), one of the three sensors that comprise the Magnetosphere Imaging Instrument (MIMI) on the Cassini/Huygens mission to Saturn and Titan, images the ENA emissions in the Saturnian magnetosphere. This study focuses on the ENA imaging of Titan (for 20-50 keV H ENAs), with the example of the Ta Titan flyby (26 October 2004): our objective is to understand the positioning of the ENA halo observed around Titan. Thus we investigate the main ENA loss mechanisms, such as the finite gyroradii effects for the parent ions, or the charge stripping with exospheric neutrals. We show that multiple stripping and charge exchange reactions have to be taken into account to understand the ENA dynamics. The use of an analytical approach, taking into account these reactions, combined with a reprocessing of the INCA data, allows us to reproduce the ENA images of the Ta flyby and indicates a lower limit for ENA emission around the exobase. However, the dynamics of energetic particle through the Titan atmosphere remains complex, with an inconsistency between the ENA imaging at low and high altitudes.
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| 39. |
- Gelencser, A., et al.
(författare)
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Source apportionment of PM2.5 organic aerosol over Europe: Primary/secondary, natural/anthropogenic, and fossil/biogenic origin
- 2007
-
Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 112:D23, s. 12-
-
Tidskriftsartikel (refereegranskat)abstract
- On the basis of a 2-year comprehensive data set obtained within the CARBOSOL project, seasonal source apportionment of PM2.5 aerosol is attempted for five rural/remote sites in Europe. The approach developed combines radiocarbon measurements with bulk measurements of organic carbon (OC), elemental carbon (EC), and two organic tracers ( levoglucosan and cellulose). Source types are lumped into primary emissions from fossil fuel combustion and biomass burning, bioaerosol, and secondary organic aerosol from precursors emitted by fossil and nonfossil sources. Bulk concentration ratios reported for these source types in the literature are used to estimate the source contributions which are constrained by measured radiocarbon concentrations. It has been found that while fossil-related sources predominate EC throughout the year at all sites, the sources of OC are primarily biogenic and markedly different between summer and winter. In winter biomass burning primary emission is the main source, with sizable additional contribution from fossil fuel combustion. In contrast, in summer secondary organic aerosol (SOA) from nonfossil sources becomes predominant (63-76% of TC), with some contribution of SOA from fossil fuel combustion. The results agree well with recent findings of other authors who established the predominance of biogenic SOA for rural sites in summer in Europe. An uncertainty analysis has been conducted, which shows that the main conclusions from this study are robust.
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| 40. |
- Golovchanskaya, I. V., et al.
(författare)
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Ballooning instability at the plasma sheet-lobe interface and its implications for polar arc formation
- 2006
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 111:A11, s. A11216-
-
Tidskriftsartikel (refereegranskat)abstract
- Huang et al. ( 1987, 1989) reported hot filaments of plasma sheet origin filling the magnetospheric lobes during northward interplanetary magnetic field ( IMF). On the other hand, cold plasma transients of presumably lobe origin are often observed in the plasma sheet. These features can be interpreted in terms of plasma exchange at the plasma sheet - lobe interface (PSLI) proceeding in a filamentary manner. We present a description of this process within ballooning destabilizing of the near-Earth curved segment of the PSLI. Although the basic ballooning instability condition is not typically met inside the plasma sheet, it is satisfied at this segment. The PSLI always separates the cold lobe population from the hot plasma sheet, thus providing a pressure gradient favorable for the instability; its near-Earth part has a nonnegligible magnetic curvature. A solution for the least stable ballooning harmonics is found, which satisfies the finite conductivity boundary condition in the ionosphere and the outgoing Alfven wave condition at the tailward end of the near-Earth curved segment of the plasma sheet boundary. We show that this part of the PSLI may be a generator region launching filamentation. The background convection is imposed on the ballooning motions. The large-scale convection associated with southward IMF B-z suppresses hot filament progression into the lobes, while promoting penetration of lobe transients into the plasma sheet. However, during northward IMF, the convection favors at certain magnetic local times the extension of plasma sheet filaments into the lobes and their subsequent protrusion toward noon. This process is signified in the ionosphere by the occurrence of nightside originating polar arcs. Several polar arc events are shown that develop from the nightside oval boundary into the polar cap on timescales of approximately 10 - 15 min, consistent with the growth rates of the studied instability.
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| 41. |
- Gong, D.-Y., et al.
(författare)
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Weekly cycle of aerosol-meteorology interaction over China
- 2007
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Ingår i: Journal of Geophysical Research - Atmospheres. - 0148-0227 .- 2156-2202. ; :D22202
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Tidskriftsartikel (refereegranskat)abstract
- Weekly cycles of the concentration of anthropogenic aerosols have been observed in many regions around the world. The phase and the magnitude of these cycles, however, vary greatly depending on region and season. In the present study the authors investigated important features of the weekly cycles of aerosol concentration and the covariations in meteorological conditions in major urban regions over east China, one of the most polluted areas in the world, in summertime during the period 2001–2005/2006. The PM10 (aerosol particulate matters of diameter < 10 μm) concentrations at 29 monitoring stations show significant weekly cycles with the largest values around midweek and smallest values in weekend. Accompanying the PM10 cycle, the meteorological variables also show notable and consistent weekly cycles. The wind speed in the lower troposphere is relatively small in the early part of the week and increases after about Wednesday. At the same time, the air temperature anomalies in low levels are positive and then become negative in the later part of the week. The authors hypothesize that the changes in the atmospheric circulation may be triggered by the accumulation of PM10 through diabatic heating of lower troposphere. During the early part of a week the anthropogenic aerosols are gradually accumulated in the lower troposphere. Around midweek, the accumulated aerosols could induce radiative heating, likely destabilizing the middle to lower troposphere and generating anomalously vertical air motion and thus resulting in stronger winds. The resulting circulation could promote ventilation to reduce aerosol concentrations in the boundary layer during the later part of the week. Corresponding to this cycle in anthropogenic aerosols the frequency of precipitation, particularly the light rain events, tends to be suppressed around midweek days through indirect aerosol effects. This is consistent with the observed anthropogenic weather cycles, i.e., more (less) solar radiation near surface, higher (lower) maximum temperature, larger (smaller) diurnal temperature range, and fewer (more) precipitation events in midweek days (weekend).
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| 42. |
- Goren, L, et al.
(författare)
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Ductile Deformation of Passive Margins : A New Mechanism for Subduction Initiation
- 2008
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113:B8, s. B08411-
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Tidskriftsartikel (refereegranskat)abstract
- [1] The onset of subduction at passive margins has been extensively investigated and debated. However, the force constellations and mechanisms that enable the development of a subduction system from a passive margin remain unclear. This study presents new insights into the conditions and processes by which lateral density differences between oceanic and continental lithospheres in passive margins may lead to initiation of a low-angle subduction system. The presented study consists of (1) analytical calculations of flow fields generated in passive margins, and (2) analogue experiments of mature passive margins performed in a centrifuge. The analytical formulation predicts temporal and spatial evolution of an interface between the oceanic and continental lithospheres, and demonstrates that oceanic underthrusting may occur by rotation of this interface. The analogue experiments show that incipient subduction may develop by ductile deformation within the lithosphere, involving no sliding along the ocean-continent interface, so that the frictional resistance between the plates need not be overcome. The force induced by the negative buoyancy of the oceanic plate with respect to the asthenosphere, is found to be in some cases irrelevant to subduction nucleation. Results of both the analogue experiments and the analytical calculation are compared to the south-east Australian passive margin, and show an excellent fit to its geometry and stress distribution. The proposed mechanism is also applied to the only two cases of subduction of Atlantic tectonic system, the Lesser-Antilles and the South Sandwich subduction systems.
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| 43. |
- Haefele, A., et al.
(författare)
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Validation of ground-based microwave radiometers at 22 GHz for stratospheric and mesospheric water vapor
- 2009
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 114:23
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Tidskriftsartikel (refereegranskat)abstract
- We present a detailed intercomparison of five ground-based 22 GHz microwave radiometers for stratospheric and mesospheric water vapor. Four of these instruments are members of the Network for the Detection of Atmospheric Composition Change (NDACC). The global measurements of middle atmospheric water vapor of the Microwave Limb Sounder (MLS) onboard the Aura satellite serve as reference and allow intercomparison of the ground-based systems that are located between 45 degrees S and 57 degrees N. The retrievals of water vapor profiles from the ground-based radiation measurements have been made consistent to a large extent: for the required temperature profiles, we used the global temperature measurements of MLS and we agreed on one common set of spectroscopic parameters. The agreement with the reference measurements is better than +/- 8% in the altitude range from 0.01 to 3 hPa. Strong correlation is found between the ground-based and the reference data in the mesosphere with respect to seasonal cycle and planetary waves. In the stratosphere the measurements are generally more noisy and become sensitive to instrumental instabilities toward lower levels (pressures greater than 3 hPa). We further present a compilation of a NDACC data set based on the retrieval parameters described herein but using a temperature climatology derived from the MLS record. This makes the ground-based measurements independent of additional information and allows extension of the data set for years in a homogeneous manner.
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| 44. |
- Hasegawa, H., et al.
(författare)
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Kelvin-Helmholtz waves at the Earth's magnetopause : Multiscale development and associated reconnection
- 2009
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 114:12, s. A12207-
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Tidskriftsartikel (refereegranskat)abstract
- We examine traversals on 20 November 2001 of the equatorial magnetopause boundary layer simultaneously at similar to 1500 magnetic local time (MLT) by the Geotail spacecraft and at similar to 1900 MLT by the Cluster spacecraft, which detected rolled-up MHD-scale vortices generated by the Kelvin-Helmholtz instability (KHI) under prolonged northward interplanetary magnetic field conditions. Our purpose is to address the excitation process of the KHI, MHD-scale and ion-scale structures of the vortices, and the formation mechanism of the low-latitude boundary layer (LLBL). The observed KH wavelength (>4 x 10(4) km) is considerably longer than predicted by the linear theory from the thickness (similar to 1000 km) of the dayside velocity shear layer. Our analyses suggest that the KHI excitation is facilitated by combined effects of the formation of the LLBL presumably through high-latitude magnetopause reconnection and compressional magnetosheath fluctuations on the dayside, and that breakup and/or coalescence of the vortices are beginning around 1900 MLT. Current layers of thickness a few times ion inertia length similar to 100 km and of magnetic shear similar to 60 degrees existed at the trailing edges of the vortices. Identified in one such current sheet were signatures of local reconnection: Alfvenic outflow jet within a bifurcated current sheet, nonzero magnetic field component normal to the sheet, and field-aligned beam of accelerated electrons. Because of its incipient nature, however, this reconnection process is unlikely to lead to the observed dusk-flank LLBL. It is thus inferred that the flank LLBL resulted from other mechanisms, namely, diffusion and/or remote reconnection unidentified by Cluster.
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| 45. |
- Hasegawa, H., et al.
(författare)
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Reconstruction of a bipolar magnetic signature in an earthward jet in the tail : Flux rope or 3D guide-field reconnection?
- 2007
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 112:A11, s. A11206-
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Tidskriftsartikel (refereegranskat)abstract
- Southward-then-northward magnetic perturbations are often seen in the tail plasma sheet, along with earthward jets, but the generation mechanism of such bipolar B-z ( magnetic flux rope created through multiple X-line reconnection, transient reconnection, or else) has been controversial. At similar to 2313 UT on 13 August 2002, Cluster encountered a bipolar B-z at the leading edge of an earthward jet, with one of the four spacecraft in the middle of the current sheet. Application to this bipolar signature of Grad-Shafranov ( GS) reconstruction, the technique for recovery of two-dimensional ( 2D) magnetohydrostatic structures, suggests that a flux rope with diameter of similar to 2 R-E was embedded in the jet. To investigate the validity of the GS results, the technique is applied to synthetic data from a three-dimensional ( 3D) MHD simulation, in which a bipolar B-z can be produced through localized ( 3D) reconnection in the presence of guide field B-y ( Shirataka et al., 2006) without invoking multiple X-lines. A flux rope-type structure, which does not in fact exist in the simulation, is reconstructed but with a shape elongated in the jet direction. Unambiguous identification of a mechanism that leads to an observed bipolar B-z thus seems difficult based on the topological property in the GS maps. We however infer that a flux rope was responsible for the bipolar pulse in this particular Cluster event, because the recovered magnetic structure is roughly circular, suggesting a relaxed and minimum energy state. Our results also indicate that one has to be cautious about interpretation of some ( e. g., force-free, or magnetohydrostatic) model-based results.
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| 46. |
- Hedfors, Jim, et al.
(författare)
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Clouds and Be-7: Perusing connections between cosmic rays and climate
- 2006
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 111:D2, s. D02208-
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Tidskriftsartikel (refereegranskat)abstract
- [1] Time series data on Be-7, precipitation, temperature, and satellite imagery of cloud cover over Scandinavia, together with cosmic ray and sunspot activity, were used to elucidate the relationship between cosmic rays and clouds, and ultimately climate change. The results indicate a coherent negative correlation between total cloud cover and Be-7 on intraseasonal, seasonal, and decadal scales. Although the reasons behind this correlation are unclear, a full-scale implication of this feature is in the possible use of Be-7 and Be-10 records for proxy paleo-reconstruction of total cloud cover. This is a strongly needed, but generally difficult to quantify parameter in climate models.
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| 47. |
- Hieronymus, C. F., et al.
(författare)
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A dynamical model for generating Eurasian lithospheric stress and strain rate fields : Effect of rheology and cratons
- 2008
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113:B7, s. B07404-
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Tidskriftsartikel (refereegranskat)abstract
- For most continents, stress models driven by plate boundary forces have successfully reproduced the main characteristics of the stress field. However, Eurasia has remained a challenge due to its large areas of intraplate deformation. We present a set of three-dimensional models of the upper mantle lithosphere system for a simplified geometry of the Eurasian plate where we try to match the first-order characteristics of the stress and strain rate fields simultaneously. For typical elastic, viscous, or plastic rheologies, high stress levels are required in order to produce realistic convergence rates between India and Asia. Our models show robustly that such stresses are transmitted throughout most of the plate, dominating locally generated stresses even in distal regions such as Europe in a manner that is not compatible with observations. Cratons with roots that extend deep into the mantle are unable to provide a significant stress-shielding effect unless the viscosity contrast between the asthenosphere and the underlying mantle is around 100 or greater. A damage rheology for the lithosphere with history-dependent behavior and material softening by a viscosity reduction of several orders of magnitude is shown to eliminate this conundrum. Continental convergence at high velocity but low stress is facilitated by the formation of long-lived shear zones similar to those observed north of the Himalayas. The low stress associated with the collision, together with the decoupling effect of the shear zones, causes the distal stress field in Europe to be controlled by the effects of the neighboring boundaries in agreement with observations.
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| 48. |
- Hobara, Y., et al.
(författare)
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Cluster observations of electrostatic solitary waves near the Earth's bow shock
- 2008
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 113:A5, s. A05211-
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Tidskriftsartikel (refereegranskat)abstract
- Using a period of internal burst mode data from the Cluster Electric Field and Wave instrument a number of electrostatic solitary structures have been identified in the foot region of Earth's quasi-perpendicular bow shock. The four individual probe potential measurements are utilized to investigate the fundamental characteristics of the solitary wave structures such as wave propagation vector, propagation velocity, scale-size and potential amplitude. Two classes of waves are observed. Bipolar solitary waves typically propagate in the solar wind direction toward the shock but at a significant angle from the ambient magnetic field. Unipolar/tripolar solitary waves tend to propagate along the ambient magnetic field. The wave amplitude-scale size relation is similar to that obtained for similar structures observed in the auroral zone. The structures lie in the theoretically allowed region in width-amplitude space to be consistent with the BGK ion holes. Using a period of internal burst mode data from the Cluster Electric Field and Wave instrument a number of electrostatic solitary structures have been identified in the foot region of Earth's quasi-perpendicular bow shock. The four individual probe potential measurements are utilized to investigate the fundamental characteristics of the solitary wave structures such as wave propagation vector, propagation velocity, scale-size and potential amplitude. Two classes of waves are observed. Bipolar solitary waves typically propagate in the solar wind direction toward the shock but at a significant angle to the ambient magnetic field in contrast to most previous studies which assume parallel propagation to the ambient magnetic field. In contrast, unipolar/tripolar solitary waves tend to propagate along the ambient magnetic field. The wave amplitude-scale size relation is similar to that obtained for structures observed in the auroral zone. The structures lie in the theoretically allowed region in width-amplitude space to be consistent with the BGK (Bernstein-Greene-Kruskal) ion holes. The two classes of observed solitary waves may greatly influence the ambient plasma dynamics around the shock. The bipolar solitary waves do not exhibit a large net potential difference but may still play an important role in plasma thermalisation by particle scattering. Unipolar/tripolar solitary waves exhibit a remarkable net potential difference that may be responsible for the plasma energisation along the ambient magnetic field.
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| 49. |
- Hong, Gang, et al.
(författare)
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Scattering database in the millimeter and submillimeter wave range of 100-1000 GHz for nonspherical ice particles
- 2009
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 114, s. D06201-
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Tidskriftsartikel (refereegranskat)abstract
- The inference of ice cloud properties from spaceborne sensors is sensitive to the retrieval algorithms and satellite sensors used. To approach a better understanding of ice cloud properties, it is necessary to combine satellite measurements from multiple platforms and sensors operating in visible, infrared, and millimeter and submillimeter-wave regions of the electromagnetic spectrum. The single-scattering properties of ice particles with consistent ice particle models are the basis for estimating the optical and microphysical properties of ice clouds from multiple satellite sensors. In this study, the single-scattering properties (extinction efficiency, absorption efficiency, single-scattering albedo, asymmetry factor, and scattering phase matrix) of nonspherical ice particles, assumed to be hexagonal solid and hollow columns, hexagonal plates, 3D bullet rosettes, aggregates, and droxtals, are computed from the discrete dipole approximation method for 21 millimeter and submillimeter-wave frequencies ranging from 100 to 1000 GHz. A database of the single-scattering properties of nonspherical ice particles are developed for 38 particle sizes ranging from 2 to 2000 μm in terms of particle maximum dimension. The bulk scattering properties of ice clouds consisting of various ice particles, which are the fundamental to the radiative transfer in ice clouds, are developed on the basis of a set of 1119 particle size distributions obtained from various field campaigns.
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| 50. |
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