| 1. |
- Aldahan, Ala, et al.
(författare)
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Atmospheric impact on beryllium isotopes as solar activity proxy
- 2008
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 35:L21812
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Tidskriftsartikel (refereegranskat)abstract
- Reconstructing solar activity variability beyond the time scale of actual measurements provides invaluable data for modeling of past and future climate change. The 10 Be isotope has been a primary proxy archive of past solar activity and cosmic ray intensity, particularly for the last millennium. There is, however, a lack of direct high-resolution atmospheric time series on 10 Be that enable estimating atmospheric modulation on the production signal. Here we report quasi-weekly data on 10 Be and 7 Be isotopes covering the periods 1983-2000 and 1975-2006 respectively, that show, for the first time, coherent variations reflecting both atmospheric and production effects. Our data indicate intrusion of stratosphere/upper troposphere air masses that can modulate the isotopes production signal, and may induce relative peaks in the natural 10 Be archives (i.e., ice and sediment). The atmospheric impact on the Be-isotopes can disturb the production signals and consequently the estimate of past solar activity magnitude. Citation: Aldahan, A., J. Hedfors, G. Possnert, A. Kulan, A.-M. Berggren, and C. Soderstrom (2008), Atmospheric impact on beryllium isotopes as solar activity proxy, Geophys. Res. Lett., 35, L21812, doi: 10.1029/2008GL035189.
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| 2. |
- Aldahan, Ala, et al.
(författare)
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Distribution of I-127 and I-129 in preciptitation at high European latitudes
- 2009
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 36, s. L11805-
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Tidskriftsartikel (refereegranskat)abstract
- We here present the most extensive data set on the distribution of I-127 and I-129 in precipitation (rain and snow) covering the period 2000-2006 and European latitudes 55 degrees N-68 degrees N. Our results indicate a wide variation in the concentrations and fluxes of the two isotopes associated with generally higher values at near coastal sites compared to the inland ones. Total wet-related annual deposition of I-127 and I-129 on Sweden and Denmark is estimated at about 1.2 x 10(9) g and 60 g respectively. The average annual I-129 wet deposition accounts for <1% and <0.05% of the total annual gaseous and liquid, respectively, discharges from the Sellafiled and La Hague Facilities. The I-127 annual wet deposition represents < 1% of the estimated global oceanic iodine flux. Air mass trajectories suggest that events of enhanced I-129 in precipitation are closely related to southwesterly weather fronts from regions of elevated concentrations.
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| 3. |
- Backrud, Marie, et al.
(författare)
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Interferometric Identification of Ion Acoustic Broadband Waves in the Auroral Region : CLUSTER Observations
- 2005
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 32:21
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Tidskriftsartikel (refereegranskat)abstract
- [1] We determine the phase velocity and k vector for parallel and oblique broadband extremely low frequency, ELF, waves on nightside auroral magnetic field lines at altitudes around 4.6 RE. We use internal burst mode data from the EFW electric field and wave instrument onboard the Cluster spacecraft to retrieve phase differences between the four probes of the instrument. The retrieved characteristic phase velocity is of the order of the ion acoustic speed and larger than the thermal velocity of the protons. The typical wavelength obtained from interferometry is around the proton gyro radius and always larger than the Debye length. We find that in regions with essentially no suprathermal electrons above a few tens of eV the observed broadband waves above the proton gyro frequency are consistent with upgoing ion acoustic and oblique ion acoustic waves.
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| 4. |
- Becerra Garcia, Marley, et al.
(författare)
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On the velocity of positive connecting leaders associated with negative downward lightning leaders
- 2008
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 35:2
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Tidskriftsartikel (refereegranskat)abstract
- A self-consistent leader propagation model is used to estimate the velocity of upward connecting positive leaders initiated from a tall tower under the influence of downward negative lightning leaders. The propagation of upward connecting leaders has been found to be influenced not only by the average velocity of the downward leader but also by the prospective return stroke current, the lateral position of the downward leader channel as well as by the ambient electric field. This result show that the velocity and propagation time of upward connecting positive leaders change from flash to flash due to the variations in these parameters.
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| 5. |
- Berggren, Ann-Marie, 1969-, et al.
(författare)
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A 600-year annual 10Be record from the NGRIP ice core, Greenland
- 2009
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 36, s. L11801-
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Tidskriftsartikel (refereegranskat)abstract
- Despite the extensive use of 10Be as the most significant information source on past solar activity, there has been only one record (Dye-3, Greenland) providing annual resolution over several centuries. Here we report a new annual resolution 10Be record spanning the period 1389-1994 AD, measured in an ice core from the NGRIP site in Greenland. NGRIP and Dye-3 10Be exhibits similar long-term variability, although occasional short term differences between the two sites indicate that at least two high resolution 10Be records are needed to assess local variations and to confidently reconstruct past solar activity. A comparison with sunspot and neutron records confirms that ice core 10Be reflects solar Schwabe cycle variations, and continued 10Be variability suggests cyclic solar activity throughout the Maunder and Spörer grand solar activity minima. Recent 10Be values are low; however, they do not indicate unusually high recent solar activity compared to the last 600 years.
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| 6. |
- Berthet, G., et al.
(författare)
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Nighttime chlorine monoxide observations by the Odin satellite and implications for the ClO/Cl2O2 equilibrium
- 2005
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Ingår i: Geophysical Research Letters. - 1944-8007 .- 0094-8276. ; 32:11, s. 1-5
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Tidskriftsartikel (refereegranskat)abstract
- We use measurements of chlorine monoxide (ClO) by the SMR instrument onboard the Odin satellite to study the nighttime thermal equilibrium between ClO and its dimer Cl2O2. Observations performed in the polar vortex during the 2002-2003 Arctic winter showed enhanced amounts of nighttime ClO over a wide range of stratospheric temperatures (185
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| 7. |
- Bertucci, C., et al.
(författare)
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Structure of Titan's mid-range magnetic tail : Cassini magnetometer observations during the T9 flyby
- 2007
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 34:24, s. L24S02-
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Tidskriftsartikel (refereegranskat)abstract
- We analyze the magnetic structure of Titan's mid-range magnetic tail (5-6 Titan radii downstream from the moon) during Cassini's T9 flyby. Cassini magnetometer (MAG) measurements reveal a well-defined, induced magnetic tail consisting of two lobes and a distinct central current sheet. MAG observations also indicate that Saturn's background magnetic field is close to the moon's orbital plane and that the magnetospheric flow has a significant component in the Saturn-Titan direction. The analysis of MAG data in a coordinate system based on the orientation of the background magnetic field and an estimation of the incoming flow direction suggests that Titan's magnetic tail is extremely asymmetric. An important source of these asymmetries is the connection of the inbound tail lobe and the outbound tail lobe to the dayside and nightside hemispheres of Titan, respectively. Another source could be the perturbations generated by changes in the upstream conditions.
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| 8. |
- Brandefelt, Jenny, et al.
(författare)
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Equilibration and variability in a Last Glacial Maximum climate simulation with CCSM3
- 2009
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 36
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Tidskriftsartikel (refereegranskat)abstract
- We present results from a 1862 year simulation of the Last Glacial Maximum (LGM) with the Community Climate System Model version 3 (CCSM3). A quasi steady state is reached after approximately 100 years of integration when the initial cooling trend in the annual global mean atmospheric surface temperature (T-s) levels off and even reverses. After another 150 years of integration the climate continues to cool and reaches a new equilibrium after a total of 800 years of integration. The cause of the continued adjustment of the climate to LGM forcing and boundary conditions is found in the abyssal ocean which is cooling at a rate decreasing from 0.15 degrees C per century until the new equilibrium is reached. The new equilibrium differs substantially from the first quasi steady state with 1.1 degrees C colder global mean Ts and regional differences of 5-15 degrees C in the North Atlantic region and a 30% reduction of the strength of the Atlantic meridional overturning circulation (AMOC). Further, the variability in the global mean Ts is significantly larger in the new equilibrium. This variability is associated with coupled ocean-atmosphere-sea ice variations in the North Atlantic region. Citation: Brandefelt, J., and B. L. Otto-Bliesner (2009), Equilibration and variability in a Last Glacial Maximum climate simulation with CCSM3, Geophys. Res. Lett., 36, L19712, doi: 10.1029/2009GL040364.
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| 9. |
- Brethouwer, Geert, et al.
(författare)
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Passive scalars in stratified turbulence
- 2008
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 35:6
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Tidskriftsartikel (refereegranskat)abstract
- Statistics of a passive scalar (or tracer) with a horizontal mean gradient in randomly forced and strongly stratified turbulence are investigated by numerical simulations. We observe that horizontal isotropy of the passive scalar spectrum is satisfied in the inertial range. The spectrum has the form E-theta(k(h)) = C-theta epsilon theta epsilon(-1/3)(K) k(h)(-5/3), where epsilon(theta), epsilon(K) are the dissipation of scalar variance and kinetic energy respectively, and C-theta similar or equal to 0.5 is a constant. This spectrum is consistent with atmospheric measurements in the mesoscale range with wavelengths 1 - 500 km. We also calculate the fourth-order passive scalar structure function and show that intermittency effects are present in stratified turbulence.
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| 10. |
- Buehler, Stefan, et al.
(författare)
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Scan asymmetries in AMSU-B data
- 2005
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 32:24, s. L2481-
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Tidskriftsartikel (refereegranskat)abstract
- A simple method of averaging measurements for different scan positions was used to quantify scan asymmetries in AMSU-B brightness temperatures for the sensors on the satellites NOAA 15, 16, and 17. The method works particularly well for the sounding channels 18 to 20. The asymmetries are small in most cases. In particular, asymmetries for Channel 18 are below 1.90, −0.53, and 0.49 K for NOAA 15, 16, and 17, respectively. On the other hand, it was found that the instrument on NOAA 15 has significant asymmetries for Channels 19 and 20, which seem to be related to the known radio frequency interference problem for this instrument. The use of the appropriate set of interference correction coefficients significantly reduces the asymmetry.
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