| 1. |
- Al-Wardy, W, et al.
(author)
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Effective stress law for the permeability of clay-rich sandstones
- 2004
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 109:4, s. 1-10
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Journal article (peer-reviewed)abstract
- Two models of clay-rich sandstones are analyzed to explain the relative sensitivity ofpermeability to pore pressure and confining pressure. In one model the clay lines the entire pore wall in a layer of uniform thickness, and in the second model the clay is distributed in the form of particles that are only weakly coupled to the pore walls. Equations of elasticity and fluid flow are solved for both models, giving expressions for theeffective stress coefficients in terms of clay content and the elastic moduli of the rock andclay. Both models predict that the permeability will be much more sensitive to changes in pore pressure than to changes in confining pressure. The clay particle model gives somewhat better agreement with data from the literature and with new data on a Staintonsandstone having a solid volume fraction of 8% clay.
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| 2. |
- Andersson, L, et al.
(author)
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Electron signatures and Alfven waves
- 2002
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 107:A9
-
Journal article (peer-reviewed)abstract
- [1] We identify two distinct electron populations associated with Alfven waves in the Freja data set using the high time resolution state of the art electron detector. One of the populations, detected together with an Alfven wave, is field-aligned and can be seen as trapped within the wave. The other electron population is detected before the wave and consists of electrons which have left the wave at a point with a velocity higher than the local Alfven speed. In the paper, the electrons leaving wave are modeled for different density profiles and are compared with the observed data. Depending on the density profile, the model can produce the same energy-time and pitch angle-time dispersion that is observed in the Freja data. The conclusion of the paper is that the Alfven wave can explain the observed particle signatures. It is shown that the Alfven wave acceleration can create electron signatures similar to inverted-V structures. The density distribution along a flux tube has an important role in the type of particle signatures that can be detected at low altitudes.
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| 3. |
- Ayarza, P, et al.
(author)
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Integrated geological and geophysical studies in the SG4 borehole area, Tagil Volcanic Arc, Middle Urals: Location of seismic reflectors and source of the reflectivity
- 2000
-
In: Journal of Geophysical Research. - : AMER GEOPHYSICAL UNION. - 0148-0227 .- 2156-2202. ; 105:B9, s. 21333-21352
-
Journal article (peer-reviewed)abstract
- Near-vertical incidence reflection seismic data acquired in the Tagil Volcanic Arc (Middle Urals) show the upper crust to be highly reflective. Two intersecting seismic lines located near the ongoing ∼5400 m deep SG4 borehole show that the main reflectivity strikes approximately N-S and dips ∼35°–55° to the east. Prominent reflections intercept the borehole at ∼1000, ∼1500, 2800–2900, ∼3400, and between ∼4000 and 5400 m, which correspond to intervals of low velocity/low density/low resistivity. The surface projections of these reflections lie parallel to the strike of magnetic anomaly trends. Multioffset vertical seismic profile (VSP) data acquired in the SG4 borehole show a seismic response dominated by P to S reflected converted waves from the moderately east dipping reflectivity and from a set of very steep east dipping reflectors not imaged by the surface data. Modeling of the VSP data constrains the depth at which reflectors intercept the borehole and suggests that the P to S conversions are best explained by low-velocity porous intervals rather than higher-velocity mafic material. The most prominent east dipping reflection on the surface seismic data is only imaged on VSP shots that sample the crust closer to the E-W seismic line. This discrepancy between the VSP and the surface seismic data is attributed to rapid lateral changes in the physical properties of the reflector. Surface and borehole data suggest that the low-velocity/low-density/low-resistivity intervals are the most important source of reflectivity in the SG4 borehole area, although lithological contrasts may also play a role. Drill cores from the these zones contain hydrothermal alteration minerals indicating interaction with fluids. Tectonic criteria suggest that they might represent imbricated fracture zones often bounding different lithologies and/or intrusions. Some of them might also represent high-porosity lava flows or pyroclastic units, common in island arc environments.
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| 4. |
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| 5. |
- Buehler, Stefan, et al.
(author)
-
Comparison of microwave satellite humidity data and radiosonde profiles : a case study
- 2004
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 109:13, s. S13103-
-
Journal article (peer-reviewed)abstract
- This article documents a case study comparing radiosonde humidity data to Advanced Microwave Sounding Unit (AMSU) satellite humidity data. The study had two goals: first, to develop a robust method for such a comparison, and second, to check the quality and mutual consistency of radiosonde data, radiative transfer model, and AMSU data. The radiosonde data used are Vaisala RS80 data from the station Lindenberg of the German Weather Service (DWD), which have been subject to several corrections compared to the standard data processing. The radiative transfer model is the Atmospheric Radiative Transfer Simulator ( ARTS), and the AMSU data are those of the satellites NOAA 15 and 16 for the time periods 2001 and 2002. The comparison was done in radiance space, using a radiative transfer model to simulate AMSU radiances from the radiosonde data. The overall agreement is very good, with radiance biases below 1.5 K and standard deviations below 2 K. The main source of "noise'' in the comparison is atmospheric inhomogeneity on the 10-km scale. While the radiosonde correction performed at Lindenberg significantly reduces the bias between simulated and measured AMSU radiance, there still remains a slope in the radiance difference. Possible reasons for this were investigated. Most likely, the radiosondes underestimate the relative humidity under extremely dry conditions, showing 0 % RH when the true value is 2 - 4 % RH.
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| 6. |
- Cumnock, J. A., et al.
(author)
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Interplanetary magnetic field control of theta aurora development
- 2002
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In: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 107:A7
-
Journal article (peer-reviewed)abstract
- [1] We ascertain the influence of the B-y component of the interplanetary magnetic field (IMF) on theta aurora evolution. During most cases where a transpolar arc is observed to move across the polar region, and form a theta aurora, there are brief (minutes) southward excursions of IMF B-z, however northward IMF is required prior to theta aurora formation. Observations show that theta aurora can form during strictly northward IMF with its motion consistent with a change in sign of IMF B-y. It is important to note that since transpolar arcs can persist for 20-30 min after the IMF turns southward, errors will occur in assigning instantaneous IMF conditions to snapshots'' of particular auroral patterns. We consider the entire evolution of the theta aurora and the changing IMF conditions. The influence of IMF B-y is best illustrated by examples which occur during steady northward IMF as compared to times when the IMF is northward on average. We show examples, provided by the Polar UV imager, when the IMF is steady northward. For one case, DMSP F13 and F14 provide in situ measurements of precipitating particles, ionospheric plasma flows and ion density. This unique data set enables us to analyze in detail the evolution of a theta aurora, in one case crossing the entire polar region. No sign change in B-z is needed for theta aurora formation.
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| 7. |
- Dieckmann, Mark E, 1969-, et al.
(author)
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Particle-in-cell simulations of electron acceleration by a simple capacitative antenna in collisionless plasma
- 2004
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 109:A12
-
Journal article (peer-reviewed)abstract
- We examine the electron acceleration by a localized electrostatic potential oscillating at high frequencies by means of particle-in-cell (PIC) simulations, in which we apply oscillating electric fields to two neighboring simulation cells. We derive an analytic model for the direct electron heating by the externally driven antenna electric field, and we confirm that it approximates well the electron heating obtained in the simulations. In the simulations, transient waves accelerate electrons in a sheath surrounding the antenna. This increases the Larmor radii of the electrons close to the antenna, and more electrons can reach the antenna location to interact with the externally driven fields. The resulting hot electron sheath is dense enough to support strong waves that produce high-energy sounder-accelerated electrons (SAEs) by their nonlinear interaction with the ambient electrons. By increasing the emission amplitudes in our simulations to values that are representative for the ones of the sounder on board the OEDIPUS C (OC) satellites, we obtain electron acceleration into the energy range which is comparable to the 20 keV energies of the SAE observed by the OC mission. The emission also triggers stable electrostatic waves oscillating at frequencies close to the first harmonic of the electron cyclotron frequency. We find this to be an encouraging first step of examining SAE generation with kinetic numerical simulation codes.
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| 8. |
- Dieckmann, Mark E, 1969-, et al.
(author)
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Plasma sounding at the upper hybrid frequency
- 2000
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 105:A6, s. 13103-13117
-
Journal article (peer-reviewed)abstract
- A sounder measures the density of plasmas in various parts of the solar system. The sounder emits wave pulses into the ambient plasma and listens to the response. Intensity peaks in the wave response are typically related to two mechanisms. One is provided by waves that are reflected off plasma inhomogeneities and propagate back to the emitting antenna, where they are then detected. The second is provided by waves propagating with the same group velocity as that of the receiving antenna. In the second case the waves stay close to the antenna and thus yield a long-lasting response. Response peaks to sounding at the upper hybrid (UH) frequency have, in most cases, been related to reflected waves. In this work we examine if accompanying waves can give rise to the UH response peak. We examine quantitatively how the plasma response to sounding at the UH frequency depends on the plasma density, on the electron temperature, and on the emission amplitude. For the first two parameters this is done by solving the linear dispersion relation. The well-known property of the UH waves to change from having a zero group velocity to propagating waves, depending on how the electron density compares to the electron cyclotron frequency, is applied to Alouette sounder data. It is discussed how the change in the group velocity may affect the spectral profile of the UH resonance. We present results from numerical particle in cell (PIC) simulations which show that in the case of nonpropagating UH waves, energy can be coupled into the plasma even though the vanishing group velocity of the UH waves should not allow this. The PIC simulations and sounder data from the Alouette mission show that in the case of propagating UH waves the response duration to sounding may be used to determine the electron temperature. Emission amplitudes that are typical for plasma sounders are also shown to suppress the generation of certain electron cyclotron harmonic waves.
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| 9. |
- El Amraoui, L., et al.
(author)
-
Assimilation of Odin/SMR and O3 and N2O Measurements in a Three-dimensional Chemistry Transport Model
- 2004
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 109:D22, s. 1-9
-
Journal article (peer-reviewed)abstract
- A method for assimilating observations of long-lived species such as ozone (O-3) and nitrous oxide (N2O) in a three-dimensional chemistry transport model (3D-CTM) is described. The model is forced by the temperature and wind analyses from the European Centre for Medium-Range Weather Forecasts (ECMWF). The O-3 and N2O fields used in this study are obtained from the Sub-Millimeter Radiometer (SMR) aboard the Odin satellite. The assimilation technique used is the sequential statistical interpolation approach. The parametrization of the error covariance matrix of the model forecast field is described. A sensitivity study of the system parameters is done in terms of the OMF (observation minus forecast) vector also called "innovation'' vector and in terms of the chi(2) (chi-square) test. The effect of the correlation distances is critical for the assimilated field. The RMS ( root mean square) of the OMF for the correlation distances is minimal for values of 1500 km in the meridional direction and 500 km in the zonal direction for both O-3 and N2O. The treatment of the meridional distance as a function of latitude does not reveal an important improvement. The chi(2) diagnostic shows that the asymptotic value of the model error ( the model error of saturation) is optimal for the value of 12.5% for O-3 and 18% for N2O. We demonstrate the applicability of the developed assimilation method for the Odin/SMR data. We also present first results of the assimilation of Odin/SMR ozone and nitrous oxide for the period from 22 December 2001 to 17 January 2002.
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| 10. |
- Emde, Claudia, et al.
(author)
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A polarized discrete ordinate scattering model for simulations of limb and nadir long-wave measurements in 1-D/3-D spherical atmospheres
- 2004
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 109:24, s. 24207-
-
Journal article (peer-reviewed)abstract
- This article describes one of the scattering algorithms of the three-dimensional polarized radiative transfer model ARTS (Atmospheric Radiative Transfer Simulator) which has been implemented to study for example the influence of cirrus clouds on microwave limb sounding. The model uses the DOIT (Discrete Ordinate Iterative) method to solve the vector radiative transfer equation. The implementation of a discrete ordinate method is challenging due to the spherical geometry of the model atmosphere which is required for the simulation of limb radiances. The involved numerical issues, which are grid optimization and interpolation methods, are discussed in this paper. Scattering simulations are presented for limb- and down-looking geometries, for one-dimensional and three-dimensional spherical atmospheres. They show the impact of cloud particle size, shape, and orientation on the brightness temperatures and on the polarization of microwave radiation in the atmosphere. The cloud effect is much larger for limb radiances than for nadir radiances. Particle size is a very important parameter in all simulations. The polarization signal is negligible for simulations with completely randomly oriented particles, whereas for horizontally aligned particles with random azimuthal orientation the polarization signal is significant. Moreover, the effect of particle shape is only relevant for oriented cloud particles. The simulations show that it is essential to use a three-dimensional scattering model for inhomogeneous cloud layers.
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| 11. |
- Eriksson, Stefan, et al.
(author)
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Comparing a spherical harmonic model of the global electric field distribution with Astrid-2 observations
- 2002
-
In: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 107:A11
-
Journal article (peer-reviewed)abstract
- [1] Electric field measurements provided by the double probe instrument on the Astrid-2 satellite are compared with the empirical Weimer electric field model for all magnetic local times, except between 11 and 13 MLT, and poleward of 55degrees corrected geomagnetic latitude (CGLat). We focus the model evaluation on its ability to predict the latitudinal locations of the convection reversal boundaries for two-cell convection patterns and to estimate the magnitude of the electric field above 55degrees CGLat. A total number of 780 polar cap passes are employed from the Northern Hemisphere between January and July 1999. The measured average electric field magnitude in the dawn-dusk meridian plane above 55degrees CGLat is generally 25% larger than the predicted field independent of the interplanetary magnetic field (IMF) direction. The model shows a better correspondence with the observed electric field for southward IMF than for northward IMF, with most cases centered around B-z = -1.5 nT and r = 0.88. However, the agreement for northward IMF is promising, and a few examples are shown to corroborate this fact. The observed and predicted convection reversal boundary locations along the satellite track for southward IMF are on the average found 2-3degrees CGLat apart in the dawn-dusk meridian plane but may be as far apart as 9degrees CGLat. An initial investigation of the relative timing of a 20-min averaging window for the IMF along the 20-25 min polar cap crossing suggests that a time-dependent transfer function may be found that applies a higher weight to the input solar wind data early in the pass and a lower weight later in the pass for an IMF window that corresponds to the first half of the crossing and the opposite weight versus time dependence for an IMF window corresponding to the last half of the crossing.
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| 12. |
- Eriksson, Stefan, et al.
(author)
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Lobe cell convection and field-aligned currents poleward of the region 1 current system
- 2002
-
In: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 107:A8
-
Journal article (peer-reviewed)abstract
- [1] We present a case and statistical study of plasma convection in the Northern Hemisphere during summer conditions using electric field, magnetic field, and particle data taken during dawn-dusk directed orbits of the FAST satellite. To our knowledge, this set provides the most comprehensive combination of data as yet presented in support of lobe cell convection from an ionospheric perspective this far from the noon sector. In particular, we study the current systems and convection patterns for all passes in July 1997 that show evidence for six large-scale field-aligned currents (FACs) rather than the usual system of four FACs associated with the region 1/region 2 current systems. A total of 71 passes out of 232 in the study had the extra pair of FACs. The extra pair of FACs in 30 of the 71 cases lies either on the dawnside or on the duskside of the noon-midnight meridian, and their position is strongly correlated with the polarity of the IMF By (negative and positive, respectively). This is consistent with the IMF dependence of a three-cell convection pattern of coexisting merging, viscous, and lobe-type convection cells. The occurrence of the asymmetric FAC pair was also strongly linked to conditions of IMF |B-y/B-z | > 1. The extra pair of FACs in these cases was clearly associated with the lobe cell of the three-cell convection system. The remaining 41 cases had the pair of FACs straddling the noon-midnight meridian. The extra pair of FACs was often (20 cases out of 30) observed at magnetic local times more than three hours away from noon, rather than being confined to regions near noon and the typical location of the cusp. Such a current system consisting of a pair of FACs poleward of the nearest region 1 current is consistent with the IMF B-y-dependent global MHD model developed by Ogino et al. [1986] for southward IMF conditions, as well as with other magnetospheric and ionospheric convection models that include the effects of merging occuring simultaneously at both low-latitude dayside and high-latitude lobe and flank magnetopause reconnection sites. Finally, the presence of the additional FACs and three-cell convection well away from noon show that the entire dayside ionosphere is affected by IMF-dependent processes, rather than only a limited region around noon.
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| 13. |
- Haley, C. S., et al.
(author)
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Retrievals of stratospheric O3 and NO2 profiles from Odin Optical Spectrograph and InfraRed Imager System (OSIRIS) limb-scattered sunlight measurements
- 2004
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 109:D16
-
Journal article (peer-reviewed)abstract
- Scientific studies of the major environmental questions of global warming and ozone depletion require global data sets of atmospheric constituents with relevant temporal and spatial resolution. In this paper global number density profiles of O3 and NO2 are retrieved from Odin/OSIRIS limb-scattered sunlight measurements, using the Maximum A Posteriori estimator. Differential Optical Absorption Spectroscopy is applied to OSIRIS radiances as an intermediate step, using the wavelength windows 571-617 nm for O3 and 435-451 nm for NO2. The method is computationally efficient for processing OSIRIS data on an operational basis. Results show that a 2-3 km height resolution is generally achievable between about 12 km and 45 km for O3 with an estimated accuracy of 13\% at the peak and between about 15 km and 40 km for NO2 with an estimated accuracy of 10\% at the peak. First validations of the retrieved data indicate a good agreement both with other retrieval techniques applied to OSIRIS measurements and with the results of other instruments. Once the validation has reached a confident level, the retrieved data will be used to study important stratospheric processes relevant to global environmental problems. The unique NO2 data set will be of particular interest for studies of nitrogen chemistry in the middle atmosphere.
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| 14. |
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| 15. |
- Hamrin, Maria, et al.
(author)
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A statistical study of wave properties and electron density at 1700 km in the auroral region
- 2002
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 107:A8, s. SIA 21-1-SIA 21-13
-
Journal article (peer-reviewed)abstract
- We present a comprehensive overview of the electron density and six different wave types in the range similar to1 Hz to similar to1 MHz, and we investigate their occurrence, average wave frequency and amplitude as a function of location, Kp index, and solar illumination. Twenty-one months of Freja observations from the Northern Hemisphere obtained at similar to1700 km altitude and invariant latitudes 40degrees-75degrees are used. We find that waves around the lower hybrid frequency occur in one low-latitude dayside band and one high-latitude nightside band. The latter band correlates with precipitating auroral electrons and coexists with electromagnetic ion cyclotron (EMIC) waves. This indicates the importance of energetic electrons for the wave generation. Both broadband ELF waves and broadband high-frequency whistler mode waves are found at high latitudes, but whistler mode emissions are most common in regions of high electron densities on the dayside, while broadband ELF waves are found where the density is reduced on the nightside. Moreover, the average density in the presence of broadband ELF waves is more reduced when the ionosphere is dark than when it is sunlit. However, broadband whistler mode waves, Langmuir waves, and waves with an upper cutoff just below the proton gyrofrequency coincide with density enhancements when the ionosphere is dark. Ion heating correlated with auroral electrons coexists with EMIC waves and the high-latitude band of waves around the lower hybrid frequency. Furthermore, ion heating not correlated with downgoing electrons coexists with broadband ELF waves.
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| 16. |
- Hamrin, Maria, et al.
(author)
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Inhomogeneous transverse electric fields and wave generation in the auroral region : A statistical study
- 2001
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 106:A6, s. 10803-10816
-
Journal article (peer-reviewed)abstract
- We use data from the Freja satellite to investigate the importance of localized transverse DC electric fields for the generation of broadband waves responsible for ion heating in the auroral region. Theoretical models indicate that shear in the plasma Row perpendicular to the geomagnetic field can generate waves in a broad range around the ion gyrofrequency for parallel currents significantly below the threshold of the current-driven electrostatic ion cyclotron instability. We compare in situ data with laboratory measurements and theoretical predictions, and we find that inhomogeneous electric fields might well be important for the generation of waves in the auroral region.
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| 17. |
- Hieronymus, C.F. 1967-, et al.
(author)
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A theoretical model of hotspot volcanism: Control on volcanic spacing and patterns via magma dynamics and lithospheric stresses
- 2001
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 106:B1, s. 683-702
-
Journal article (peer-reviewed)abstract
- Many linear island chains are thought to be the result of the steady motion of the lithospheric plates over stationary hotspots. The occurrence of discrete, nearly regularly spaced volcanoes, rather than continuous ridges, are assumed to be caused by the interaction of flexural stresses (due to the volcanic loads) with magma percolation. A parameter study is performed on a simple model that incorporates these effects in addition to dike wall erosion. It is demonstrated that the strength of the erosional feedback determines whether the model generates discrete volcanoes or a continuous ridge. The intervolcanic spacing depends not only on the elastic thickness of the lithosphere but also on the magma pressure at the base of the lithosphere. The size of the eruptive region of the individual volcanoes is controlled by the elastic response of the lithosphere to magma overpressurization. If an initial off-axis edifice is introduced, the model is able to preserve this asymmetry and produce an alternating series of volcanoes. A small initial perturbation grows over time, resulting in double lines or wider patterns depending on the width of the magma source region. Single lines of volcanoes therefore indicate very narrow magma source regions.
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| 18. |
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| 19. |
- Johansson, Jan, 1960, et al.
(author)
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Continuous GPS measurements of postglacial adjustment in Fennoscandia - 1. Geodetic results
- 2002
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 107:B8
-
Journal article (peer-reviewed)abstract
- Project BIFROST (Baseline Inferences for Fennoscandian Rebound Observations, Sea-level, and Tectonics) combines networks of continuously operating GPS receivers in Sweden and Finland to measure ongoing crustal deformation due to glacial isostatic adjustment (GIA). We present an analysis of data collected between August 1993 and May 2000. We compare the GPS determinations of three-dimensional crustal motion to predictions calculated using the high-resolution Fennoscandian deglaciation model recently proposed by Lambeck et al. [1998a, 1998b]. We find that the maximum observed uplift rate (~10 mm/yr) and the maximum predicted uplift rate agree to better than 1 mm yr-1. The patterns of uplift also agree quite well, although significant systematic differences are evident. The root-mean-square residual rate for a linear error model yields estimates of rate accuracy of 0.4 mm/yr for east, 0.3 mm/yr for north, and 1.3 mm/yr for up; these figures incorporate model errors, however. We have also compared the values for the observed radial deformation rates to those based on sea level rates from Baltic tide gauges. The observational error for the vertical GPS rates required to give a reduced chi^2 of unity is 0.8 mm/yr. The time series do exhibit temporal variations at seasonal frequencies, as well as apparent low-frequency noise. An empirical orthogonal function analysis indicates that the temporal variations are highly correlated among the sites. The correlation appears to be regional and falls off only slightly with distance. Some of this correlated noise is associated with snow accumulation on the antennas or, for those antennas with radomes, on the radomes. This problem has caused us to modify the radomes used several times, leading to one of our more significant sources of uncertainty.
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| 20. |
- Karig, D. E., et al.
(author)
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Geological perspectives on consolidation of clay-rich marine sediments
- 2003
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 108:B4, s. 2197-
-
Journal article (peer-reviewed)abstract
- Experimental consolidation of uncemented clay-rich marine sediments provides information concerning their stress history. A main finding is that some of the well-known behavior of soft sediment deformation in geotechnical applications cannot validly be extrapolated to sediments that have been subjected to higher stresses and longer times of geologic conditions. This study confirms that the yield stress of the uncemented sediment accurately reflects its previous maximum consolidation state. Furthermore, we have identified a new phase of post-yield strain that is associated with higher values of the modified compression index (the slope of the porosity versus the logarithm of effective vertical stress) than that of elastic deformation, but with much lower values than that for primary consolidation. This post-yield behavior is a linear, non-elastic deformation, and is termed tertiary consolidation. Yield stress appears independent of creep time or strain rate, whereas the tertiary-primary consolidation transition is sensitive to these parameters. During post-yield creep (secondary consolidation) the slope of the porosity versus the logarithm of time curve, or the secondary consolidation index, is generally assumed constant. However, this is not valid for claystones at effective vertical stresses above about 1 MPa, where the secondary consolidation index increases with stress. At a given effective vertical stress, the secondary consolidation index also increases with creep times greater than about 105 s (28 h).
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| 21. |
- Kaufmann, M., et al.
(author)
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Satellite observations of daytime and nighttime ozone in the mesosphere and lower thermosphere
- 2003
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 108:9
-
Journal article (peer-reviewed)abstract
- The global distribution of mesospheric and lower thermospheric ozone 9.6 μm infrared emissions was measured by the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) experiment during two Space Shuttle missions in November 1994 and August 1997. The radiances measured by CRISTA have been inverted to O3 number densities in the 50-95 km range by using a nonlocal thermodynamic equilibrium model. A detailed sensitivity study of retrieved O3 number densities has been carried out. The ozone abundance profiles show volume mixing ratios of 1-2 ppmv at the stratopause, 0.5 ppmv or less around 80 km, and typically 1 ppmv during daytime and 10 ppmv during nighttime at the secondary maximum. The agreement with other experiments is typically better than 25%. The global distribution of upper mesospheric ozone shows significant latitudinal gradients and an enhancement in the equatorial upper mesosphere. At the polar night terminator a third ozone maximum is observed. Three-dimensional model results indicate that the latitudinal gradients are significantly influenced by solar tides.
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| 22. |
- Kullen, Anita, et al.
(author)
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On the relation between solar wind, pseudobreakups, and substorms
- 2004
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In: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 109:A12
-
Journal article (peer-reviewed)abstract
- A statistical study of pseudobreakups and substorms is performed using Polar UV images from a 3-month period in winter 1998-1999. Data from the ACE solar wind monitor are examined in order to determine the influence of solar wind parameters on the occurrence of different substorm and pseudobreakup types. The results confirm that the IMF clock angle and the amount of solar wind energy flux control the strength of a substorm. The majority of large substorms appear when the IMF is strongly southward and the solar wind energy flux is high. Most small substorms occur during weakly positive or zero IMF B-z and low solar wind energy flux values. Pseudobreakups are associated with even lower energy fluxes than small substorms and appear typically for weakly positive IMF B-z. These results are in agreement with the scenario that pseudobreakups essentially are very weak substorms. Pseudobreakups appear during quiet times and during the growth phase or the recovery phase of weak or medium strong substorms. Time periods of enhanced geomagnetic activity with recurrent substorms are devoid of pseudobreakups. A detailed analysis of the different pseudobreakup types reveals that quiet time pseudobreakups appear predominantly during northward IMF. At least 20 percent of these appear at the poleward oval boundary. Optically, they do not differ much from very weak substorms. Growth phase pseudobreakups develop typically at the end of a 1 to 2 hour long excursion from northward to weakly southward IMF and are followed by quite weak substorms. A large majority of recovery phase pseudobreakups occur at a strongly polewardly displaced oval boundary at the end of a very active recovery phase. A considerable decrease of the polar cap size during the preceding substorm is connected to a northward turning of the IMF.
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| 23. |
- Kullen, A., et al.
(author)
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Solar wind dependence of the occurrence and motion of polar auroral arcs : A statistical study
- 2002
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In: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 107:A11
-
Journal article (peer-reviewed)abstract
- [1] Polar UV images from a 3-month period in winter 1998-1999 are used for a statistical study of polar arcs. The study covers all auroral arcs that are located poleward of the northern auroral oval, and which are detectable by the UV imager. The arcs are examined with respect to their spatial and temporal behavior as well as to a possible connection to solar wind parameters using ACE satellite data. It is found that the majority of polar arcs appear during northward IMF, strong IMF magnitude, and high solar wind speed. A modified Akasofu-Perreault epsilon parameter with a cosine function instead of a sine function (nuB(2) cos(4) (theta/2)(l(0)(2)/mu(0))) combines these results. It correlates well with the occurrence frequency of polar arcs for long timescales. The location of polar arcs is strongly dependent on the sign of the IMF B-y component. Static polar arcs occur in the Northern Hemisphere on the dawn (dusk) side of the oval for negative (positive) IMF B-y, whereas poleward-moving arcs separate from the opposite side of the oval, and then move in the direction of IMF B-y. All polar arcs are sorted into five different categories according to their spatial structure and evolution: oval-aligned, bending, moving, midnight, and multiple arcs. Each polar arc type occurs for a characteristic combination of solar wind parameters. IMF clock angle changes seem to have a strong influence on what type of arc occurs. Oval-aligned arcs appear mainly during steady IMF, bending arcs after an IMF B-z sign change, and moving arcs after an IMF B-y sign change. For the rare midnight and multiple arc events, no characteristic IMF clock angle dependence has been found. The different types of clear polar arcs are discussed in the context of existing observational studies and transpolar arc models.
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| 24. |
- Magyari-Kope, B., et al.
(author)
-
High-pressure structure of ScAlO3 perovskite
- 2002
-
In: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 107:B7
-
Journal article (peer-reviewed)
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| 25. |
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| 26. |
- Meier, H E Marcus, et al.
(author)
-
A multiprocessor coupled ice-ocean model for the Baltic Sea: Application to salt inflow
- 2003
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 108:C8, s. 3273-
-
Journal article (peer-reviewed)abstract
- Within the Swedish Regional Climate Modeling Program, SWECLIM, a three-dimensional (3-D) coupled ice-ocean model for the Baltic Sea has been developed to simulate physical processes on timescales of hours to decades. The code has been developed based on the massively parallel version of the Ocean Circulation Climate Advanced Modeling (OCCAM) project of the Bryan-Cox-Semtner model. An elastic-viscous-plastic ice rheology is employed, resulting in a fully explicit numerical scheme that improves computational efficiency. An improved two-equation turbulence model has been embedded to simulate the seasonal cycle of surface mixed layer depths as well as deepwater mixing on decadal timescale. The model has open boundaries in the northern Kattegat and is forced with realistic atmospheric fields and river runoff. Optimized computational performance and advanced algorithms to calculate processor maps make the code fast and suitable for multi-year, high-resolution simulations. As test cases, the major salt water inflow event in January 1993 and the stagnation period 1980-1992, have been selected. The agreement between model results and observations is regarded as good. Especially, the time evolution of the halocline in the Baltic proper is realistically simulated also for the longer period without flux correction, data assimilation, or reinitialization. However, in particular, smaller salt water inflows into the Bornholm Basin are underestimated, independent of the horizontal model resolution used. It is suggested that the mixing parameterization still needs improvements. In addition, a series of process studies of the inflow period 1992/1993 have been performed to show the impact of river runoff, wind speed, and sea level in Kattegat. Natural interannual runoff variations control salt water inflows into the Bornholm Basin effectively. The effect of wind speed variation on the salt water flux from the Arkona Basin to the Bornholm Basin is minor.
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| 27. |
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| 28. |
- Milne, G.A., et al.
(author)
-
Continuous GPS measurements of postglacial adjustment in Fennoscandia: 2. Modeling results
- 2004
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 109:B2
-
Journal article (peer-reviewed)abstract
- [1] Data collected under the auspices of the BIFROST GPS project yield a geographically dense suite of estimates of present-day, three-dimensional (3-D) crustal deformation rates in Fennoscandia [Johansson et al., 2002]. A preliminary forward analysis of these estimates [Milne et al., 2001] has indicated that models of ongoing glacial isostatic adjustment (GIA) in response to the final deglaciation event of the current ice age are able to provide an excellent fit to the observed 3-D velocity field. In this study we revisit our previous GIA analysis by considering a more extensive suite of forward calculations and by performing the first formal joint inversion of the BIFROST rate estimates. To establish insight into the physics of the GIA response in the region, we begin by decomposing a forward prediction into the three contributions associated with the ice, ocean, and rotational forcings. From this analysis we demonstrate that recent advances in postglacial sea level theory, in particular the inclusion of rotational effects and improvements in the treatment of the ocean load in the vicinity of an evolving continental margin, involve peak signals that are larger than the observational uncertainties in the BIFROST network. The forward analysis is completed by presenting predictions for a pair of Fennoscandian ice histories and an extensive suite of viscoelastic Earth models. The former indicates that the BIFROST data set provides a powerful discriminant of such histories. The latter yields bounds on the ( assumed constant) upper and lower mantle viscosity (nu(UM), nu(LM)); specifically, we derive a 95% confidence interval of 5 x 10(20) less than or equal to nu(UM) less than or equal to 10(21) Pa s and 5 x 10(21) less than or equal to nu(LM) less than or equal to 5 x 10(22) Pa s, with some preference for (elastic) lithospheric thickness in excess of 100 km. The main goal of the ( Bayesian) inverse analysis is to estimate the radial resolving power of the BIFROST GPS data as a function of depth in the mantle. Assuming a reasonably accurate ice history, we demonstrate that this resolving power varies from similar to 200 km near the base of the upper mantle to similar to 700 km in the top portion of the lower mantle. We conclude that the BIFROST data are able to resolve structure on radial length scale significantly smaller than a single upper mantle layer. However, these data provide little constraint on viscosity in the bottom half of the mantle. Finally, elements of both the forward and inverse analyses indicate that radial and horizontal velocity estimates provide distinct constraints on mantle viscosity.
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| 29. |
- Morgan, Julia K., et al.
(author)
-
Consolidation state and strength of underthrust sediments and evolution of the décollement at the Nankai accretionary margin: Results of uniaxial reconsolidation experiments
- 2004
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 109:B3
-
Journal article (peer-reviewed)abstract
- Uniaxial reconsolidation experiments conducted on Ocean Drilling Program drill cores along the Muroto Transect of the Nankai accretionary margin demonstrate complex yield and postyield behavior and provide evidence for enhanced strengths within sediments beneath the décollement zone. Tests were conducted on samples collected from similar stratigraphic levels below the décollement and its seaward projection. Consolidation state of the samples increased landward and with depth and was tracked closely by sediment yield stress for all but one of the samples. The sediments, however, exhibited substantial postyield strength: up to 2.8 times the predicted in situ effective vertical stress beneath the protothrust zone. This enhanced strength results from diagenesis that leads to matrix cementation during stable effective stress conditions within the underthrust section. The close correspondence between yield stress and predicted in situ effective stress suggests that despite the cemented state, the sediment matrix remains sensitive to in situ stress conditions. The low yield stress of one sample, collected within ∼40 m of the décollement fault at Site 808, may reflect diagenesis under reduced effective stress conditions, due to postconsolidation increases in pore pressure along the décollement. Due to their cemented state, the strong underthrust sediments resist décollement downcutting beneath the toe of the prism, leading to preferential incorporation of weaker, continuously deforming accreted sediments during shear. Seismogenic slip along the décollement at depth may create stresses in excess of sediment strength, causing shear failure and rapid release of trapped pore fluids feeding high pore fluid pressures along the décollement zone.
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| 30. |
- Rutgersson, Anna, et al.
(author)
-
The use of conventional stability parameters during swell
- 2001
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 106:C11, s. 27117-27134
-
Journal article (peer-reviewed)abstract
- The situation with swell is of climatological importance over the Baltic Sea since swell is present during as much as 40% of the time. In this study, two periods with unstable and two periods with stable stratification and wind following swell are investigated. Data are taken at a small flat island in the Baltic Sea. During unstable stratification the turbulent structure shows great resemblance to the free convective boundary layer and scales with the boundary layer height. Since surface heat flux is too small to support the high levels of turbulence present, inactive turbulence is probably the dominating source. For the stably stratified layer, there are smaller differences between data with and without swell. The turbulence is mainly transported upward into the atmosphere with the aid of pressure fluctuations induced by the waves. For most of the data with swell the gradients are smaller than for growing sea. During unstable conditions the wind gradients are negative, indicating the presence of a wave-driven wind. The gradients increase with increasing height above the surface. The drag coefficient is smaller than is usually found for both stable and unstable stratification and varies very little with wind and stratification. There are only small variations in the heat transfer coefficients with changing stratification, but they are significantly different for stable and unstable stratification.
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| 31. |
- Rönnmark, Kjell, et al.
(author)
-
Auroral Electron acceleration by Alfven waves and electrostatic fields
- 2000
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 105:A11, s. 25333-25344
-
Journal article (peer-reviewed)abstract
- We present a two-dimensional numerical model for the formation of discrete auroral arcs. This model describes the evolution of shear Alfven waves generated by a growing force near the equatorial plane, and the transition to electrostatic fields when the force becomes stationary. The parallel electric fields on auroral field lines may be regarded as shear Alfven waves driven by a magnetospheric generator at zero frequency. In our collisionless model, precipitating auroral electrons are accelerated to an energy of 350 eV when the upward current is 3.1 mu Am-2. We also find that the electrostatic potential drop is proportional to the square of the current density.
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| 32. |
- Shukla, Padma Kant, et al.
(author)
-
Stimulated scattering of intense radio waves in partially ionized space dusty plasmas
- 2004
-
In: Journal of Geophysical Research. - Washington : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 109:A3
-
Journal article (peer-reviewed)abstract
- The nonlinear interaction between intense ordinary mode (O-mode) radio waves and modified magnetoacoustic waves in partially ionized space dusty plasmas is considered, including the combined action of the radio wave pressure and the electron Joule heating caused by the O-mode electric field. A set of equations containing the wave equation for the radio waves and the electron density and temperature fluctuations associated with the modified magnetoacoustic waves is derived. The set is then used to investigate decay and modulational instabilities of a constant amplitude O-mode radio wave in the Earth's ionosphere and mesosphere.
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| 33. |
- Silevitch, M B, et al.
(author)
-
O+ phase bunching as a source for stable auroral arcs
- 2000
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 105, s. 10739-10749
-
Journal article (peer-reviewed)abstract
- We propose a model to explain how ion dynamics create an Alfven wave generator in the equatorial region that can be applied to the stable are problem. For example, in the earthward drifting magnetotail plasma, phase bunching of O+ ions land to a much lesser extent of the H+ ions) can be caused by a weak (similar to 1x10(-9) Vm(-2)) electric field gradient [Rothwell et al., 1994]. This leads to density striations in the GSM frame. O+ density striations in the earthward drifting plasma frame are seen as a tailward propagating source of Alfven waves where the hydrogen ions provide the polarization current of the wave. A transformation to the CSM frame will yield a static, oblique wave structure similar to that previously treated. The waves propagate from the equatorial region to both ionospheres where they are reflected. The ionospheric boundary condition when combined with a magnetospheric boundary condition allows a solution of the wave amplitudes in terms of the striation structure. The frequency of the Alfven wave and the associated wavelengths are also determined by the striation driver. We find that the magnitude of the parallel current density at the ionosphere has a spatial resonance when the distance between the ionosphere and the equatorial plane is equal to a quarter wavelength along B-o. In that case, the magnitude of the parallel current density at the ionosphere is of the order of 10 mu A m(-2) and peaks for striation wavelengths las mapped to the ionosphere) of 10 -40 km, which is comparable to the transverse scale of auroral arcs. The associated Poynting flux incident on the ionosphere is found to be similar to 2 mWm(-2) and represents a net transfer of energy from the magnetosphere to the ionosphere as recently observed by experimenters studying substorm onsets. We find that in the steady state the power extracted from the bulk flow to power the are is balanced by energy provided by the solar wind through the cross-tail electric field.
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| 34. |
- Stenström, Petter
(author)
-
Hydraulics and mixing in the Hudson River estuary : A numerical model study of tidal variations during neap tide conditions
- 2004
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 109:C4, s. C04019-
-
Journal article (peer-reviewed)abstract
- Three-dimensional numerical modeling is performed to study intratidal and along-channel variability in stratification and mixing in the Hudson River estuary. The modeled fields show good agreement with observations, both qualitatively and quantitatively. Estuarine circulation dominates the mean fields, and intratidal variability is dominated by tidal straining that acts to strengthen the stratification during ebb and weaken it during flood. Mixing is mainly confined to a bottom layer during flood but occurs higher up in the water column during ebb. Mixing across the halocline shows marked along-channel variability due to bathymetric effects. During ebb, mixing occurs preferentially at an abrupt channel expansion seaward of a channel constriction at the George Washington Bridge, as predicted by Chant and Wilson [2000]. The salt flux across the halocline in this region, averaged over ebb, exceeds 5x10(-4) kg m(-2) s(-1), a factor of 3 greater than the along-channel average. Increased residence time of tracers should be expected in this region due to the strong mixing but also due to observed secondary circulation [Chant and Wilson, 1997]. Mixing across the halocline during flood is small, except for early flood, before the well-mixed bottom layer is developed. Mixing is then localized to the landward slope of sills.
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| 35. |
- Svensson, Bo, 1946-, et al.
(author)
-
Methane emission from Swedish mires : National and regional budgets and dependence on mire vegetation
- 2001
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 106:D18, s. 20847-20860
-
Journal article (peer-reviewed)abstract
- A national land use inventory program was used to estimate the flux of methane from Swedish mires. During the snow-free season in 1994, methane fluxes were monitored at 60% (n = 619) of the total number of permanent inventory plots for the four mire vegetation classes included. The methane flux was measured only once at each inventory plot using transparent static chambers, yielding 3157 (89%) accepted flux rates. The temperature during the measurement period was close to the long-term (30 year) average in the north and much warmer in the south. The precipitation was lower than the long-term average. Average flux rates, specific for each mire vegetation class and geographical region, varied between 8 and 238 mg CH4 m-2 d-1. The national averages ▒ SE (mg CH4 m-2 d-1) were hummock communities, 24 ▒ 6, transitional fens, 15 ▒ 2, short sedge fens, 49 ▒ 4, and tall sedge fens, 97 ▒ 22. To derive the annual flux, the number of days with a daily average air temperature over 5░C was used. The annual methane fluxes for each mire vegetation class and region varied between 1.5 and 40 g CH4 m-2 yr-1. The annual methane fluxes (average ▒ SE in g m-2 yr-1) for each mire vegetation class for the whole of Sweden were hummock sites, 4.9 ▒ 1.3, transitional fens, 2.5 ▒ 0.5, low sedge fens, 8.2 ▒ 0.8, and tall sedge fens, 16.5 ▒ 3.6. The methane flux (▒95% confidence limits) in 1994 from the monitored mire types in Sweden was 0.29 ▒ 0.05 Tg CH4. The low and tall sedge mires accounted for 96% of the methane emitted. Copyright 2001 by the American Geophysical Union.
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| 36. |
- Verdes, C., et al.
(author)
-
Pointing and temperature retrieval from millimeter-submillimeter limb soundings
- 2002
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 107:16, s. 4299-
-
Journal article (peer-reviewed)abstract
- Passive microwave limb sounding instruments like the Millimeter-Wave Atmospheric Sounder (MAS) or the Microwave Limb Sounder (MLS) observe dedicated oxygen lines for the derivation of temperature and pointing information, since these quantities are essential for the quality of the retrieval of the trace gas mixing ratio. Emission lines of oxygen are chosen because the volume mixing ratio (VMR) profile is known. In this paper, we demonstrate the capabilities of a new and innovative method by means of which accurate temperature and pointing information can be gathered from other strong spectral features like ozone lines, without including accurate knowledge of the VMR profile of these species. For this purpose, retrievals from two observation bands with a bandwidth of about 10 GHz each, one including an oxygen line, have been compared. A full error analysis was performed with respect to critical instrument and model parameters, such as uncertainties in the antenna pattern, calibration uncertainties, random pointing error, baseline ripples, baseline discontinuities, and spectroscopic parameters. The applied inversion algorithm was the optimal estimation method. For the selected scenario and instrumental specifications we find that the retrieval of a pointing offset and the atmospheric temperature profile can be achieved with a good accuracy. The retrieval precision of the pointing offset is better than 24 m. The retrieval precision of the temperature profile is better than 2 K for altitudes ranging from 10 to 40 km. Systematic errors (due to model parameter uncertainties) are somewhat larger than these purely statistical errors. Investigations carried out for different atmospheric states or different instrumental specifications show similar results.
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| 37. |
- von Engeln, Axel, et al.
(author)
-
One-dimensional variational (1-D Var) retrieval of temperature, water vapor, and a reference pressure from radio occultation measurements : A sensitivity analysis
- 2003
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 108:11, s. 4337-
-
Journal article (peer-reviewed)abstract
- A 1-D Var retrieval study of simulated radio occultation measurements is presented. Temperature and a water vapor profile are retrieved along with a reference pressure to generate the pressure profile by applying the hydrostatic equation. High-resolution European Center for Medium-Range Weather Forecasts (ECMWF) atmospheric fields are used by a ray tracing tool to calculate the exact positions of the tangent point. The 1-D atmospheric profiles following the calculated tangent point trajectory in the 3-D ECMWF fields are used to simulate bending angle measurements with a 1-D forward model. Assimilation of these bending angles in a 1-D Var tool employing the same 1-D forward model is performed. We analyze the sensitivity of the retrieval to changes in vertical resolution, horizontal smearing of the tangent point trajectory, and the assumption of hydrostatic equilibrium for a nonvertical atmospheric scan. We find that retrievals calculated without adequate vertical resolution can have significant errors in temperature and water vapor. Errors in the retrieval by assuming hydrostatic equilibrium for a nonvertical scan generally cause only minor errors in the retrieved profiles. A study into the occurrence of rays curving down toward the Earth surface indicates that about 5–10% of the profiles could experience so-called critical refraction at altitudes between 0.5 km and 2 km in case of the applied high-resolution ECMWF data.
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| 38. |
- von Engeln, Axel, et al.
(author)
-
Temperature profile determination from microwave oxygen emissions in limb sounding geometry
- 2002
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 107:19, s. 12-1
-
Journal article (peer-reviewed)abstract
- We present a theoretical investigation of the temperature profile retrieval capabilities of oxygen emission lines in the microwave. The main focus is on two strong lines, both allowing temperature retrieval throughout the mesosphere. One is within the oxygen cluster at 61.15 GHz, the other one is isolated at 118.75 GHz. A thorough comparison of these two lines is presented. Several instrumental parameters such as system noise temperature, antenna beam width, filter width, and coverage of the line are assessed, as well as the possible impact of an error in the spectroscopic parameters. The instrumental setup follows roughly the specifications for the Millimeter Wave Acquisitions for Stratosphere/Troposphere Exchange Research (MASTER) instrument, serving as a basis for a modern passive microwave instrument. The instrumental parameters have also been varied in order to allow comparisons with two instruments that use the 118.75-GHz line for temperature profile determination, the Odin submillimeter radiameter (SMR) and the EOS microwave limb sounder (MLS). Simultaneous retrieval of temperature and pointing bias is performed with the Optimal Estimation Method. We find temperature retrieval errors of
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| 39. |
- Yurganov, L.N., et al.
(author)
-
A Quantitative Assessment of the 1998 Carbon Monoxide Emission Anomaly in the Northern Hemisphere Based on Total Column and Surface Concentration Measurements
- 2004
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 109:15, s. D15305-
-
Journal article (peer-reviewed)abstract
- Carbon monoxide abundances in the atmosphere have been measured between January 1996 and December 2001 in the high Northern Hemisphere (HNH) (30degrees-90degreesN) using two different approaches: total column amounts of CO retrieved from infrared solar spectra and CO mixing ratios measured in situ at ground-based stations. The data were averaged, and anomalies of the CO HNH burden ( deviations of the total tropospheric mass between 30degreesN and 90degreesN from the mean seasonal profile, determined as the 5 year average) were analyzed. The anomalies obtained from in situ and total column data agree well and both show two maxima, by far the largest in October 1998 and a lower one in August 1996. A noticeable decrease of the positive 1998 summer anomaly with increasing height was found. A box model was applied, and anomalies in source rates were obtained under the assumption of insignificant interannual sink variations. In August 1998 the HNH emission anomaly was estimated to be 38 Tg month(-1). The annual 1998 emission positive anomaly was 96 Tg yr(-1). Nearly all excess CO may be attributed to the emissions from boreal forest fires. According to available inventories, biomass burning emits around 52 Tg yr(-1) during the "normal'' years; therefore total biomass emissions in 1998 were as large as 148 Tg yr(-1). In August 1998, CO contribution from the biomass burning was twice as large as that from fossil fuel combustion. The results were compared to available emission inventories.
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| 40. |
- Öquist, Mats, et al.
(author)
-
Vascular plants as regulators of methane emissions from a subarctic mire ecosystem
- 2002
-
In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 107:21, s. 4580-4590
-
Journal article (peer-reviewed)abstract
- Vascular plant functions as controlling mechanisms of methane emissions were investigated at two contrasting habitat types at a subarctic peatland ecosystem in northern Sweden. One of the habitats was ombrotrophic (vegetation dominated by Eriophorum vaginatum and Carex rotundata), while the other was minerotrophic (vegetation dominated by Eriophorum angustifolium). Through shading manipulations we successfully reduced the gross photosynthetic rates of the vascular plant communities. At the ombrotrophic site a 25% reduction in gross photosynthesis lead to a concomitant 20% reduction in methane emission rates, indicating a strong substrate-based coupling between the vascular plant community and the methanogenic populations. At the minerotrophic site, methane emission rates were unaffected, although plant photosynthesis was reduced by almost 50%. However, the methane emission rates at the minerotrophic site were significantly correlated with the number of vascular plants. We conclude that at the minerotrophic site the vegetation influences methane emission rates by facilitating methane transportation between the soil and the atmosphere, while at the ombrotrophic site the relationship between the vascular plant community and methane emissions is mediated by substrate-based interactions regulated by plant photosynthetic activity. Copyright 2002 by the American Geophysical Union.
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| 41. |
- Rakov, Vladimir A., et al.
(author)
-
M-component mode of charge transfer to ground in lightning discharges
- 2001
-
In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 106:D19, s. 22817-22831
-
Journal article (peer-reviewed)abstract
- The M-component mode of charge transfer to ground is examined using (1) multiple-station measurements of electric and magnetic fields at distances ranging from 5 to ∌ 500 m from triggered-lightning channels and (2) measured currents at the channel base. Data have been obtained in 1997, 1999, and 2000 at the International Center for Lightning Research and Testing at Camp Blanding, Florida, for (1) “classical†M-components that occur during the continuing currents following return strokes and (2) impulsive processes that occur during the initial stage of rocket-triggered lightning and are similar to the “classical†M components. All lightning events considered here effectively transported negative charge to ground. For one triggered-lightning event the electric field 45 km from the lightning channel was measured together with the current and close fields. The shapes and magnitudes of the measured close electric and magnetic fields are generally consistent with the guided-wave mechanism of the lightning M component. Specifically, the M-component electric field peak exhibits logarithmic distance dependence, ln(kr−1), which is indicative of a line charge density that is zero at ground and increases with height. Such a distribution of charge is distinctly different from the more or less uniform charge density that is characteristic of the dart leaders in triggered lightning, as inferred from close electric field measurements. The M-component magnetic field peak decreases as the inverse distance (i.e., r−1), which is generally consistent with a uniform current within the lowest kilometer or so of channel. The M-component electric field at 45 km appeared as a bipolar, microsecond-scale pulse that started prior to the onset of the M-component current at the channel base. M-component-type processes can produce acoustic signals with peak pressure values of the same order of magnitude as those from the leader/return stroke sequences in triggered lightning.
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| 42. |
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| 43. |
- Thottappillil, Rajeev, 1958-, et al.
(author)
-
On different approaches to calculating lightning electric fields
- 2001
-
In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 106:D13, s. 14191-14205
-
Journal article (peer-reviewed)abstract
- Three different approaches to the computation of lightning electric fields are compared. These approaches are the traditional dipole (Lorentz condition) technique and two versions of the monopole (continuity equation) technique. The latter two techniques are based on two different formulations of the continuity equation, one used by Thottappillil et al. [1997] and the other by Thomson [1999], the difference between the formulations being related to different treatments of retardation effects. The three approaches involve the same expression for the vector potential but different expressions for the scalar potential. It is analytically shown that the three different expressions for the scalar potential are equivalent and satisfy the Lorentz condition. Further, the three approaches yield the same total fields and the same Poynting vectors. However, expressions in the three approaches for the individual electric field components in the time domain, traditionally identified by their distance dependence as electrostatic, induction, and radiation terms, are different, suggesting that explicit distance dependence is not an adequate identifier. It is shown that the so identified individual field components in the electric field equation in terms of charge density derived by Thottappillil et al. [1997] are equivalent to the corresponding field components in the traditional equation for electric field in terms of current based on the dipole technique. However, the individual field components in the electric field equation based on Thomson’s [1999] approach are not equivalent to their counterparts in the traditional dipole technique equation. Further, in Thottappillil et al.’s [1997] technique and in the traditional dipole technique, the gradient of scalar potential contributes to all three electric field components, while in Thomson’s [1999] technique it contributes only to the electrostatic and induction components. Calculations of electric fields at different distances from the lightning channel show that the differences between the corresponding field components identified by their distance dependence in different techniques are considerable at close ranges but become negligible at far ranges.
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| 44. |
- Von Clarmann, T., et al.
(author)
-
A blind test retrieval experiment for infrared limb emission spectrometry
- 2003
-
In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 108:D23
-
Journal article (peer-reviewed)abstract
- The functionality and characteristics of six different data processors (i.e., retrieval codes in their actual software and hardware environment) for analysis of high-resolution limb emission infrared spectra recorded by the space-borne Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) have been validated by means of a blind test retrieval experiment based on synthetic spectra. For this purpose a self-consistent set of atmospheric state parameters, including pressure, temperature, vibrational temperatures, and abundances of trace gases and aerosols, has been generated and used as input for radiative transfer calculations for MIPAS measurement geometry and configuration. These spectra were convolved with the MIPAS field of view, spectrally degraded by the MIPAS instrument line shape, and, finally, superimposed with synthetic measurement noise. These synthetic MIPAS measurements were distributed among the participants of the project “Advanced MIPAS level-2 data analysis” (AMIL2DA), who performed temperature and species abundance profile retrievals by inverse radiative transfer calculations. While the retrieved profiles of atmospheric state parameters reflect some characteristics of the individual data processors, it was shown that all the data processors under investigation are capable of producing reliable results in the sense that deviations of retrieved results from the reference profiles are within the margin that is consistent with analytical error estimation.
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| 45. |
- Anderson, Leif G, 1951, et al.
(author)
-
Variability in river runoff distribution in the Eurasian Basin of the Arctic Ocean
- 2004
-
In: Journal of Geophysical Research-Oceans. - 0148-0227. ; 109:C1
-
Journal article (peer-reviewed)abstract
- The distribution of freshwater within the Arctic Ocean and its export from it are intimately involved in climate and climate change processes both within and outside the Arctic Ocean. River runoff in the Arctic Ocean constitutes a major part of the Arctic Ocean freshwater budget. Within the Arctic Ocean, variability in the distribution of river runoff will be reflected in the location of the cold halocline that isolates the sea ice from the warm Atlantic Layer. Outside the Arctic Ocean, such variability will impact on the salinity of North Atlantic waters (Great Salinity Anomaly) and on deep convection areas of the North Atlantic Ocean, and thereby potentially on global thermohaline circulation. Rivers entering the Arctic Ocean have high levels of total alkalinity that contribute significantly to the total alkalinity of the surface Polar Mixed Layer. We exploit total alkalinity data to trace river runoff in the surface Polar Mixed Layer and to observe variability in the river runoff distribution in the Eurasian Basin over the period 1987-2001. The river runoff front changed from a position over the Gakkel Ridge in 1987 and 1991 to over the Lomonosov Ridge in 1996, and returned to a midpoint between the two ridges in 2001. Wind field changes as characterized by the Arctic Oscillation index are considered to be a major factor in determining ice and surface water flow. We note a correlation with 4-6 years delay between changes in river runoff distribution and the Arctic Oscillation index. We show that the delay can be inferred from a geostrophic flow calculation.
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| 46. |
- Bigelow, NH, et al.
(author)
-
Climate change and Arctic ecosystems: 1. Vegetation changes north of 55 degrees N between the last glacial maximum, mid-Holocene, and present
- 2003
-
In: Journal of Geophysical Research. - 2156-2202. ; 108:D19
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Research review (peer-reviewed)abstract
- [1] A unified scheme to assign pollen samples to vegetation types was used to reconstruct vegetation patterns north of 55degreesN at the last glacial maximum (LGM) and mid-Holocene (6000 years B. P.). The pollen data set assembled for this purpose represents a comprehensive compilation based on the work of many projects and research groups. Five tundra types (cushion forb tundra, graminoid and forb tundra, prostrate dwarf-shrub tundra, erect dwarf-shrub tundra, and low- and high-shrub tundra) were distinguished and mapped on the basis of modern pollen surface samples. The tundra-forest boundary and the distributions of boreal and temperate forest types today were realistically reconstructed. During the mid-Holocene the tundra-forest boundary was north of its present position in some regions, but the pattern of this shift was strongly asymmetrical around the pole, with the largest northward shift in central Siberia (similar to200 km), little change in Beringia, and a southward shift in Keewatin and Labrador (similar to200 km). Low- and high-shrub tundra extended farther north than today. At the LGM, forests were absent from high latitudes. Graminoid and forb tundra abutted on temperate steppe in northwestern Eurasia while prostrate dwarf-shrub, erect dwarf-shrub, and graminoid and forb tundra formed a mosaic in Beringia. Graminoid and forb tundra is restricted today and does not form a large continuous biome, but the pollen data show that it was far more extensive at the LGM, while low- and high-shrub tundra were greatly reduced, illustrating the potential for climate change to dramatically alter the relative areas occupied by different vegetation types.
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| 47. |
- Boberg, Fredrik, et al.
(author)
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Solar mean magnetic field variability: A wavelet approach to Wilcox Solar Observatory and SOHO/Michelson Doppler Imager observations
- 2002
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In: Journal of Geophysical Research. - 2156-2202. ; 107:A10: 1318
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Journal article (other academic/artistic)abstract
- Solar mean magnetic field (SMMF) measurements from the Wilcox Solar Observatory and with the SOHO/MDI instrument are described and analyzed. Even though two completely different methods of observation are used, the two data sets obtained show a strong similarity. Using continuous wavelet transforms, SMMF variability is found at a number of temporal scales. Detected SMMF signals with a 1–2 year period are considered to be linked to variations in the internal rotation of the Sun. Intermediate SMMF oscillations with a period of 80–200 days are probably connected to the evolution of large active regions. We also find evidence for 90 min variations with coronal mass ejections as a suggested origin.
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| 48. |
- Bolzan, JF, et al.
(author)
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Reconstruction of the undiffused seasonal oxygen isotope signal in central Greenland ice cores
- 2000
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In: JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS. - 0148-0227. ; 105:C9, s. 22095-22106
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Journal article (peer-reviewed)abstract
- The climatologically important seasonal signal in ice core oxygen isotope records is altered by diffusion with time. Here we use the diffusion model of Whillans and Grootes [1985] to calculate the effects of isotopic diffusion in nine shallow ice cores recovered over a 150 x 150 km grid centered on Summit, Greenland. Two different computational methods give similar results for the back-diffused delta(18)O seasonal extrema. Although back diffusing the ice core delta(18)O records improves the correlations between the individual ice core oxygen isotope and Greenland coastal temperature records; the correlations remain poor, particularly for the low-accumulation sites. Stacking the back-diffused isotopic records from several cores results in much higher correlations, with a strong winter linkage between west coast stations and central Greenland. These correlations are also significantly greater than the correlations between the stacked measured (diffused) isotopic profiles and coastal temperatures. Our results indicate that the use of ice core isotopic records to identify anomalous seasons requires that diffusion effects be accounted for.
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| 49. |
- Carozzi, TD, et al.
(author)
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Full polarimetry measurements of stimulated electromagnetic emissions: First results
- 2001
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In: JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS. - : AMER GEOPHYSICAL UNION. - 0148-0227. ; 106:A10, s. 21395-21407
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Journal article (peer-reviewed)abstract
- A theory and technique for the measurement of the full state of polarization (SOP) for wideband electromagnetic HF emissions from the ionosphere are introduced. The technique was employed in a recent experiment at the Sura ionospheric HF pumping facility
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| 50. |
- Eriksson, Stefan, et al.
(author)
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The Cross-Polar Potential Drop and its Correlation to the Solar Wind
- 2000
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In: Journal of Geophysical Research - Space Physics. - 0148-0227. ; 105, s. 18639-18653
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Journal article (peer-reviewed)abstract
- The cross-polar potential drop Phi(pc) as derived from the FAST satellite is used to study the average magnetospheric response to changes in the solar wind as monitored by the Wind spacecraft. The coupling of the solar wind with the magnetosphere is examined using the interplanetary magnetic field (IMF) B-z, the model reconnection electric field vB(t)sin(k)(theta/2) for k=3, 4, and the Akasofu-Perreault epsilon parameter. Initial results comprising 37 cases of Phi(pc) show one major response of the magnetosphere to the solar wind forcing at 15 min time lag followed by two minor pulses at 55 min and 105 min, respectively, during times when the IMF polarity was mainly southward and the geomagnetic activity was moderate to low. Phi(pc) shows a very good correlation with typical models of the reconnection electric field at 15 min time lag, reaching a maximum linear correlation coefficient of r=0.95 for vB(t)sin(3)(theta/2). In order to reach an understanding of the importance of individually calculated correlation coefficients, we introduce the statistical. bootstrap algorithm of Efron and Tibshirani [1993], which allows us to estimate a correlation coefficient standard error. In defining a quality measure based on this method, the significance coefficient s, we are able to interpret a resulting correlation coefficient time lag series in terms of a linear prediction filter similar to earlier techniques. The results on the magnetospheric response for Phi(pc) are further compared with those obtained using the geomagnetic indices Dst, SYM-H, and ASY-H. The similar magnetospheric response to the solar wind electric field for Phi(pc) and ASY-H at time delays of more than 40 min together with a high correlation coefficient between Phi(pc) and ASY-H suggest that these magnetospheric parameters couple to one another. The average dynamic response of the ionospheric convection to the solar wind electric field is in essential agreement with those reported by Klimas et al. [1994] on a normal magnetospheric mode with a recurrence frequency of 50 min.
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