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[en] Complete text of publication follows. The recent years have brought enormous progress for our understanding of the characteristics and the variability of the thermosphere. This is mainly based on the high-resolution measurements of the accelerometer on board the CHAMP satellite. It sampled the air drag from the solar maximum in 2001 to the minimum in 2008. This long data set enabled a lot of dedicated studies. One important finding is the close relation of thermospheric features to the geomagnetic field geometry. Here the ionised component has to take the role of the mediator between magnetic field and neutral gas. Examples of that are the equatorial mass density anomaly, the cusp density anomaly and the high-speed wind channel along the dip equator. The thermosphere reacts quite differently to solar and magnetic forcing. This is particularly evident when comparing the mass density on the day and night side at low latitudes. We were able to delineate the variations due to solar flux, season, and magnetic activity. It turns out that the controlling parameters, e.g. F10.7, Day-of-Year or Am influence the thermosphere in a linear fashion. Recently a lot of interest has been put in the tidal modulation of the high atmosphere. Also in the thermosphere at 400 km altitude the signatures of migrating and non-migrating tides have been identified. These are again well correlated with ionospheric signatures. On the evening and morning side terminator waves are identified which still wait for their proper interpretation. Representative examples of the listed effects will be presented.
[en] Complete text of publication follows. Based on a comprehensive catalogue with more than 4000 substorm entries from the years 2000--2005, the spatial distribution of the substorm-related magnetic signature at mid and low latitudes around local midnight is investigated. Superposed epoch analysis of a larger number of recent observatory data from mid and low latitudes revealed a field strength increase that is consistent with the results of earlier studies. For the first time, the magnetic signature of the substorm current wedge formation is studied also in near-Earth satellite data from CHAMP. The average maximal deflection measured on board the satellite is smaller by a factor of 2 than that determined from ground observations. The near-Earth and ground-based magnetic field observations cannot be described adequately by a simple current wedge model. A satisfactory agreement between model results and observations at satellite height and on ground can be achieved only if the current reconfiguration scenario combines the following four elements: (1) a gradual decrease of the tail lobe field; (2) a re-routing of a part of the cross-tail current through the ionosphere; (3) eastward ionospheric currents at low and mid latitudes driven by Region-2 field-aligned currents; and (4) a partial ring current connected to these Region-2 FACs.
[en] Complete text of publication follows. The Harang discontinuity is known as the region in the near magnetospheric tail where the earthward plasma flow is divided into the parts passing the Earth on the dawnward or duskward sides. The importance of this bifurcation point for the substorm onset has long been discussed controversially. Presently there is more and more observational evidence provided that the intense field-aligned current (FAC) associated with the substorm break-up is connected to the Harang discontinuity. Based on a catalogue of more than 4000 substorms a statistical study of the onset characteristics has been performed. For about 50 cases nearby CHAMP observations are available. They show systematically the presence of a strong upward FAC in the center flanked by weaker downward currents on the poleward and equatorward sides. In certain cases the magnetic field variations exhibit some helical signatures. We will try to explain the observations in a global picture.
[en] Complete text of publication follows. The recent magnetic field missions have brought about an enormous progress in the understanding of near-Earth external field structures and the description of their sources. Based on observations from Oersted, CHAMP and SAC-C a rather comprehensive picture of their temporal-spatial variability could be achieved. It is the first time that magnetic effects from the ring current and the magnetospheric tail currents could be separated. With that it became evident that part of the Sq current signature on ground is caused by the tail current. In recent years more and more evidence has been presented for the significance of currents at F region altitudes where ion-neutral collisions are less important. These currents are driven predominantly by plasma processes. Among others, gravity and pressure gradient forces are acting on the charged particles. In particular, plasma irregularities like plasma bubbles cause detectable magnetic signatures. These F region currents are shown to flow also on the night side, and they cause magnetic fields of order 5 nT. Techniques have been developed for correcting to for first order the magnetic effect of plasma gradient currents. At low latitudes the effect of the F-region dynamo could be verified experimentally. This wind-driven generator sets up meridional current systems which peak around noon and sunset. An appropriate method would be to model all the ionospheric currents self-consistently and predict the magnetic field corrections. The constellation mission Swarm is expected to provide the necessary observational input for such an approach.
[en] Complete text of publication follows. A solar terminator wave has been revealed in thermospheric wind and density simultaneously observed by CHAMP. The wind terminator wave is out of phase with the density terminator wave. But both have wavefronts about 30 circ$ inclined to the terminator line at low latitudes, and wavelengths ranging between 3000-5000 km. They show a clear dawn-dusk asymmetry, with more pronounced wave signatures forming at dusk. Terminator wave is indiscernible in the dawnside wind. Most wave structures are observed at night, with some extension to the sunlit region around solstices. The midnight density maximum is seen to be closely connected to terminator wave structures, hence indicating a possible role of terminator waves in its formation.
[en] Complete text of publication follows. Four years (2002-2005) of continuous accelerometer measurements taken onboard the CHAMP satellite (orbit altitude ∼400 km) present a unique opportunity to investigate the thermospheric zonal wind on a global scale. Recently we were able to relate the identified wave-4 structure in the zonal wind at equatorial latitudes to the influence of nonmigrating tides and in particular to the eastward propagating diurnal tide with zonal wavenumber 3 (DE3). The DE3 tide is primarily excited by latent heat release in the tropical troposphere in deep convective clouds and thus was not expected to be found at 400 km altitude. In order to investigate the mechanisms that couple the tidal signals all the way to the upper thermosphere we started a comparison with the thermosphere-ionosphere-mesosphere-electrodynamics general circulation model (TIME-GCM) developed at the National Center for Atmospheric Research (NCAR). Therefore, the model output was processed the same way as the satellite data. Initial results for June solstice show a good agreement between the model and the satellite data for the westward propagating tides. Yet the model is underestimating the eastward propagating zonal wavenumber 2 diurnal tide (DE2) which is quite prominent in the CHAMP data. Furthermore, the model predicts a solar flux dependence of the tides with increasing (decreasing) amplitudes for the westward (eastward) propagating tides with increasing solar flux level. We can confirm the dependence on the solar flux level for the nonmigrating tidal signatures in CHAMP data as well.
[en] Complete text of publication follows. We analyse Field-Aligned Current (FAC) measurements on board the CHAMP satellite, conducted on days 30.06-02.07.2008. In general, there are 91 crossings over polar regions (46 in the Northern hemisphere and 45 - in the Southern hemisphere). The data are gathered under quiet geomagnetic conditions (Kp < 1, Vsw < 450 km/s, and IMF Bz is within ±3.5nT). On these days most of the FAC measurements fall in the noon (MLT =10-14, Mlat = 55-85) and midnight (MLT =16-12, Mlat = 55-85) sectors. Our preliminary analysis reveals a multitude of alternating medium-scale (at least 1-2 degrees) FAC sheets of reverse sign and with increasing amplitude. Determination of FAC by single satellite measurement however is accompanied with the following uncertainties: i) underestimation of the FAC density and ii) appearance of false FAC structures. Consecutively we apply several procedures to reduce the occurrence of such errors. First, FAC structures of density less than 0.1 μmA/m2 were neglected; secondly, all FAC obtained at attack angles (the angle between the satellite trajectory and polar oval) considerably different from 90 degrees (e.g. for angle less than 65 degrees) were discarded. Despite these methodological constraints, the existence of multiple FAC close to the noon-midnight meridian under quiet geomagnetic conditions remains. The multiple FAC structure is more frequent in the night sector, with as much as 7 sheets of alternate sign for IMF Bz > 0. This issue is verified for both Northern (summer) and Southern (winter) hemispheres. The number of all FAC sheets observed in the Southern hemisphere, however, is less than in the Northern one. FAC sheets poleward of FAC Region 1 were also detected. Possible mechanisms of multiple FAC structures under quiet geomagnetic conditions are considered.
[en] Complete text of publication follows. Parameters of Pc2-3 pulsations, measured from auroral to middle latitudes on the ground surface at the MM100 meridional chain in the F-layer of the Earth's ionosphere by CHAMP satellite are analyzed. The combination of ground and ionospheric measurements at different geomagnetic latitudes allows to discriminate between temporal and spatial variations of pulsations' wave field, and the contribution of pulsations of different frequencies, polarization and spatial scales to the observed picture. The active role of the Earth's ionosphere in generation of local Pc2/3 disturbances is also discussed. It is found that typical signal spectra is enriched by higher frequencies in the ionosphere in comparison with the ground measured signal. Typical for Alfvenic resonance frequency dependence on latitude is sufficiently weaker in the ionosphere, than on the ground. Partly this can be explained by the short wavelengths of the pulsations in the ionosphere which are effectively damped by the ionospheric E-layer and not seen on the ground surface. Day to night amplitude ratio is low for the pulsations, measured in the ionosphere, in comparison with those, measured on the ground surface. Multiharmonic pulsations are more often in the ionosphere. The observed difference in main morphological features of pulsations on the ground and the ionosphere shows that along with extra-magnetospheric parameters and Alfvenic resonances, that control mostly the amplitude and frequency of big-scale MHD waves, registered on the ground as Pc3, for ionospheric pulsations with smaller spatial scales fine parameters of the field aligned distribution of plasma in the magnetosphere and ionosphere and, probably, intra-magnetospheric local sources are important.