ALBERT

Description: Field-aligned currents (FACs) are an important mechanism for transporting energy and momentum from the solar wind into the upper atmosphere at high latitudes. The efficiency of energy dissipation depends on the ionospheric conductivity and the transverse scale size of the field-aligned current circuits. According to Vogt (2002) the conversion of electric current into heat should be most effective for scales of the order of some 10 km when typical ionospheric conductivities are considered. In order to test this conclusion we have performed a statistical analysis with FACs of various scale sizes. This study is based on observations that are derived from CHAMP magnetic field measurements taken during the years 2002-2005. FAC densities within the medium-scale wavelength band of 15-150 km and large-scale FACs (〉150 km) are considered separately and their distributions are compared. The thermospheric mass density recorded simultaneously by CHAMP is used as a measure for the heat deposited in the ionospheric E-layer. The relation between current strength of the two FAC types and thermospheric heating is determined in a superposed epoch analysis. The analysis is performed separately for each season and covers all local time sectors.

Description: After decades of intensive study the question of the origin of dayside Pc3 (ULF waves in the 20-100 mHz band) activity still has not been fully answered. Kelvin-Helmholtz instability at the magnetopause, cavity resonances in different regions of the magnetosphere were proposed as possible drivers of dayside field line resonances, which are regularly observed both in the magnetosphere and on the ground along closed field lines. In the last decade the ULF waves generated in the upstream region (called upstream ULF waves) received little attention, although the relation of ground and magnetospheric Pc3 pulsations to interplanetary conditions (solar wind speed, IMF strength, IMF cone angle) was discovered early in the 1970’s, and also the theory of upstream wave generation was developed. In a recent study we demonstrated a clear correspondence between upstream- related Pc3 events observed at LEO by CHAMP and by the MM100 ground magnetometer array (Heilig et al. 2007). In the case studies presented here dayside Pc3s, observed simultaneously in the terrestrial foreshock (by CLUSTER), in the topside ionosphere (by CHAMP) and on the ground (by MM100 array), are compared. We find, in accordance with theory, that the frequencies of these waves are governed by the IMF strength, and that the wave energy depends on the IMF cone angle. However, a direct correspondence between foreshock ULF events and dayside Pc3s could not be established. Narita et al. (2004), who studied foreshock waves observed by CLUSTER during the months (February-March 2002), found that the majority of these waves are Alfvén waves propagating upstream along IMF and that their period is proportional to the IMF strength. Our combined results strongly support the upstream origin of the considered dayside pulsations. However, it remained an open question how the upstream wave energy couples into the magnetosphere.

Description: The effect of non-migrating tidal waves has recently been identified in many atmospheric and ionospheric parameters. Here we focus on the diurnal, eastward propagating wavenumber 3, DE3, mode. The satellite CHAMP with its sensitive accelerometer on board provides the opportunity to investigate the thermospheric dynamics in great detail. On its near-polar, low-Earth orbit (about 400 km) it is well suited to map the air density and winds along the track. In this talk we concentrate on the equatorial region. Special attention is paid to longitudinal variations of the thermospheric zonal wind. During certain seasons and local times a clear wave-4 longitudinal pattern is evident in the wind speed. From the phase change of this wave over an LT day this pattern can be related to the DE3 tidal mode. So far it has been assumed that the non-migrating wave signal is dissipating at heights above 120 km. Therefore it is still not clear how the wave-4 pattern is coupled into the upper thermospheric zonal wind. At E-layer altitudes the DE3 tides cause modulations of the electric field. These modulations are well recognisable in the longitudinal variations of the equatorial electrojet (EEJ). High-resolution CHAMP magnetic field recordings have been used to investigate the EEJ variations on a statistical basis. From them the response of the E-layer to DE3 tides could clearly be identified. We are going to compare the phase relation and the annual amplitude variation of the current signals at 110 km and the winds at 400 km altitude. From the results suggestions for the coupling mechanisms are deduced.

Description: The input of energy and momentum from the magnetosphere is most efficiently coupled into the high latitude ionosphere, thermosphere. There are, however, several mechanisms which channel part of the disturbances to low and mid latitudes. Some of these mechanisms cause a prompt response, for example, the penetration electric field. Other processes require hours before the perturbation has reached the equator (e.g. travelling atmospheric disturbances). Here we will present some recent observations primarily derived from the CHAMP satellite. With its sensitive accelerometer it can measure the air density and zonal winds. During magnetic storms the thermospheric density is enhanced first at high latitudes and some hours later the bulge reaches the equator. The response time is shorter on the dayside than on the night side. None of these thermospheric responses to magnetospheric inputs are reproduced well by present-day atmospheric models. Another topic to be addressed will be the low latitude ionosphere/thermosphere response to substorm onsets. Based on a large number of substorm events the average response to this kind of disturbance was deduced. We compare ground-based and satellite observations. This allows us to distinguish between ionospheric and magnetospheric currents. There is increasing evidence for significant ionospheric currents even during the night. As a consequence of that many of the interpretations based solely on observatory data from the night side have to be reconsidered.

Description: In this study, we investigate two global climatological data sets; the occurrence rate of Equatorial SpreadF (ESF), associated with equatorial plasma irregularities, at ∼400 km altitude obtained from CHAMP observations, and the evening equatorial vertical plasma drift, vz , from ROCSAT-1 measurements. First, as retrieved for a solar flux level of F10.7=150, the longitudinal variation of the two independently derived quantities correlates between 84% and 93% in the seasons December solstice, equinox and June sol- stice. The highest correlation is found for the solstice seasons when vz is integrated over local time around the prereversal enhancement (PRE) and displaced 6◦ towards east. The integrated vz is a suitable estimate of the ionospheric height at the time just after the PRE and the 6◦ displacement is con- sistent with ESF eastward drift during 2 h which is assumed between creation and detection at satellite altitudes. Second, our analyses reveal a global threshold vz which is required to observe ESF at satellite altitudes. This threshold depends linearly on solar flux with correlations of 97%. Both results bring global evidence on the linear relations between ESF and the vertical plasma drift which have been proven only by local observations so far. This paper includes the first global map of the seasonal/longitudinal variation of the ESF occurrence rate over local time being valid for high solar flux years 2001–2004. The map reveals, e.g. a longitudinal dependence of the persistence of the plasma irregularities indicating that longitude dependent mechanisms other than the PRE determine the ESF lifetime.

Description: The Equatorial Ionization Anomaly (EIA) is a signicant feature of the low latitude ionosphere. During daytime, the eastward electric field drives a vertical plasma fountain at the magnetic equator creating the EIA. Since the eastward electric field is also the driving force for the Equatorial Electrojet (EEJ), the latter is positively correlated with the EIA strength. We investigate the correlation between the zonal electric field and the EIA in the Peruvian sector, and compare the results with correlations of the EEJ versus EIA strength. Analyzing 5 years of CHAMP electron density measurements, plasma drift readings from the JULIA radar, and magnetic field observations at Huancayo and Piura, we find the EEJ strength and the zonal electric field to be suitable proxies for the EIA intensity. Both analyses re- veal high correlation coe±cients of cc 〉 0:8. A typical response time of the EIA to variations in the zonal electric field is »1-2 hours and it is »2- 4 hours after EEJ strength variations. Quantitative expressions are provided, which directly relate the EIA parameters to both proxies. From these rela- tions, we infer that an EIA develops also during weak Counter Electrojets (CEJ), but no EIA forms when the vertical plasma drift is zero. For posi- tive EEJ magnetic signatures to form, a minimum eastward electric field of 0.2 mV/m is required on average. The above mentioned delay between EIA and EEJ variations of »3 hours is further confirmed by the investigation of the EIA response to transitions from CEJ to EEJ, e.g., during late morning hours.