ALBERT

Description: CHAMP passes the geographic poles at a distance of 2.7° in latitude, thus providing a large number of magnetic readings of the dynamic auroral regions. The data of these numerous overflights were used for a detailed statistical study on the level of activity. A large number of tracks with very low rms of the residuals between the scalar field measurements and a high degree field model were singled out over both the northern and southern polar regions, independently. Low rms values indicate best model fits and are therefore regarded as a measure of low activity, although we are aware that this indicator also has its limitations. The occurrence of quiet periods is strongly controlled by the solar zenith angle at the geomagnetic poles, indicating the importance of the ionospheric conductivity. During the dark polar season, about 30% of the passes can be qualified as quiet. The commonly used magnetic activity indices turn out not to be a reliable measure for the activity state in the polar region. Least suitable is the Dst index, followed by the Kp. Slightly better results are obtained with the PC and the IMAGE-AE indices. The latter is rather effective within a time sector of ±4 hours of magnetic local time around the IMAGE array. The orientation of the interplanetary magnetic field (IMF) is an important controlling factor for the activity. This is also supported by the prevailing FAC distribution during quiet times, which resembles the typical NBZ (northward Bz) pattern. In a superposed epoch analysis we show that the merging electric field is a suitable geoeffective solar wind parameter. Based on the size of this electric field and the solar zenith angle at the geomagnetic poles, a prediction method for quiet auroral region periods is proposed. This may, among others, be useful for the data selection in main field modelling approaches.

Description: Simultaneous whistler records of one station and geomagnetic pulsation (Pc3) records at three stations were compared. In a previous study correlation was found between occurrence and L value of propagation/excitation for the two phenomena. The recently investigated simultaneous records have shown that the correlation is better on longer time scales (days) than on shorter ones (minutes), but the L values of the propagation of whistlers/excitation of pulsations are correlated, i.e. if whistlers propagate in higher latitude ducts, pulsations have periods longer than in the case when whistlers propagate in lower latitude ducts.Key words: Electromagnetics (wave propagation) - Magnetospheric physics (magnetospheric configuration and dynamics; MHD waves and instabilities)

Description: One important contribution to the magnetic field measured at satellite altitude and at ground level comes from the external currents. We used the total field data sampled by the Overhauser Magnetometer on CHAMP and the horizontal magnetic field measurements of the IMAGE ground-based magnetometer network to study the ionospheric Hall current system in the auroral regions. For the CHAMP data a current model consisting of a series of lines and placed at a height of 110km is fitted to the magnetic field signature sampled on the passage across the polar region. The derived current distributions depend, among others, on season and on the local time of the satellite track. At dawn/dusk the auroral electrojets can be detected most clearly in the auroral regions. Their intensity and location are evidently correlated with the A E activity index. For a period of almost two years the results obtained from space and the currents determined from ground-based observations are studied. For the full IMAGE station array a newly-developed method of spherical elementary current systems (SECS) is employed to compute the 2-D equivalent current distribution, which gives a detailed picture of an area covering latitudes 60° – 80° N and 10° – 30° E in the auroral region. Generally, the current estimates from satellite and ground are in good agreement. The results of this survey clearly show the average dependence of the auroral electrojet on season and local time. This is particularly true during periods of increased auroral activity. The correlation coefficient of the results is close to one in the region of sizeable ionospheric current densities. Also the ratio of the current densities, as determined from above and below the ionosphere, is close to unity. It is the first time that the method of Hall current estimate from a satellite has been validated quantitatively by ground-based observations. Among others, this result is of interest for magnetic main field modelling, since it demonstrates that ground-based observations can be used to predict electrojet signatures in satellite magnetic field scalar data. Key words. Ionosphere (auroral Ionosphere; electric fields and currents; ionosphere-magnetosphere interactions)