During the IMF rotation, GEOTAIL was in the duskside magnetosphere. At 21:28 UT, GEOTAIL suddenly moved into a denser plasma environment, most probably the LLBL. Afterwards, oscillatory perturbations appear in many variables, which persist over several hours.
Figure 5 shows an example of GEOTAIL data for the interval of 22:39-23:00 UT. The bottom panel shows the potential difference between an electric field probe and the satellite which is related to the ambient electron density[Laakso and Pedersen, 1997]; the more negative , the lower the density. When -5 volts, the spacecraft is well inside the magnetosphere, whereas it is in the magnetosheath when -2 volts. When is between these two regimes, the spacecraft is usually in the LLBL. Therefore the observed 's suggest that during 21:30-24:00 UT, GEOTAIL encountered the magnetopause a few times, otherwise the spacecraft was either in the LLBL or in the plasma sheet, as illustrated in Figure 5. The top three panels in Figure 5 show magnetic field vectors in three planes, indicating that magnetic field perturbations are dominated by large-scale vortices, particularly whenever the spacecraft is in the LLBL. In the plasma sheet, both magnetic field vortices and compressional waves were observed. The strongest signals were observed at 1-15 mHz.
well correlated so that large 's occur at large densities. Figure 6 shows the power spectral densities of (solid line) and (dashed line) at 1-20 mHz for the 22:05-22:56 UT interval. A good correlation between these two variables suggests that the fluctuations are compressional perturbations; possible sources are surface waves moving the spacecraft between two plasma regions and fast mode waves generated by the K-H instability. Lowest frequencies in Figure 6 are related to surface waves and are in agreement with the power spectra of ULF waves observed with the high-latitude ground-based magnetometers [Clauer et al., 1997].