ULF Waves Simultaneously Observed by SuperDARN and GEOTAIL Around LLBL
Y. TONEGAWA, T. SAKURAI, Y. MATSUI [1], N. SATO, H. YAMAGISHI, A. YUKIMATSU [2], SuperDARN/GEOTAIL Research Group
[1] Tokai University, Japan
[2] National Institute of Polar Research, Japan
We have carried out coordinated observations of GEOTAIL, SuperDARN, and ground magnetometers in order to investigate energy flow of ULF waves from the outer magnetosphere to the ground through the ionosphere. During GEOTAIL was skimming the dayside magnetopause, the ionosphere near the magnetic footprints of the satellite was searched by SuperDARN radars with high time resolution mode. This kind of special operation of SuperDARN has been carried out several times since 1998. In general, GEOTAIL observes transverse Pc 5 waves mainly in the morning sector, and compressional Pc 5 waves in the dusk side LLBL. The transverse Pc 5 waves are also detected in a wide region of the ionosphere and on the ground. On the other hand, the compressional waves are not observed on the ground. Interesting two events in which transverse and compressional Pc 5 waves were successively excited in the dusk side LLBL were observed by GEOTAIL on January 25, and 30, 1999. In both cases, transverse waves were observed simultaneously in the ionosphere and on the ground. Latitudinal polarization characteristics shown in the IMAGE magnetometers suggests that the transverse waves are propagating eastward and resonating with the field line around 70º magnetic latitude. On the other hand, the compressional waves were simultaneously observed only in the ionosphere, not on the ground. This result suggests a screening effect by the ionosphere for the compressional waves, so that the spatial scale of the waves is to be small. However, the phase analysis of a longitudinal radar beam shows that the m number of the compressional waves is about 13. Therefore, we need another explanation why the compressional waves were not observed on the ground. Looking over additional events found in the coordinated observations, we discuss generation and propagation mechanisms of Pc 5 waves.
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