Coordinated observations of Pc3 pulsations near cusp latitudes

[1] In spite of accumulated observations of ULF pulsations, there are no satisfactory explanations for the generation and transmission mechanisms of Pc3 pulsations at very high latitudes. We studied the characteristics of the high-latitude Pc3 pulsations by using data acquired on the ground by magnetometer arrays, in the ionosphere by the Super Dual Auroral Radar Network (SuperDARN) of HF radars, and in space by the Geotail satellite. The observations revealed that narrowband Pc3 pulsations are occasionally observed simultaneously in magnetic field data on the ground and in Doppler velocity data acquired by the HF radars in the intervals of small interplanetary magnetic field (IMF) cone angles. We find that there is a clear peak of the Pc3 power near the cusp latitudes and that the power of pulsations decreases very steeply with latitude. At times, there are periods of high coherence between the pulsations in the dawn magnetosheath and on the ground, although the spectra of the magnetosheath magnetic field variations exhibit broadband signatures. The Poynting flux in the Pc3 band increases in the magnetosheath when narrowband Pc3 pulsations are observed in the cusp region. The results suggest that the driving source is located in the magnetosheath and that part of the MHD energy in the magnetosheath propagates to the ionospheric cusp. We suggest that the transmission of Pc3 pulsations in the cusp region requires not only a non-MHD mechanism as suggested (e.g., modulated precipitation of trapped electrons) but also an MHD wave incident into the ionosphere.