ULF PULSATIONS OBSERVED BY THE ETS-VI SATELLITE - SUBSTORM ASSOCIATEDAZIMUTHAL PC-4 PULSATIONS ON THE NIGHTSIDE

The magnetic field data from the Engineering Test Satellite-VI (ETS-VI ) have been analyzed to investigate the occurrence distributions of pu lsations in Pc 3-5 frequency ranges in the magnetosphere. The observat ion of ETS-VI covered the invariant latitude (ILAT) range of 64.5 degr ees-69 degrees ILAT near the geomagnetic equator (-10 degrees-20 degre es magnetic latitude) at all magnetic local time (MLT). Magnetic pulsa tions were selected by the Fast Fourier Transform method and checked b y visual scanning if they have continuous waveforms. From the occurren ce distributions of pulsations, we have found distinctive features in the following pulsations: (1) azimuthal Pc 5 pulsation; (2) azimuthal Pc 3 pulsation; (3) radial Pc 4 pulsation on the dayside; (4) azimutha l Pc 4 pulsations on the nightside, In respect of the first three type s of pulsations (i.e., the azimuthal Pc 5 pulsation, the azimuthal Pc 3 pulsation, and the radial Pc 4 pulsation on the dayside), the result s presented in this study confirm the previous results obtained by oth er satellite observations. The azimuthal Pc 4 pulsations on the nights ide were observed in continuous waveforms lasting for about 10 minutes . Although the azimuthal Pc 4 pulsations on the nightside start at alm ost the same time as substorm onsets, they are different from Pi 2 pul sations in the magnetosphere. They are observed frequently in the MLT range of 23-04MLT with an occurrence peak at 01-02MLT. We suggest that the azimuthal Pc 4 pulsations on the nightside are excited through co upling to the fast mode Alfven waves which were launched at substorm o nset.