ULF pulsations observed by the ETS-VI satellite: Substorm associated azimuthal 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 pulsations in Pc 3–5 frequency ranges in the magnetosphere. The observation of ETS-VI covered the invariant latitude (ILAT) range of 64.5°−69°ILAT near the geomagnetic equator (−10°−20° magnetic latitude) at all magnetic local time (MLT). Magnetic pulsations were selected by the Fast Fourier Transform method and checked by visual scanning if they have continuous waveforms. From the occurrence 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) azimuthal Pc 4 pulsations on the nightside. In respect of the first three types of pulsations (i.e., the azimuthal Pc 5 pulsation, the azimuthal Pc 3 pulsation, and the radial Pc 4 pulsation on the dayside), the results presented in this study confirm the previous results obtained by other satellite observations. The azimuthal Pc 4 pulsations on the nightside were observed in continuous waveforms lasting for about 10 minutes. Although the azimuthal Pc 4 pulsations on the nightside start at almost the same time as substorm onsets, they are different from Pi 2 pulsations 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 coupling to the fast mode Alfvén waves which were launched at substorm onset.