Comparative study of Geomagnetic Sudden Commencement (SC) between Oersted and ground observations at different local times

Abstract

Oersted is a low‐altitude (638–849 km) polar‐orbiting satellite. Using vector magnetic field measurements obtained from Oersted, we identified more than 20 geomagnetic sudden commencement (SC) events on both dayside (0600–1800 MLT) and nightside (1800–0600 MLT). The unique properties reflected by these events have never been reported before. The SCs observed by Oersted in the B// (compressional) component on the nightside had the nearly same waveforms as those observed on the ground in the H (northward) component. We suggest that the SCs observed by Oersted on the nightside were dominantly caused by the enhanced magnetopause currents, which were transmitted by the compressional hydromagnetic waves, and the effects of the ionospheric current (IC) were negligible on the nightside. The SC waveforms observed by Oersted on the dayside were apparently different from those observed on the ground. Near the dayside dip equator (DDE), corresponding to preliminary reverse impulses (PRIs) observed in the ground H component, Oersted always observed positive impulses in the B// component, which suggest that the PRIs at the DDE are generated by westward ICs. On the dayside, corresponding to positive main impulses (MIs) of SCs observed in the ground H component, the Oersted B// component always presented clear decreases, which implies that an eastward IC was excited after the PRI. The generation mechanism for the westward and eastward ICs are discussed according to previously proposed models. On the dayside, we suggest that the waveforms observed both on the ground and at Oersted during the time period of PRI and MI were superposition of the incident compressional waves and the disturbance fields caused by the ICs. The features observed by Oersted just above the ionosphere are significant complementary to our empirical knowledge for SCs.