Spacecraft potential variations inside the magnetopause during transient events: Geotail observations

We examine the spacecraft potential (SP) variations near the magnetopause. In the magnetosphere the Geotail satellite often observed transient SP increase accompanied by magnetic field enhancements. This signature (SP increase and magnetic field increase) was also observed inside the magnetopause during the outbound (from the magnetosphere to the magnetosheath) and/or inbound (from the magnetosheath to the magnetosphere) magnetopause crossings. For the interval of the SP increase, the plasma density and temperature were intermediate between those of the magnetosheath and the magnetosphere, and strong enhancements of the field-aligned bidirectional electron fluxes were observed mainly in the medium energy (∼300-450 eV) range. These observations are consistent with previous studies in the inner part of the low-latitude boundary layer (LLBL). Thus we suggest that the transient SP increase in the magnetosphere may be a good indicator of the entry into the inner LLBL.