Identi®cation of magnetosheath mirror modes in Equator-S magnetic ®eld data

Between December 1997 and March 1998 Equator-S made a number of excursions into the dawnside magnetosheath, over a range of local times between 6:00 and 10:40 LT. Clear mirror-like structures, characterised by compressive ̄uctuations in |B| on occasion lasting for up to 5 h, were observed during a signi®cant fraction of these orbits. During most of these passes the satellite appeared to remain close to the magnetopause (within 1±2 Re), during sustained compressions of the magnetosphere, and so the characteristics of the mirror structures are used as a diagnostic of magnetosheath structure close to the magnetopause during these orbits. It is found that in the majority of cases mirror-like activity persists, undamped, to within a few minutes of the magnetopause, with no observable ramp in |B|, irrespective of the magnetic shear across the boundary. This suggests that any plasma depletion layer is typically of narrow extent or absent at the location of the satellite, at least during the subset of orbits containing strong magnetosheath mirror-mode signatures. Power spectra for the mirror signatures show predominately ®eld aligned power, a well de®ned shoulder at around 3± 10 ́ 10)2 Hz and decreasing power at higher frequencies. On occasions the ̄uctuations are more sinusoidal, leading to peaked spectra instead of a shoulder. In all cases mirror structures are found to lie approximately parallel to the observed magnetopause boundary. There is some indication that the amplitude of the compressional ̄uctuations tends to be greater closer to the magnetopause. This has not been previously reported in the Earth's magnetosphere, but has been suggested in the case of other planets.