Identification of magnetosheath mirror modes in Equator-S magnetic field 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 a nd 10:40 LT. Clear mirror-like structures, characterised by compressive flu ctuations in \B\ on occasion lasting for up to 5 h, were observed during a significant fraction of these orbits. During most of these passes the satel lite appeared to remain close to the magnetopause (within 1-2 Re), during s ustained compressions of the magnetosphere, and so the characteristics of t he mirror structures are used as a diagnostic of magnetosheath structure cl ose to the magnetopause during these orbits. It is found that in the majori ty of cases mirror-like activity persists, undamped, to within a few minute s 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 satel lite, at least during the subset of orbits containing strong magnetosheath mirror-mode signatures. Power spectra for the mirror signatures show predom inately field aligned power, a well defined shoulder at around 3-10 x 10(-2 ) Hz and decreasing power at higher frequencies. On occasions the fluctuati ons are more sinusoidal, leading to peaked spectra instead of a shoulder. I n all cases mirror structures are found to lie approximately parallel to th e observed magnetopause boundary. There is some indication that the amplitu de of the compressional fluctuations tends to be greater closer to the magn etopause. This has not been previously reported in the Earth's magnetospher e, but has been suggested in the case of other planets.