Mirror mode structures observed in the dawn-side magnetosheath by Equator-S

The Equator-S satellite was ideally positioned to make magnetic field obser vations in the dawn-side magnetosheath, relatively close to the magnetopaus e. The magnetosheath data were particularly rich in compressional signature s, consistent with mirror mode structures, which occurred during similar to 30% of orbits crossing into the magnetosheath. In most, although not all c ases, strongly compressive signatures extended up to the magnetopause bound ary, with no increase in the underlying magnetic field magnitude on the tim e scale of ten to thirty minutes. The proximity and character of mirror-lik e fluctuations near the magnetopause suggest that in the dawn-side magnetos heath the plasma depletion layer (PDL) is of narrower extent than is genera lly observed closer to the subsolar point, or is absent.