Sudden compression of the outer magnetosphere associated with an ionospheric mass ejection

On September 24, 1998 at 2345 UT the magnetosphere was suddenly compressed as the dynamic pressure of the solar wind rapidly rose from 2 to 15 nPa. At the Polar spacecraft, at high altitudes above the center of the northern polar cap, a remarkably smooth increase in the field strength occurred while the plasma properties changed abruptly, as described in an accompanying paper. Comparisons with models and an examination of the wave amplitudes during the compression indicate that the initial change in plasma properties was most probably due to convection of pre‐existing boundary layer plasma to the location of Polar rather than due to local heating by betatron acceleration and ion cyclotron waves. The smoothness of the increase in field strength is attributed to the very high velocity of compressional waves in the tail that outrun the advancing solar wind disturbance. The signatures as measured by GOES 10 at 1444 LT and at GOES 8 at 1846 LT in low latitude geosynchronous orbit are the more familiar sudden jump on the dayside, where the density is high and the compressional wave velocity low, and a weak change on the nightside, where tail current changes oppose the effects of the dayside magnetopause currents. This event is an ideal candidate for collaborative investigation of the effects of a classical sudden storm commencement on the magnetosphere.