Polar cusp

Polar saw different signatures at high altitudes on these two days. When passing through the high altitude region near local noon, Polar normally sees the depressed magnetic field associated with the plasma entry into the cusp. On May 14, the magnetic field depression was clearly seen from 1230 to 1452 UT (from 82.5^o to 85.6^o magnetic latitudes at the Polar ionospheric footprints), but the cusp signature was completely absent on May 11 as Polar passed through the same region at the same altitude. This indicates that the energy density of the entering plasma in the cusp was much smaller than usual, consistent with the greatly reduced solar wind number density on May 11 with presumably a typical temperature in the magnetosheath.