Sub-kilometer thermal plasma structure near 1750 km altitude in the polar cusp/cleft

We present Freja Cold Plasma Analyzer (CPA) measurements from an encounter with the low altitude (∼1750 km) polar cusp during which the CPA measured 2-D images of the thermal (0–16 eV) particle distributions at 1.2 s time resolution, and simultaneously made rapid estimates (600/s) of integrated thermal particle flux into the instrument. The high resolution data show bursty ion flux enhancements of the order of tens of percent on time scales of tens of ms, or alternatively, hundreds of m spatial scales. The flux of electrons from 0–16 eV also varied by tens of percent and on temporal/spatial scales comparable to those in the ion cases. There is some evidence that the thermal particle flux variations are associated with intense low-frequency electromagnetic fluctuations with temporal/spatial scales identical to those seen by the CPA (tens of ms, hundreds of m).