Cusp energetic particle events measured by POLAR spacecraft

The Charge and Mass Magnetospheric Ion Composition Experiment (CAMMICE) on board the POLAR spacecraft observed more than 70 cusp energetic particle (C EP) events in 1996. All of these events were associated with a decrease in the magnitude of the local magnetic field measured by the Magnetic Field Ex periment (MFE) on POLAR. This is an important discovery, which reveals six interesting features for the CEP events: (1) They all were detected in the dayside at high latitude near apogee; (2) their energies were in excess of 2.4 MeV; (3) an individual. CEP event could last for hours; (4) the event-a veraged intensity of 1-200 keV/e helium was anticorrelated with the magnitu de of the local geomagnetic field but correlated with the turbulent magneti c energy density; (5) the events were associated with an enhancement of the low frequency electromagnetic noise; and (6) a possible seasonal variation was found for the occurrence rate of the events with a maximum in Septembe r. The measured high charge state of helium and oxygen ions in the CEP even ts indicates a solar source for these particles. A possible explanation is that the energetic helium ions are energized from lower energy helium by a local acceleration mechanism associated within the high-altitude dayside cu sp. These observations represent a discovery of a major acceleration region of the magnetosphere. (C) 1998 Elsevier Science Ltd. All rights reserved.