CUSP ENERGETIC PARTICLE EVENTS - IMPLICATIONS FOR A MAJOR ACCELERATION REGION OF THE MAGNETOSPHERE

The Charge and Mass Magnetospheric Ion Composition Experiment (CAMMICE ) on board the Polar spacecraft observed 75 energetic particle events in 1996 while the satellite was at apogee. All of these events were as sociated with a decrease in the magnitude of the local magnetic field measured by the Magnetic Field Experiment (MFE) on Polar. These new ev ents showed several unusual features: (1) They were detected in the da yside polar cusp near the apogee of Polar with about 79% of the total events in the afternoonside and 21% in the morningside; (2) an individ ual event could last for hours; (3) the measured helium ion had energi es up to and many times in excess of 2.4 MeV; (4) the intensity of 1-2 00 KeV/e helium was anticorrelated with the magnitude of the local geo magnetic field but correlated with the turbulent magnetic energy densi ty; (5) the events were associated with an enhancement of the low-freq uency magnetic noise; the spectrum of which typically extends from a f ew hertz to a few hundreds of hertz as measured by the Plasma Wave Ins trument (PWI) on Polar; and (6) a seasonal variation was found for the occurrence rate of the events with a maximum in September. These char acterized a new phenomenon which we are calling cusp energetic particl e (CEP) events. The observed high charge state of helium and oxygen io ns in the CEP events indicates a solar source for these particles. Fur thermore, the measured 0.52-1.15 MeV helium flux was proportional to t he difference between the maximum and the minimum magnetic field in th e event. A possible explanation is that the energetic helium ions are energized from lower energy helium by a local acceleration mechanism a ssociated with the high-altitude dayside cusp. These observations repr esent a potential discovery of a major acceleration region of the magn etosphere.