Phys. Chem. Earth(C), Vol. 24, No. 1-3, pp. 135-140, 1999
© 1998 Elsevier Science Ltd

Cusp Energetic Particle Events Measured by POLAR Spacecraft

T.A. Fritz1, Jiasheng Chen1, Robert B. Sheldon1, Harlan E. Spence1, Joseph F. FennellMsup>2, Stefano Livi3, Christopher T. Russell4, and Jolene S. Pickett5

1Center for Space Physics, Boston University, 725 Commonwealth Ave, Boston, MA 02215 U.S.A.
2The Aerospace Corporation, Los Angeles, CA, 90009 U.S.A.
3Max-Planck-Institut fur Aeronomie, 37189 Katlenburg-Lindau, Germany
4Institute of Geophysics and Planetary Physics, Unversity of California, Los Angeles, CA90095-1567 U.S.A.
5Department of Physics and Astronomy, The University of Iowa, Iowa Cita, IA 52242 U.S.A.

Abstract. The Charge and Mass Magnetospheric Ion Composition Experiment (CAMMICE) on board the POLAR spacecraft observed more than 70 cusp energetic particle (CEP) 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 Experiment (MFE) on POLAR. This is an important discovery, which reveals six interesting features for the CEP events: (1) They were all 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-averaged intensity of 1-200 keV/e helium was anti-correlated with the magnitude of the local geomagnetic field but correlated with the turbulent magnetic field 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 September. The measured high charge state of helium and oxygen ions in the CEP events 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 cusp. These observations represent a discovery of a major acceleration region of the magnetosphere.