Features of the cusp energetic particle events

In 1996, the POLAR spacecraft discovered a new phenomenon called a cusp energetic particle (CEP) event, in which the He ions had energies up to 8 MeV. A total of 75 CEP events have been reported (Chen et al., 1998). The CEP events showed six unusual features: (1) They were detected in the high altitude dayside cusp; (2) large fluxes of energetic (> 20 keV) ions were observed for periods of hours where such "trapped" ions should not be stably trapped; (3) the associated magnetic field was turbulent and greatly depressed appearing to reach a null value in many cases; (4) the 1-200 keV/e helium ions are He++; (5) compared to O>+2, the O<+3 is negligible; and (6) at 1-200 keV/e, the He++ particles are the dominant heavy ions with an intensity of about one order of magnitude larger than the oxygen ions. During periods when the GEOTAIL was around the bow shock and the POLAR was in the cusp, the simultaneous observations indicated that the ion fluxes in the CEP events were higher than that in the bow shock. We have also compared the proton flux in the October 14, 1996 CEP event with that in the equatorial radiation belt (L = 6.6) at midnight, and found that both fluxes were comparable. These new features and observations imply that the discovery of the CEP events may open a new avenue for understanding the magnetosphere. (C) 1999 COSPAR. Published by Elsevier Science Ltd.