The discovery of a new acceleration and possible trapping region of the magnetosphere Original Research Article

A new acceleration and possible trapping region of the magnetosphere has been discovered by the POLAR spacecraft through the observations of the cusp energetic particle (CEP) events. The events were detected in the dayside high-altitude cusp and were associated with a dramatic decrease and large fluctuations in the local magnetic field strength. These events could last for hours, in which the measured helium ions had energies up to 8 MeV (Chen, 1997a, 1998). A fundamental question to be addressed is the origins of the cusp energetic ions. Simultaneous observations indicated that the ion fluxes in the CEP events were higher than that in both upstream and downstream from the bow shock. We have determined the power spectra of the local magnetic field turbulence calculated over the September 18, 1996 CEP event periods for fluctuations in the ultra-low frequency (ULF) ranges, corresponding to periods of about 0.33–500 seconds. It is found that the mirror parameter, defined as the ratio of the square root of the integration of the parallel turbulent spectral component over the ULF ranges to the local mean field, is correlated with the intensity of the MeV helium flux. A statistical study of the CEP events indicated that the MeV helium flux was also proportional to the difference between the maximum and the minimum magnetic field in the event. These results represent a discovery that the high-altitude dayside cusp is a new acceleration and possible trapping region of the magnetosphere.