Correlation of cusp MeV helium with turbulent ULF power spectra and its implications

Abstract

A new magnetospheric phenomenon called a cusp energetic particle (CEP) event was observed by the POLAR spacecraft. CEP events were detected in the dayside polar cusp near the apogee of POLAR and could last for hours, in which the measured helium ions had energies up to 8 MeV [ Chen et al., 1997a, 1998]. A fundamental question is where do the cusp MeV ions come from? To answer this question, we have compared the ion flux in the CEP events with that in both the upstream and the downstream from the bow shock and found that bow shock acceleration cannot explain the measured ion flux in the CEP events. We have further 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 integrated power of the turbulent spectra over the ULF ranges is correlated with the intensity of the MeV helium flux. These new results provides the first direct observation evidence that the high‐altitude dayside cusp is a new acceleration region of the magnetosphere.