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The purpose of our study is to investigate the way particles are accelerated up to supra-thermalenergies in the cusp diamagnetic cavities. For this reason we have examined a number of Clustercusp crossings, originally identified by Zhang et al. (2005), for the years 2001 and 2002 using datafrom RAPID, STAFF, EFW, CIS, PEACE, and FGM experiments. In the present study we focus on twoparticular cusp crossings on 25 March 2002 and on 10 April 2002 which demonstrate in a clear waythe general characteristics of the events in our survey. Both events exhibit very sharp spatialboundaries seen both in CNO (primarily single-charged oxygen of ionospheric origin based on CISobservations) and H+ flux increases within the RAPID energy range with the magnetic fieldintensity being anti-correlated. Unlike the first event, the second one shows also a moderateelectron flux increase. The fact that the duskward electric field Ey has relatively low values<5 mV/m while the local wave activity is very intense provides a strong indication that particleenergization is caused primarily by wave-particle interactions. The wave power spectra andpropagation parameters during these cusp events are examined in detail. It is concluded that thehigh ion fluxes and at the same time the presence or absence of any sign of energization in theelectrons clearly shows that the particle acceleration depends on the wave power near the localparticle gyrofrequency and on the persistence of the wave-particle interaction process beforeparticles escape from cusp region. Furthermore, the continuous existence of energetic O+ ionssuggests that energetic O+ populations are of spatial nature at least for the eight events thatwe have studied so far

annales geophysicae

Cluster observations of particle acceleration up to supra-thermal energies in the cusp region related to low-frequency wave activity – possible implications for the substorm initiation process