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Ann. Geophys., 26, 653-669, 2008
www.ann-geophys.net/26/653/2008/
© European Geosciences Union 2008
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
I. I. Vogiatzis1, T. E. Sarris1, E. T. Sarris1, O. Santolík2, I. Dandouras3, P. Robert4, T. A. Fritz5, Q.-G. Zong6, and H. Zhang5
1Space Research Laboratory, Dept. of Electrical and Computer Engineering, Democritus University of Thrace, Xanthi, Greece
2Institute of Atmospheric Physics, ASCR, Prague and Charles University, Prague, Czech Republic
3Centre d' Etude Spatialedes Rayonnements, Toulouse, France
4CETP/CNRS/IPSL, Vélizy, France
5Center for Space Physics, Department of Astronomy, Boston University, Boston, MA, USA
6Center for Atmospheric Research, University of Massachusetts Lowell, Lowell, MA, USA
Abstract. The purpose of our study is to investigate the way particles are accelerated up to supra-thermal
energies in the cusp diamagnetic cavities. For this reason we have examined a number of Cluster
cusp crossings, originally identified by Zhang et al. (2005), for the years 2001 and 2002 using data
from RAPID, STAFF, EFW, CIS, PEACE, and FGM experiments. In the present study we focus on two
particular cusp crossings on 25 March 2002 and on 10 April 2002 which demonstrate in a clear way
the general characteristics of the events in our survey. Both events exhibit very sharp spatial
boundaries seen both in CNO (primarily single-charged oxygen of ionospheric origin based on CIS
observations) and H+ flux increases within the RAPID energy range with the magnetic field
intensity being anti-correlated. Unlike the first event, the second one shows also a moderate
electron 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 particle
energization is caused primarily by wave-particle interactions. The wave power spectra and
propagation parameters during these cusp events are examined in detail. It is concluded that the
high ion fluxes and at the same time the presence or absence of any sign of energization in the
electrons clearly shows that the particle acceleration depends on the wave power near the local
particle gyrofrequency and on the persistence of the wave-particle interaction process before
particles escape from cusp region. Furthermore, the continuous existence of energetic O+ ions
suggests that energetic O+ populations are of spatial nature at least for the eight events that
we have studied so far.
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