Statistical Study of EMIC Waves and Related Proton Distributions
Observed by the Arase Satellite
Abstract
We performed a statistical study of electromagnetic ion cyclotron (EMIC)
wave distributions and their coupling with energetic protons in the
inner magnetosphere using the Arase satellite data from May 2017 to
December 2020. We investigated the energetic proton pitch-angle
distributions and partial thermal pressures associated with EMIC waves
using inter-calibrated proton data in the energy range of 30 eV/q-187
keV/q. With a cold plasma approximation, we computed the proton minimum
resonance energies using the observed EMIC wave frequency and plasma
density values. We found that the EMIC waves had left-handed
polarization near the magnetic equator close to the threshold of proton
cyclotron instability, and propagated to higher latitudes along the
field line with polarization reversal. H-EMIC waves showed two peak
occurrence regions in the morning and noon sectors at L=7.5-9 outside
the plasmasphere. The flux enhancements associated with morning side
H-EMIC waves appeared at E<1 keV/q among all pitch angles,
while H-EMIC waves in the noon sector exhibited flux enhancement in
field-aligned directions at E=1-100 keV/q. He-EMIC waves showed a broad
occurrence region from 12 to 20 magnetic local time at L=5.5-8.5 inside
the plasmasphere with strong flux enhancements at all pitch-angle ranges
at E=1-100 keV/q. The proton minimum resonance energy using the obtained
central frequency was consistent with the observed flux enhancements at
different peak occurrence regions. We conclude that the free energy
sources of EMIC waves in different geomagnetic environments drive the
two different types of EMIC waves, and they interact with energetic
protons at different energy ranges.