Hot Plasma Effects on the Pitch-Angle Scattering of Ring Current Protons by EMIC Waves

Abstract

<p>Via cyclotron resonant interactions, electromagnetic ion cyclotron (EMIC) waves play an important role in the loss of ring current protons. In this study, by calculating the proton bounce-averaged pitch angle diffusion coefficients using both the cold and hot plasma dispersion relations, we investigate the hot plasma effects on the EMIC wave-induced scattering loss of ring current protons. Our results show that, for H<sup>+</sup> band (He<sup>+</sup> band) EMIC waves, inclusion of hot protons results in significant decrease of pitch angle diffusion coefficients of ~ 10 - 60 keV (4 - 30 keV) protons, while the scattering efficiency of higher energy protons increases at low pitch angles and decreases at relatively high pitch angles. We also find that the cold plasma approximation seriously underestimates the loss timescales of protons at energies from a few keV to tens of keV but overestimate that of higher energy protons. The differences in proton loss timescales caused by hot plasmas are generally less than a factor of ~ 5 for H<sup>+</sup> band but can exceed an order of magnitude for He<sup>+</sup> band, showing a strong dependence on ,  and <em>L</em>-shell. This study confirms that hot plasma effects play a crucial role in the EMIC wave driven loss of ring current protons and should be included in future modeling of ring current dynamics.</p>