Ion Bernstein instability as a possible source for oxygen ion cyclotron harmonic waves

TitleIon Bernstein instability as a possible source for oxygen ion cyclotron harmonic waves
Publication TypeJournal Article
Year of Publication2017
AuthorsMin, K, Denton, RE, Liu, K, S. Gary, P, Spence, HE
JournalJournal of Geophysical Research: Space Physics
Date Published05/2017
KeywordsO+ Bernstein instability; O+ harmonic waves; O+ ring distribution; Van Allen Probes
AbstractThis paper demonstrates that an ion Bernstein instability can be a possible source for recently reported electromagnetic waves with frequencies at or near the singly ionized oxygen ion cyclotron frequency, inline image, and its harmonics. The particle measurements during strong wave activity revealed a relatively high concentration of oxygen ions (∼15%) whose phase space density exhibits a local peak at energy ∼20 keV. Given that the electron plasma-to-cyclotron frequency ratio is inline image, this energy corresponds to the particle speed inline image, where vA is the oxygen Alfvén speed. Using the observational key plasma parameters, a simplified ion velocity distribution is constructed, where the local peak in the oxygen ion velocity distribution is represented by an isotropic shell distribution. Kinetic linear dispersion theory then predicts unstable Bernstein modes at or near the harmonics of inline image and at propagation quasi-perpendicular to the background magnetic field, B0. If the cold ions are mostly protons, these unstable modes are characterized by a low compressibility ( inline image), a small phase speed (vph∼0.2vA), a relatively small ratio of the electric field energy to the magnetic field energy (between 10−4 and 10−3), and the Poynting vector directed almost parallel to B0. These linear properties are overall in good agreement with the properties of the observed waves. We demonstrate that superposition of the predicted unstable Bernstein modes at quasi-perpendicular propagation can produce the observed polarization properties, including the minimum variance direction on average almost parallel to B0.
Short TitleJ. Geophys. Res. Space Physics

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