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

Abstract

This 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, Omega(+)(O), and its harmonics. The particle measurements during strong wave activity revealed a relatively high concentration of oxygen ions (similar to 15%) whose phase space density exhibits a local peak at energy similar to 20 keV. Given that the electron plasma-to-cyclotron frequency ratio is omega(pe)/Omega(e) greater than or similar to 1, this energy corresponds to the particle speed v/v(A) . greater than or similar to 0.3, where v(A) is the oxygen Alfven 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 Omega(+)(O) and at propagation quasi-perpendicular to the background magnetic field, B-0. If the cold ions are mostly protons, these unstable modes are characterized by a low compressibility (vertical bar delta B-parallel to vertical bar(2)/vertical bar delta B vertical bar(2) less than or similar to 0.01), a small phase speed (v(ph) similar to 0.2v(A)), 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 B-0. 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 B-0.