Linear instabilities in multicomponent plasmas and their consequences for the auroral zone

Abstract

Ion conics are commonly observed along auroral zone field lines and involve all major terrestrial ion species, including hydrogen, helium, and oxygen. It is believed that low‐frequency plasma waves, driven unstable by field‐aligned currents, transversely heat the ion distributions via wave‐particle interactions, creating the ion conics. Considered here are low‐frequency, oblique, electrostatic instabilities found in a mixed plasma that includes an electron beam streaming through a background of electrons, hydrogen, and oxygen. The addition of oxygen not only modifies the lower hybrid frequency, but allows the existence of an ion‐ion (Buchsbaum) hybrid mode with a frequency between the hydrogen and oxygen gyrofrequencies. Because of its low frequency, the ion‐ion hybrid instability can be effective in transversely heating heavy ions. When the electron beam drift speed is greater than 3 times the background electron thermal velocity(Vo > 3vte) and the electron gyrofrequency to plasma frequency ratio is less than 10 (Ωe/ωpe < 10), the lower hybrid instability dominates. However, for Ωe/ωpe > 20, which is a condition commonly found in the auroral zone night side region, the ion‐ion instability has the largest growth rates; in these regions, heavy ion transverse heating can occur. When 10 < Ωe/ωpe < 20 the hydrogen to oxygen density ratio determines which instability dominates.