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Proton velocity ring‐driven instabilities and their dependence on the ring speed: Linear theory
Author(s) -
Min Kyungguk,
Liu Kaijun,
Gary S. Peter
Publication year - 2017
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
eISSN - 2169-9402
pISSN - 2169-9380
DOI - 10.1002/2017ja023944
Subject(s) - physics , cyclotron , proton , phase velocity , pitch angle , atomic physics , electron , ring (chemistry) , ion , magnetic field , scattering , computational physics , optics , nuclear physics , quantum mechanics , chemistry , organic chemistry , geophysics
Linear dispersion theory is used to study the Alfvén‐cyclotron, mirror and ion Bernstein instabilities driven by a tenuous (1%) warm proton ring velocity distribution with a ring speed, v r , varying between 2 v A and 10 v A , where v A is the Alfvén speed. Relatively cool background protons and electrons are assumed. The modeled ring velocity distributions are unstable to both the Alfvén‐cyclotron and ion Bernstein instabilities whose maximum growth rates are roughly a linear function of the ring speed. The mirror mode, which has real frequency ω r =0, becomes the fastest growing mode for sufficiently large v r / v A . The mirror and Bernstein instabilities have maximum growth at propagation oblique to the background magnetic field and become more field‐aligned with an increasing ring speed. Considering its largest growth rate, the mirror mode, in addition to the Alfvén‐cyclotron mode, can cause pitch angle diffusion of the ring protons when the ring speed becomes sufficiently large. Moreover, because the parallel phase speed, v ∥ p h , becomes sufficiently small relative to v r , the low‐frequency Bernstein waves can also aid the pitch angle scattering of the ring protons for large v r . Potential implications of including these two instabilities at oblique propagation on heliospheric pickup ion dynamics are discussed.