Stochastic Growth of Mirror Mode and Ion Cyclotron Waves in Earth's Magnetosheath

Physics – Plasma Physics

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Scientific paper

Spacecraft magnetometer data routinely show the presence of mirror mode and electromagnetic ion cyclotron (EMIC) waves in the magnetosheaths of Earth and other planets. Recent observational work by Anderson, Fuselier, Gary, and Denton shows that these waves quantitatively affect the magnetosheath plasma, limiting the proton perpendicular/parallel temperature anisotropy near the marginal stability boundary predicted by linear theory. The waves are bursty, vary widely in amplitude, and persist throughout much of the magnetosheath and plasma depletion layer. These properties are inconsistent with the standard model for wave growth in plasmas, of exponential amplification with constant growth rate until saturation by nonlinear processes. Stochastic growth theory (SGT) and self-organized criticality (SOC) are statistical theories describing the growth of waves in inhomogeneous plasmas. In SGT, the waves and driving particles interact self-consistently in a prescribed plasma background and lead to log-normal wave statistics; plasma inhomogeneities introduce favored sites for wave growth, injecting spatio-temporal fluctuations into the particle distribution which evolve toward marginal stability and introduce non-local physics. SOC instead involves the waves, particles and ambient plasma all interacting self-consistently, resulting in power-law wave statistics. In this paper the probability distributions of wave fields are calculated for both EMIC and mirror mode waves in Earth's magnetosheath. The observed distributions are shown to be in excellent quantitative agreement with the functional form predicted by SGT, with very good statistical significance. In contrast, the distributions are inconsistent with SOC and the standard model. SGT can thus explain the statistics and persistence of the waves. This is the first application of SGT to magnetic waves, to waves driven by protons, and to waves driven by a temperature anisotropy instability. In conjunction with earlier analyses, this work shows that SGT applies in four distinct space plasmas, to multiple wave modes ranging from almost electrostatic waves near the electron plasma frequency to electromagnetic waves near the proton cyclotron frequency, and to both electron and ion instabilities. SGT should thus be considered widely applicable to space and (most likely) astrophysical plasmas.

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