Theoretical construction of anisotropy-beta relation using EMIC, mirror and fire-hose instabilities

Physics – Plasma Physics

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[2149] Interplanetary Physics / Mhd Waves And Turbulence, [2159] Interplanetary Physics / Plasma Waves And Turbulence, [2164] Interplanetary Physics / Solar Wind Plasma, [7867] Space Plasma Physics / Wave/Particle Interactions

Scientific paper

As the solar wind expands the density and magnetic decreases radially, thus leading to parallel temperature anisotropy (T-parallel > T-perp). However, the measured temperature anisotropy near the Sun and 1 AU indicates that sometimes T-perp > T-parallel, while at other times, the measured T-parallel/T-perp is much more mild than predicted by adiabatic theory [Kasper et al., 2002; Matteini et al., 2007; Bale et al., 2009]. Physical reasons for the observation remain poorly understood at present. Regardless of the mechanisms that maintain the various observed temperature anisotropies, it is known that for perpendicular temperature anisotropy (meaning perpendicular temperature exceeding parallel proton temperature) electromagnetic ion-cyclotron (EMIC) and mirror instabilities are excited for T-perp>T-parallel, while for parallel temperature anisotropy (where parallel proton temperature is greater than perpendicular temperature), both the parallel and oblique fire-hose instabilities are excited. In the present paper we discuss theoretical construction of the anisotropy-beta relation observed in the solar wind by means of quasilinear theories of EMIC and mirror instabilities for the case of perpendicular temperature anisotropy, and (parallel) fire-hose instability for the situation in which parallel temperature anisotropy is dominant. We shall compute saturated wave amplitudes corresponding to various unstable modes computed on the basis of quasilinear theory, and compare the outcome with the various anisotropy-beta relations published in the literature and from recent observations.

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