A plasma instability associated with thermal anisotropies in the solar wind

Recent solar wind measurements on the Pioneer 6 spacecraft reveal that, in general, the proton velocity distribution is anisotropic with T∥ > T⊥, where the subscripts refer to direction with respect to the interplanetary magnetic field. Production of the anisotropy by magnetic moment conservation is discussed, and the consequences of this particle distribution with respect to stability of the plasma are analyzed. It is shown that a generalized form of the firehose instability must occur, with growth of magnetosonic (whistler mode) waves near the ion cyclotron frequency. Growth rates are computed for a range of wave frequencies and distances from the sun, and these predictions are compared with available interplanetary power spectra.

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