Proton resonant firehose instability: Temperature anisotropy and fluctuating field constraints

Details Proton resonant firehose instability: Temperature anisotropy and fluctuating field constraints Proton resonant firehose instability: Temperature anisotropy and fluctuating field constraints

Jul 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998jgr...10314567g&link_type=abstract

Journal of Geophysical Research, Volume 103, Issue A7, p. 14567-14574

Physics

Plasma Physics

32

Space Plasma Physics: Waves And Instabilities

Scientific paper

The electromagnetic proton firehose instability may grow in a plasma if the proton velocity distribution is approximately bi-Maxwellian and T∥p>T⊥p, where the directional subscripts denote directions relative to the background magnetic field. Linear Vlasov dispersion theory in a homogeneous electron-proton plasma implies an instability threshold condition at constant maximum growth rate 1-T⊥p/T∥p=Sp/β∥pαp over 1<β∥p<=10 where β∥p≡8πnpT∥p/B02 and B0 is the background magnetic field. Here Sp and αp are fitting parameters and αp~=0.7. One- and two-dimensional initial value hybrid simulations of this growing mode are carried out under proton cyclotron resonant conditions in a homogeneous plasma on the initial domain 2<~β∥p<=100. The two-dimensional simulations show that enhanced fluctuations from this instability impose a bound on the proton temperature anisotropy of the form of the above equation with the fluid theory result αp~=1.0. On this domain both one- and two-dimensional simulations yield a new form for the upper bound on the fluctuating field energy density from the proton resonant firehose instability |δB|2/B02=SB+αBln(β∥p) where SB and αB are empirical parameters which are functions of the initial growth rate. This logarithmic behavior is qualitatively different from a fluid theory prediction and, like the anisotropy bound, should be subject to observational verification in any sufficiently homogeneous plasma in which the proton velocity distribution is approximately bi-Maxwellian.

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