Resonant electron firehose instability: Particle-in-cell simulations

Abstract

Consider a collisionless, homogeneous plasma in which the electron velocity distribution is a bi-Maxwellian with T⊥<T∥, where the subscripts refer to directions relative to the background magnetic field B0. If this anisotropy is sufficiently large and the electron β∥ is sufficiently greater than one, linear dispersion theory predicts that a cyclotron resonant electron firehose instability is excited at propagation oblique to B0 with growth rates less than the electron cyclotron frequency |Ωe| and zero real frequency. This theory at constant maximum growth rate yields threshold conditions for this growing mode of the form 1−T⊥e/T∥e=Se′/β∥eαe′, where the two fitting parameters satisfy 1≲Se′≲2 and αe′≲1.0 over 2.0⩽β∥e⩽25.0. The first particle-in-cell computer simulations of the resonant electron firehose instability are described here. These simulations show that enhanced magnetic field fluctuations reach a maximum value of |δB|2/B02 which increases with β∥e. These enhanced fields scatter the electrons, reducing their anisotropy approximately to a linear theory threshold condition and yielding a dimensionless scattering rate which increases as β∥e increases. These results are consistent with the general principle that, for a given plasma species, scattering by enhanced fluctuations from anisotropy-driven electromagnetic instabilities acts to make the velocity distribution more nearly isotropic as the β∥ of that species increases.