Resonant electron firehose instability: Particle-in-cell simulations

Consider a collisionless, homogeneous plasma in which the electron velocity distribution is a bi-Maxwellian with T-perpendicular to<T-parallel to, where the subscripts refer to directions relative to the background magnetic field B-0. If this anisotropy is sufficiently large and the electron beta(parallel to) is sufficiently greater than one, linear dispersion theory predicts that a cyclotron resonant electron firehose instability is excited at propagation oblique to B-0 with growth rates less than the electron cyclotron frequency parallel toOmega(e)parallel to and zero real frequency. This theory at constant maximum growth rate yields threshold conditions for this growing mode of the form 1-T-perpendicular toe/T-parallel toe=S-e(')/beta(parallel toe)(e)(alpha)('), where the two fitting parameters satisfy 1less than or similar toS(e)(')less than or similar to2 and alpha(e)(')less than or similar to1.0 over 2.0less than or equal tobeta(parallel toe)less than or equal to25.0. 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 parallel todeltaBparallel to(2)/B-0(2) which increases with beta(parallel toe). 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 beta(parallel toe) 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 betaparallel to of that species increases. (C) 2003 American Institute of Physics.