Large-scale numerical simulations of ion beam instabilities in unmagnetized astrophysical plasmas

Collisionless quasiperpendicular shocks with magnetoacoustic Mach numbers e xceeding a certain threshold are known to reflect a fraction of the upstrea m ion population. These reflected ions drive instabilities which, in a magn etized plasma, can give rise to electron acceleration. In the case of shock s associated with supernova remnants (SNRs), electrons energized in this wa y may provide a seed population for subsequent acceleration to highly relat ivistic energies. If the plasma is weakly magnetized, in the sense that the electron cyclotron frequency is much smaller than the electron plasma freq uency omega (p), a Buneman instability occurs at omega (p). The nonlinear e volution of this instability is examined using particle-in-cell simulations , with initial parameters which are representative of SNR shocks. For simpl icity, the magnetic field is taken to be strictly zero. It is shown that th e instability saturates as a result of electrons being trapped by the wave potential. Subsequent evolution of the waves depends on the temperature of the background protons T-i and the size of the simulation box L. If T-i is comparable to the initial electron temperature T-e, and L is equal to one B uneman wavelength lambda (0), the wave partially collapses into low frequen cy waves and backscattered waves at around omega (p). If, on the other hand , T-i much greater thanT(e) and L = lambda (0), two high frequency waves re main in the plasma. One of these waves, excited at a frequency slightly low er than omega (p), may be a Bernstein-Greene-Kruskal mode. The other wave, excited at a frequency well above omega (p), is driven by the relative stre aming of trapped and untrapped electrons. In a simulation with L = 4 lambda (0), the Buneman wave collapses on a time scale consistent with the excita tion of sideband instabilities. Highly energetic electrons were not observe d in any of these simulations, suggesting that the Buneman instability can only produce strong electron acceleration in a magnetized plasma. [S1070-66 4X(00)02712-9].