Turbulent Heating and Quenching of the Ion Sound Instability

Abstract

Turbulent heating and stabilization of the ion sound instability is investigated by twodimensional computer simulation. Quasilinear rather than nonlinear effects determine the evolution of the instability. The instability is quenched by flattening of the electron distribution and the formation of a high-energy ion tail.Numerous stabilization mechanisms have been proposed for the current-driven ion sound instability. 1 "* 5 We have done extensive simulation studies in order to provide a test for these basic predictions. 6 The two-dimensional code has been described previously. 7 Specifically for the purpose of testing nonlinear theories of stabilization, we have made runs in which the ratio between drift velocity and electron thermal velocity was kept constant, in addition to runs with constant current. In the same vein we discuss the case of a current perpendicular to a weak magnetic field (fy/o> e = 0.04). The magnetic field (perpendicular to the plane of computation) has a very small effect on wave dispersion, but keeps the electron distribution isotropic. (In the case of a current along a magnetic field, further complicated dynamical effects are added by the formation of an electron runaway tail.)We find that for a wide range of initial parameters the growth phase of the instability is followed by the decay of the wave energy W , the return of the fluctuation level W/nT e to the thermal level, 8 and termination of heating in typically (100-200)a) i " 1 . Clearly, in the case of constant current, the growth phase of the instability must terminate at the latest when the phase velocity reaches the drift velocity, w«c s = (T e /M) 1/2 , The runs with constant u/c s , however, show quench-ing in much the same way; see Fig. 1. It is seen that in this case the plasma enters a regime in which the macroscopic parameters remain constant.Nonlinear theories of stabilization generally determine a quasisteady fluctuation level W/nT e as a function of m/M, u/v ey and T e /T t from the condition that the nonlinear damping just balances the linear growth rate, y = y L + y NL = 0. Actually, for (*/dt)ln(W/nT e ) = W/W-T e /T, = 0 9 W/W=2y must be balanced by the electron heat-FIG. 1. Wave energy W, fluctuation level W/nT e , T e , and T i /T e for a typical run. M/m=100, (T^T^ = 0.02, w/v e = 0.75. 1231

Suprathermal electron environment of comet 67P/Churyumov-Gerasimenko: Observations from the Rosetta Ion and Electron Sensor

“…Electron VDFs measured by IES exhibit features that are consistent with previous electron acceleration studies (Thomsen et al 1986;Feldman et al 1983b;Hall et al 1986;Bingham et al 1988). Table 1 summarizes the values of p found in this study and previous Feldman et al (1983a) 4−7.5 Flat-topped distributions in interplanetary shocks Feldman et al (1983a) 3.6−4 Ion acoustic heating in laboratory studies Dum et al (1974) 2.5−13 Comet G-Z Thomsen et al (1986) 2−15 Comet 67P This study works. Here, we briefly introduce the various mechanisms that have been shown to accelerate electrons and discuss the context of our observations.…”

“…We therefore fit the suprathermal component of the electron distribution using (Thomsen et al 1986;Feldman et al 1983b,a;Dum et al 1974)…”

Suppression of Energetic Electron Transport in Flares by Double Layers

“…Multi-dimensional computer simulations show that current-driven ion-acoustic turbulent spectrum is flared out within about 45 • of the current direction (Biskamp & Chodura 1971;Biskamp et al 1972;Dum et al 1974). The presence of a background magnetic field, as in solar flare systems, will likely inhibit scattering perpendicular to the field, making it less effective in suppressing the transport of hot electrons.…”

A nonextensive approach for the instability of current-driven ion-acoustic waves in space plasmas

“…When examining the investigations on the current-driven ion-acoustic instability, one can find that nearly all the works have employed the Maxwellian velocity distribution as the statistical description for the stationary distribution of the ions and electrons. However, computer simulations revealed that the electron distribution was not purely Maxwellian [1]. In some physical situations of either laboratory plasma or space plasma, the velocity distributions are not Maxwellian one but behave the power-law.…”