Stochastic electron acceleration by temperature anisotropy instabilities under solar flare plasma conditions

Abstract

Using 2D particle-in-cell plasma simulations, we study electron acceleration by temperature anisotropy instabilities, assuming conditions typical of above-the-loop-top sources in solar flares. We focus on the long-term effect of T ( e,perpendicular to) > T ( e,parallel to) instabilities by driving the anisotropy growth during the entire simulation time through imposing a shearing or a compressing plasma velocity (T ( e,perpendicular to) and T ( e,parallel to) are the temperatures perpendicular and parallel to the magnetic field). This magnetic growth makes T ( e,perpendicular to)/T ( e,parallel to) grow due to electron magnetic moment conservation, and amplifies the ratio omega (ce)/omega (pe) from similar to 0.53 to similar to 2 (omega (ce) and omega (pe) are the electron cyclotron and plasma frequencies, respectively). In the regime omega (ce)/omega (pe) less than or similar to 1.2-1.7, the instability is dominated by oblique, quasi-electrostatic modes, and the acceleration is inefficient. When omega (ce)/omega (pe) has grown to omega (ce)/omega (pe) greater than or similar to 1.2-1.7, electrons are efficiently accelerated by the inelastic scattering provided by unstable parallel, electromagnetic z modes. After omega (ce)/omega (pe) reaches similar to 2, the electron energy spectra show nonthermal tails that differ between the shearing and compressing cases. In the shearing case, the tail resembles a power law of index alpha ( s ) similar to 2.9 plus a high-energy bump reaching similar to 300 keV. In the compressing runs, alpha ( s ) similar to 3.7 with a spectral break above similar to 500 keV. This difference can be explained by the different temperature evolutions in these two types of simulations, suggesting that a critical role is played by the type of anisotropy driving, omega (ce)/omega (pe), and the electron temperature in the efficiency of the acceleration.