Electron acceleration by pressure anisotropy instabilities under solar flare plasma conditions

We use particle-in-cell (PIC) simulations to show that pressure anisotropy instabilities can stochastically accelerate electrons in plasmas with temperatures, magnetic fields and densities suitable to solar/stellar flares. Using a setup where the global magnetic field grows, we self-consistently produce the growth of electron pressure anisotropy, driving different electron scale plasma modes unstable (whistler and z-modes). In the regime $\omega_{ce}/\omega_{pe} \sim 1$ (where $\omega_{ce}$ and $\omega_{pe}$ are the electron cyclotron and plasma frequencies, respectively), and after the instabilities have reached their non-linear, saturated regime (after the global magnetic field has been amplified by a factor $\sim 3$), the electron energy spectrum can develop a power-law tail with indices between $\sim 2$ and $3$, and reach $\sim$MeV energies.