Simultaneous Nonthermal Acceleration of Both Electrons and Protons in 3D Magnetic Reconnection

Abstract:

Solar flare observations have suggested that magnetic reconnection in the non-relativistic low-beta regime efficiently accelerates particles (both ions and electrons). The resulting nonthermal energy spectra from acceleration often take a power-law. Remarkably, the acceleration of protons and electrons are strongly correlated, indicating that they are accelerated by closely related, if not the same mechanism.

By using 3D particle-in-cell (PIC) simulations with large domain sizes and long simulation durations, here we show, for the first time, that protons and electrons are both accelerated into nonthermal power-law distribution for the plasma regime relevant to solar flares. We show that kink instability of reconnection flux ropes disrupts the close flux surfaces and enhances the separation of magnetic field lines in the low-guide-field regime. Energetic particles following these field lines are efficiently transported out of the flux ropes, facilitating the acceleration of high-energy particles. Based on the criterion of kink instability (safety factor at the edge of flux ropes q<1), we find a threshold for the domain size in the guide field direction for efficient acceleration and power-law formation. These results may explain the acceleration of both electrons and protons by magnetic reconnection in solar flares.