Particle Acceleration in Solar Flare Reconnection

Solar flares are explosive space weather events that rapidly convert stored magnetic energy into bulk motion, plasma heating, and particle acceleration via magnetic reconnection. Recent theory and modeling investigations have revealed that plasmoids, coherent magnetic structures generated by reconnection, play a key role in driving nonthermal particles via a first-order Fermi process. The efficiency of this mechanism is highly sensitive to the strength of the reconnection guide field. While direct observation of reconnection in the corona is highly challenging, key insights can be derived from ‘flare ribbons’ that trace the footpoints of reconnected field lines and ‘flare loops’, filled with hot plasma, that reveal the morphology of the reconnected magnetic field lines. We present new high-resolution MHD simulations of three-dimensional reconnection in an eruptive flare and compare to recent data. We derive analogues of flare ribbons and show that they highly structured and exhibit many ‘whorl’ patterns that are linked to turbulent plasmoids in the reconnecting current sheet. Such flare ribbon fine structure reveals crucial information about the fundamental turbulent vs. laminar nature of the reconnection. We also show that the guide field weakens more than an order of magnitude over the course of the flare, and instantaneously varies over a similar range along the three-dimensional current sheet. We demonstrate how the guide field may be inferred from observations of sheared (tilted) flare loops. Interestingly, we find that the number of plasmoids in the flare reconnecting current sheet increases with weakening guide field, underscoring the important role of the guide field in particle acceleration. We discuss the implications for understanding particle acceleration via reconnection the solar corona and throughout the universe.