Particle Acceleration during Collisionless Magnetic Reconnection

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Magnetic reconnection is a fundamental process underlying important phenomena in nature, including solar flares, magnetospheric substorms and disruptions in laboratory fusion experiments. Classical resistivity is not adequate to explain the observed energization of particles during reconnection events. An important recent discovery is that for a given set of plasma parameters, reconnection can proceed in two distinct modes: a slow mode known as Sweet-Parker reconnection, and a fast mode that involves electric and magnetic fields produced by the Hall effect. The catastrophic transition between these two modes can explain the sudden onset of fast reconnection in resistive systems such as the solar corona and laboratory reconnection experiments. This project will examine the structure of the reconnection region near the magnetic x-line and determine how the structure is related to the rate of magnetic reconnection. It will focus on the development of a new model of particle heating in which electrons circulating in contracting magnetic islands undergo Fermi-like acceleration. The model will link the energy gain by electrons to the magnetic energy released during reconnection. Ions, whose thermal speeds are typically comparable to or much smaller than the Alfven speed, are too slow to undergo Fermi reflection and some other process must be invoked to explain their energization. This project will explore a seed mechanism in which super-Alfvenic ions entering in reconnection exhausts act like pickup particles and gain a thermal energy equal to the exhaust velocity. Kinetic particle, Hall magnetohydrodynamic (MHD) and test particle simulations will be used to address the key issues related to electron and ion acceleration during reconnection. Collaborations will be continued with scientists working on laboratory reconnection experiments and with satellite data to benchmark the theoretical predictions with observations. The exploration of the dynamics of reconnection and particularly the development of a predictive capability for energetic particle spectra has broad importance for assessing both the safety of astronauts and our country's space-based technological assets. The active involvement of undergraduate students in research under this program and support for female graduate students will further the educational goals of NSF.