Enhanced Fermi acceleration in multiscale MHD reconnection

Reconnection provides myriad mechanisms for the acceleration of plasma particles to high energies. Fermi acceleration within compressing plasmoids in particular has been used to explain energetic particle distributions throughout a range of reconnection parameters. Unfortunately, investigations of this process which focus on the individual particle acceleration are few. We present a detailed investigation of Fermi acceleration in 2D MHD anti-parallel plasmoid reconnection based on the guiding center approximation of test particle orbits. We show that accounting only for the variation in the excursion width of particle orbits around a magnetic island, energization obeys a d_tε_||~ε_||^p power law where p=1+2/χ with χ typically between 3 and 3.6. Including further effects of the non-constant E⨉B drift allows for acceleration rates as large as p≥2, with p decreasing as plasmoid size at injection increases. As a result, plasmoid sizes are likely to be coupled to the distributions of energetic particles within. We discuss the implications this has for global energetic particle distributions in multi-island reconnection, as well as the relevance of these results to the collisionless plasmoid instability, where additional particle drifts and non-MHD effects are present.