A nonthermal particle acceleration mechanism involving the interaction of a charged particle with multiple magnetic islands is proposed. The original Fermi acceleration model, which assumes randomly distributed magnetic clouds moving at random velocity $V_c$ in the interstellar medium, is known to be of second-order acceleration of $O(V_c/c{)}^2$ owing to the combination of head-on and head-tail collisions. In this Letter, we reconsider the original Fermi model by introducing multiple magnetic islands during reconnection instead of magnetic clouds. We discuss that the energetic particles have a tendency to be distributed outside the magnetic islands, and they mainly interact with reconnection outflow jets. As a result, the acceleration efficiency becomes first order of $O(V_A/c)$, where $V_A$ and $c$ are the Alfvén velocity and the speed of light, respectively.

• Received 29 July 2011

DOI:https://doi.org/10.1103/PhysRevLett.108.135003