Solar particle acceleration at reconnecting 3D null points

¹ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
e-mail: stanier@jb.man.ac.uk
² Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK

Received: 20 January 2012
Accepted: 14 March 2012

Abstract

Context. The strong electric fields associated with magnetic reconnection in solar flares are a plausible mechanism to accelerate populations of high energy, non-thermal particles. One such reconnection scenario, in a fully 3D geometry, occurs at a magnetic null point. Here, global plasma motion can give rise to strong currents in the spine axis or fan plane.

Aims. We aim to understand the mechanism of charged particle energy gain in both the external drift region and the diffusion region associated with 3D magnetic reconnection. In doing so we aim to evaluate the efficiency of resistive spine and fan models for particle acceleration, and find possible observables for each.

Methods. We used a full orbit test particle approach to study proton trajectories within electromagnetic fields that are exact solutions to the steady and incompressible magnetohydrodynamic equations. We studied the acceleration physics of single particle trajectories and found energy spectra from many particle simulations. The scaling properties of the accelerated particles with respect to field and plasma parameters was investigated.

Results. For fan reconnection, strong non-uniform electric drift streamlines can accelerate the bulk of the test particles. The highest energy gain is for particles that enter the current sheet, where an increasing “guide field” stabilises particles against ejection. The energy is only limited by the total electric potential energy difference across the fan current sheet. The spine model has both slow external electric drift speed and weak energy gain for particles reaching the current sheet.

Conclusions. The electromagnetic fields of fan reconnection can accelerate protons to the high energies observed in solar flares, gaining up to 0.1 GeV for anomalous values of resistivity. However, the spine model, which gave a harder energy spectrum in the ideal case, is not an efficient accelerator after pressure constraints in the resistive model are included.