Particle acceleration at a reconnecting magnetic separator

We present first results of test particle orbit calculations in a time-dependent electro-magnetic field configuration which models separator reconnection in three dimensions. The test particle orbits are calculated using the relativistic guiding-centre approximation. The test particles are accelerated to high kinetic energies by the parallel electric field generically associated with three-dimensional magnetic reconnection, with the final energy depending on the choice of model parameters. We present a discussion of how the test-particle orbits and the energy gain depend on the initial conditions, for both electrons and protons, and how observations (for example, of solar flares) may be used to constrain our model parameters. Introduction & motivation Understanding the physical processes causing acceleration of large numbers of charged particles to high energies during solar flares is one of the biggest unsolved problems in solar physics to date[1]. Magnetic reconnection is essential to facilitate magnetic restructuring during a flare, crucially allowing stored magnetic energy of the solar corona to produce the observed array of accelerated particles. Due to the vast difference in length and time scales between the macroscopic (MHD) description of reconnection in solar flares and the microscopic description of particle acceleration, most studies use a test particle approach. 9th Aug 2011 solar flare, credit: AIA/SDO (NASA) Many past studies of particle acceleration by magnetic reconnection focus on acceleration in 2D or 2.5D reconnection models. Typically, these are either 2D null point configurations[2] and/or current sheets[3−4]; significant progress has also been made using reconnecting 3D null point configs[5]. Recent work[6−10] has shown that special field lines, called separators, are common sites of reconnection in 3D. Locally, separators intersect four distinct topological domains of magnetic flux and are prone to current sheet formation. Evidence of separator reconnection has been found both in EUV observations of the solar corona[11] and via in situ measurements of magnetic field topology within Earth’s magnetosphere[12]. The role that these separators play in the acceleration of particles is, as yet, unknown. Model setup: global field & particle dynamics Separator reconnection model[13]: nulls at (0,0,±z0), separator [green] marks intersection of fan planes [orange/blue], electric field induced by B-field [dashed rings] along separator controlled by length (l) & width (a). The equations are solved using an RK4 scheme with adaptive step-size. We use a simple kinematic model[13], where an electric field is induced along a separator by a magnetic flux ring (see left): B0 = b0 L2 [ x(z − 3z0)x + y(z + 3z0)ŷ + 1 2 (z2 0 − z2 + x2 + y2)ẑ ]