Abstract
The conversion of magnetic energy into other forms (such as plasma heating, bulk plasma flows, and non-thermal particles) during solar flares is one of the outstanding open problems in solar physics. It is generally accepted that magnetic reconnection plays a crucial role in these conversion processes. In order to achieve the rapid energy release required in solar flares, an anomalous resistivity, which is orders of magnitude higher than the Spitzer resistivity, is often used in magnetohydrodynamic (MHD) simulations of reconnection in the corona. The origin of Spitzer resistivity is based on Coulomb scattering, which becomes negligible at the high energies achieved by accelerated particles. As a result, simulations of particle acceleration in reconnection events are often performed in the absence of any interaction between accelerated particles and any background plasma. This need not be the case for scattering associated with anomalous resistivity caused by turbulence within solar flares, as the higher resistivity implies an elevated scattering rate. We present results of test particle calculations, with and without pitch angle scattering, subject to fields derived from MHD simulations of two-dimensional (2D) X-point reconnection. Scattering rates proportional to the ratio of the anomalous resistivity to the local Spitzer resistivity, as well as at fixed values, are considered. Pitch angle scattering, which is independent of the anomalous resistivity, causes higher maximum energies in comparison to those obtained without scattering. Scattering rates which are dependent on the local anomalous resistivity tend to produce fewer highly energised particles due to weaker scattering in the separatrices, even though scattering in the current sheet may be stronger when compared to resistivity-independent scattering. Strong scattering also causes an increase in the number of particles exiting the computational box in the reconnection outflow region, as opposed to along the separatrices as is the case in the absence of scattering.
Highlights
Solar flare energy release is commonly attributed to magnetic reconnection, during which magnetic energy is converted into other forms, such as plasma heating, bulk plasma flows, and non-thermal accelerated particles
To study the effect of pitch angle scattering on particle behaviour, we initialise test particle orbits in the MHD snapshot shown in Fig. 1b, and integrate the governing equations for their evolution, detailed in Sect. 3, until the orbit leaves the computational domain
In previous studies which included the effects of collisional scattering (e.g. Numata & Yoshida 2002; Burge et al 2014), it was found that repeated crossings of the reconnection region by test particles in the presence of scattering could lead to a higher energy gain than in the absence of scattering, but that the effect on energy spectra was not significant
Summary
Solar flare energy release is commonly attributed to magnetic reconnection, during which magnetic energy is converted into other forms, such as plasma heating, bulk plasma flows, and non-thermal accelerated particles (for reviews of solar flare observations and theory see e.g. Priest & Forbes 2002; Benz 2008; Shibata & Magara 2011). The introduction of anomalous resistivity, caused by turbulent processes, could account for the rate of energy release during flares Vlasov or particlein-cell simulations are required in order to capture the physics of turbulent processes in magnetic reconnection. These simulations are too computationally expensive to model the whole of a solar flare, so an MHD approach is often used. In this paper we complement previous work by presenting the results of test particle simulations including pitch angle scattering in fields generated by two-dimensional (2D) MHD simulations. We examine the impact of pitch angle scattering, with varying dependencies of the scattering rate on the velocity of the particle and anomalous resistivity used in the MHD simulations.
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