Particle trajectories and acceleration during 3D fan reconnection

TL;DR

This study compares particle acceleration and trajectories during 3D fan and spine magnetic reconnection at null points, revealing differences in efficiency, escape mechanisms, and spectral evolution relevant to solar flare energy release.

Contribution

It provides the first detailed comparison of particle acceleration dynamics in fan and spine reconnection regimes at 3D null points using numerical simulations.

Findings

Fan reconnection is less efficient than spine reconnection in providing seed particles.

Steady-state spectra take twice as long to establish in fan reconnection.

Particle escape mechanisms differ: jets in spine, ribbon-like in fan.

Abstract

Context. The primary energy release in solar flares is almost certainly due to magnetic reconnection, making this a strong candidate as a mechanism for particle acceleration. While particle acceleration in 2D geometries has been widely studied, investigations in 3D are a recent development. Two main classes of reconnection regimes at a 3D magnetic null point have been identified: fan and spine reconnection Aims. Here we investigate particle trajectories and acceleration during reconnection at a 3D null point, using a test particle numerical code, and compare the efficiency of the fan and spine regimes in generating an energetic particle population. Methods. We calculated the time evolution of the energy spectra. We discuss the geometry of particle escape from the two configurations and characterise the trapped and escaped populations. Results. We find that fan reconnection is less efficent than spine reconnection in providing seed particles to the region of strong electric field where acceleration is possible. The establishment of a steady-state spectrum requires approximately double the time in fan reconnection. The steady-state energy spectrum at intermediate energies (protons 1 keV to 0.1 MeV) is comparable in the fan and spine regimes. While in spine reconnection particle escape takes place in two symmetric jets along the spine, in fan reconnection no jets are produced and particles escape in the fan plane, in a ribbon-like structure.