Particle acceleration at null-point current sheets in the solar corona

Abstract

Energetic events in the solar corona include solar flares and coronal mass ejections (CMEs), which can have a significant effect on Earth’s orbital infrastructure. Learning how non-thermal energy is built-up, stored and released in the corona is crucial for understanding and predicting these events. There are several candidate mechanisms for coronal energy release including magnetic reconnection, whereby magnetic field lines break, reform and rearrange into a lower energy geometry. MHD simulations can be used to create analytical geometries of reconnecting fields within which particle acceleration can be simulated. A code was written to use a predictor-corrector algorithm to model the full motion of particles in such fields, switching to a Runge-Kutta guiding-centre scheme (adapted from the the St Andrews party orb code1) where appropriate. These methods were applied to simulate particle motion in two MHD-generated evolving fields: an isolated tearing null-point current sheet (Wyper and Pontin, 2014a,b) and a twisting coronal jet developed by Peter Wyper (similar to structures shown by Masson et al. (2009); Pariat et al. (2009)). The results of these simulations were then compared to observations of corresponding coronal field structures.

The simulations demonstrated significant non-thermal acceleration of protons and electrons in both field geometries. The impact positions of high energy charged particles in the coronal jet fields form a curved arc where the dome structure intersects the photosphere, corresponding to observed circular f lare ribbons. The highest energy particles are guiding along flux rope field lines, replicating the bright ‘knots’ observed in circular flare ribbons and a bright point where the spine field lines intersect the photosphere. High energy protons and electrons demonstrate significant divergence in their trajectories, with both impacting the photosphere in broadly separate distributions composing the circular flare ribbon. Similarly, protons and electrons ejected alongside the outgoing jet split into broadly separate distributions as the jet evolves and spine-adjacent field lines diverge. Both of these divergences are a result of protons and electrons being initially accelerated in different directions in the current sheet, leading to them aligning with different field-line groups.

Date of Award	2021
Original language	English
Awarding Institution	University of Dundee
Sponsors	Science and Technology Facilities Council
Supervisor	David Pontin (Supervisor) & Alexander Russell (Supervisor)