Kinetic Modeling of Particle Acceleration in a Solar Null-Point Reconnection Region

Abstract

The primary focus of this paper is on the particle acceleration mechanism in solar coronal threedimensional reconnection null-point regions. Starting from a potential field extrapolation of a Solar and Heliospheric Observatory (SOHO ) magnetogram taken on 2002 November 16, we first performed magnetohydrodynamics (MHD) simulations with horizontal motions observed by SOHO applied to the photospheric boundary of the computational box. After a build-up of electric current in the fan-plane of the null-point, a sub-section of the evolved MHD data was used as initial and boundary conditions for a kinetic particle-in-cell model of the plasma. We find that sub-relativistic electron acceleration is mainly driven by a systematic electric field in the current sheet. A non-thermal population of electrons with a power-law distribution in energy forms in the simulated pre-flare phase, featuring a power-law index of about -1.78. This work provides a first step towards bridging the gap between macroscopic scales on the order of hundreds of Mm and kinetic scales on the order of cm in the solar corona, and explains how to achieve such a cross-scale coupling by utilizing either physical modifications or (equivalent) modifications of the constants of nature. With their exceptionally high resolutionup to 135 billion particles and 3.5 billion grid cells of size 17.5 km -these simulations offer a new opportunity to study particle acceleration in solar-like settings.

Flare particle acceleration in the interaction of twisted coronal flux ropes

“…The recovered power law indices lie well within the ranges described in Gordovskyy & Browning (2012). While relatively hard, similar index values have also been recovered in studies of a range of particle acceleration mechanisms (Baumann et al 2013;Stanier et al 2012;Threlfall et al 2015), and indeed agree with selected observational cases (Crosby et al 1993;Krucker et al 2007;Hannah et al 2011). However, many more cases exist where such values are exceedingly low: in one example, Hannah et al (2008) analysed all available microflare spectra using RHESSI (Lin et al 2003), commonly finding indices in the range of 4 − 10 with a median of 7.…”

Relative magnetic helicity as a diagnostic of solar eruptivity

“…Despite tremendous advances in numerical modeling of solar eruptions, few studies have used numerical simulations to advance the search for eruptivity criteria. This is partly due to the fact that numerical models are strongly driven by either having a high level of realism (Lukin & Linton 2011;Baumann et al 2013;Pinto et al 2016;Carlsson et al 2016) or focus on case-by-case studies of observed events (Jiang et al 2012;Inoue et al 2014;Rubio da Costa et al 2016). Little effort is spent on performing systematic parametric simulations which would allow the determination of eruptivity criteria.…”

Particle dynamics in a non-flaring solar active region model

“…Acceleration in more complex 3D models, such as twisted coronal loops (Gordovskyy & Browning 2011;Gordovskyy et al 2014), have also been investigated. A significant step forward from the analysis of a localised reconnection site was taken by Baumann et al (2013), who combined localised particle-in-cell (PIC) analysis in the vicinity of a reconnecting null-point with a magnetohydrodynamic (MHD) model of the surrounding active region (AR). This allowed the global impact sites of the particles to be identified.…”