The Role of Fast Magnetosonic Waves in the Release and Conversion via Reconnection of Energy Stored by a Current Sheet

Abstract

Using a simple two-dimensional, zero-β model, we explore the manner by which reconnection at a current sheet releases and dissipates free magnetic energy. We find that only a small fraction (3%-11% depending on currentsheet size) of the energy is stored close enough to the current sheet to be dissipated abruptly by the reconnection process. The remaining energy, stored in the larger-scale field, is converted to kinetic energy in a fast magnetosonic disturbance propagating away from the reconnection site, carrying the initial current and generating reconnectionassociated flows (inflow and outflow). Some of this reflects from the lower boundary (the photosphere) and refracts back to the X-point reconnection site. Most of this inward wave energy is reflected back again and continues to bounce between X-point and photosphere until it is gradually dissipated, over many transits. This phase of the energy dissipation process is thus global and lasts far longer than the initial purely local phase. In the process, a significant fraction of the energy (25%-60%) remains as undissipated fast magnetosonic waves propagating away from the reconnection site, primarily upward. This flare-generated wave is initiated by unbalanced Lorentz forces in the reconnection-disrupted current sheet, rather than by dissipation-generated pressure, as some previous models have assumed. Depending on the orientation of the initial current sheet, the wave front is either a rarefaction, with backward-directed flow, or a compression, with forward-directed flow.

A Reconnection-Driven Model of the Hard X-Ray Loop-Top Source From Flare 2004 February 26

“…This elimination is meant to represent those processes which stop the tube, including the emission of fast magnetosonic waves or even formation of a fast magnetosonic termination shock. Longcope & Tarr (2012) argued, using a crude two-dimensional MHD calculation, that the process of eliminating a CS by reconnection could produce fast magnetosonic waves carrying roughly this fraction of the released energy.…”

Spontaneous reconnection at a separator current layer: 1. Nature of the reconnection

“…Everywhere else the current is very low. In contrast to Longcope and Priest [] and Longcope and Tarr [], (zero‐beta models), the numerical models of Fuentes‐Fernández et al [] (high beta) and Fuentes‐Fernández et al [] (low beta) find that the reconnection process converted most of the magnetic energy (stored in the 2‐D null current layer configuration) directly into internal energy, via Ohmic dissipation, with only a little being converted initially into kinetic energy and then damped due to viscosity. Additionally, Fuentes‐Fernández et al [] found that the value of the magnetic diffusivity affects not only the reconnection rate but also the amount of magnetic energy converted into kinetic and internal energy.…”

Solar flares and focused energy transport by MHD waves

“…Waves and Poynting flux are also receiving renewed attention in reconnection theory, where studies by Longcope & Priest (2007), Birn et al (2009), Kigure et al (2010), and Longcope & Tarr (2012) have highlighted their role carrying signals and energy away from the reconnection site into the surrounding medium. These aspects are particularly apparent if reconnection is time-dependent, conditions are low-beta, waves are allowed to freely propagate away from the reconnection site without being reflected by artificial boundaries, and resistivity is small or localised to the reconnection site so that waves are not artificially overdamped.…”