The mechanism that accelerates particles to the energies required to produce the observed high-energy impulsive emission in solar flares is not well understood. Drake et al. (2006) proposed a mechanism for accelerating electrons in contracting magnetic islands formed by kinetic reconnection in multi-layered current sheets. We apply these ideas to sunward-moving flux ropes (2.5D magnetic islands) formed during fast reconnection in a simulated eruptive flare. A simple analytic model is used to calculate the energy gain of particles orbiting the field lines of the contracting magnetic islands in our ultrahigh-resolution 2.5D numerical simulation. We find that the estimated energy gains in a single island range up to a factor of five. This is higher than that found by Drake et al. for islands in the terrestrial magnetosphere and at the heliopause, due to strong plasma compression that occurs at the flare current sheet. In order to increase their energy by two orders of magnitude and plausibly account for the observed high-energy flare emission, the electrons must visit multiple contracting islands. This mechanism should produce sporadic emission because island formation is intermittent. Moreover, a large number of particles could be accelerated in each magnetohydrodynamic-scale island, which may explain the inferred rates of energetic-electron production in flares. We conclude that island contraction in the flare current sheet is a promising candidate for electron acceleration in solar eruptions.
Authors: S. E. Guidoni, C. R. DeVore, J. T. Karpen, B. J. Lynch
Publication Status: Accepted for publication in The Astrophysical Journal (2016)
Last Modified: 2016-03-16 11:28
Supra-arcade downflows (SADs) and supra-arcade downflowing loops (SADLs)descending from reconnection regions toward solar post-flare arcades seem to betwo different observational signatures of retracting, isolated reconnected fluxtubes with irreducible three-dimensional geometries. This dissertationdescribes work in refining and improving a novel model of patchy reconnection,where only a small bundle of field lines is reconnected across a current sheetand forms a reconnected thin flux tube. Traditional models have not been ableto explain why some of the observed SADs appear to be hot and relatively devoidof plasma. The present work shows that plasma depletion naturally occurs influx tubes that are reconnected across nonuniform current sheets and slidetrough regions of decreasing magnetic field magnitude. Moreover, through adetailed theoretical analysis of generalized thin flux tube equations, we showthat the addition to the model of pressure-driven parallel dynamics, as well astemperature-dependent, anisotropic viscosity and thermal conductivity isessential for self-consistently producing gas-dynamic shocks inside reconnectedtubes that heat and compress plasma to observed temperatures and densities. Theshock thickness can be as long as the entire tube and heat can be conductedalong tube's legs, possibly driving chromospheric evaporation. We developed acomputer program that solves numerically the thin flux tube equations thatgovern the retraction of reconnected tubes. Simulations carried out with thisprogram corroborate our theoretical predictions. A comparison of thesesimulations with fully three-dimensional magnetohydrodynamic simulations ispresented to assess the validity of the thin flux tube model. We also presentan observational method based on total emission measure and mean temperature todetermine where in the current sheet a tube was reconnected.
Authors: Silvina E. Guidoni
Publication Status: Published -- Ph.D. Dissertation. Montana State University - Bozeman
Last Modified: 2011-06-09 08:58