Context. Solar flares are associated with intense soft X-ray emission generated by the hot flaring plasma in coronal magnetic loops.
Kink unstable twisted flux-ropes provide a source of magnetic energy which can be released impulsively and account for the heating of the plasma in flares.
Aims. We investigate the temporal, spectral and spatial evolution of the properties of the thermal continuum X-ray emission produced in such kink-unstable magnetic flux-ropes and we discuss the results of the simulations with respect to solar flare observations.
Methods. We compute the temporal evolution of the thermal X-ray emission in kink-unstable coronal loops based on a series of MHD numerical simulations. The numerical setup used consists of a highly twisted loop embedded in a region of uniform and untwisted background coronal magnetic field. We let the kink instability develop, compute the evolution of the plasma properties in the loop (density, temperature) without accounting for mass exchange with the chromosphere. We then deduce the X-ray emission properties of the plasma during the whole flaring episode.
Results. During the initial (linear) phase of the instability plasma heating is mostly adiabatic (due to compression). Ohmic diffusion takes over as the instability saturates, leading to strong and impulsive heating (up to more than 20 MK), to a quick enhancement of X-ray emission and to the hardening of the thermal X-ray spectrum. The temperature distribution of the plasma becomes broad, with the emission measure depending strongly on temperature. Significant emission measures arise for plasma at temperatures higher than 9 MK. The magnetic flux-rope then relaxes progressively towards a lower energy state as it reconnects with the background flux. The loop plasma suffers smaller sporadic heating events but cools down globally by thermal conduction. The total thermal X-ray emission slowly fades away during this phase, and the high temperature component of emission measure distribution converges to the power-law distribution EM ∝ T-4.2. The amount of twist deduced directly from the X-ray emission patterns is considerably lower than the maximum magnetic twist in the simulated flux-ropes.
Authors: R. F. Pinto, N. Vilmer, A. S. Brun
Publication Status: A&A (accepted)
Last Modified: 2015-01-12 08:20