A Model for Gradual Phase Heating Driven by MHD Turbulence in Solar Flares

Authors: William Ashfield IV (Montana State University), Dana Longcope (Montana State University)

Coronal flare emission is commonly observed to decay on timescales longer
than one-dimensional flare loop models typically predict. This discrepancy
is most apparent during the gradual phase, where emission from impulsively
driven models decays over minutes, in contrast to the hour or more often ob-
served. Magnetic reconnection is invoked as the energy source of a flare, but
should deposit energy into a given loop within a matter of seconds. Models
which supplement this impulsive energization with a long, persistent ad hoc
heating have successfully reproduced long-duration emission, but without pro-
viding a clear physical justification. Here we propose a model for extended
flare heating by the slow dissipation of turbulent Alfvén waves initiated dur-
ing the retraction of newly-reconnected flux tubes through a current sheet.
Using one-dimensional simulations, we track the production and evolution of
MHD wave turbulence trapped by reflection from high density-gradients in the
transition region. Turbulent energy dissipates through non-linear interaction
between counter-propagating waves, modeled here using a phenomenological
one-point closure model. AIA EUV lightcurves synthesized from the simula-
tion were able to reproduce emission decay on the order of tens of minutes.
We find this simple model offers a possible mechanism for generating the ex-
tended heating demanded by observed coronal flare emissions self-consistently
from reconnection-powered flare energy release.