Is flare-ribbon fine structure related to tearing in the flare current sheet?
2021
Observations of solar flare ribbons show significant fine structure in the form of breaking wave-like
perturbations and spirals. The origin of this structure is not well understood, but one possibility is
that it is related to the tearing instability in the flare current sheet. Here we study this connection
by constructing an analytical three-dimensional magnetic field representative of an erupting flux rope
with a flare current sheet below it. We introduce small-scale flux ropes representative of those formed
during a tearing instability in the current layer, and use the squashing factor on the solar surface to
identify the shape of the presumed flare ribbons and fine structure. Our analysis suggests there is a
direct link between flare-ribbon fine structure and flare current sheet tearing, with the majority of the
ribbon fine structure related to oblique tearing modes. Depending upon the size, location and twist of
the small-scale flux ropes, breaking wave-like and spiral features within the hooks and straight sections
of the flare ribbon can be formed that are qualitatively similar to observations. We also show that the
handedness of the spirals/waves must be the same as the handedness of the hooks of the main ribbon.
We conclude that tearing in the flare current layer is a likely explanation for spirals and wave-like features in flare ribbons.
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