Abstract
Solar flares may be the best-known examples of the explosive conversion
of magnetic energy into bulk motion, plasma heating, and particle
acceleration via magnetic reconnection. The energy source for all flares
is the highly sheared magnetic field of a filament channel above a
polarity inversion line (PIL). During the flare, this shear field
becomes the so-called reconnection guide field (i.e., the
non-reconnecting component), which has been shown to play a major role
in determining key properties of the reconnection including the
efficiency of particle acceleration. We present new high-resolution,
three-dimensional, magnetohydrodynamics simulations that reveal the
detailed evolution of the magnetic shear/guide field throughout an
eruptive flare. The magnetic shear evolves in three distinct phases:
shear first builds up in a narrow region about the PIL, then expands
outward to form a thin vertical current sheet, and finally is
transferred by flare reconnection into an arcade of sheared flare loops
and an erupting flux rope. We demonstrate how the guide field may be
inferred from observations of the sheared flare loops. Our results
indicate that initially the guide field is larger by about a factor of 5
than the reconnecting component, but it weakens by more than an order of
magnitude over the course of the flare. Instantaneously, the guide field
also varies spatially over a similar range along the three-dimensional
current sheet. We discuss the implications of our results for
understanding observations of flare particle acceleration.