1.4 The big picture
The flare observations may be ordered in a scenario or a sequence of processes in which the flare energy
is released in the corona by reconnecting magnetic fields. The process heats the plasma in the reconnection
region to temperatures of tens of millions of degrees Kelvin (MK), but also efficiently accelerates electrons
to super-thermal energies peaking below some 20 keV and extending sometimes to several tens of
MeV. Reconnection most likely takes place also in the interior of the Sun and the lower solar
atmosphere. The impulsiveness of the flare energy release is possibly caused by a sudden change in
resistivity by high-frequency (collisionless) wave turbulence and is probably related to the capability
of particle acceleration. Both require long collision times, i.e., low density. Not surprisingly,
impulsive flare reconnection is generally assumed to take place in the corona, consistent with
observations.
The energy then propagates from the corona into the dense chromosphere along a magnetic loop by
thermal conduction or free-streaming non-thermal particles, depending on the flare and the flare
phase. The chromospheric material is heated to tens of million degrees and expands into the
corona. The upward motion fills up existing coronal loops, but the motion may continue in an
expansion of these loops. In some cases, the flare may be only a minor part of a much larger
destabilization of the corona, when the magnetic confinement of a considerable part of the corona is
broken up. It expands and is expelled by magnetic forces in a coronal mass ejection (CME,
Figure 4
). The shock front associated with this motion is also a site of particle acceleration,
particularly of high-energy solar cosmic rays observed near Earth or at ground level. Note that a
CME is not simply the explosive result of a flare, but has its own magnetic driver. A CME is a
different plasma physical process and may even lead to the conditions for reconnection, causing a
flare.
