Abstract
Radio imaging observation of the solar flare on January 2, 1993 showed a
formation process of
radio sources over the flare loop (Shibasaki, 1996). In the present
paper, we interpret this
overtopping radio sources as magnetic islands (or balloons) produced by
a ballooning instability
due to high pressure (or high beta value) in the flare loop.
The overtopping radio sources appeared several moments in the flare: 1)
When a rising plasma
cloud hit the apex of the loop, 2) when the growing flare loop reached
at its maximum height, and
3) a couple of times after the well developed bright flare loop was
formed. At these moments, the
plasma beta value was high, due to local enhancement of the pressure by
the collision of the rising
plasma cloud with the curved magnetic field at the loop apex 1), and by
the collision of the
evaporated plasma flow through both legs of the loop at the loop top 2).
After the formation of the
bright flare loop, the loop was filled with the evaporated high density
plasma 3).
Extensive studies have been done of the stability of plasma loops in the
solar corona (e.g. Bray et
al., 1991). In the field of fusion in magnetically confined plasma such
as Tokamak, detailed studies
of the stability of the confined plasma have been developed. Main
difference between these two
cases is line-tying of the solar coronal magnetic fields. They are
anchored at the photosphere.
Instabilities with low wave numbers (large scale) are suppressed due to
line-tying effect.
However, instabilities with high wave numbers can develop. Outer
boundary of high density
(beta) flare loops are unstable against localized interchange
instability (ballooning mode) because
the outer boundary has unfavorable magnetic field curvature (convex
outwards). Appearances of
overtopping microwave sources satisfy the above condition of the
ballooning instability. The
overtopping sources are interpreted as magnetic islands (or balloon)
produced by the ballooning
instability.
Recent development of numerical simulation made it possible to simulate
nonlinear full 3D
behavior of the plasma in Tokamak machines under various conditions. The
growth of the
ballooning mode under high beta condition was simulated and was compared
with the experiment
by Park et al.(1995). In the experiment, a strong local pressure bulge
destroyed the flux surface
and most of the confined plasma escaped. During the disruption,
nonthermal emission was
observed. Large magnetic islands were formed after the recovery of the
magnetic surface. This
scenario is very attractive to solar flare scenario, although the
physical conditions are very
different between the two. It is necessary
to do similar MHD simulation (nonlinear, full 3D, resistive) for loops
in the corona with high beta.
With this scenario, high temperature regions above flare loops, loop top
hard X-ray sources and
high density plasmoid ejections which were found by YOHKOH could be
explained without
reconnection above the flare loop.