Abstract
Soft and hard X-ray observations from Yohkoh have shown that the solar
corona changes its
shape much more dynamically than previously expected. Coronal magnetic
structures tend to
intermittently expand and restructure themselves through magnetic
reconnection. Dynamical
phenomena, presumably related to this type of magnetic activity, include
a wide variety that
ranges from tiny transient brightenings (microflares) and jets, through
active-region-sized flares,
to gigantic large-scale arcade formations. An interesting finding in
this context is that both flares
(irrespective of whether they are of impulsive or long-duration type)
and large-scale arcade
formations (believed to be intimately associated with CMEs) show
features such as
``loop-with-a-cusp'' structure and ``plasmoid/filament ejection'' in
common, which is suggestive
that all these phenomena, listed above, can be interpreted by a unified
view of magnetic
reconnection.
The above-mentioned view needs to be confirmed, however, since X-ray
observations do not
necessarily reveal cool materials nor magnetic fields that do not
contain hot plasmas. The
Nobeyama Radioheliograph,
operating at 17 GHz (plus at 34 GHz since
autumn 1995) with high
spatial and temporal resolution, provides supplemental information to
the X-ray observations;
radio emission is more sensitive than X-rays to magnetic fields,
high-energy electrons, and also
cool materials. Detailed comparisons of radio and X-ray images have
revealed that, more than
one systems of magnetic loops are involved in a flaring process. Without
doubt their interaction
with each other is a key factor that leads to an explosive release of
magnetic energy in a form of
particle acceleration.