  Solar flares signify the sudden release of magnetic energy and are sources of
so called space weather. The fine structures (below 500 km) of flares are
rarely observed and are accessible to only a few instruments world-wide. Here
we present observation of a solar flare using exceptionally high resolution
images from the 1.6~m New Solar Telescope (NST) equipped with high order
adaptive optics at Big Bear Solar Observatory (BBSO). The observation reveals
the process of the flare in unprecedented detail, including the flare ribbon
propagating across the sunspots, coronal rain (made of condensing plasma)
streaming down along the post-flare loops, and the chromosphere's response to
the impact of coronal rain, showing fine-scale brightenings at the footpoints
of the falling plasma. Taking advantage of the resolving power of the NST, we
measure the cross-sectional widths of flare ribbons, post-flare loops and
footpoint brighenings, which generally lie in the range of 80-200 km, well
below the resolution of most current instruments used for flare studies.
Confining the scale of such fine structure provides an essential piece of
information in modeling the energy transport mechanism of flares, which is an
important issue in solar and plasma physics.

Solar and Stellar Astrophysics