OPTICAL IMAGING SPECTROSCOPY (Invited)

During the recent solar maximum the combination of imaging and spectroscopy in the visible part of the spectrum became a powerful tool for observational study of flares primarily because of the development of two-dimensional charge-coupled-device (CCD) arrays. In combination with appropriate new operational methods, this has led to the ability to observe, for the first time, the preflare and impulsive-phase physical processes associated with spatially resolved features of flare loops. As a result of concurrent theoretical developments, modeling progressed from an empirical to a physical level. This made it possible to interpret imaging spectra in terms of coronal pressure and heat flux, particle beam heating, chromospheric evaporation, and explosive chromospheric dynamics at the footpoints of flare loops. There is clear potential for further advances in the near future, taking advantage of improvements in digital recording speed ("-10-fold),number of photosensitive elements per array (_10-fold), real-time data pre-reduction (potentially 10to 100-fold), and using multiple CCD arrays. By the time of the next solar maximum I expect imaging spectroscopy to achieve spatial resolution _<1 arc s, temporal resolution <_5 s, and simultaneous critically-sampled spectroscopy of several lines and continua. As a result, we can anticipate continued increase in our understanding of the physical processes and configurations of solar flares in the chromosphere, temperature minimum region, and photosphere. Even greater progress toward a more global understanding of flares will obviously come about when we have simultaneous optical, X-ray, and 3,-ray imaging spectroscopy.