Transition region emission and energy input to thermal plasma during the impulsive phase of solar flares

The energy released in a solar flare is partitioned between thermal and nonthermal particle energy and lost to thermal conduction and radiation over a broad range of wavelengths. It is difficult to determine the conductive losses and the energy radiated at transition region temperatures during the impulsive phases of flares. We use UVCS measurements of O vi photons produced by five flares and subsequently scattered by O vi ions in the corona to determine the 5.0 <= log T <= 6.0 transition region luminosities. We compare them with the rates of increase of thermal energy and the conductive losses deduced from RHESSI and GOES X-ray data using areas from RHESSI images to estimate the loop volumes, cross-sectional areas, and scale lengths. The transition region luminosities during the impulsive phase exceed the X-ray luminosities for the first few minutes, but they are smaller than the rates of increase of thermal energy unless the filling factor of the X-ray-emitting gas is similar to 0.01. The estimated conductive losses from the hot gas are too large to be balanced by radiative losses or heating of evaporated plasma, and we conclude that the area of the flare magnetic flux tubes is much smaller than the effective area measured by RHESSI during this phase of the flares. For the 2002 July 23 flare, the energy deposited by nonthermal particles exceeds the energy radiated in X-rays, the energy radiated at transition region temperatures, and the rate of increase of the thermal energy.