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The energy released in a solar flare is partitioned between thermal andnon-thermal particle energy and lost to thermal conduction and radiation over abroad range of wavelengths. It is difficult to determine the conductive lossesand the energy radiated at transition region temperatures during the impulsivephases of flares. We use UVCS measurements of O VI photons produced by 5 flaresand 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 ofincrease of thermal energy and the conductive losses deduced from RHESSI andGOES X-ray data using areas from RHESSI images to estimate the loop volumes,cross-sectional areas and scale lengths. The transition region luminositiesduring the impulsive phase exceed the X-ray luminosities for the first fewminutes, but they are smaller than the rates of increase of thermal energyunless the filling factor of the X-ray emitting gas is ~ 0.01. The estimatedconductive losses from the hot gas are too large to be balanced by radiativelosses or heating of evaporated plasma, and we conclude that the area of theflare magnetic flux tubes is much smaller than the effective area measured byRHESSI during this phase of the flares. For the 2002 July 23 flare, the energydeposited by non-thermal particles exceeds the X-ray and UV energy losses andthe rate of increase of the thermal energy.Comment: 20 pages, 3 figures To appear in Ap

Transition Region Emission and Energy Input to Thermal Plasma during the Impulsive Phase of Solar Flares