Evolution of the Loop‐Top Source of Solar Flares: Heating and Cooling Processes

We present a study of the spatial and spectral evolution of the loop-top (LT)sources in a sample of 6 flares near the solar limb observed by {\it RHESSI}. Adistinct coronal source, which we identify as the LT source, was seen in eachof these flares from the early ``pre-heating'' phase through the late decayphase. Spectral analyses reveal an evident steep power-law component in thepre-heating and impulsive phases, suggesting that the particle accelerationstarts upon the onset of the flares. In the late decay phase the LT source hasa thermal spectrum and appears to be confined within a small region near thetop of the flare loop, and does not spread throughout the loop, as is observedat lower energies. The total energy of this source decreases usually fasterthan expected from the radiative cooling but much slower than that due to theclassical Spitzer conductive cooling along the flare loop. These resultsindicate the presence of a distinct LT region, where the thermal conductivityis suppressed significantly and/or there is a continuous energy input. Wesuggest that plasma wave turbulence could play important roles in both heatingthe plasma and suppressing the conduction during the decay phase of solarflares. With a simple quasi-steady loop model we show that the energy input inthe gradual phase can be comparable to that in the impulsive phase anddemonstrate how the observed cooling and confinement of the LT source can beused to constrain the wave-particle interaction.Comment: 32 pages, 14 figures, submitted to Ap