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3.6 Thermal flare

Most of the flare energy is thermalized in the solar atmosphere, some of which is heated to high temperatures. This part is visible in soft X-rays (Figure 22Watch/download Movie). In the absence of further energy release the plasma cools by thermal conduction to the chromosphere and by radiating X-rays. At high temperature and low density, conductive cooling dominates, radiative cooling in the opposite case (Cargill et al., 1995). If conductive cooling leads to evaporation of chromospheric material, the cooling time becomes longer as the energy remains in the loop.

Flare loop cooling has been investigated by several authors. Radiative cooling and conduction losses have been found to balance approximately (e.g., Jiang et al., 2006). In the late phase, radiative cooling usually dominates, but considerable heat input is frequently observed (e.g., Milligan et al., 2005).

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Figure 22: mpg-Movie (1139 KB) Soft X-ray images observed by Hinode on April 30, 2007. It shows an active region during two hours. Some small flares or microflares occurred; the largest was of class B2.6 (prepared by A. Savcheva).

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