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Session 33 - Dynamics of Solar Magnetic Fields.
Oral session, Tuesday, June 11
We analyze simultaneous H _\alpha images (from Big Bear Solar Observatory), soft and hard X-ray images and spectra (from Yohkoh during the first three minutes of the 1994 June 30 flare. The strong blueshifts observed in the Ca XIX soft X-ray line are interpreted as evidence of chromospheric evaporation, with maximum up--flow velocities occurring two minutes prior to the hard X--ray emission peak. In this paper, we search for moving sources in H_\alpha, soft and hard X--ray images that correspond to the blueshifted component. The chromospheric evaporation in this flare is divided into two phases: an early phase with up-flow velocities of 300-450 km s^-1, and a later phase (during the hard X--ray peak) characterized by velocities of 100-200 km s^-1. During the first chromospheric evaporation phase, the footpoints of a loop seen in HXT maps are seen to move towards the loop top source. No source displacement is observed in SXT images. The hard X--ray spectra of individual sources, obtained from HXT maps, display a very steep slope (\gamma\sim 10-12). Thermal fitting of the spectra yield temperatures of 20-50 MK. Images of the later phase of chromospheric evaporation show the magnetic configuration to have changed. The early HXT loop is no longer visible and HXT maps during this time display the two footpoints of a new loop also visible in SXT images. Now the HXT sources are stationary and a SXT footpoint source is seen to move toward the loop top. We interpret the observed displacement of footpoint sources in HXT (early phase) and SXT (later phase) maps to be the images of the evaporating front projected onto the solar disk, while the up--flow velocities (inferred from the blueshifts) are due to the movement of the same evaporating material along the line of sight. By combining the up--flow velocities with the proper motion of the footpoint sources seen in the maps, we constructed a 3-D view of the magnetic loop for each chromospheric evaporation phase. The early loop is almost semi--circular with a height of 1.7\times 10^9 cm, whereas the later magnetic loop is more elongated (height of 2.3\times 10^9 cm) and asymmetric with its apex closer to the footpoint where most of the evaporation took place. The implications of these magnetic configurations and the distinct evaporation phases are discussed.
Program listing for Tuesday