Imaging the chromospheric evaporation of the 1994 june 30 solar flare

We analyze simultaneous Hα images (from the Big Bear Solar Observatory), soft and hard X-ray images and spectra (from the soft X-ray telescope [SXT], the Bragg Crystal Spectrometer [BCS], and the hard X-ray telescope [HXT] on Yohkoh), and radio time profiles (from the Owens Valley Radio Observatory) during the first 3 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 2 minutes prior to the hard X-ray emission peak. In this study, we search for moving sources in Hα, 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 350-450 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 low-energy maps are seen to move toward the loop-top source. No source displacement is observed in SXT images at this time. Images of the later phase of chromospheric evaporation show a change in the source morphology. The early HXT loop is no longer visible, and HXT maps during this time display the two footpoints of a new loop 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 three-dimensional view of the magnetic loop for each chromospheric evaporation phase. The early loop is almost semicircular, with a height of 1.7 × 10⁹ cm, whereas the later magnetic loop is more elongated (a height of 3.2 × 10⁹ cm), 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.