Morphology and Physical Parameters for Two Long-Duration Solar Flares: Observations from YOHKOH

We have analyzed the Yohkoh soft X-ray telescope images and Bragg crystal spectrometer spectra of two long-duration X-class events (LDEs), obtained from instruments on the Japanese Yohkoh spacecraft. The two events are a limb flare that occurred on 1992 November 2 near 03 UT and a disk flare that occurred on 1992 February 27 near 10 UT. The spatial resolution of the images is about 2"5, and the time resolution is 2 s. The emission originates from plasma at temperatures ≥ 10⁷ K.

The morphology of the LDEs is quite complicated, but the most intense emission consists of a small number of loops (usually one or two prominent loops). The brightest emission regions are located at the tops of loops for most of the flares' duration, as was found in earlier studies of compact flares observed by Yohkoh In the case of the 1992 November 2 limb flare, the brightening at the loop top is visible for a period of 24 hr after flare onset. This implies that the heating mechanism must act over a period of tens of hours for these long-duration events, that the energy is deposited at the top of the loop, and that the hot plasma is confined at the top of the loop.

We derive emission measures and temperatures from the soft X-ray telescope and Bragg crystal spectrometer data. Lower limit estimates of electron density of the bright regions at the loop tops are derived from the morphology of the regions and the emission measures. Fractional ion abundances for highly ionized iron are deduced from the spectrometer data.

We relate the Yohkoh observations to previous observations from the solar instruments on the Skylab manned space station.