EFFECTIVE GEOMETRICAL THICKNESS AND ELECTRON-DENSITY OF A FLARE OF 1991 DECEMBER 2 OBSERVED WITH THE SOFT-X-RAY TELESCOPE OF YOHKOH AND CORONAGRAPH

A very small geometrical thickness of 1000 km was found for an M3.7 flare of 1991 December 2, which occurred beyond the limb. While the Yohkoh soft X-ray telescope gives the emission measure, a new coronagraph which can observe continuum images at 6630-angstrom gives the electron column density originating from electron scattering of a 10(7) K flare plasma. The reasoning for the latter is because [Fe XIV] 5303 angstrom images show a much different shape compared with X-ray and the 6630-angstrom continuum, and there was no Halpha emission. From these we obtained an electron density of 4 x 10(10) cm-3 and the above-mentioned small length in the line of sight. Since the apparent width of a flaring plasma both in soft X-ray and continuum images measured parallel to the limb is approximately 4 x 10(4) km at a height of 7 x 10(4) km, the smallness of the effective length of 1000 km is striking, and may have significant bearing on the energy conversion of the flare. A brief discussion concerning the origin of the mass and magnetic morphology is given.