We examined five flares, observed by the Hard X-Ray Imaging Spectrometer aboard the Solar Maximum Mission, for the occurrence of coronal explosions and found that these occur only if (a) the flare shows distinct single impulsive hard X-ray bursts and (b) it shows upward (convective) motions during the initial part of the impulsive phase. Coronal explosions are therefore explained as a manifestation of plasma streaming laterally out of the flare kernel(s). There is some evidence that streaming occurs into a number of cylindrical fluxtubes which spread over a larger area, thus supporting the 'spaghetti-bundle model' for the flaring region.
I. Description of Coronal Explosions; Working Procedure
The phenomenon investigated in this paper, coronal explosions, was discovered by de Jager and Boelee (1984) and was interpreted in that paper as a density wave sweeping over the surface of the Sun in the low corona. After the first publication another explosion was described by de Jager etal. (1984) and a description of the three explosions known so far was given by de Jager (1985a). The information then available, combined with data on the thermal behaviour of the plasma in and around the flare kernel(s) showed that the observations must probably refer to the motion of hot plasma streaming out of the flare kernels and spreading over the solar surface, thereby being confined by the magnetic field distribution in the flaring area (de Jager, 1985b).
Coronal explosions were discovered in flare images in soft X-rays. They appear as one or more brightness waves that originate from an area near or at the flare footpoint(s), and move outward with velocities in the range of 100 to 500 l~n s -~ and more, which tend to decrease in a few minutes time. They were detected by drawing isochrones connecting all pixels in which local brightness maximum is reached simultaneously. The observational material consisted of images of flares obtained by the Hard X-Ray Imaging Spectrometer (HXIS) aboard the Solar Maximum Mission (for a description of the instrument cf. Van Beck et aL, 1980). HXIS produced images of the Sun in a nearly circular area, diameter 2'40", with a spatial resolution of 8" (~5600km), a time resolution of 1.5 s during the flare and 7.5 s before and after, and in six energy bands, spaced quasi-logarithmically between 3.5 and 30 keV.
The aim of the present investigation was to find more evidence for the existence of the phenomenon and to check the model given by de Jager (1985b). For this purpose we examined five more cases, namely the flares of