FIRST IMAGES OF A SOLAR-FLARE AT MILLIMETER WAVELENGTHS

We present the first high spatial resolution images of a solar flare a t millimeter wavelengths. On 1994 August 17, a GOES soft X-ray class M 1 flare was observed by the Berkeley-Illinois-Maryland Array at 86 GHz by the Nobeyama 17 GHz array and by the Yohkoh spacecraft. The flare displayed both a prominent impulsive phase in microwaves and a gradual phase that lasted over 30 minutes. The millimeter data were taken onl y during the gradual phase. The millimeter images show a source with a size of similar to 8 '', a peak brightness temperature of similar to 10(6) K, and maximum optical depth of 0.09. At both X-ray and radio wa velengths, the emitting region appeared to be compact (less than or si milar to 20 ''). In soft X-ray, the images are resolved into two sourc es: one located at a footpoint and the other at the top of the flaring loop. The millimeter emission is consistent with the predicted free-f ree flux from an isothermal temperature (similar to 14 MK) loop-top so urce, a multitemperature footpoint source with a hot (similar to 22 MK ), and a cold (similar to 12 MK) component. Most (80%) of the millimet er flux density originates from the top of the magnetic loop, and the footpoint contribution is only 20%.