First Images of Impulsive Millimeteremission and Spectral Analysis of the 1994 August 18 Solar

We present here the rst images of impulsive millimeter emission of a are. The are on 1994 August 18 was simultaneously observed at millimeter (86 GHz), microwave (1-18 GHz), and soft and hard X{ray wavelengths. Images of millimeter, soft and hard X{ray emission show the same compact (< 8 00) source. Both the impulsive and the gradual phases are studied in order to determine the emission mechanisms. During the impulsive phase, the radio spectrum was obtained by combining the millimeter with simultaneous microwave emission. Fitting the nonthermal radio spectra as gyrosynchrotron radiation from a model with constant magnetic eld yield the physical properties of the aring source, that is, total number of electrons, power{law index of the electron energy distribution, and the nonthermal source size. These results are then compared to those obtained from the hard X{ ray spectra. The energy distribution of the energetic electrons inferred from the hard X{ray and radio spectra is found to follow a double power{law with slope 6-8 below 30-50 keV and 4 above those energies. The temporal evolution of the electron energy spectrum and its implication for the acceleration mechanism are then discussed. Comparison of millimeter and soft X{ray emissions during the gradual phase implies that the millimeter emission is free{free radiation from the same hot soft X{ray emitting plasma, and further suggests that the are source contains multiple temperatures.