3~2~ Rio. Vol.4, No.7, pp. 247—23O, 1984 0273—1177/84 ~0.O0 .30 Printed in Great Britain. All rights reserved. Copyright C COSPAR SIMULTANEOUS DUAL WAVELENGTH OBSERVATIONS OF AN IMPULSIVE MICROWAVE BURST USING THE V.L.A. R. K. Shevgaonkar* and M. R. Kundu Astronomy Program, University of Maryland, College Park, MD 20742, U.S.A. ABSTRACT Simultaneous observations of a microwave burst at 2 and 6 cm wavelengths were carried out with the Very Large Array (VLA). The 6 cm burst source is located close to a magnetic neutral line, presumably near the top of a flaring loop, while the 2 cm emission originates from the footpoints of the loop. It is concluded that the 6 cm emission is dominated by gyrosynchrotron radiation of the thermal electrons in the bulk heated plasma at a temperature of 4 x 1O7 K, while the 2 cm emission is due to nonthermal particles released and accelerated during the flare process. From the observed low degree of polarization and the lack of the 2 cm source cospatiality with the 6 cm source a magnetic field of 200—350 C and 6 ~ 4 are estimated in the flare energy release site. A DC electric field flare model is invoked to explain the long delay between the peaks at the two wavelengths. From the delay, the strength of the electric field is estimated to be 0.2—4 i statvolt cm1 in the flaring region. INTRODUCTION In a flare the energy release region is usually located in the lower corona and therefore the microwave observations can be used to derive the physical parameters in the flaring region such as the electron density, the magnetic field strength and the number of gyro— synchrotron emitting electrons. With the high resolution of a few arc seconds as presently available with the VLA, it is possible to locate the flare energy release site within the complex magnetic structure of the flaring active region. The observations conducted in the past (e.g. /1, 7, 8, 9, 11/) indicate that the radio bursts are generally located over the magnetic neutral line whereas their Ho counterparts are located near the footponts of the flaring loop. The close association of hard X—ray burst with the microwave bursts /6/ suggests a common origin of energetic electrons radiating in the two spectral domains. Simultaneous multifrequency observations in microwaves and hard X—rays are needed to properly understand the radiation mechanisms operating in different spectral domains and are therefore important in modelling a flare. OBSERVATIONS AND RESULTS Simultaneous microwave burst observations at 6 and 2 cm were carried Out on 13 November 1981 with the VIA in C—configuration by dividing the 27 antennas into two subarrays. The visibilities were recorded with a receiver bandwidth of 12.5 MHz and 10 sec integration time. The maps produced by Fourier transforming the visibilities were CLEANed and the CLEANed maps were restored with a Gaussian beam of appropriate size, namely ~x 3” at 6 cm and 2” x 1.6” at 2 cm, respectively. Figure 1 shows the time profiles of the burst at 6 cm, 2 cm and in hard X—rays. At 6 and 2 cm, the shortest baseline of — 2 kA was used to obtain the time profile. The burst consists of two peaks; the first peak is much stronger than the second one. The hard X—ray and the 2 cm emissions peak more or less simultaneously but the 6 cm peak is delayed by 15 sec. Figure 2 shows the total intensity maps at 6 and 2 cm around the peak of the burst. The circular polarization maps are not presented here because due to low degree of polarization the quality of maps is not good. From total intensity maps it is clear that the burst sources at 6 and 2 cm are not co— spatial. The source at 6 cm has its total intensity maximum lying between the two isolated bright components of the 2 cm source. Comparing the source locations with the magnetic structure of the quiescent active region it is apparent that the 6 cm source more or less *On leave of absence from Indian Institute of Astrophysics, Bangalore, India. 247