First high spatial resolution interferometric observations of solar flares at millimeter wavelengths

We present the first high spatial resolution interferometric observations of solar flares at millimeter wavelengths, carried out with the Berkeley-Illinois-Maryland Array (BIMA). The observations were made at 3.3 mm wavelength during the very active period of 1989 March, using one or three baselines with fringe spacings of 2″5″. The observations represent an improvement of an order of magnitude in both sensitivity and spatial resolution compared with previous solar observations at these wavelengths. Most of the flares occurring within the field of view during the observations were detected by BIMA, including both very impulsive and longer duration events. The longer flares generally seem to have been larger in size than the fringe spacings available, whereas some of the impulsive events may have had sizes comparable to our resolution. However, it appears that millimeter burst sources are not much smaller than microwave sources. The most intense bursts imply brightness temperatures of over 10⁶ K and are due to nonthermal gyrosynchrotron emission or possibly thermal free-free emission. If the emission in the flash phase is predominantly due to gyrosynchrotron emission, we can rule out thermal gyrosynchrotron models for the radio emission because the flux at millimeter wavelengths is too high.