A spatial and spectral maximum entropy method as applied to OVRO solar data

Abstract

We present first results of applying a Maximum Entropy Method (MEM) algorithm that acts in both the spatial and spectral domains to data obtained with the frequency-agile solar interferometer at Owens Valley Radio Observatory (OVRO) taken at 45 frequencies in the range 1 – 18 GHz. The traditional MEM algorithm does not exploit the spatial information available at adjacent frequencies in the OVRO data, but rather applies separately to each frequency. We seek an algorithm that obtains a global solution to the visibilities in both the spatial and spectral domains. To simplify the development process, the algorithm is at present limited to the one-dimensional spatial case. We apply our 1-d algorithm to observations taken with the OVRO frequency-agile interferometer of active region AR 5417 near the solar limb on March 20, 1989 (vernal equinox). The interferometer's two 27 m antennas and 40 m antenna were arranged in a linear east-west array, which at the vernal equinox gives a good match to the 1-d algorithm. Our results show that including the spectral MEM term greatly improves the dynamic range of the reconstructed image compared with a reconstruction without using this information. The derived brightness temperature spectra show that for AR 5417 the dominant radio emission mechanism is thermal gyroresonance and we use this information to deduce the spatial variation of electron temperature and magnetic field strength in the corona above the active region.