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

Abstract

We present rst 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 ad- jacent 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 spa- tial 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 inter- ferometer 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 includ- ing the spectral MEM term greatly improves the dynamic range of the reconstructed image compared with a re- construction without using this information. The derived brightness temperature spectra show that for AR 5417 the dominant radio emission mechanism is thermal gyrores- onance and we use this information to deduce the spa- tial variation of electron temperature and magnetic eld strength in the corona above the active region.