Allen Telescope Array Multi-Frequency Observations of the Sun

We present the first observations of the Sun with the Allen Telescope Array (ATA). We used up to six frequencies, from 1.43 to 6 GHz, and baselines from 6 to 300 m. To our knowledge, these are the first simultaneous multifrequency full-Sun maps obtained at microwave frequencies without mosaicing. The observations took place when the Sun was relatively quiet, although at least one active region was present each time. We present multi-frequency flux budgets for each sources on the Sun. Outside of active regions, assuming optically thin bremsstrahlung (free--free) coronal emission on top of an optically thick ~10 000 K chromosphere, the multi-frequency information can be condensed into a single, frequency-independent, "coronal bremsstrahlung contribution function" [EM/sqrt(T)] map. This technique allows the separation of the physics of emission as well as a measurement of the density structure of the corona. Deviations from this simple relationship usually indicate the presence of an additional gyroresonance-emission component, as is typical in active regions.Comment: 16 pages, 11 figures. Accepted for publication in Solar Physic

Two-dimensional mapping of the sun with the ratan-600

We present two-dimensional solar maps at 2.7, 3.2, 4, and 8.2 cm computed from one-dimensional observations with the RATAN-600, using Earth rotation aperture synthesis techniques. Before the calculation of maps, the position of each scan was corrected with respect to the center of the solar disk and the scans were calibrated. The circular polarization scans were corrected for polarization cross-talk between the I and V channels. Subsequently, the quiet-Sun background emission was subtracted. After all corrections, a dirty map was computed by combining the scans at different position angles. The last step of the processing was an attempt to free the dirty map of the sidelobes, using the standard CLEAN procedure. The resolution of the clean maps at 2.7 cm was 0.5′ by 6′. Both active regions which were present on the solar disk were mapped. We studied the flux spectra of different types of sources: one was associated with a sunspot, the second was located over the neutral line of an active region, and the other was associated with the plage. The emission mechanism of the former was attributed to the gyroresonance process, while the short wavelength emission of the others was attributed to the free-free process. For the sunspot-associated source we estimated the magnetic field strengths at the base of the transition region and in the low corona. © 1996 Kluwer Academic Publishers