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 multi-frequency 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 of the 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” $[\mathrm{EM}/\sqrt{T}]$ 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.