Imaging spectroscopy of solar microwave radiation. I. Flaring emission

We present observations of an impulsive microwave burst on the Sun with both high spatial and spectral resolution, made with the Solar Array at the Owens Valley Radio Observatory. The burst was imaged in total intensity, as well as both right and left circular polarization, at 24 frequencies distributed logarithmically over the range 2.4-14.0 GHz, with spatial resolution ranging from ∼5″-29″. This powerful new technique for studying microwave bursts shows that: (1) even a relatively weak burst with a simple temporal and spectral morphology in total power can have a complex spatial structure, comprising three distinct sources; (2) the burst structure changes with frequency so that images at widely spaced frequencies show a different number and/or location of sources, whereas images at closely spaced frequencies reveal the relationship between sources at different frequencies; and (3) the brightness temperature spectrum of each source is different, so that the composite total-power spectrum is not representative of the spectrum of any individual source. We used the measured brightness temperature spectrum to infer the emission process responsible for each microwave source, and to derive physical conditions in the source region. We confirmed our predictions using soft X-ray measurements from GOES, soft X-ray images from Yohkoh, and Ha flare images together with sunspot and magnetogram images from the Big Bear Solar Observatory. The primary microwave source, located close to but not coincident with either the lone flaring soft X-ray kernel or primary Ha kernel, had a peak brightness temperature of only ∼ 10⁷ K, but nevertheless was produced by nonthermal gyrosynchrotron emission. The two secondary sources, both without any soft X-ray or Hα counterparts, had lower turnover frequencies but also were produced by nonthermal gyrosynchrotron emission. These sources were connected to the primary flare site by magnetic loops visible in soft X-rays, suggesting that their nonthermal electrons escaped from the primary microwave source.