Sharp edges in solar microwave spectra - Neutral current sheets or cyclotron lines?

Two solar active regions have been mapped using the VLA at three closely spaced frequencies (4496, 4716, and 4996 MHz) in an attempt to determine the origin of the steep spectra (index gamma equal to about -5 to -8) sometimes observed with large single telescopes. One of the regions observed indeed shows an anomalously large slope (gamma equal to about -6) compared to the usual (gamma equal to about -2 to -2.5). The other region shows a similar slope (gamma equal to about -5) but with a larger range of statistical error. Two possible explanations for such steep edges in solar spectra are (1) transmission effects of neutral current sheets, and (2) the appearance of cyclotron lines. The internal evidence of the microwave maps and simultaneous optical observations favor an explanation in terms of cyclotron lines.