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

Abstract

We have mapped two solar active regions using the VLA at three closely spaced frequencies (4496, 4716, and 4996 MHz) in an attempt to determine the origin of the steep spectra (indexγ ≈ −5 to −8) sometimes observed with large single telescopes. One of the regions observed indeed shows an anomalously large slope (γ ≈ −6) compared to the usual (γ ≈ −2 to −2.5). The other region shows a similar slope (γ ≈ −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.