Correlation of solar microwave and soft X-ray radiation: 2. The burst component

Abstract

A study of the histories of solar flares observed at 2 cm at the North Liberty Radio Observatory of the University of Iowa and observed at X-ray wavelengths with Mariner 5 (2–9 A) and Explorers 33 and 35 (2–12 A) shows that ‘post-burst increase’ and ‘gradual rise and fall’ events are concurrent microwave and soft X-ray phenomena. The correlation between the X-ray flux and the radio flux is high but nonlinear. The character of the correlation is consistent with a thermal flare theory in which the volume emissivity at X-ray wavelengths is of the spectral form (dE/dν) ∼ exp(-hν/kT) and the radio flux is from the same region that is optically thick with a temperature T. The correlation yields the peak flare temperature, Tp, and the flare solid angle in terms of the fractional increase in temperature relative to the peak temperature, F = δT/Tp. Comparing flare sizes with those obtained by other means (e.g., X-ray telescopes) shows F > 0.2. If free-free emission is assumed responsible for both the X-ray and the radio emissions, an F ∼ 0.5 is consistent with the initial assumptions, whereas an F ∼ 1.0 is not. Thus, temperatures typically double during a flare. Of twenty cases studied, an F = 0.5 yielded a mean peak temperature of 4 million degrees Kelvin and a mean effective diameter of 32 arc seconds.