Extreme ultraviolet flashes of solar flares observed via sudden frequency deviations: Experimental results

Abstract

Properties of solar-flare EUV flashes measured via a type of ionospheric event, called a sudden frequency deviation (SFD), are presented. SFD's are sensitive to bursts of radiation in the 1–1030 A wavelength range. He ii 303.8 A, O v 629.7 A, HL γ 972.5 A and C iii 977.0 A have essentially the same impulsive time dependence as the 1–1030 A flash responsible for SFD's. Soft X-rays (2–20 A) and certain EUV lines have a much slower time dependence than the 1–1030 A flash. Most SFD's have some fine structure, but marked quasi-periodicity in EUV flashes is quite rare. EUV flashes are closely associated with hard X-ray bursts, white-light emission, microwave radio bursts and small bright impulsive kernels in the Hα flare. The intensity of EUV flashes depends on the central meridian distance of the Hα flare location; the intensity decreases at the limb. The total energy radiated in the 10–1030 A flash for the largest events observed is about 1031 ergs.