Ultraviolet brightenings in active regions as observed from OSO-8

Physics

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Chromosphere, Plasma Spectra, Satellite Observation, Solar Activity Effects, Solar Spectra, Ultraviolet Spectra, Carbon, H Alpha Line, High Temperature Plasmas, Luminous Intensity, Oso-8, Plasma Resonance, Silicon, Spectroheliographs

Scientific paper

Repeated raster images of solar active regions taken at the line centers of the Si IV and C IV resonance lines using an ultraviolet spectrometer aboard OSO-8 reveal dramatic transient brightenings of up to factors of 10. These brightenings last several minutes and frequently show a repetitive character. Inspection of simultaneous H-alpha flare patrol records show that these transition-zone events are often associated with subflarelike brightenings in the chromosphere. These observations indicate that direct excitation or heating of material already at transition-zone temperatures caused by nonthermal particle streams is inadequate to explain the degree of brightening of these lines. The measurements suggest that some process that enhances the density of material at temperatures of the order of 100,000 K is occurring during these events.

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