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Observations of Coronal Hard X-ray Sources: Insights from Historical and Recent Studies

The corona, typically known for its tenuous nature and inefficiency in bremsstrahlung, has presented intriguing evidence of coronal hard X-ray sources above 20 keV despite expectations of darkness in hard X-ray emissions. Initial observations date back to a limb-occulted flare in 1969. Key features include impulsive phases and unique X-ray spectra during various flare events. This review highlights significant findings, including non-thermal sources, emissions from escaping electrons, and the role of coronal thick targets, enhancing our understanding of solar flare dynamics.

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Observations of Coronal Hard X-ray Sources: Insights from Historical and Recent Studies

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  1. Coronal hard X-ray sources • The corona is tenuous and bremsstrahlung is inefficient => it should be dark in hard X-rays • Nevertheless several clear observations of coronal sources >20 keV have been reported

  2. The first clear evidence? Coronal hard X-ray source from a limb-occulted flare in 1969 (Frost & Dennis, 1971). Note: • Coronal height ~30,000 km • Impulsive phase evident • X-ray spectrum ~hn-2 in later extended phase Review by Cliver et al., ApJ 305, 920 (1986)

  3. Coronal sources of hard X-rays? • Late non-thermal sources (radio type IV?) • Masuda “above-the-loop-top” hard X-rays • Direct emission from escaping electrons (counterpart of Type III bursts) • Coronal thick targets • Early-phase hard spectrum sources (e.g. July 23 2002)

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