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Hard X-ray emission from the solar corona  

S Krucker   Submitted: 2008-08-12 13:55

This review surveys hard X-ray emissions of non-thermal electrons in the solar corona. These electrons originate in flares and flare-related processes. Hard X-ray emission is the most direct diagnostic of electron presence in the corona, and such observations provide quantitative determinations of the total energy in the non-thermal electrons. The most intense flare emissions are generally observed from the chromosphere at footpoints of magnetic loops. Over the years, however, many observations of hard X-ray and even gamma-ray emission directly from the corona have also been reported. These coronal sources are of particular interest as they occur closest to where the electron acceleration is thought to occur. Prior to the actual direct imaging observations, disk occultation was usually required to study coronal sources, resulting in limited physical information. Now RHESSI has given us a systematic view of coronal sources that combines high spatial and spectral resolution with broad energy coverage and high sensitivity. Despite the low density and hence low bremsstrahlung efficiency of the corona, we now detect coronal hard X-ray emissions from sources in all phases of solar flares. Because the physical conditions in such sources may differ substantially from those of the usual `footpoint' emission regions, we take the opportunity to revisit the physics of hard X-radiation and relevant theories of particle acceleration.

Authors: Krucker Säm, Battaglia M., Cargill P.J., Fletcher L., Hudson H.S, MacKinnon, A.L., Masuda S., Sui L., Tomczak T., Veronig A.M., Vlahos l., White S.M.
Projects: RHESSI

Publication Status: in press, A&A Rev
Last Modified: 2008-09-23 20:51
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Solar Flare Electron Spectra at the Sun and near the Earth  

S Krucker   Submitted: 2007-07-02 17:22

We compare hard X-ray (HXR) photon spectra observed by the RHESSI with the spectra of the electrons in the associated solar impulsive particle events observed near 1 AU by the WIND 3D Plasma and Energetic Particle (3DP) instrument. For prompt events, where the inferred injection time at the Sun coincides with the HXR burst, the HXR photon power-law spectral index γ and the in situ observed electron spectral index δ measured above 50 keV show a good linear fit, δ = γ + 0.1(?0.1), with correlation coefficient of 0.83, while for delayed events (inferred injection >10 minutes after the HXR burst) only a weak correlation with a coefficient of 0.43 is seen. The observed relationship for prompt events is inconsistent, however, with both the thin target case, where the escaping electrons come from the X-ray-producing electron population, and the thick target case where some of the accelerated source population escapes to 1 AU and the rest produce the HXRs while losing all their energy to collisions. Furthermore, the derived total number of escaping electrons correlates with the number of electrons required to produce observed X-ray flux but is only about ∼0.2% of the number of HXR-producing electrons.

Authors: Säm Krucker, E. P. Kontar, S. Christe, and R. P. Lin
Projects: RHESSI

Publication Status: The Astrophysical Journal, 663:L109-L112, 2007 July 10
Last Modified: 2007-07-03 09:43
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Hard X-ray emission from the solar corona
Solar Flare Electron Spectra at the Sun and near the Earth

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