YOHKOH remnants: partially occulted flares in hard X-rays

Abstract

Flares being partially occulted by the solar limb, are the best reservoir of our knowledge about hard X-ray loop-top sources. Recently, the survey of partially occulted flares observed by the RHESSI has been published (Krucker & Lin 2008). The extensive YOHKOH database still awaits such activities. This work is an attempt to fill this gap. Among from 1286 flares in the YOHKOH Hard X-ray Telescope Flare Catalogue, for which the hard X-ray images had been enclosed, we identified 98 events that occurred behind the solar limb. We investigated their hard X-ray spectra and spatial structure. We found that in most cases the hard X-ray spectrum of partially occulted flares consists of two components, non-thermal and thermal, which are co-spatial. The photon energy spectra of the partially occulted flares are systematically steeper than spectra of the non-occulted flares. Such a difference we explain as a consequence of intrinsically dissimilar conditions ruling in coronal parts of flares, in comparison with the footpoints which usually dominate the hard X-ray emission of disk flares. At least two reasons of the difference should be taken into consideration: (1) stronger contamination of hard X-rays with emission of thermal plasma, (2) different mechanism in which non-thermal electrons radiate their energy. A schematic picture, in which thin-target mechanism is responsible for hard X-ray emission of loop-top sources and thick-target mechanism -- for emission of footpoint sources, can be modified by the presence of some coronal thick-target sources. At least a part of them suggests a magnetic trapping. Investigated flares do not respond the global magnetic configuration of the solar corona. For their characteristics conclusive is rather the local magnetic configuration in which they were developed.

16 pages, 12 figures, 7 tables, submitted to A&A

References in corpus (10)

• Double Coronal Hard and Soft X-ray Source Observed by RHESSI: Evidence for Magnetic Reconnection and Particle Acceleration in Solar Flares
• Looptop Hard X-Ray Emission in Solar Flares: Images and Statistics
• Relations between concurrent hard X-ray sources in solar flares
• Hard X-ray Spectra and Positions of Solar Flares observed by RHESSI: photospheric albedo, directivity and electron spectra
• A Statistical Survey of Hard X-ray Spectral Characteristics of Solar Flares with Two Footpoints
• Spectral Hardening of Large Solar Flares
• Exploring the connection between coronal and footpoint sources in a thin-thick target solar flare model
• Observational evidence for return currents in solar flare loops
• Footpoint versus loop-top hard X-ray emission sources in solar flares
• Non-thermal recombination - a neglected source of flare hard X-rays and fast electron diagnostic

Cited by in corpus (11)

• Deducing Electron Properties From Hard X-Ray Observations
• Electron Power-Law Spectra in Solar and Space Plasmas
• Hard X-Ray Emission from Partially Occulted Solar Flares: RHESSI Observations in Two Solar Cycles
• Determination of Stochastic Acceleration Model Characteristics in Solar Flares
• Impulsive Phase Coronal Hard X-ray Sources in an X3.9 Class Solar Flare
• Investigation of quasi-periodic varaiations in hard X-rays of solar flares
• Density Enhancements and Voids following Patchy Reconnection
• Coronal Electron Confinement by Double Layers
• Dynamics of double layers, ion acceleration and heat flux suppression during solar flares
• Inverse Compton X-rays from relativistic flare electrons and positrons
• A Catalogue of Solar X-ray Plasma Ejections observed by the Soft X-ray Telescope onboard YOHKOH