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Unusual ribbon-like hard X-ray sources were found with the RHESSI observation of a 2B/M8.0 flare on 2005 May 13. We use this unique observation to investigate the spatial distribution of the hard X-ray intensity along the ribbons and compare it with the local magnetic reconnection rate and energy release rate predicted by the standard magnetic reconnection model for two ribbon flares. In the early phase of the flare, the hard X-ray sources appear to be concentrated in strong field regions within the Ha ribbons, which is explicable by the model. At and after the flare maximum phase, the hard X-ray sources become spatially extended to resemble Ha ribbons in morphology, during which the spatial distribution of hard X-ray intensity lacks a correlation with that of the local magnetic reconnection rate and energy release rate predicted by the model. We argue that the magnetic reconnection during this event may involve the rearrangement of magnetic field along the magnetic arcade axis that is inevitably overlooked by the two-dimensional model and suggest that this type of three-dimensional reconnection will be best seen in so-called sigmoid-to-arcade transformations.

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The structure and properties of a newly emerged solar active region (NOAA Active Region 7985) are discussed using the Coronal Diagnostic Spectrometer (CDS) and the Extreme-Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory. CDS obtained high-resolution EUV spectra in the 308È381 and 513È633 wavelength ranges, while EIT recorded full-disk EUV images in A A the He II (304 Fe IX/X (171 Fe XII (195 and Fe XV (284 bandpasses. Electron density mea-A ), A ), A ), A ) surements from Si IX, Si X, Fe XII, Fe XIII, and Fe XIV line ratios indicate that the region consists of a central high-density core with peak densities of the order of 1.2 ] 1010 cm~3, which decrease monotonically to D5.0 ] 108 cm~3 at the active region boundary. The derived electron densities also vary systematically with temperature. Electron pressures as a function of both active region position and temperature were estimated using the derived electron densities and ion formation temperatures, and the constant pressure assumption was found to be an unrealistic simpliÐcation. Indeed, the active region is found to have a high-pressure core (1.3 ] 1016 cm~3 K) that falls to 6.0 ] 1014 cm~3 K just outside the region. CDS line ratios from di †erent ionization stages of iron, speciÐcally Fe XVI (335.4and Fe XIV A ) (334.4 were used to diagnose plasma temperatures within the active region. Using this method, peak A ), temperatures of 2.1 ] 106 K were identiÐed. This is in good agreement with electron temperatures derived using EIT Ðlter ratios and the two-temperature model of Zhang et al. The high-temperature emission is conÐned to the active region core, while emission from cooler (1È1.6) ] 106 K lines originates in a system of loops visible in EIT 171 and 195 images. Finally, the three-dimensional geometry of the A active region is investigated using potential Ðeld extrapolations from a Kitt Peak magnetogram. The combination of EUV and magnetic Ðeld extrapolations extends the "" core-halo ÏÏ picture of active region structure to one in which the core is composed of a number of compact coronal loops that conÐne the hot, dense, high-pressure core plasma while the halo emission emerges from a system of cooler and more extended loops.