High-Resolution Imaging of Solar Flare Ribbons and its Implication on the Thick-Target Beam Model
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We report on high-resolution optical and hard X-ray observations of solar are ribbons seen during the GOES X6.5 class white-light flare of 2006 December 6. The data consist of imaging observations at 430 nm (the Fraunhofer G-band) taken by the Hinode Solar Optical Telescope (SOT) with the hard X-rays observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The two sets of data show closely similar ribbon structures, strongly suggesting that the flare emissions in white light and in hard X-rays have physically linked emission mechanisms. While the source structure along the ribbons is resolved at both wavelengths (length ~30´´), only the G-band observations resolve the width of the ribbon, with values between ~ 0.5´´ and ~ 1.8´´. The unresolved hard X-ray observations reveal an even narrower ribbon in hard X-rays (the main footpoint has a width perpendicular to the ribbon of < 1.1´´ compared to the G-band width of ~ 1.8´´) suggesting that the hard X-ray emission comes from the sharp leading edge of the G-band ribbon. Applying the thick-target beam model, the derived energy deposition rate is > 5£1012 erg s¡1 cm¡2 provided by an electron flux of 1 x 1020 electrons s¡1 cm¡2 above 18 keV. This requires that the beam density of electrons above 18 keV be at least 1 x 1010 cm¡3. Even if field lines converge towards the chromospheric footpoints, the required beam in the corona has a too high density to be described as a dilute tail population on top of a Maxwellian core. We discuss this issue and others associated with this extreme event, which poses serious questions to the standard thick target beam interpretation of solar flares.