Evolutionary characteristics of Hα line profiles in a kernel of the 2B/X6.9 flare and semi-empirical models of the flare chromosphere
The time evolution of the Hα line in a flare kernel during the 2011 August 9 white-light flare of importance 2B/X6.9 was investigated. At the rising stage of the impulsive phase the Hα emission line with a central reversal, extended emission wings and strong red asymmetry arises but changes to a single-peaked profile at the time of soft X-ray maximum. There is an evidence for Stark broadening of the Hα emission wings. The temporal correlation between Hα light curve and time profile of microwave emission flux is found. These results, according to Canfield et al` (1984) model, indicate that the Hα profiles from this kernel are a consequence of the flare chromosphere heating by the nonthermal electrons and the coronal pressure grows subsequently with the time. The spectra also show the strong downward motion at the start of the impulsive phase which is noticeably reduced in the future.
The semi-empirical models of the flare chromosphere in a knot are calculated for three moments on the basis of the observed profiles of Hα line and with regard to the absence of continuous emission. It was obtained that for explanation of the observed details it is necessary to suppose that the flare has a subtelescopic structure with small bright elements that have intensity of Hα 8−15 units of continuum and occupy 8−12 % of knot area, but the rest of the flare has low intensity. The averaged profile of such structure has broad wings and the continuous intensity is about 1 %. Asymmetry and displacement of Hα profiles are explained by the presence of radial velocities. The atmospheric structure of the investigated flare knot differ from the ones of kernels with continuum emission: there is no deep heating of the chromosphere (including temperature minimum region), but there is a region of high density (log Nh = 14−15) with thickness of 40−60 km in the upper chromosphere.
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