Observational characteristics of the white-light flare of 2011 August 9
We present observational data for the 2011 August 9 2B/X6.9 white-light flare (WLF) including Hα images and spectra within the range 6549–6579 Å. The morphology and evolution of the Hα flare, location, temporal behavior, spectrum and energetics of the WLF kernels were investigated. The following results have been obtained: the flare erupted inside the regions of interaction of new rapidly growing and pushing magnetic flux and preexistent strong magnetic fields, that leads to superposition of two bipolar active regions. The structure of the WLF was complex. At least five individual kernels of continuum emission could be identified before and during the impulsive phase. The pre-impulsive and impulsive WLF kernels belong to different types: II and I, respectively. For the impulsive phase of white light emission there was a good time correlation between the maximum of continuum emission and the peak of microwave radiation. Peak surface flux, luminosity and total energy in optical continuum of the brightest impulsive WLF kernel were 1.4 ⋅ 1010erg cm-2s-1, 1.5 ⋅ 1027 erg/s и 5 ⋅ 1029 erg, respectively. The Hα profiles have a broad emission wings (overall extensions up to 26 Å, halfwidths up to 9 Å) and central reversal. Broad-wing profiles are symmetrical and shifted to the red as a whole, indicating the downward motion of the Hα emission region with balk velocities some tens km/s. The Hα broad wings are not the result of Stark broadening. They are broadened by nonthermal turbulent motion with velocities 150–300 km/s. The observed Hα profiles have been analyzed in terms of Hα profiles calculations for strong nonthermal electron beam heating of the chromosphere and formation of a downward moving chromospheric condensation. We must conclude that the flare shows spectral signatures that are difficult to explain within the context of electron beam heating.
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