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Temporal variations of the CaXIX spectra in solar flares
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Robert Falewicz Submitted: 2009-09-25 13:10
Standard model of solar flares comprises a bulk
expansion and rise of abruptly heated plasma (the chromospheric evaporation). Emission from plasma ascending along loops rooted on the visible solar disk should be often dominated, at least temporally, by a blue-shifted emission. However, there is only a very limited number of published observations of solar flares having spectra in which the blue-shifted component dominates the stationary
one. In this work we compare observed X-ray spectra of three solar flares recorded during their impulsive phases and relevant synthetic spectra calculated using one-dimensional hydro-dynamic numerical model of these flares. The main aim of the work was to explain why
numerous flares do not show blue-shifted spectra. We synthetised time series of BCS spectra of three solar flares in various moments of their evolution from the beginning of the impulsive phases beyond maxima of the X-ray emissions using 1D numerical model of the solar
flares and standard software to calculate BCS synthetic spectra of the flaring plasma. The models of the flares were calculated using observed energy distributions of the non-thermal electron beams injected into the loops, initial values of the main physical parameters of the plasma confined in the loops and geometrical properties loops' estimated using available observational data. The synthesized BCS spectra of the flares were compared with the
relevant observed BCS spectra. Taking into account the geometrical dependences of the line-of-sight velocities of the plasma moving along the flaring loop inclined toward the solar surface as well as a distribution of the investigated flares over the solar disk, we conclude that stationary component of the spectrum should be observed almost for all flares during their early phases of
evolution. In opposite, the blue-shifted component of the spectrum could be not detected in flares having plasma rising along the flaring loop even with high velocity due to the geometrical dependences only. Our simulations based on realistic heating rates of plasma by non-thermal electrons indicate also that the upper chromosphere is
heated by non-thermal electrons a few seconds before beginning of noticeable high-velocity bulk motions, and before this time plasma emits stationary component of the spectrum only. After the start of the up-flow motion, the blue-shifted component dominate temporally the synthetic spectra of the investigated flares at their early phases.
Authors: R. Falewicz, P. Rudawy, and M. Siarkowski
Projects: None
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Publication Status: accepted (A&A)
Last Modified: 2009-09-28 09:47
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