Temporal variations of the CaXIX spectra in solar flares

¹ Astronomical Institute of Wrocław University, 51-622 Wrocław, ul. Kopernika 11, Poland e-mail: [falewicz;rudawy]@astro.uni.wroc.pl
² Space Research Centre, Polish Academy of Sciences, 51-622 Wrocław, ul. Kopernika 11, Poland e-mail: ms@cbk.pan.wroc.pl

Received: 29 June 2009
Accepted: 18 September 2009

Abstract

Aims. The standard model of solar flares comprises a bulk expansion and a rise of abruptly heated plasma (chromospheric evaporation). Emission from plasma ascending along loops rooted in the visible solar disk often should be 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 is to explain why many flares do not show blue-shifted spectra.

Methods. We synthetised time series of Bragg Crystal Spectrometer (BCS) spectra of three solar flares at various moments of their evolution from the beginning of the impulsive phases to beyond maxima of the X-ray emission using a 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 of the loops estimated using available observational data. The synthesized BCS spectra of the flares were compared with the relevant observed BCS spectra.

Results. 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 the stationary component of the spectrum should be observed for almost all flares during their early phases of evolution. On the contrary, the blue-shifted component of the spectrum could not be detected in flares having plasma rising along the flaring loop even with high velocity due to the geometric dependences only. Our simulations based on realistic heating rates of plasma by non-thermal electrons indicate that the upper chromosphere is heated by non-thermal electrons a few seconds before the beginning of noticeable high-velocity bulk motion, and before this time plasma emits the stationary component of the spectrum only. After the start of the upward flow, the blue-shifted component temporally dominates the synthetic spectra of the investigated flares in their early phases.