Non-LTE diagnostics of velocity fields during the gradual phase of a solar flare

¹ Observatoire de Paris, Section de Meudon, LESIA, 92195 Meudon Principal Cedex, France e-mail: arkadiusz.berlicki@obspm.fr
² Astronomical Institute of the Wrocław University, ul. Kopernika 11, 51-622 Wrocław, Poland
³ Astronomical Institute, Academy of Sciences of the Czech Republic, 25165 Ondřejov, Czech Republic
⁴ Institute of Theoretical Astrophysics, University of Oslo, Blindern, 0315 Oslo, Norway

Received: 14 May 2004
Accepted: 13 September 2004

Abstract

We perform an analysis of the velocity field within the Hα ribbons during the gradual phase of an M 1.0 solar flare observed on October 22, 2002. We use spectroscopic observations performed with the German VTT (Vacuum Tower Telescope) working in the MSDP (Multichannel Subtractive Double Pass spectrograph) observing mode. From these observations the Hα line profiles in chosen areas of the flare ribbons were reconstructed and these observational profiles were compared with a grid of synthetic Hα line profiles calculated by the non-LTE radiative-transfer code. This code allows us to calculate different models of the chromosphere with a prescribed velocity field. By optimising the best fit between the observed and synthetic profiles we find the most appropriate models of the chromosphere and vertical structure of the velocity field in the analysed areas of the flare ribbons. By means of the non-LTE radiative-transfer calculations we show that in most analysed areas of the Hα flare ribbons the chromospheric plasma exhibited upward motion with a mean velocity of a few . These results are consistent with previous estimates and support the scenario of a gentle evaporation during the gradual phase.