Author(s): , ,
Institution(s): 1. Astronomical Institute, Czech Academy of Sciences, 2. Astronomical Institute, University of Wroclaw
It is well known that during solar flares the heated chromospheric plasma emits the ultraviolet (UV) radiation in spectral lines and continua. UV space telescopes (e.g. TRACE, Solar Dynamic Observatory/Atmospheric Imaging Assembly - SDO/AIA) provide images of solar flares where the complicated fine structure of the flaring atmosphere is well visible. However, these broadband images contain the mixture of line and continuum UV emission and it is not possible to disentangle between these two contributions. E.g. solar flare brightening observed in SDO/AIA 160 nm channel can be visible both due to the strong line emission in C IV 154.8 nm or due to the continuum increase. Spectral lines and continua are formed in plasma of different temperature and location. Therefore, in order to obtain the parameters of plasma we should know quantitatively the contribution from lines and continua.
Such analysis would be possible using the UV spectrograph, where we can see the intensity of lines and continua separately. For example Interface Region Imaging Spectrograph (IRIS) provides such spectra of flares, where the enhanced lines and continuum is visible both in NUV (near UV) and FUV (far UV) channels.
In our work we used IRIS flare observations in FUV channel to determine the intensity of spectral lines, including Si IV 139.4 nm line. Then, using CHIANTI procedures and the observed intensities of some FUV lines we computed the synthetic UV spectrum for given flare model. IRIS spectrum does not include C IV line, which contaminates the SDO/AIA images, but this line can be calculated in the theoretical CHIANTI spectrum. In this way, we can subtract the C IV (and other lines) emission from SDO/AIA images and obtain the pure UV continuum intensity. Finally, the obtained continuum emission can used to study the physics of heated chromosphere of flares.