The intensity ratio variation of the Si IV 1394/1403 & ANGS; lines during solar flares Resonance scattering and opacity effects

Context. The Si IV lines at 1394 & ANGS; and 1403 & ANGS; form in the solar atmosphere at a temperature of & SIM;10(4.8) K. They are usually considered optically thin, but their opacity can be enhanced during solar flares. Traditionally, the intensity ratio of these lines are used as an indicator of the optical thickness. However, observations have shown a wavelength-dependent intensity ratio profile r(& UDelta;& lambda;) of the 1394 & ANGS; to 1403 & ANGS; lines.Aims. We aim to study the variation of the intensity ratio profile in solar flares and the physical reasons behind it.Methods. The Si IV lines and their intensity ratio profiles were calculated from the one-dimensional radiative hydrodynamics flare model with nonthermal electron heating.Results. During flares, r(& UDelta;& lambda;) is smaller than two at the line core but larger than two at the line wings. We attribute the deviation of the ratio from two to the following two effects: the resonance scattering effect and the opacity effect. Resonance scattering increases the population ratio of the upper levels of the two lines, and, as a result, increases r(& UDelta;& lambda;) in all wavelengths. The opacity effect decreases r(& UDelta;& lambda;), especially at the line core where the opacity is larger. These two effects compete with each other and cause the U shape of r(& UDelta;& lambda;).