Formation of Si IV due to the interaction of an electron beam with the flaring solar chromosphere

Astronomical Institute of the Czech Academy of Sciences, Fričova 298, Ondřejov, Czech Republic

^⋆ Corresponding author: jaroslav.dudik@asu.cas.cz

Received: 18 June 2025
Accepted: 26 August 2025

Abstract

Aims. We modeled Si IV emission at the footpoints of flaring loops to evaluate the effects of the electron beam, non-equilibrium ionization, and suppression of dielectronic recombination.

Methods. The radiation-hydrodynamic code FLARIX was used to model the properties of plasma in solar flare ribbons. Energy was deposited via short-duration electron beams with a triangular profile in time, peaking at 1, 3, or 5 s, and with an energy flux ranging from 1 to 9 × 10¹⁰ cm⁻² s⁻¹. Subsequently, we calculated the non-equilibrium ionization states of silicon, taking into account the non-Maxwellian electron distribution containing the electron beam component. The Si IV line intensities and their evolution in time were modeled using a 161-level CHIANTI ion model.

Results. We found that the maximum of Si IV emissivity occurs in a “hot bubble” formed at the location of the maximum of beam energy deposition. The non-equilibrium abundances of Si IV were found to differ from those of the equilibrium by more than one order of magnitude. The ionization by the electron beam has only a small effect on the Si IV emissivity; typically below 10%. However, the effect of density suppression of dielectronic recombination is much more important, as the emissivities calculated without it can be several times lower than when this effect is included. Overall, the main contribution to the total intensity of Si IV lines along the flare loop comes from the hot bubble in optically thin cases or from plasma above this bubble when the plasma is optically thick.

Conclusions. Non-equilibrium ionization and density suppression of dielectronic recombination are found to be important effects influencing the modeled ionization states of the flaring plasma and the resulting Si IV emissivities. For short-duration electron beams, the maximum of the emissivity corresponds to the position of the hot bubble.