Modeling magnetohydrodynamics and non-equilibrium SoHO/UVCS line emission of CME shocks

Abstract

Aims: We provide a guideline to interpret the UltraViolet Coronograph Spectrometer (UVCS) emission lines (in particular O VI and Si XII) during shock wave propagation in the outer solar corona.Methods. We use a numerical magnetohydrodynamic (MHD) model performing a set of simulations of shock waves generated in the corona and from the result we compute the plasma emission for the O VI and Si XII, including the effects of Non Equilibrium Ionization (NEI). We analyze the radiative and spectral properties of our model with the support of a detailed radiation model, including Doppler dimming and an analytical model for shocks, and, finally, we synthesize the expected O VI 1032 Åline profile.Results. We explain several spectral features of the observations like the absence of discontinuities in the O VI emission during the shock passage, the brightening of Si XII emission, and the width of the lines. We also use our model to give very simple and general predictions for the strength of the line wings due to the ions shock heating and on the line shape for Limb Coronal Mass Ejections (CMEs) or Halo CMEs. Conclusions: The emission coming from the post-shock region in the solar corona roughly agrees with the emission from a simple planar and adiabatic shock, but the effect of thermal conduction and the magnetic field may be important depending on the event parameters. Doppler dimming significantly influences the O VI emission while Si XII line brightens mainly because of the shock compression. Significant shock heating is responsible for the wide and faint component of the O VI line usually observed, which may be taken as a shock signature in the solar corona. © 2008 ESO.