The influence of thermal pressure gradients and ionization (im)balance on the ambipolar diffusion and charge-neutral drifts

Abstract

Solar partially ionized plasma is frequently modelled using single-fluid (1F) or two-fluid (2F) approaches. In the 1F case, charge-neutral interactions are often described through ambipolar diffusion, while the 2F model fully considers charge-neutral drifts. Here, we expand the definition of the ambipolar diffusion coefficient to include inelastic collisions (ion/rec) in two cases: a VAL3C one-dimensional model and a 2F simulation of the Rayleigh–Taylor instability (RTI) in a solar prominence thread based on [Lukin et al. 2024 Phil. Trans. R. Soc. A 382 , 20230417. ( doi:10.1098/rsta.2023.0417 )]. On one side, we evaluate the relative importance of the inelastic contribution, compared to elastic and charge-exchange collisions. On the other side, we compare the contributions of ion/rec, thermal pressure, viscosity and magnetic forces to the charge-neutral drift velocity of the turbulent flow of the RTI. Our analysis reveals that the contribution of inelastic collisions to the ambipolar diffusion coefficient is negligible across the chromosphere, allowing the classical definition of this coefficient to be safely used in 1F modelling. However, in the transition region, the contribution of inelastic collisions can become as significant as that of elastic collisions. Furthermore, we ascertain that the thermal pressure force predominantly influences the charge-neutral drifts in the RTI model, surpassing the impact of the magnetic force. This article is part of the theme issue ‘Partially ionized plasma of the solar atmosphere: recent advances and future pathways’.

Partially ionized plasma of the solar atmosphere: recent advances and future pathways

“…Gómez Míguez et al . [ 2 ] made a detailed comparison between this approximation for partially ionized plasma modelling and the more detailed two-fluid model (solving for both the dynamics of neutrals and charges), highlighting the importance of the extra force terms that are not readily captured in single-fluid approximations. Taking a different perspective, Hillier [ 3 ] looked at how to relax one of the underlying assumptions behind the development of single-fluid partially ionized plasma modelling showing that when the velocity difference between charges and neutrals gets large enough this has a negative feedback on the rate at which neutrals drift across the magnetic field.…”