This work aims to provide an accurate description and calculations of collision frequencies in conditions relevant to the solar atmosphere. To do so, we focus on the detailed description of the collision frequency in the solar atmosphere based on a classical formalism with Chapman–Cowling collision integrals, as described by Zhdanov. These collision integrals allow linking the macroscopic transport fluxes of multifluid models to the kinetic scales involved in the Boltzmann equations. In this context, the collision frequencies are computed accurately while being consistent at the kinetic level. We calculate the collision frequencies based on this formalism and compare them with approaches commonly used in the literature for conditions typical of the solar atmosphere. To calculate the collision frequencies, we focus on the collision integral data provided by Bruno et al., which is based on a multicomponent hydrogen–helium mixture used for conditions typical for the atmosphere of Jupiter. We perform a comparison with the classical formalism of Vranjes & Krstic and Leake & Linton. We highlight the differences obtained in the distribution of the cross sections as functions of the temperature. Then, we quantify the disparities obtained in numerical simulations of a 2.5D solar atmosphere by calculating collision frequencies and ambipolar diffusion. This strategy allows us to validate and assess the accuracy of these collision frequencies for conditions typical of the solar atmosphere.

The ponderomotive force has been suggested to be the main mechanism to produce the so-called first ionization potential (FIP) effect—the enrichment of low-FIP elements observed in the outer solar atmosphere, in the solar wind, and in solar energetic events. It is well known that the ionization of these elements occurs within the chromosphere. Therefore, this phenomenon is intimately tied to the plasma state in the chromosphere and the corona. For this study, we combine IRIS observations, a single-fluid 2.5D radiative magnetohydrodynamics (MHD) model of the solar atmosphere, including ion–neutral interaction effects and nonequilibrium (NEQ) ionization effects, and a novel multifluid multispecies numerical model (based on the Ebysus code). Nonthermal velocities of Si iv measured from IRIS spectra can provide an upper limit for the strength of any high-frequency Alfvén waves. With the single-fluid model, we investigate the possible impact of NEQ ionization within the region where the FIP may occur, as well as the plasma properties in those regions. These models suggest that regions with strongly enhanced network and type II spicules are possible sites of large ponderomotive forces. We use the plasma properties of the single-fluid MHD model and the IRIS observations to initialize our multifluid models to investigate the multifluid effects on the ponderomotive force associated with Alfvén waves. Our multifluid analysis reveals that collisions and NEQ ionization effects dramatically impact the behavior of the ponderomotive force in the chromosphere, and existing theories may need to be revisited.