Time-dependent Lattice Cross Sections and Line Ratios for Solar Wind Charge Exchange: Bare Ne Incident on Atomic H and He

Abstract

Charge exchange (CX) between highly charged ions and neutrals within the geocorona and heliosphere introduces a problematic background for every X-ray observation. For H-like ions the ℓ states within each n shell are degenerate. The majority of CX spectral models rely on calculated n-resolved cross sections, and the distribution of ℓ states is assumed to follow one of several analytical expressions. Laboratory data to validate these distributions at solar wind velocities is not available. Explicitly velocity-dependent calculations of n ℓ-resolved CX cross sections using the time-dependent lattice (TDL) method for Ne10+ incident on atomic H and He at 1–5 keV amu−1 are reported. The cross sections are incorporated into a radiative cascade model, and the influence of n- and n ℓ-resolved cross sections on possibly diagnostic emission line ratios is investigated. Comparisons of the ℓ distribution extracted from the TDL cross sections to the predominantly used analytical distributions reveals a consensus among n and n ℓ-resolved data at solar wind velocities, suggesting a statistical distribution is most appropriate for CX collisions involving atomic H. No consensus is found for atomic He targets, and the ℓ distribution extracted from n ℓ-resolved cross sections appears most appropriate. Open questions related to CX are discussed with emphasis on experimental methods to resolve them. Spectral measurements at an electron beam ion trap facility are planned to benchmark the presented cross section data.