Three-dimensional distribution of ions and electrons in the lunar ionosphere originated from the photochemical reactions

Abstract

ABSTRACT Using a fluid-based time-dependent numerical photochemical model, the three-dimensional distribution of ions and electrons in the lunar ionosphere, originated purely from photochemical reactions, is investigated. The photochemical model includes the production and recombination of 16 ions, namely CO$_2^+$, H2O+, H3O+, OH+, O$_2^+$, O+, Ar+, Ne+, He+, H+, H$_2^+$, CH$_3^+$, CH$_4^+$, and CH$_5^+$. The model also includes the interaction of solar wind with lunar plasma and calculates electron density profiles from the surface to 200 km altitude for the entire latitudes and longitudes. Model runs suggest that the surface electron density at the Moon could be as high as 1.2 × 105 cm−3 over the mid-latitudes if dynamical interaction between the solar wind and lunar plasma is not accounted for. The dominant ions, in this case, would be Ar+, Ne+, and He+. The absence of any intrinsic magnetic field however leads the ionosphere at the Moon to interact continuously with the solar wind and result in the removal of positive ions. This, in turn, leads to a negligible presence of plasma in the lunar ionosphere with a maximum electron density of ∼1600 cm−3. The electron density is maximum during the midnight and post-midnight periods at all the latitudes, and the maximum is centred around the polar region. Though solar wind acts as a strong removal agent, the electron density distribution is controlled by photochemistry, and ions are molecular in origin.