The Role of High Energy Photoelectrons on the Dissociation of Molecular Nitrogen in Earth's Ionosphere

Abstract

Soft x-ray radiation from the sun is responsible for the production of high energy photoelectrons in the D and E regions of the ionosphere, where they deposit most of their ionization energy. The photoelectrons created by this process are the main drivers for dissociation of Nitrogen molecule ($N_2$) below 200 km. The dissociation of N2 is one of main mechanisms of the production of Nitric Oxide (NO), an important minor constituent at these altitudes. In order to estimate the dissociation rate of N2 we need its dissociation cross-sections. The dissociation cross-sections for N2 by photoelectrons are primarily estimated from the cross-sections of its excitation states using predissociation factors and dissociative ionization channels. The lack of cross-sections data, particularly at high electron energies and of higher excited states of $N_2$ and $N_2^+$, introduces uncertainty in the dissociation rate calculation, which subsequently leads to uncertainties in the NO production rate from this source. In this work, we have fitted updated electron impact cross-sections data and by applying predissociation factors obtained, updated dissociation rates of N2 due to high energy photoelectrons. The new dissociation rates of N2 are compared to the dissociation rates obtained from Solomon and Qian [2005]. The new dissociation cross-sections and rates are estimated to be about 30% lower than the Solomon and Qian [2005] model. Simulations using a parameterized version of the updated dissociation rates in the Atmospheric Chemistry and Energetics (ACE1D) model leads to a $20%$ increase in NO density at the altitudes below 100 km is observed.