Effects of Solar Soft X-rays on Earth's Atmosphere

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Virginia Tech

The soft x-rays (wavelengths less than 30 nm) emitted by the sun are responsible for the production of high energy photoelectrons in the D and E regions of the ionosphere, where they deposit most of their energy. The photoelectrons created by this process are the main drivers for dissociation of nitrogen (N2) molecules in the altitude range below 200 km. The dissociation of N2 is one of main mechanisms responsible for the production of nitric oxide (NO) at these altitudes. These processes are important to understand because NO plays a critical role in controlling the temperatures of various regions of Earth's atmosphere. In order to estimate the dissociation rate of N2 we need its dissociation cross-sections. The dissociation cross-sections of N2 due to inelastic collisions with electrons is primarily es- timated from the cross-sections of its excitation states (using predissociation factors) and dissociative ionization channels. Predissociation is the transition without emission of radi- ation from a stable excited state to an unstable excited state of a molecule that leads to dissociation. Unfortunately, the lack of cross-section data, particularly at high electron en- ergies and of higher excited states of N 2 and N 2 + , introduces uncertainty in the dissociation cross-section and subsequently the dissociation rate calculation, which leads to uncertainties in the NO production rate. We have updated a photoelectron model with thoroughly-revised electron impact cross- section data of all major species and experimentally determined predissociation factors. The dissociation rates of N2 using this model are compared to the dissociation rates obtained using another existing (Solomon and Qian [2005]) model. A parameterized version of the updated dissociation rates are used in a one-dimensional global average thermospheric/ ionospheric model, ACE1D (Atmospheric Chemistry and Energetics), to obtain the updated production rates of NO. In the final chapter, we use the ACE1D model to show that the energies deposited by the solar soft x-rays in the lower thermosphere at altitudes between 100 -150 km affect the temperature of the Earth's thermosphere at altitudes well above 300 km. By turning off the input solar flux in the different wavelength bins of the model iteratively, we are able to demonstrate that the maximum change in exospheric temperature is due to changes in the soft solar x-ray bins. We also show, using the thermodynamic heat equation, that the molecular diffusion via non-thermal photoelectrons is the main source of heat transfer to the upper ionosphere/thermosphere. Moreover, these temperature changes and heating effects of the solar soft x-rays are comparable to that of the much stronger He II 30.4nm emission. Finally, we show that the uncertainties in the solar flux irradiance at these soft x-rays wavelengths result in corresponding uncertainties in the modeled exospheric temperature, and these uncertainties increase substantially with increased solar activity.

Electron impact ionization and excitation cross-sections of major species N2, O2 and O, global average NO; Effects of soft x-rays on thermospheric temperature.