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 ($N_2$) molecules in the altitude range below 200 km. The dissociation of $N_2$ 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 $N_2$ we need its dissociation cross-sections. The dissociation cross-sections of $N_2$ 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 $N_2$ 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. / Doctor of Philosophy / The radiation from the sun covers a wide range of the electromagnetic spectrum. The soft x-rays with wavelengths less than 30 nm are the most energetic and variable part of the spectrum, and would have detrimental effects on humans were they not absorbed by the atmosphere. The absorption of soft x-rays by the Earth's atmosphere at altitudes near 100- 150 km creates ionized and energized particles. These energetic changes can affect and even damage the satellites in low Earth orbit, and can cause radio communication blackouts and radiation storms (large quantities of energetic particles, protons and electrons accelerated by processes at and near the Sun). Therefore, we need to have good models that can quantify these changes in order to correctly predict their effects on our atmosphere, and help to mitigate any harmful effects.
The soft x-rays and the extreme ultraviolet (EUV) are responsible for ionization of the major neutral species, $N_2$ , $O_2$ and O, in the Earth's atmosphere, which leads to the production of ions and energetic photoelectrons. These high energy photoelectrons can cause further ion- ization, excitation and dissociation. We study the dissociation of $N_2$ by these photoelectrons to create neutral N atoms. The N atoms created via this process combine with the $O_2$ in the atmosphere to produce nitric oxide (NO), which is one of the most important minor constituents because of its role in regulating atmospheric heating/cooling. The production of NO peaks near 106 km altitude, where most of the energy of the soft x-rays are deposited.
However, they also affect the temperature of the upper atmosphere well above this altitude.
This is because the energy of the photoelectrons is conducted to the upper atmosphere by collisions of electrons and ions with ambient neutral atoms and molecules, thus increasing their temperature.
In this study, we use modeling of soft x-ray irradiance, photoelectron ionization, excitation and dissociation rates and atmospheric neutral temperature to quantify the effects of soft x-rays on the Earth's atmosphere.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/113691 |
| Date | 06 February 2023 |
| Creators | Samaddar, Srimoyee |
| Contributors | Electrical Engineering, Bailey, Scott M., Earle, Gregory D., England, Scott L., Stilwell, Daniel J., Barnes, Edwin Fleming |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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