Nitric oxide (NO) is a minor species in the Earth’s atmosphere whose densities have been measured to closely reflect solar energy deposition above 100 km. It is an efficient emitter in the infrared where the thermosphere is optically thin, and serves as an important source of radiative cooling between 100 - 200 km. The primary mechanism of this cooling involves the conversion of kinetic energy from the background atmosphere into vibrational energy in NO, followed by the radiative de-excitation of the NO molecule. This results in the production of a 5.3 µm photon which escapes the thermosphere and results in a net cooling of the region. While this process causes the excitation of ground state NO to its first vibrational level, nascent vibrational excitation to the (v≥ 1) levels may also occur from the reactions that produce NO in the thermosphere. The NO(v≥ 1) molecules produced from this secondary process can undergo a radiative cascade and emit multiple photons, thus forming a significant fraction of the 5.3 µm emission from NO in the thermosphere.
Existing thermospheric models consider the collisional excitation of NO to be the only source of the 5.3 µm emission and assume the contribution from nascent excitation to be negligible. These models also tend to use a rate coefficient for the collisional excitation that is significantly larger than the values suggested in literature in order to obtain a temperature profile that is in agreement with empirical data. We address these discrepancies by presenting an updated calculation of the chemically produced emission by accounting for the v ≤ 10 level populations. By incorporating this process into a three dimensional global upper atmospheric model, it is shown that the additional emission contributes between 5 − 40% of the daytime emission from nitric oxide under quiet solar conditions, and is a significant source of energy loss during periods of enhanced solar energy deposition. Accounting for this process however does not resolve the model-data discrepancy seen with regards to the recovery times of thermospheric densities following geomagnetic storms, suggesting that an improved treatment of nitric oxide chemistry is required to resolve this issue.
In order to improve our understanding of the thermospheric energy budget, we also develop the Atmospheric Chemistry and Energetics (ACE) 1D model using up-to-date aeronomic results. The model self-consistently solves the 1D momentum and energy equations to produce a global average profile of the coupled thermosphere and ionosphere system in terms of its constituent densities and temperatures. The model calculations of neutral densities and exospheric temperatures are found to be in good agreement with empirical data for a wide range of solar activity.
It is concluded from the present work that while the magnitude of the chemically produced emission from nitric oxide has previously been underestimated, its effect on the thermospheric energy budget is relatively small. Including the secondary emission in thermospheric models results in an average reduction of 3% in the exospheric temperatures, which does not completely offset the change introduced by using a smaller rate coefficient for the collisional excitation of NO. However, thermospheric temperatures can still be accurately modeled by including these changes as part of broader improvements to calculations of the thermospheric energy budget. / Ph. D. / Nitric oxide (NO) is a molecule that is produced in the Earth’s thermosphere (the region of the atmosphere above 100 kilometers) as a consequence of solar energy deposition. As an important source of radiative cooling, its presence significantly influences the temperature structure of the region. An accurate understanding of the associated energetics is thus vital towards the development of numerical models used to describe the thermosphere.
Energy loss from the thermosphere due to nitric oxide begins with the vibrational excitation of the molecule either due to collisions or chemical processes, followed by the emission of one or more infrared photons which returns the molecule to the ground state. The photons produced escape the thermosphere resulting in a net energy loss from this region of the atmosphere.
Existing thermospheric models generally account for the vibrational excitation of nitric oxide only via collisions, and have assumed chemical processes to be a negligible source of thermospheric energy loss. These models also assume a rate of collisional excitation that is significantly larger than the values suggested in literature in order to obtain a temperature profile that is in agreement with empirical data. The present work demonstrates that the chemical excitation in fact contributes to between 5 − 40% of the total energy loss due to nitric oxide under quiet solar conditions on the dayside of the Earth, and is also an important energy loss mechanism during periods of enhanced solar activity. However, including this mechanism into existing models does not resolve outstanding model-data discrepancies regarding the rate at which the thermosphere returns to equilibrium following sudden enhancements in solar energy deposition. This suggests the need for an improved treatment of the nitric oxide chemistry in current thermospheric models.
This work also presents the Atmospheric Chemistry and Energetics (ACE) 1D model, a new one dimensional upper atmospheric model developed in order to obtain a better understanding of the thermospheric energy budget. The model includes the effects of the chemically produced emissions from nitric oxide, and also uses a collisional cooling rate that is in line with the value suggested in literature. The model calculations of thermospheric densities and temperatures are shown to be in good agreement with empirical data over a wide range of solar activity.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/84400 |
| Date | 25 July 2018 |
| Creators | Venkataramani, Karthik |
| Contributors | Electrical Engineering, Bailey, Scott M., Zhu, Yizheng, Baker, Joseph B. H., Earle, Gregory D., Srinivasan, Bhuvana |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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