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A significant part of my graduate research has revolved modeling nitric oxide (NO) chemistry in the Earth's thermosphere. In particular, I spent a lot of time trying to understand the role played by the higher vibrational levels of NO in removing energy from the upper atmosphere following events of enhanced solar activity. My work has been published in the Journal for Geophysical Research. My Master's thesis can be found here, and my PhD dissertation over here.
Nitric Oxide in the Thermosphere
Nitric Oxide in the Thermosphere
Nitric oxide is minor species in the Earth's thermosphere (above 100 km), where it contributes to the formation of the ionosphere and is an indicator of solar energy deposition both in the form of extreme ultraviolet (EUV) energy and energetic particle precipitation.
Despite being many orders of magnitude lower in density compared to the background atmosphere (molecular and atomic oxygen, molecular nitrogen), nitric oxide plays an important role in the determining the structure and energetics of the upper atmosphere.
The plot on the right shows the densities of the major atmospheric species (molecular nitrogen and molecular oxygen, as well as atomic oxygen), as well as the odd nitrogen species - nitric oxide (NO), ground state atomic nitrogen N(4S), and the first excited state of atomic nitrogen N(2D). Also shown is the bulk temperature of the thermosphere (black dashed line).
(Side note : N(2D) and N(4S) are read as "N-doublet-D" and "N-quartet-S".)
Data from the Student Nitric Oxide Explorer (SNOE) satellite mission. Nitric oxide densities exhibit a peak near 106 km across all latitudes, as a result of various processes that deposit energy in the thermosphere.
NO production is driven by the Sun's EUV flux at low latitudes, while energetic particles channeled along the Earth's magnetic field lines drives production at high latitudes. Nitric oxide produced in the thermosphere can either be transported vertically (down into the mesosphere) or horizontally - in which case we are generally interested in transport from higher to lower latitudes.
Cooling of the thermosphere by infrared emissions
Cooling of the thermosphere by infrared emissions
Being a heteronuclear molecule, NO is capable of undergoing vibrational transitions. The fundamental (1-0) vibrational transition corresponds to a 5.3 micron emission in the infrared, to which the thermosphere is optically thin.
Nitric oxide undergoes vibrational excitation by collisions with atomic oxygen, where kinetic energy from the latter is converted into vibrational energy in the former. The consequent infrared emission from the NO molecule escapes the thermosphere, resulting in a net cooling of the thermosphere. Thus nitric oxide is an important contributor to the energy budget of the upper atmosphere.
An example of this is shown on the right. The top row is the 5.3 micron column emission as observed by SABER, and the the following row shows the percentage enhancement in the emission over "quiet time" measurements.
The bottom two column shows measurements of the solar X-ray flux and the direction of the "Bz" component of the solar wind. It can be seen that the emission enhancements coincide with the periods when Bz < 0 - this is known to be a coupling mechanism between the solar wind and the Earth's magnetic field, which results in energy deposition in the Earth's atmosphere.
The role of higher vibrational levels of NO
The role of higher vibrational levels of NO
NO may also be vibrationally excited as it is produced in the thermosphere. Exothermic reactions producing NO may excite the higher (v>1) vibrational levels, leading to a cascade process emitting multiple photons in the infrared. Detailed modeling of this process has shown that under quiescent solar conditions these vibrational levels may contribute to more than 30% of the infrared emissions from nitric oxide from the thermosphere. However this contribution will dominate the energy loss processes under enhanced solar activity when NO production in the thermosphere is increased. Thus the NO helps remove energy from the thermosphere as it is being produced/energy deposition occurs. My research explores the link between this additional source of energy loss to the unexplained phenomena of thermospheric overcooling seen in the wake of geomagnetic storms.
My PhD work also involved working with the atmospheric models (such as the NCAR TIE-GCM, and models built from ground up) and data sets (SABER, CHAMP) to quantify the importance of the production of nitric oxide to the energetics of the thermosphere.
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