The project will develop a 2-D time-dependent nonlinear chemistry-dynamics model of the mesosphere/lower thermosphere region to simulate the response of two airglow emissions to atmospheric gravity waves. Gravity waves are important because they transport energy and momentum from the lower atmosphere to the upper atmosphere, where they can deposit their energy and momentum through wave-mean flow interactions. These interactions can result in perturbations to the distributions of atmospheric species by the combined effects of chemical perturbations and vertical transport; exothermic heating variations may also be induced. In addition to gravity waves, airglow in the mesosphere/lower thermosphere region constitutes another important research topic in the atmospheric science community: variations in airglow intensity can oftentimes be used to deduce characteristics of gravity waves or other types of waves that cause the variations. The project includes both elements, gravity waves and airglow emissions, by constructing a model that includes a monochromatic gravity wave dynamics code and a chemistry code that simulates two airglow emissions: OH and the O2 atmospheric (0,1) band. The model will be used to study the following topics: 1) gravity wave effects on the OH and O2(b1) airglow emission layers in a windless atmosphere and for a constant background wind condition; 2) the airglow response to a variety of different gravity wave characteristics; 3) wave-induced secular variations of the airglow intensities and the intensity-weighted temperatures of the two airglow layers; 4) the amplitude growth factor associated with the various simulations of the airglow layers; 5) wave-induced secular variations of gas species concentrations; and 6) wave-induced exothermic heating associated with the passage of different gravity waves. Comparisons with observations will be made to test and validate the model. Undergraduate students will participate in the project by analyzing the simulated airglow emissions to determine the effects of gravity waves, co-authoring the papers describing the work, and presenting some of the results at professional meetings such as the annual CEDAR meeting. The model resulting from this research will provide a unique tool to the community.
A 2-D, time-dependent, nonlinear Multiple-Airglow Chemistry-Dynamics (MACD) model was developed. Our findings from the research topics in the project have provided significant insights to the wave effects on dynamics coupled with airglow chemistry, energy and momentum budget, nightglow chemistry in the occurrence of lightning in the MLT region, data interpretation and data analysis of the observations. Airglow is very sensitive to ambient environment (like temperature, atmospheric gas concentrations), so its intensity variations on different time scales can be used to investigate the change of the atmosphere arising from different causes. On the millisecond time scale, they can be used for lightning induced airglow intensity enhancement study; on the hourly time scale, they can be used for gravity wave and earthquake studies; on the decadal time scale, they can used for global climate change study. There are altogether 6 refereed papers published, one currently under review, one in preparation, one book chapter, one conference proceedings, and 34 presentations of which 9 were invited talks resulting from our investigations. Broader Impacts Three undergraduate students worked under the guidance of the PI during the award period. The students attended and presented at the local and regional research symposia and CEDAR meetings to enrich their research and learning experiences. There were collaborations with researchers from international and national institutions. A graduate student and a software engineer of National Cheng Kung University in Taiwan were mentored by the PI. The research topics, sponsored by NSF, were conducted by the PI who is a member of underrepresented groups. Listed below are summaries of research outcomes addressing intellectual merit. Gravity Wave Effects on Airglow and Exothermic Heating: Our simulation results of the wave effects by a 30-km wave packet show that it induces a significant amount of secular increase in OH airglow intensity, O2(0,1) atmospheric band, and O(1S) greenline, of which O(1S) shows the largest increase. This indicates that for this wave packet, the higher altitude the airglow layer locates the larger effect it will have on the airglow. Our study shows that airglow intensity is very sensitive to wave activity. However, the effects of the wave packet on airglow temperatures are quite different - they are too small to be detected. Our simulation results of the wave-induced exothermic heating variations show that the wave packet induces a large secular increase in the number densities of the minor species in the OH chemistry, and the ultimate driver for these increases is the wave-driven downward transport of O. We find that the total exothermic heating rates have increased by a significant amount by the wave packet. The major reactions contributing to exothermic heating rates are the three-body recombination O + O +M and the H + O3 reaction. Lightning-Induced Transient Emissions (LITEs) in the Airglow Layers: Our study of lightning-induced transient emissions (LITEs) in the OH, O(1S) greenline, and O2(0,0) airglow layers using the model simulations and observations from the FORMOSAT-II satellite has indicated that lightning in the troposphere can induce airglow emissions in addition to the N21P emission in the MLT region. Further, from the statistical analysis it suggests that most of the LITEs are induced by the elves-producing Electromagnetic Pulses (EMPs) and not by the sprite-producing mechanism. ISUAL’s Global Observations of OH and O(1D): OH airglow observations by the ISUAL (Imager of Sprites and Upper Atmospheric Lightning) instrument onboard the FORMOSAT 2 satellite were analyzed to derive its peak height and latitudinal distribution. OH airglow can be correctly derived with its average peak height of 89 ± 2.1 km. ISUAL data reveal detailed structures of equatorial OH airglow distribution such as the existence of a few secondary maxima within the equatorial region. From the data analysis of ISUAL’s global observations of O(1D) red line, we found that the midnight airglow brightness was controlled by different factors at different locations. The factors are the ionospheric annual anomaly from May to July, the MTM effect, and winter anomaly in which the neutral wind plays a role in its formation. It is necessary to take into account the locations and seasons when explaining the mechanism of midnight airglow brightness occurrence. Examining Methods Used in Density Trends Attributed to Global Warming: We have conducted numerical experiments to examine which method is most effective to remove or minimize solar variations in the derived thermospheric density data. Our results indicate that when the amplitude of density variations caused by solar variability is small, the methods used in previous trend analysis yield trends closer to the true trend. However, when the amplitude is large, the trends obtained by the methods other than the Difference method deviate from the true trend. The severity increases with increasing C value.
