The objective of this research is to determine the amount of energy sprites deposit in the mesosphere. Sprites are large (~50km scale size) luminous events seen above thunderstorms. The investigators will make altitude resolved optical spectra at millisecond and higher time resolution. The observations will determine how sprites modify the mesosphere. Most sprites are associated with positive cloud to ground lighting strikes, and luminosity may extend from cloud tops up to ionospheric heights. Sprites lasts a few millisecond (ms) to 10's of ms with initiation commonly at an altitude of 70-75 km followed by both upward and downward propagation of luminosity. Sprites are predominantly red, but tendrils at low altitude are blue. Spectral measurements at TV frame rates (33 ms resolution) show the emissions to be from molecular nitrogen. Blue emissions from ionized nitrogen are expected to be present, but this has not been confirmed. The data available so far have not been able to answer firmly the central questions of how much energy is deposited into the mesosphere, and the resulting modification of the mesosphere by sprites. To address the time development of the energy deposition in sprites, the investigators will make spectral observations with ms time resolution. The primary instruments on a common mount will be an imaging slit spectrograph and two 16-channel multi-anode spectral photometers together with a TV imager to provide spatial context. The imaging spectrograph will provide better spectral resolution, while the spectral photometers will provide better temporal resolution. In addition, an imager will be deployed at a separate location to provide triangulated altitude information. The measurements will be compared to first principles models of sprite emissions to determine the energy deposition as a function of altitude and time. The investigators will also determine the amount of ionization and resulting changes in conductivity within the mesosphere caused by sprites. Two extended ground campaigns are planned. This research will provide clear understanding of sprite formation and propagation as a function of time and altitude. Spectral measurement will provide an unambiguous record of the neutral and ionized nitrogen emissions from sprites. Comparison of data obtained from this project with models of sprite emission will provide the time history of sprite energetics. The research will provide a balance of experimental and theoretical development suitable for a student working towards a PhD. The research plan also includes undergraduate cadets from the Air Force Academy in the calibration of the spectral photometers, and the gathering and analysis of the data. These experiences will provide the undergraduates a first hand look at real research, hopefully interesting them in further study at the graduate level. In addition the investigators plan a vigorous outreach program directed at high school students and teachers.

National Science Foundation (NSF)
Division of Atmospheric and Geospace Sciences (AGS)
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Cassandra G. Fesen
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University of Alaska Fairbanks Campus
United States
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