Remote Sensing of the Upper Atmosphere Using Ground Based Imaging Spectroscopy
Abstract
Complex processes in the Earth’s upper atmosphere give rise to optical emissions in the form of airglow and aurora. These emissions are diagnostics of photochemical and collisional processes in the upper atmosphere and specifically the ionosphere-thermosphere (IT, 90-600 km) region. Simultaneous observations of selected emissions allow one to - a) determine the characteristics of the precipitating particles that produce auroras, b) infer compositional changes in the upper atmosphere. A versatile ground-based spectrograph known as High Throughput and Multi-slit Imaging Spectrograph (HiT&MIS) has been developed to observe airglow and auroral emissions simultaneously at six selected wavelengths on a round-the-clock basis. The spectral features used in this thesis are - OI 557.7 nm (green line), OI 630.0 nm (red line), OI 777.4 nm and N2+ 427.8 nm.For this dissertation, two questions are addressed. First, can the characteristic energy and energy flux of auroral electron precipitation be simultaneously derived using multispectral measurements? Second, did the total solar eclipse of August 21, 2017 cause the brightness perturbation observed in nighttime red and green lines?During auroral events, the brightnesses of different auroral emissions change differently based on the energy of the precipitating electrons, the density structure of the atmospheric constituents producing these emissions and their exact excitation mechanisms. Hence, simultaneous measurements of optical emissions can be utilized to infer the energy characteristics of the precipitating electrons. Auroral emission brightnesses can be modeled by using electron transport and chemical reaction models in terms of atmospheric composition, geophysical parameters and the energy and energy flux of the precipitating particles. The method developed in this dissertation derives the energy and flux of auroral electrons by comparing modeled brightnesses with simultaneous measurements of three auroral emissions. To answer the first question, the results obtained from this method are compared to results obtained from another method which is similar to methods used in previous studies.Multispectral measurement of airglow emissions can be used to study and char-acterize atmospheric waves, known as Atmospheric Gravity Wave (AGWs). AGWs are produced by various mechanisms in the IT system. These include forcing from the lower atmosphere, heating due to increased ionospheric currents and temperature changes induced by total solar eclipses. Airglow emissions depend on the density structure of the neutral atmosphere. Since the processes causing AGWs alter the neutral density structure, they cause brightness perturbations in airglow emissions. This relationship can be used to derive the wave characteristics of AGWs. AGWs manifest as Traveling Ionospheric Disturbances (TIDs) via collisional coupling in the ionosphere. Thus, AGWs also have signatures in ionospheric measurements in form of plasma density perturbations (TIDs). Approximately 8 hours after the total solar eclipse on August 22, 2017, wavelike brightness perturbations with a dominant period of approximately 1.5 hours were observed in both red and green lines by HiT&MIS. Previous studies have shown that solar eclipse leaves the upper atmosphere disturbed. However, it is not completely understood how long the eclipse’s effect on the upper atmosphere lasts for and these perturbations could be the eclipse’s aftereffect. To study this possibility, first, the wave properties of the AGWs (and associated TIDs) are derived. Then, measurements from HiT&MIS and other instruments are compared with an Ionosphere-Thermosphere model to estimate the eclipse’s effect on the IT system to answer the second question.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 2019
- Bibcode:
- 2019PhDT........36A
- Keywords:
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- Aeronomy