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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
11

Spatial characteristics of the midnight temperature maximum and equatorial spread F from multi-instrument and magnetically conjugate observations

Hickey, Dustin A. 13 November 2018 (has links)
The upper atmosphere, a region above ~85 km called the ionosphere and thermosphere, has been studied extensively for over one hundred years. Measurements were often considered in isolation, but today, advances in technology and ground-based distributed arrays have allowed concurrent multi-instruments measurements. In this dissertation, I combine measurements from all-sky imagers (ASIs), coherent scatter radars, incoherent scatter radars (ISRs), and Fabry-Perot interferometers (FPIs). I focus on two phenomena, the midnight temperature maximum (MTM) and equatorial spread F (ESF), using observations from equatorial to mid-latitudes. The spatial characteristics of these phenomena are not fully understood. I combine observations at various latitudes and longitudes to extend MTM detection to mid-latitudes. I present the first simultaneous detections of the MTM at multiple altitudes and latitudes over North America and the first observations below the F-region peak using the Millstone Hill Observatory ISR in a south pointing, low-elevation mode. The MTM can also be observed with an ASI and I present concurrent measurements of the MTM with an ASI and ISR. The Whole Atmosphere Model, a global circulation model, was found to be consistent with these observations. This further verifies that the MTM is partially created by lower atmospheric tides, demonstrating coupling between the lower and upper atmosphere. In addition to the MTM, I investigate different aspects of ESF using ASIs concurrently with other instruments. I compare various scale sizes (sub-meter to kilometers) using coherent scatter radar and an ASI and conclude that the lower hybrid drift instability causes radar echoes to occur preferentially on the western wall of large-scale depletions. The source of day-to-day variability in ESF is not fully known but I show that one driver may be large-scale wave structures (~400 km) that modulate the development of ESF. Finally, I compare concurrent observations of ESF plasma depletions with ASIs at magnetically-conjugate foot points and show how the magnitude and structure of the Earth’s magnetic field is responsible for differences in the morphology and velocity of these depletions. In summary, I have used multi-instrument observations of ESF and the MTM to provide a deeper understanding of the dynamics of the upper atmosphere.
12

Feasibility for Orbital Life Extension of a CubeSat Flying in the Lower Thermosphere

Martinez, Nicolas 29 July 2015 (has links)
"Orbital flight of CubeSats in extremely Low Earth Orbit, defined here as an altitude between 150 – 250 km, has the potential to enable a wide range of missions in support of atmospheric measurements, national security, and natural resource monitoring. In this work, a mission study is presented to demonstrate the feasibility of using commercially available sensor and electric thruster technology to extend the orbital lifetime of a 3U CubeSat flying at an altitude of 210 km. The CubeSat consists of a 3U configuration and assumes the use of commercially available sensors, GPS, and electric power systems. The thruster is a de-rated version of a commercially available electrospray thruster operating at 2 W, 0.175 mN thrust, and an Isp of 500 s. The mission consists of two phases. In Phase I the CubeSat is deployed from the International Space Station orbit (414 km) and uses the thruster to de-orbit to the target altitude of 210 km. Phase II then begins during which the propulsion system is used to extend the mission lifetime until propellant is fully expended. A control algorithm based on maintaining a target orbital energy is presented in which simulated GPS updates are corrupted with measurement noise to simulate state data which would be available to the spacecraft computer. An Extended Kalman Filter is used to generate estimates of the orbital dynamic state between the 1 Hz GPS updates, allowing thruster control commands at a frequency of 10 Hz. For Phase I, operating at full thrust, the spacecraft requires 25.21 days to descend from 414 to 210 km, corresponding to a ΔV = 96.25 m/s and a propellant consumption of 77.8 g. Phase II, the primary mission phase, lasts for 57.83 days, corresponding to a ΔV = 119.15 m/s during which the remaining 94.2 g of propellant are consumed. "
13

Vertical motions in the mesophere /

Murphy, Damian John. January 1984 (has links) (PDF)
Thesis (M. Sc.)--University of Adelaide, Dept. of Physics, 1985. / Includes bibliographical references (leaves 74-78).
14

Polar middle atmosphere dynamics

Dowdy, Andrew James. January 2005 (has links)
Thesis (Ph.D.) -- University of Adelaide, School of Chemistry and Physics, Discipline of Physics, 2005. / Includes author's previously published papers. Includes bibliographical references. Also available in a print form.
15

The PMC Turbo Experiment: Design, Development, and Results

Kjellstrand, Carl Bjorn January 2021 (has links)
In the middle and upper atmosphere, dynamics of scales from tens of meters to thousands of kilometers primary arise due to the influence of gravity waves propagating from lower altitudes. In order to understand the structure and variability of these regions of our planet's atmosphere, we must understand the propagation, influences, and dissipation of gravity waves. However, gravity waves and their influences are difficult to measure. Their largest and most observable effects occur in the remote mesosphere and lower thermosphere and the relevant spatial scales extend across many orders of magnitude. The EBEX group discovered a novel method to observe polar mesospheric clouds, which are a sensitive tracer of gravity waves and their associated dynamics. This discovery motivated the Polar Mesospheric Cloud Turbulence (PMC Turbo) experiment. Polar mesospheric clouds form an extremely thin but bright layer at roughly 80 kilometer altitude in which we can observe brightness fluctuations created by gravity wave dynamics and the resulting instabilities. PMC Turbo included seven pressure vessels, each of which contained an optical camera, hard drives, and computers that controlled the image capture, flight control, and communication with ground stations. The cameras captured spatial scales from gravity waves with wavelengths of roughly 10-100 kilometers, instability dynamics at scales from about 1-10 kilometers, and the fine structure at the inner scale of turbulence down to 20 meters. PMC Turbo flew at 38 kilometer altitude and remained afloat for nearly six days. During this time, it travelled from Esrange Space Center in Sweden to the Northwest Passage in Canada. Complementary data from other instruments provides additional atmospheric context to the PMC Turbo measurements. During flight, the PMC Turbo cameras captured images of polar mesospheric clouds tracing Kelvin-Helmholtz instabilities with a high signal-to-noise ratio. Kelvin-Helmholtz instabilities play major roles in energy dissipation and structure of geophysical fluids, and they have a close relationship with gravity waves. The PMC Turbo images include complicated interactions and secondary instabilities leading to turbulence. These dynamics provide insight into the atmospheric conditions and rate of energy dissipation in the mesosphere and lower thermosphere.
16

The Sun's Influence on the vertical structure of the ionospheres of Venus and Mars

Girazian, Zachary 13 February 2016 (has links)
The ionospheres of Venus and Mars are important components of the planet-space boundary that play a major role in atmospheric escape processes. Characterization of these regions reveals the physical processes that control them and provides a foundation for more detailed studies of chemistry, dynamics, and energetics. At both planets the ionospheres contain two layers: the main layer, which is formed by photoionization from extreme ultraviolet radiation (EUV, λ<120 nm), and the lower layer, which is formed by photoionization from soft X-rays (SXRs, λ<10 nm) and subsequent electron impact ionization. In this dissertation I investigate how the solar EUV and SXR irradiance controls these layers at Venus and Mars. First, I develop an empirical model of the ultraviolet (UV, λ<190 nm) solar spectrum as a function of F10.7, which is a commonly used proxy of the UV irradiance. I derive power-law relationships between F10.7 and the ionizing irradiance for five neutral species and show that the relationships are nonlinear. These relationships can be used to estimate the EUV irradiance when no solar spectrum measurements are available. Second, I show that the peak electron densities in the ionospheres of Venus and Mars are proportional to the square-root of the ionizing irradiance, which is in contrast to previous studies that have used F10.7 as their representation of the UV irradiance. This finding ameliorates a discrepancy between theory and observations and is in agreement with the prediction that dissociative recombination is the main ion loss mechanism near the ionospheric peaks at Venus and Mars. Third, using a numerical model and electron density profiles from Venus Express, I examine the behavior of the peak altitude, peak density, and morphology of the lower layer at Venus. I show that the peak altitudes and densities in the lower and main layers vary similarly with solar zenith angle (SZA). This implies that neutral and electron thermal gradients at these altitudes vary little with SZA. I also show that, compared to the main layer, the lower layer morphology and peak density varies more over the solar cycle due to the hardening of the solar spectrum.
17

A radar study of the thermosphere

Emery, Barbara Ann January 1975 (has links)
Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Meteorology, 1975. / Bibliography: leaves 94-96. / by Barbara A. Emery. / M.S.
18

Seasonal wind variations in the mid-latitude neutral thermosphere.

Emery, Barbara Ann January 1977 (has links)
Thesis. 1977. Sc.D.--Massachusetts Institute of Technology. Dept. of Meteorology. / Microfiche copy available in Archives and Science. / Vita. / Bibliography : leaves 255-261. / Sc.D.
19

The dependence of the circulation of the thermosphere on solar activity

Babcock, Richard Robert January 1978 (has links)
Thesis. 1978. Ph.D.--Massachusetts Institute of Technology. Dept. of Meteorology. / Microfiche copy available in Archives and Science. / Bibliography: leaves 182-191. / by Richard Robert Babcock, Jr.. / Ph.D.
20

Modeling the Energetics of the Upper Atmosphere

Venkataramani, Karthik 25 July 2018 (has links)
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.

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