## All Dissertations

May 2021

Dissertation

#### Degree Name

Doctor of Philosophy (PhD)

#### Department

Physics and Astronomy

#### Committee Member

Stephen R Kaeppler

Miguel F Larsen

Jens Oberheide

#### Abstract

The Earth's atmosphere is a complicated environment. Different physical processes affect it depending on the altitude and latitude, among other factors. Three different aspects of the Earth's upper atmosphere are investigated here, using two different techniques. These investigations are: the mid-latitude midnight temperature maximum (MTM), the mesosphere and low-thermosphere Kelvin-Helmholtz instability (KHI), and the advective acceleration in the E-region. All of these studies occur in the Earth's thermosphere and expand our understanding of these phenomena that represent different ways in which energy is transferred throughout the Earth's atmosphere. Observing and characterizing these energy transfer pathways is crucial to further our knowledge of these geophysical processes.

The MTM is typically understood as an equatorial phenomenon that has a characteristic temperature increase around midnight due to the constructive interference between tidal components. While this phenomenon has been studied thoroughly in latitudes $<\pm$20$^\circ$ and modeled to reach $\sim$60$^\circ$; previous observations of temperature and winds had not confirmed its occurrence in latitudes $>$20$^\circ$ N. In \citet{Mesquita2018} and Chapter \ref{chap:MTM} the following scientific question is addressed: What are the characteristics of the mid-latitude MTM? To answer it, a technique was developed to observe the phenomenon and estimate its amplitude between 32$^\circ$ N and 42$^\circ$ N. This investigation used the North American Thermosphere Ionosphere Observing Network (NATION) containing 5 Fabry-Perot interferometers (FPI). Its data set includes a total of 846 nights of observations over a period of approximately 5 years. The new approach for calculating the MTM amplitude was developed by using a series of fits to determine the tidal background. Removing this background from the temperatures and applying an inversion algorithm allowed for the construction of two-dimensional temperature and wind maps, which illustrated the effects of the MTM on the wind field. A statistical analysis of the feature proved that both MTM peaks oscillate with semi-annual and annual periods.

The KHI has been observed and characterized in the mesosphere (statically unstable region). However, the few observations of this phenomena in the low thermosphere (statically stable region) were not detailed and did not show evidence of turbulence above the mesopause. The following scientific questions were still unanswered: What is the triggering mechanism of KHIs in statically stable regions and how does it evolve? These questions are addressed by \citet{Mesquita2020} and in Chapter \ref{chap:SS}. The triangulation of vapor traces from sounding rockets showed the KHI in great detail above 100 km. Characterizing the KHI development in three dimensions revealed wavelength, eddy diameter, and vertical length scale of 9.8, 5.2, and 3.8 km, respectively, centered at 102 km altitude. Further analysis of dimensionless numbers -- such as Richardson, Reynolds, and Froude numbers -- illustrated that the presence of strong and sustained shears was the mechanism involved in generating KHIs in the thermosphere.

Advection has been modeled to be an important acceleration in high-latitude. However, observations of this forcing mechanism have been scarce. Moreover, previous studies investigated the effects of the Hall drag on the Coriolis parameter without including the centrifugal force in the analysis. Chapter \ref{chap:adv} addresses the following scientific question: How does geomagnetic activity affect the vertical distribution of forces (including advection) and the modified Coriolis parameter in the E-region? Triangulation of vapor traces released from sounding rockets was used to calculate the meridional advective acceleration. The observations took place during 5 different geomagnetic conditions for the JOULE II, HEX II, MIST, Auroral Jets, and Super Soaker launches. The instantaneous Lorentz acceleration, which is often considered a dominant force in high-latitude active conditions, was calculated by using the Poker Flat Incoherent Scatter Radar (PFISR) data. These calculations showed that advection can become a dominant term depending on the geomagnetic activity level. The analysis of modified Coriolis parameter $\Phi$, which includes the centrifugal acceleration, revealed that in strong geomagnetic activity an air parcel tends to remain in the auroral oval (channel of enhanced Lorentz acceleration) for an extended period of time. This potentially provides an explanation for why winds are enhanced in the low thermosphere above 115 km during strong geomagnetic activity.

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