This dissertation comprises multiple studies surrounding the middle atmosphere's chemistry, composition, and dynamics. The middle atmosphere refers to the region from ~ 10 km to ~ 100 km and consists of the Stratosphere, Mesosphere, and Lower Thermosphere. The Stratosphere, Mesosphere, and Thermosphere are bounded by pauses where the strongest changes in chemical composition, movement, density, and thermal behavior take place. While several studies in the past have investigated the chemical composition of the middle atmosphere and quantified the distribution of various species from the stratosphere to the lower thermosphere, seasonal variations and redistribution of species resulting from transport events make it important to continuously monitor the middle atmosphere. Dynamic events such as Sudden Stratospheric Warmings (SSW) impact the temperature gradient and the zonal mean wind pattern in the stratopause. Descent events triggered by SSWs result in enhanced transport of species from the lower thermosphere to the stratosphere. Temperature increments during SSWs have an important impact on Polar Stratospheric Clouds (PSCs), resulting in Antarctic ozone enhancement and a smaller ozone hole. The middle atmosphere is, thus, home to a diverse range of dynamics and chemistry, making it a critical subject that warrants attention from the science community. The continuous monitoring of the middle atmosphere is important to this end. Several satellite missions in the past have been dedicated to monitoring the middle atmosphere and collecting data for decades. However, continual revisions and revaluations of measurement approaches and the introduction of novel space instruments are necessary to compensate for the limitations associated with existing missions, expand the extant specimen database, and improve phenomenon-centric observations. The Solar Occultation for Ice Experiment (SOFIE) is one of the two instruments on the Aeronomy of Ice in the Mesosphere (AIM) spacecraft. The studies presented in this dissertation primarily focus on the use of SOFIE observations combined with results from other science missions, an atmospheric model, and other datasets. Chapter I is an overview of the research goals and the motivations that propelled this research. In Chapter II, a validation study of the Version 1.3 SOFIE ozone data against the Atmospheric Chemistry Experiment (ACE) and the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) ozone data is presented. The SOFIE-ACE and SOFIE-MIPAS data pairs demonstrate similar variability in the ozone vertical profile. SOFIE vertical ozone profiles agree best with ACE from 30 - 70 km and MIPAS from 30-64 km. The mean difference values averaged over all seasons and both hemispheres are typically < 24% with ACE and < 20 % with MIPAS. Atomic oxygen is an important species in the mesopause region (~ 80 – 100 km) that impacts the region's ozone photochemistry and radiative balance. In Chapter III, SOFIE ozone measurements used to derive daytime atomic oxygen are compared to coincident retrievals from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument and the Naval Research Laboratory Mass Spectrometer Incoherent Scatter radar (NRLMSIS 2.0) model. The datasets agree qualitatively. Results indicate a strong seasonal variation of atomic oxygen with summer and wintertime maxima at ~ 84 km and 94 km, respectively. The middle atmospheric composition is redistributed by the transport of species during SSWs. In Chapter IV, the 2019 SSW in the northern hemisphere that triggered a large transport event from the lower thermosphere to the stratosphere is evaluated using SOFIE, ACE, and the Modern-Era Retrospective analysis for Research and Applications (MERRA-2) observations. The event was similar to the major SSW-triggered descent events in the northern hemisphere since 2004 and led to the enhancement of nitric oxide produced by Energetic Particle Precipitation, attributed to unusual meteorology. The transport peak descended by ~ 5-6 km every 10 days. An SSW event occurred in the southern hemisphere in 2019 and led to enhanced ozone in the stratosphere. In Chapter V, satellite instruments, ground station data, and measurements from NASA Ozone Watch are used to conclude that large temperature increments evaporated PSCs, resulting in the lower conversion of halogen reservoir species into ozone-destroying forms. Thus, a large ozone enhancement was recorded in 2019. Chapter VI concludes all findings and Chapter VII summarizes future work. / Doctor of Philosophy / The middle atmosphere is the region between ~ 10 and 100 km in the atmosphere and is comprised of the Stratosphere, Mesosphere, and Lower Thermosphere. The middle atmosphere is a dynamic region, and the chemistry of this region is subject to variations occurring naturally or those triggered by anomalous events such as Sudden Stratospheric Warmings (SSW). Several species in the middle atmosphere need to be measured continuously or reevaluated for improved understanding. Dynamical events in the middle atmosphere are responsible for transporting and redistributing species in the middle atmosphere. Thus, the continuous monitoring of the middle atmosphere is necessary. Novel approaches with improved techniques and approaches are thus important to explore the middle atmosphere and quantify the chemistry of the region. The Solar Occultation for Ice Experiment (SOFIE) instrument is an instrument onboard the Aeronomy of Ice in the Mesosphere (AIM) spacecraft. SOFIE typically measures at high latitudes and looks at a wide range of wavelengths. This dissertation uses SOFIE and other datasets to evaluate the varying chemistry and dynamics of the middle atmosphere. The dissertation addresses four research problems and assimilates them to evaluate the middle atmosphere. Ozone is an important species in the middle atmosphere, which is present in the highest quantity in the stratosphere, followed by the lower thermosphere (~ 85 – 100 km). Ozone is important as it absorbs ultraviolet radiations and impacts the stratospheric radiative balance. Missions in the past have monitored ozone in the middle atmosphere. Novel approaches and improved observation techniques to compensate for the limitations of past missions and the continuous measurement of ozone are necessary. Thus, ozone retrievals from SOFIE are validated against independent and established datasets to demonstrate the robustness and usability of the SOFIE ozone data product within the atmospheric science community. Atomic oxygen is an important species in the mesopause region (~ 80 – 100 km) because of its role in ozone photochemistry and impact on the radiative balance of the region. It is technologically challenging to make direct measurements of atomic oxygen; thus, most conventionally, derived measurements and model results are used. To this date, atomic oxygen has been understood in a limited capacity with several inaccuracies. To improve the understanding of atomic oxygen and fill the current knowledge gaps, atomic oxygen is derived from SOFIE ozone measurements during the daytime using the Chapman equations for ozone photochemistry. Further, the derived atomic oxygen is compared to other established datasets from satellite instrument-derived measurements and model predictions. The seasonal variability of atomic oxygen is evaluated with a focus on the difference in its behavior during summer and winter. Lastly, inter-hemispheric differences in atomic oxygen distribution are evaluated. Apart from the natural atmospheric variation in species, SSW-triggered transport events redistribute species in the atmosphere. The 2019 SSW event in the northern hemisphere was similar to those in 2004, 2006, 2009, and 2013. Large quantities of nitric oxide were transported from the lower thermosphere to the stratosphere. Air poor in water vapor and methane was also transported. Atomic oxygen was transported from the lower thermosphere to several kilometers below in amounts higher than usual. The increased nitric oxide concentration in the stratosphere due to the transport catalytically destroyed the ozone in the region. The vertical transport rates were calculated to understand the speed of the descent. The low geomagnetic index in 2019, like in all years besides 2004, indicates that these events are attributed to unusual meteorology. An SSW event took place in the southern hemisphere in 2019 during the Antarctic winter. This led to a large increase in temperature, which evaporated the Polar Stratospheric Clouds (PSCs). PSCs provide their surface for converting halogen reservoir species into ozone-destroying reactive forms. The absence of PSCs during and immediately after the SSW event led to a lower conversion of halogen reservoir species into reactive forms. Satellite instrument measurements agree with theoretical expectations. The 2002 SSW in the SH led to similar outcomes and are compared to the 2019 event. Large enhancements in ozone in 2019 led to the smallest ozone hole since ~ 1982.

Abstract This thesis studies the long-term relation between the solar wind driven energetic particle forcing into the atmosphere and the tropospheric circulation in the Northern Hemisphere winter. The work covers the period of more than one hundred years since the turn of the 20th century to present. The thesis makes a statistical analysis of satellite measurements of precipitating energetic electrons, sunspot number data and geomagnetic activity, and compares them with temperature and pressure measurements made at the Earth's surface. Recent results, both observational and from chemistry climate models, have indicated significant effects in the Earth's middle atmosphere due to the energetic electrons precipitating from the magnetosphere. These effects include the formation of reactive hydrogen and nitrogen oxides in the high latitude mesosphere and the depletion of ozone caused by them. Ozone is a radiatively active and important gas, which affects the thermal structure and dynamics of the middle atmosphere. Accordingly, the depletion of ozone can intensify the large scale stratospheric circulation pattern called the polar vortex. Winter weather conditions on the surface have been shown to be dependent on the polar vortex strength. This thesis shows that there is a significant relation between the average fluxes of medium energy (ten to hundred keVs) precipitating electrons and surface temperatures in parts of the Northern Hemisphere in winter time. Temperatures are positively correlated with electron fluxes in North Eurasia and negatively correlated in Greenland during the period 1980-2010 which is covered by direct satellite observations of precipitating particles. This difference is especially notable when major sudden stratospheric warmings and the quasi-biennial oscillation (QBO), which both are known to affect the polar vortex strength, are taken into account. When extended to the late 19th century, the analysis shows that a similar temperature pattern is predominated during the declining phase of the sunspot cycle. The high speed solar wind streams and energetic particle precipitation typically maximize also at the declining phase of the solar cycle. This specific temperature pattern is related to the variability of the northern annular mode (NAM), which is the most significant circulation pattern in the Northern Hemisphere winter. Before the space era, geomagnetic activity measured by ground observations can be used as a proxy for energetic particle precipitation. Earlier studies have found a significant positive correlation between geomagnetic activity and NAM since the 1960s. We find that, when the QBO measured at 30 hPa height is in the easterly phase, a positive correlation is extended to the beginning of 1900s. We also show that high geomagnetic activity causes a stronger effect in the Northern Hemisphere winter than high sunspot activity, especially in the Atlantic and Eurasia. A comprehensive knowledge of the Earth's climate system and all its drivers is crucial for the future projection of climate. Solar variability effects have been estimated to produce only a small factor to the global climate change. However, there is increasing evidence, including the results presented in this thesis, that the different forms of solar variability can have a substantial effect to regional and seasonal climate variability. With this new evidence, the solar wind related particle effects in the atmosphere are now gaining increasing attention. These effects will soon be included in the next coupled model inter comparison project (CMIP6) as an additional solar related climate effect. This emphasizes the relevance of this thesis.