UC Berkeley

Extending from approximately 85 to 1,000 km above Earth’s surface, the terrestrial ionosphere-thermosphere (I-T) system holds growing significance as we increasingly rely on space-based infrastructure. The I-T system comprises a mixture of ionized and neutral gases whose dynamics are tightly coupled, generating a variety of complex phenomena, some of whichremain poorly characterized and understood. This dissertation presents studies investigating three of the most enigmatic I-T phenomena, each driven by a distinct energy source. Specifically, we examine: 1. rapid electrodynamic changes and plasma redistribution following explosive events in the lower atmosphere, with a particular focus on the 2022 eruption of the Hunga Tonga-Hunga Ha’apai (hereafter ‘Tonga’) volcano, 2. thermospheric wind perturbations linked to the abrupt changes in solar inputs triggered daily by the setting sun, and 3. aurora-like glows in the subauroral ionosphere associated with rapid ion flows in the upper ionosphere during geomagnetically active periods. Each study employs both observational data and theoretical modeling to examine the coupled plasma-neutral response to the energy driver and to characterize and explain the ensuing phenomenon. The first study delves into the ionospheric effects of the 2022 Tonga volcanic eruption. This eruption drove global-scale atmospheric waves that propagated into space and propelledionospheric disturbances. This dissertation investigates the ionospheric consequences of the eruption within about 5,000 km of the volcano. The study demonstrates the immediate large-scale electrodynamic effects of the eruption using observations from NASA’s Ionospheric Connection Explorer (ICON) satellite. Extreme (>100 m/s) east-west and vertical ion drifts are observed thousands of kilometers away from the volcano within an hour of the eruption, before the arrival of any known neutral atmospheric wave. The measured ion drifts are magnetically conjugate to the ionospheric E region about 400 km from Tonga. A theoretical calculation shows that the observed ion drifts are consistent with the ionospheric E region dynamo effects of an expanding neutral atmospheric wavefront with a large (>200 m/s) neutral wind amplitude. The analysis suggests that the thermospheric neutral winds initiated by the eruption interacted with the E Region ionospheric plasma and created strong electric potentials which propagated along Earth’s magnetic field via Alfv ́en waves and caused the observed plasma drifts in the opposite hemisphere. These observations are the first direct detection in space of the rapid and extreme electrodynamic consequences of a volcanic eruption and contributes to our understanding of the coupling between the lower atmosphere and I-T system following explosive events such as this eruption. The second study considers the daily effect of the setting sun on the I-T system. The moving solar terminator (ST) generates atmospheric disturbances, broadly termed solar terminator waves (STWs). Despite theoretically recurring daily, STWs remain poorly understood, partially due to measurement challenges near the ST. By presenting analysis of neutral wind data from the Michelson Interferometer for Global High-resolution Thermospheric Imaging (MIGHTI) onboard the ICON satellite, this dissertation reveals observations of STW signatures in thermospheric neutral winds, including the first observed meridional wind signatures. Seasonal analysis demonstrates that STWs are most prominent during solstices, when they intersect the ST about ∼ 20◦ latitude from the equator in the winter hemisphere and have phase fronts inclined at a ∼ 40◦ angle to the ST. This work provides the first observed STW altitude profiles, revealing large (>200 km) vertical wavelengths above 200 km. Comparing these observations to four different models suggests the STWs likely originate directly or indirectly with waves from below 97 km. These results indicate that STWs may play an under-recognized role in the daily variability of the I-T system, warranting further study. Finally, this dissertation considers aurora-like emissions which arise equatorward of the auroral oval in conjunction with extremely fast ionospheric ion flows. The ‘picket fence’ is a captivating visual phenomenon featuring vibrant green streaks. It is often observed concurrently with and at lower altitudes than the rare purpleish-white arc called STEVE (Strong Thermal Emission Velocity Enhancement). Despite its aurora-like appearance, recent studies suggest that the picket fence may not be driven by magnetospheric particle precipitation but instead by local electric fields parallel to Earth’s magnetic field. This dissertation evaluates the parallel electric fields hypothesis by quantitatively comparing picket fence spectra with the emissions generated in a kinetic model driven by local parallel electric fields energizing ambient electrons in a realistic neutral atmosphere. The results demonstrate that, at a typical picket fence altitude of 110 km, parallel electric fields between 40 and 70 Td (∼80 to 150 mV/m at 110 km) energize ambient electrons sufficiently so that, when they collide with neutrals, they reproduce the observed ratio of N2 first positive to atomic oxygen green line emissions, without producing N2+ first negative emissions, consistent with the features observed in picket fence spectra. These findings establish a quantitative connection between ionospheric electrodynamics and observable picket fence emissions, offering verifiable targets for future models and experiments. The work presented in this dissertation has contributed to ongoing I-T research as well as spawned new research directions, including providing benchmarks for more detailed modeling studies of the Tonga volcanic eruption, demonstrating the need for in-depth modeling follow-up studies to examine the origin of STWs and their effects on the ionosphere, and leading to a proposal for a rocket campaign to measure the parallel electric fields that may drive picket fence emissions for the first time.