ECE PhD Dissertation Defense: Sebastijan Mrak

Chair:TBD

Committee:Professor Luca Dal Negro, ECE; Professor Meers Oppenheim, CAS/ECE; Dr. Anthea Coster, MIT Haystack Observatory

Abstract:The Earth's ionosphere is the major source of disturbances for trans-ionospheric radio signals such as the Global Navigation Satellite Systems (GNSS). As human society has become heavily dependent on GNSS services, timely and accurate space weather characterization and forecasts are needed. This is particularly true at mid-latitudes, such as the contiguous United States (US), where population density is greatest and infrastructure most vulnerable. As a conducting layer, the ionosphere alters radio signals by means of phase delay/advance (refraction) or amplitude fluctuations (diffraction), depending on spatial scales of the irregular permittivity (plasma density). Ionospheric refraction can be used to estimate the path-integrated plasma density, referred to as the Total Electron Content (TEC). Maps of TEC constructed from ground-based receiver networks provide a global time-dependent image of the ionospheric dynamics. While refraction scales with radio-frequency and is routinely compensated by multi-frequency GNSS receivers, diffraction is stochastic and hence an impairment to GNSS services. Radio receivers, including GNSS monitors, are being used to monitor and quantify these effects, producing climatological maps of ionospheric irregularities. However, efforts have focused on low- and high-latitude regions as they are continuously perturbed by geophysical processes related to the orientation of the Earth’s magnetic field. The region in-between have a much more nuanced space-time connection to geomagnetic disturbances. As a consequence, no dedicated observatories are operating today at mid-latitudes. This dissertation provides a fundamental analysis of this underexplored territory in the burgeoning field of space weather.

In this dissertation, we developed signal processing techniques to leverage data from geodetic GNSS receivers to study ionospheric irregularities and scintillation, and their connection to spatiotemporal variations in TEC. The newly introduced data source covers areas of Central America and the Caribbean, contiguous US, and Alaska. We applied these techniques initially to study the ionospheric effects of the 2017 solar eclipse and terrestrial weather patterns. We then focused our effort on a long term study of geomagnetic storm effects at mid-latitudes. Eight years of these opportunistic data have been processed in a period of the last solar cycle (2012-2019), and nine profound space weather events were identified. The newly constructed maps were used in conjunction with the TEC maps that provide a critical spatial context for understanding the origin of the irregularities. The observations revealed several types of space weather events that affected the area, including a poleward expansion of equatorial plasma bubbles (EPB) near local midnight, a single plasma bubble expanding poleward while trailing the terminator, and a newly observed mid-latitude phenomena we termed mid-latitude density striations. We also discovered evidence for expansion into and coupling with processes in the near Earth magnetosphere. All events occurred during geomagnetic storms, with an average strength of Dst=-125 nT and Kp=6+, conditions that on average occur three times per year. The events were recorded at all seasons.

The one event with mid-latitude density striations was analyzed in greater detail using GNSS-derived products, and in-situ measurements of plasma parameters in the ionosphere and conjugate magnetosphere. While the large scale TEC projection closely resembles the expected characteristics of an EPB, we identified distinct differences in plasma parameters. Namely, the electric field peaked at the density gradients instead of in the depletion, and the density irregularities were lagging the trough formation by about one hour. Morphology of TEC irregularities measured by a ground-based GNSS receiver was compared to the first GNSS scintillation observations at mid-latitudes from 2001. We found the large scale density structures, as well as the respective location of scintillation, closely resemble the mid-latitude density striations. We suggest the narrow density, and electric field perturbations were likely caused by the penetration of a substorm-induced electric field to mid-latitudes. We conclude the dissertation by discussing the implication of such space weather events on modern technology.