UC Berkeley

Aurora is a stunning visual display of the Sun-planet coupling for planets with global magnetic fields. At Earth, the Dungey cycle describes this process: the solar wind interaction with the planet drives magnetic reconnection on the dayside, field lines are convected into the magnetotail, carrying trapped magnetic energy. The field lines subsequently reconnect in the magnetotail, releasing the stored magnetic energy. This results in the generation of electric fields parallel to the magnetic field, which accelerate electrons. Energized electrons then precipitate onto the atmosphere, whereby they interact with neutral species and produce auroral emissions.At Mars, the absence of a global magnetic field results in the localized crustal magnetic fields interacting with the Sun's magnetic field in similar ways as the Earth's dipole field. These crustal fields create miniature magnetospheres, and interact with the Sun's magnetic field at different local times as the planet rotates. Compelling evidence suggests that a small-scale version of the Dungey cycle can explain the interaction between Mars' mini-magnetospheres and the Sun's magnetic field. Several stages of this miniature Dungey cycle have been observed at Mars: dayside and nightside magnetic reconnection, auroral emissions at UV and visible wavelengths, and electron precipitation during active aurora. These important studies were limited spatially and temporally because they were mostly case studies of individual events or individual orbits. We thus conducted a statistical study to characterize the high-altitude component of the accelerated electrons themselves. The work presented in this dissertation is a piece of the puzzle showing how Mars' magnetotail dynamics couple to auroral emissions.Such a study was not possible until the Mars Atmosphere and Volatile EvolutioN Mission (MAVEN) arrived at Mars in 2014.Many individual case studies have been valuable in establishing the likelihood of a mini-Dungey cycle at Mars, but case studies are inherently limited both spatially and temporally. MAVEN enabled many of these studies of electrons associated with aurora on Mars, and was foundational in the studies of the aurora themselves. One orbit provides a detailed look of the data at one location and one time, making it difficult to establish patterns or determine the large-scale connection to upstream conditions. Asymmetries in the observed auroral emission occurrence rates, which are enhanced in the southern hemisphere, the dusk terminator, and in the +E hemisphere, are not testable with case studies. These asymmetries are however testable with a statistical study, and our goal was to test these asymmetries. By studying electron signatures and the conditions of the magnetic field environment, one can determine the relationship between magnetotail electron energization and the discrete auroral emissions themselves. It is then possible to use the abundant plasma measurements at Earth as an analogy to examine the planet-Sun coupling at all local times and altitudes with MAVEN.This dissertation presents two complementary studies of energized electrons in the Martian magnetotail. The first is a case study where accelerated electrons were observed simultaneously with multiple current sheet crossings and counter-streaming electrons. We identified an electric potential structure consistent with the presence of plasma double layers. These features arise from reconnection of the open crustal field loops in the tail, and the subsequent dipolarization of newly closed fields. The case study established a methodology for the statistical study that followed. The statistical study presented a 10-year MAVEN dataset of suprathermal electron energy spectra from MAVEN's Solar Wind Electron Analyzer (SWEA). We created an algorithm that identified electron peaked energy fluxes indicative of acceleration, across the entire magnetotail.From this dataset, we created the first spatial maps of energized electron occurrence rates in the Martian magnetotail.These maps revealed asymmetries in the distribution of energized electrons that closely mirror those observed in discrete auroral occurrence rates. Enhanced occurrence rates are present in the southern hemisphere, on the dusk terminator, and in the +E hemisphere. These conditions maximize magnetic shear between the crustal fields and the Sun's magnetic field, and are therefore conducive to enhanced magnetic reconnection. Energized electron occurrence rates were also 2-4 times greater during energetic solar events, and this is consistent with the Sun imparting additional energy into the magnetotail during such events. Linking the electron occurrence rate asymmetries to the auroral asymmetries statistically provides evidence that the proposed mini-Dungey cycle likely occurs on Mars because we studied the high altitude component of this cycle. If such a cycle is indeed present at Mars, it implies that the Mars-Sun coupling is similar to the planet-Sun coupling of planets with global magnetic fields (e.g. Earth and Mercury).The results presented in this dissertation also lay the foundation for multiple follow-up studies. First, the dependence of energized electron occurrence rates on solar wind conditions can be further constrained by coupling the dataset we created to a dataset of energetic solar events (e.g. coronal mass ejections, stream interaction regions). Such a study could utilize measurements from existing Mars assets (e.g. Tianwen-1 and Mars Express) and forthcoming assets (e.g. ESCAPADE) for upstream coverage. Coupling the upstream environment with the downtail response is another way to understand how the Sun exchanges energy and momentum with Mars' unique magnetic field environment. Second, the electron acceleration mechanisms themselves ought to be classified. Our dataset includes data from MAVEN-STATIC, providing ion densities during electron energization events; however, we did not explore these measurements in close detail in our studies. A natural follow-up study could include the ion measurements and determine the extent to which ambipolar electric fields driven by ion density enhancements within current sheets are responsible for accelerating electrons. Finally, the relationship between magnetotail current sheets and sinuous discrete aurora warrants further investigation. Combining a dataset of sinuous aurora events with our current sheet dataset and our electron dataset could elucidate the relationship between sinuous aurora and magnetotail current sheets.