11/01/2024
Grant Stephens, Physics PhD Candidate
Title: Revealing the Global Morphology of the Magnetosphere duringSubstorms using Data Mining-Driven Empirical Magnetic Field Modeling
Committee:
Robert Weigel, Dissertation Director
Jie Zhang, Committee Member
Barry Mauk, Committee Member
Erdal YiΔit, Committee Member
November 14 | 2:30 p.m. - 4:30 p.m. | Expl Hall 3301
Zoom: https://gmu.zoom.us/j/4695537525
Abstract: The Earth's magnetosphere undergoes global dynamical reconfigurations termed magnetospheric substorms in response to changes in the solar wind. Understanding how the 3D magnetic field and associated current systems evolve in time during these events is a critical component needed to understand and predict the magnetosphere system. However, modeling their description using magnetohydrodynamic (MHD) approaches is complicated because non-ideal MHD processes, such as the formation of ion-scale thin current sheets and magnetic reconnection, are key to their description. As such, several unanswered questions persist about the global morphology of the magnetosphere and its evolution during these events. (1) What is the global-scale configuration of the magnetospheric magnetic field and current systems and how do they evolve during a substorm? (2) Where does magnetic reconnection occur in the magnetotail during a substorm and what is the structure of the associated X-line? (3) Where do ion and electron isotropy boundaries (IBs) map to in the magnetotail during the substorm growth phase?
The dissertation will address these questions by empirically reconstructing the global 3D magnetic field and electric currents. For a given time, a multi-decade, multi-mission archive of magnetospheric magnetic field observations is mined to form a subset of data from other times when the magnetosphere was in a similar substorm and storm state. This subset of data is used to fit an empirical model of the magnetic field that analytically describes the key magnetospheric current systems associated with substorms. This procedure is repeated for each snapshot during an event, revealing the global morphology of the magnetosphere during substorms. The resulting model, termed SST19, represents the first empirical magnetic field model capable of capturing substorm features.
To address (1), the data mining component of SST19 characterizes the state of the magnetosphere using geomagnetic indices, their time derivatives, and a metric for the intensity of the solar wind driving. We show that the primary substorm features include the formation of an embedded thin current sheet (TCS) that stretches the magnetotail throughout the substorm growth phase. Following substorm onset, the tail undergoes a rapid reconfiguration. The TCS collapses as the tail dipolarizes and magnetic flux piles up in the near-Earth region. To address (2), we use in situ observations of tail reconnection from the Magnetospheric Multiscale (MMS) Mission and compare them to their reconstructed location using SST19. We demonstrate that the SST19 analytical structure is sufficiently flexible to resolve most tail X- and O-lines and that their modeled location generally matches the MMS observed reconnection site to < 2.0 Earth radii (RE). The reconstructed X-lines vary in length from 5 to 40 RE, with the shorter ones tending to form inside of ~20 RE while the longer ones, ~40 RE, appear beyond 25 RE. Question (3) is addressed by inferring the location of ion and electron IBs using the SST19 model, mapping their location to low altitudes, and comparing them to their observed location using precipitating particle data from the Electron Losses and Fields Investigation (ELFIN) mission. Both the observed and modeled IBs move equatorward during the growth phase and diverge in latitude after substorm onset. Further, they reveal a βcheckmarkβ pattern in energy vs. time/latitude plots indicative of an accumulation of flux in the magnetotail during the substorm growth phase.
