UCLA

Magnetic reconnection is a fundamental, ubiquitous process which dictates physical phenomenaacross Earth’s magnetosphere. Reconnection has been well investigated with spacecraft for decades, and the community’s understanding of its structure continues to evolve. The recently launched Magnetospheric Multiscale (MMS) Mission has allowed for multispacecraft observations of ion-scale and sub-ion scale structures, such as the Ion Diffusion Region (IDR). In 2016, MMS1 and 3 observed oppositely directed electron outflow jets while crossing a thin, electron-scale current sheet in Earth’s turbulent magnetosheath. The current sheet lacked the ion outflow jets expected in ion-coupled, turbulent, magnetic reconnection. As a result, this structure was dubbed “electron-only” reconnection. Current sheets labelled as electron-only reconnection have since been reported with simulation work and spacecraft data in Earth’s magnetosheath, dayside magnetopause, and magnetotail. However, due to the rarity of electron-only current sheet observations and a lack of consistency/clarity in the underlying physical properties of events, a precise definition and generation mechanism of electron-only reconnection remains unclear.

In this dissertation, we use MMS data to investigate non-reconnecting and reconnectingcurrent sheets in Earth’s magnetotail. First, we perform a statistical survey of quiet current sheets, ion-coupled reconnection, and Ion Diffusion Regions in Earth’s magnetotail. We compare the properties of these current sheets in tilted vs. equatorial current sheets. We find that all current sheet types occur equally in tilted and equatorial current sheets, and that ion and electron outflow profiles are generally unaffected by the orientation of the current sheet. Next, we define electron-only reconnection as a reconnecting current sheet that is electronscale in thickness and less than 10 de long, such that spacecraft are unable to detect ion acceleration or heating. Using 2D PIC simulations and MMS magnetotail observations of a known Electron Diffusion Region (EDR) and several electron-only reconnection candidates, we develop the following observational criteria for electron-only reconnection in Earth’s magnetotail: 1. BL Reversal, 2. Btot minimum, 3. sub-Alfv´enic ion outflow, 4. super-Alfv´enic electron outflow, 5. < 10% change in Ti,tot, 6. > 10% increase in Te,tot, 7. positive peak in J · E′, 8. deviation of ve,⊥ from E×B B2 , 9. meeting the electron tearing criterion, 10. meeting the flux transport velocity criterion, and 11. increase in agyrotropy correlated with an increase in J · E′. We report 12 electron-only reconnection candidates in Earth’s magnetotail. We utilize preliminary statistics of these candidate events to distinguish electron-only flux rope erosion from electron-only onset of reconnection, where during the onset of magnetotail reconnection, a ≈ 10 second transition phase occurs where only electrons are accelerated. We verify that some event candidates align better with a transition phase than alternative models such as turbulent secondary reconnection and flux rope erosion.

This dissertation confirms that the properties of plasma accelerated by magnetic reconnectionare largely unaffected by magnetotail orientation. In addition, our current sheet database will enable future current sheet statistical studies. Establishing universal observational criteria for electron-only reconnection has allowed the community to report electrononly reconnection in new and unexpected places. Our electron-only reconnection candidate pool also provides the first opportunity to explore how electron-only reconnection changes over time. Lastly, our preliminary electron-only statistics support the hypothesis that electron tearing is the primary driver of reconnection onset, and that electrons couple to the reconnecting fields before the ions.