SM020-0001
The onset of reconnection in Earth’s magnetotail: simultaneous observations from Magnetospheric Multiscale and the Heliophysics System Observatory

Thursday, 10 December 2020
Poster
Kevin James Genestreti1, Charles J Farrugia2, San Lu3, Tai Phan4, Daniel N Baker5, Trevor W Leonard6, Sarah K. Vines7, Ian J. Cohen8, Sam Bingham9,10, Jason R Shuster11,12, Daniel J Gershman12, Christopher Mouikis13, Anthony J Rogers14, James Burch1, Roy B Torbert13,15, Barbara L Giles12, Robert Ergun16, Narges Ahmadi16, Christopher T Russell17, Robert J Strangeway18 and Rumi Nakamura19, (1)Southwest Research Institute, San Antonio, TX, United States, (2)University of New Hampshire, Institute for the Study of Earth, Oceans, and Space, Durham, NH, United States, (3)University of California Los Angeles, Los Angeles, CA, United States, (4)University of California Berkeley, Berkeley, United States, (5)University of Colorado, Laboratory for Atmospheric and Space Physics, Boulder, CO, United States, (6)University of New Hampshire, Durham, NH, United States, (7)University of Texas at San Antonio, San Antonio, TX, United States, (8)The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, United States, (9)Johns Hopkins University Applied Physics Laboratory, (Deceased during the planning stages of the session), Laurel, MD, United States, (10)University of New Hampshire Main Campus, Durham, NH, United States, (11)University of Maryland College Park, College Park, MD, United States, (12)NASA Goddard Space Flight Center, Greenbelt, MD, United States, (13)Univ New Hampshire, Durham, NH, United States, (14)University of New Hampshire Main Campus, Physics, Durham, NH, United States, (15)Southwest Research Institute Durham, Durham, NH, United States, (16)Laboratory for Atmospheric and Space Physics, Boulder, CO, United States, (17)University of California, Los Angeles, CA, United States, (18)Univ California, Los Angeles, CA, United States, (19)Space Research Institute, Graz, Austria
Abstract:
Magnetic reconnection violently shreds the oppositely-polarized magnetic fields in Earth’s magnetic tail, generating aurorae and geomagnetic substorms. “What initiates magnetotail reconnection”? The decades-old “onset problem” is one of the biggest open questions in magnetospheric substorm physics. It is known that macroscopic forces gradually push the tail current sheet to some critical stage, after which some instability rapidly short-circuits the current in one or more microscopic reconnection sites. Prior multi-satellite missions showed that tail reconnection occurs only after the tail current sheet is thinned and stretched, but not all such current sheets reconnect. Reconnection between the solar wind and the low-latitude dayside boundary of the magnetosphere supplies forces that can thin and stretch the tail current sheet, but tail reconnection does not necessarily follow dayside reconnection. The microscopic instability that triggers reconnection is unknown, though a number of candidates have been proposed. Only NASA’s Magnetospheric Multiscale (MMS) mission is capable of resolving the microscopic onset of reconnection. MMS began surveying the tail in mid-2017. Here we present in-situ MMS observations of the microscopic initiation of tail reconnection. Simultaneously, a network of space and ground-based observatories recorded the macroscopic forces that caused the current sheet to thin. It is unambiguously demonstrated that the tail current sheet thins and stretches somewhat spontaneously by evacuating its thermal pressure without significant dayside reconnection. The thinning and stretching occur with minimal prompting from the solar wind. MMS observed the exact set of critical current sheet conditions in the moment of onset: the current sheet was thinned and stretched, with a low ion temperature and a large electron-to-ion temperature ratio. The moment the current sheet surpassed this threshold, known as the electron tearing threshold, reconnection was initiated in multiple locations. One dominant reconnection site, which engulfed the others, was quickly established.
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