Seminar: The Onset of Magnetic Reconnection in Earth's Magnetotail
Wednesday, September 30, 2020 - 3:00pm to 4:00pm
Speaker: Kevin Genestreti, Senior Research Scientist for SwRI-EOS
Abstract: "The elongated tail of our night-side magnetosphere stores energy from the solar wind. The oppositely-directed magnetic fields in the northern and southern tail are separated by a current sheet, which often thins dramatically then “short circuits”. Microscopic (~107 km3) magnetic reconnection sites violently shred the thin current sheet, rapidly dissipating stored energy. To first order, the current sheet thickness is controlled by the balance of its internal proton thermal pressure and external magnetic pressure. Reconnection at the day-side magnetosphere drives global (~1015 km3) plasma convection, which may thin the current sheet by increasing its external pressure and/or depleting its internal pressure. However, tail reconnection does not necessarily follow day-side reconnection and some thin current sheets remain stable. Causal relationships between reconnection and the current sheet instabilities that often accompany it have been predicted but not verified. Here we report in-situ observations of the of the macroscopic forces that thinned the current sheet and the subsequent microscopic initiation of tail reconnection. We find that the tail current sheet thins by evacuating its thermal pressure without significant day-side reconnection. The solar wind prompts the pressure evacuation and, eventually, initiates reconnection by momentarily compressing the tail. Reconnection was initiated in multiple locations once the current sheet surpassed the threshold for the electron-tearing instability, which requires a sufficiently thin current sheet with a weak magnetic field and a low ion-to-electron temperature ratio. One reconnection site quickly engulfs the others, becoming the dominant region that shreds the tail’s magnetic field, fitting with simple models."
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