Title : The onset of reconnection in Earth's magnetotail Speaker Name: Kevin Genestreti Speaker Institution : Southwest Research Institute
Abstract : The elongated tail of our nightside 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 periodically becomes very thin then "short circuits". Understanding the causes of magnetic reconnection - the "short circuit" mechanism - is an important yet elusive problem in space physics. We have learned a tremendous amount about reconnection by observing it while it is occurring in space, yet it is still debated how reconnection starts in the first place. Ambiguity stems from the difficulty of determining the accurate time history of a reconnection event over the vast range of relevant spatial scales. The standard picture is that reconnection of the solar wind and Earth's dayside magnetic fields drives global (~1015 km3) magnetospheric convection, which can thin the current sheet by compressing it or depleting its internal pressure. However, tail reconnection does not necessarily follow dayside reconnection and some thin current sheets remain stable. Microscopic (~107 km3) magnetic reconnection sites are formed after one of several proposed instabilities is triggered. While several instabilities accompany reconnection, causal relationships have been difficult to verify.
We start the seminar by summarizing the basic concepts of magnetospheric reconnection and the wide-ranging impacts of reconnection on near-Earth space. We then report a case study where many space and ground-based observatories witnessed magnetotail reconnection being initiated. We find that the tail current sheet became thin by evacuating its internal thermal pressure without significant dayside reconnection. The solar wind prompted the pressure evacuation and, eventually, initiated 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 engulfed the others, becoming the dominant region that shredded the tail's magnetic field, fitting with simple models.
Host: Marc Swisdak Host E-Mail: This email address is being protected from spambots. You need JavaScript enabled to view it.