Reconnection in Earths magnetotail

Abstract:

The role of magnetic reconnection in determining the structure and dynamics of the Earth’s magnetotail has been the subject of extensive study since the early years of space exploration. In the late 1960’s and early 1970’s auroral substorms were found to be closely associated with the build-up and subsequent release of energy stored in the lobes of the tail. Although highly controversial at the time, it was found that expansion phase onset was preceded by thinning of the plasma sheet and its embedded current sheet just prior to the observation of the particle acceleration and plasma heating expected to accompany near-Earth reconnection. However, it was not until the mid-1980’s that the central role of near-Earth x-lines (NEXLs) in the substorm process was confirmed by the first observations of large flux ropes, called “plasmoids,” being ejected down the tail at substorm onset. About this same time it was also discovered that sunward convection in the plasma sheet under quiescent conditions exists to downstream distances of ~ 100 to 150 R_e; marking the location of the distant x-line (DXL). The 1990s began with the first clear observations of high-speed Earthward flow from the NEXL and the determination that these flows are “bursty” with the individual events lasting only a few tens of minutes. These “bursty bulk flows” (BBFs) have been shown to dominate the sunward transport of energy and magnetic flux in the plasma sheet. Detailed investigations using space and ground-based observations beginning in the late 1990’s have revealed much about how the braking of these BBFs when they reach the inner magnetosphere couples to the high-latitude ionosphere and gives rise to the auroral substorm. The first decade of the new millennium has seen the first direct measurements of ion-scale features of the NEXL such as quadrupolar east – west magnetic fields associated with Hall currents driven by ion demagnetization. Now, approximately 50 years since the discovery of the auroral substorm, NASA’s Magnetospheric MultiScale mission will provide the electron-scale observations of the NEXL necessary to finally understand the microphysics controlling the onset, rate, and evolution of magnetic reconnection in the Earth’s magnetotail.