Electron-Only Tail Current Sheets and Their Temporal Evolution

The Earth's magnetotail is formed when the solar wind stretches the dipolar geomagnetic field in the anti-Sunward direction with a current sheet separating the anti-Sunward field of the southern lobe from the sunward-pointing northern lobe. While plasmas are to a high order of approximation charge-neutral, ions and electrons need not contribute equally to the currents. Herein, we report tail current sheets that are too thin to support magnetic gradient drift by ions and are supported by only electron currents. We examine one electron-only current sheet in detail, and briefly discuss ten others. Three of these current sheets are interpreted in terms of the time-evolution of reconnection onset. These current sheets show evidence of parallel electron heating, perpendicular ion heating, and current sheet expansion. These features are consistent with electron and ion behavior in traditional magnetotail ion and electron diffusion regions during bursty reconnection. Ground-based and in-situ data show that bursty reconnection occurs shortly after each of these three "pre-reconnection" events. This supports the hypothesis that electron-only reconnection can act as a precursor to traditional "electron-ion" reconnection. We note that five of the electron-only reconnection events occur after a period of traditional magnetic reconnection, and one event occurs without any corresponding signatures of traditional magnetic reconnection. These events suggest that electron-only reconnection is more than merely a transitory stage to ion reconnection and can form a steady-state magnetotail lobe without the support of ions other than to provide a charge-neutral plasma.