What Causes Electron Holes During Magnetic Reconnection and What Can We Learn From Them

Weak bipolar electrostatic fields are commonly observed in association with magnetic reconnection. Recent attention has focused on their origin due to nonlinear evolution of electrostatic instabilities.\footnote{Goldman, M.~V., D.~L.~Newman, and P. Pritchett, \textit{GRL}, \textbf{35}, doi:10.1029/2008GL035608 (2008).}$^,$\footnote{Newman, D.~L.~and M.~V.~Goldman, SM31B-1735, AGU Fall Meeting (2008).}$^,$\footnote{Che, H., J.~F.~Drake, M.~Swisdak, and P.~H.~Yoon, \textit{PRL}, \textbf{102}, 145004 (2009).} We present evidence from both older and new reconnection simulations for the SPATIAL dependence of electrostatically unstable electron distributions along the separatrix during guide-field magnetic reconection. Particle distributions further from the reconnection region tend to be Buneman (electron-ion) unstable, while distributions closer to the reconnection region tend to be two-stream (electron-electron) unstable. It may be possible to infer properties of the particle distributions from measurements of the speed, half-width, amplitude and aspect ratio of weak electron holes.