Evidence and theory for trapped electrons in guide field magnetotail reconnection

A previous analysis of electron distributions measured in situ by the Wind spacecraft has revealed that electrons were trapped in the electromagnetic geometry of the reconnection event encountered in the deep magnetotail. In this paper we develop a detailed theory that can account for the main anisotropic features of the electron distributions associated with trapping in reconnection. The analysis shows that electron trapping in electric fields is generic in reconnection, as it is required in order to maintain the condition of quasineutrality. In addition to the spacecraft data, evidence of trapping in numerical simulations is also presented. Trapping is effective in eliminating free-streaming electrons along magnetic fields and thereby reduces parallel electron currents. Its importance for fast reconnection is discussed and emphasized by observations in a laboratory plasma.