In Situ Discovery of an Electrostatic Potential, Trapping Electrons and Mediating Fast Reconnection in the Earth's Magnetotail
Massachusetts Institute of Technology, Plasma Science Fusion Center, Cambridge, Massachusetts 02139, USA.Physical Review Letters (Impact Factor: 7.51). 02/2005; 94(2):025006. DOI: 10.1103/PhysRevLett.94.025006
Anisotropic electron phase space distributions, f, measured by the Wind spacecraft in a rare crossing of a diffusion region in Earth's far magnetotail (60 Earth radii), are analyzed. We use the measured f to probe the electrostatic and magnetic geometry of the diffusion region. For the first time, the presence of a strong electrostatic potential (1 kV) within the ion diffusion region is revealed. This potential has far reaching implications for the reconnection process; it accounts for the observed acceleration of the unmagnetized ions out of the reconnection region and it causes all thermal electrons be trapped electrostatically. The trapped electron motion implies that the thermal part of the electron distributions are symmetric around v( parallel)=0: f(v( parallel),v( perpendicular)) approximately f(-v( parallel),v( perpendicular)). It follows that the field aligned currents in the diffusion region are limited and fast magnetic reconnection is mediated.
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- "Some electrons are trapped in the reconnection region bouncing back from separatrices regions. These kind of trajectories were found in previous studies (Egedal et al. 2005). (6) Ions are not accelerated to relativistic velocities. "
ABSTRACT: We carried out a 3D fully kinetic simulation of Earth's magnetotail magnetic reconnection to study the dynamics of energetic particles. We developed and implemented a new relativistic particle mover in iPIC3D, an implicit Particle-in-Cell code, to correctly model the dynamics of energetic particles. Before the onset of magnetic reconnection, energetic electrons are found localized close to current sheet and accelerated by lower hybrid drift instability. During magnetic reconnection, energetic particles are found in the reconnection region along the x-line and in the separatrices region. The energetic electrons are first present in localized stripes of the separatrices and finally cover all the separatrix surfaces. Along the separatrices, regions with strong electron deceleration are found. In the reconnection region, two categories of electron trajectory are identified. First, part of the electrons are trapped in the reconnection region, bouncing a few times between the outflow jets. Second, part of the electrons pass the reconnection region without being trapped. Different from electrons, energetic ions are localized on the reconnection fronts of the outflow jets.
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- "2. Adiabatic Model for Magnetized Electrons  A desirable goal in reconnection studies is to understand the dynamics of the electrons from thermal energies all the way up to relativistic energies. New insight into the behavior of the thermal electrons has been obtained through a recently derived adiabatic theory for the temperature anisotropy of the thermal electrons [Egedal et al., 2005, 2008]. Given the success of this theory in accounting for electron distribution functions measured by spacecraft [Chen et al., 2009; Egedal et al., 2010] we here explore if the high energy limit of this model is applicable to the electron distributions at super‐thermal energies. "
ABSTRACT: 1] We present a candidate mechanism for the energization of super‐thermal electrons during magnetic reconnection in the Earth's magnetotail. By analyzing in‐situ measurements of electron distribution functions we characterize the relative energy gain of the electrons as a function of energy, DE(E). For all the events considered the high energy part of DE(E) is nearly independent of E. This is the signature of energization in an acceleration potential, F k , which is caused by parallel electric fields in the vicinity of the reconnection region. The same acceleration mechanism is also documented for a kinetic simulation of reconnection.
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- "Different from the reconnection event observed by Wind in the deep magnetotail [Øieroset et al., 2001, 2002; Egedal et al., 2005], there is no obvious observed guide field during this event. The spacecraft fortuitously measured the electron pitch angle distributions at different locations (in the vicinity of the X line and the outflow region). "
ABSTRACT: 1] In this paper, we present Cluster observations of a magnetotail reconnection event without the presence of an obvious guide magnetic field and analyze electron pitch angle distributions in the vicinity of the X line and the outflow region, respectively. In the vicinity of the X line, at lower energies the distributions are highly anisotropic (field-aligned bidirectional anisotropic), while at higher energies, the electrons are observed to flow away from the X line along the magnetic field lines. The electron distributions change largely in the outflow region. At the edge of the outflow region, at lower energies, the electrons flow toward the X line, while the electrons at higher energies are directed away from the X line. When the satellites approach the center of the current sheet, at lower energies, the electrons have field-aligned bidirectional distributions, while at higher energies, the electron distributions are isotropic. The generation mechanisms of such distributions are explained by following typical electron trajectories in the electric and magnetic fields of magnetic reconnection, which are obtained in two-dimensional particle-in-cell simulations. It is shown that the observed high-energy electrons directed away from the X line both in the vicinity of the X line and in the outflow region are due to the acceleration by the reconnection electric field near the X line, and the field-aligned bidirectional distributions at lower energies are caused by the effects of the magnetic mirror in the reconnection site. The isotropic distributions at higher energies in the outflow region are the results of the electron stochastic motions when their gyroradii are comparable to the curvature radii of the magnetic field lines.