Article

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: 7.73). 02/2005; 94(2):025006. DOI: 10.1103/PhysRevLett.94.025006
Source: PubMed

ABSTRACT 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.

0 Bookmarks
 · 
101 Views
  • [Show abstract] [Hide abstract]
    ABSTRACT: [1] Magnetic reconnection is an efficient way to convert magnetic energy into particle energy. In this paper, we use Cluster thermal electron and ion measurements in the vicinity of a reconnection X line to delineate the structure of the reconnection current sheet. Multispacecraft observations made by Cluster on 18 August 2002 indicate that an X line drifted close to the spacecraft, about 3.4 RE earthward of the position where another X line had been observed earlier. Comparison of the Hall magnetic and electric field geometry and the observed properties of energetic electron beams streaming along the separatrix between the Cluster spacecraft indicates that the second X line formed within 20 s of the observation of the first X line. Repeated flow reversals and Hall field geometry together with the presence of a magnetic island embedded in the outflow region downstream of the first X line suggest that the initial current sheet was unstable, perhaps to the tearing mode. We identify a region with a thickness of 0.72 ion inertial lengths (29 electron inertial lengths, de) of super-Alfvénic electron outflow (greater than the ion in-flow Alfvén speed) during the period when the spacecraft was in the vicinity of the neutral sheet. Slightly below the neutral sheet, Cluster observed asymmetric counter-streaming electrons with a loss of axisymmetry in the electron (V⟂1,V⟂2) distribution functions over a thin boundary with a thickness of several de. This electron-scale transition layer was embedded in a much wider region where both the ion and electron Walén tests failed, and the electron super-Alfvénic bulk outflow jets with high-energy electron beams were detected. Those phenomena provide details of the substructure of the reconnection current sheet and suggest that the spacecraft traversed or skimmed the tailward edge of an elongated electron current layer. We also note that this event differs from a previously reported reconnection event in that strong electron temperature anisotropy (T∥>T⟂) is observed both in the inflow region and in the exhaust, where the anisotropy appears to be associated with the elongated electron outflow jets.
    Journal of Geophysical Research: Space Physics 07/2013; 118(7). · 3.44 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: A linear experiment dedicated to the study of driven magnetic reconnection is presented. The new device (VINETA II) is suitable for investigating both collisional and near collisionless reconnection. Reconnection is achieved by externally driving magnetic field lines towards an X-point, inducing a current in the background plasma which consequently modifies the magnetic field topology. Owing to the open field line configuration of the experiment, the current is limited by the axial sheath boundary conditions. A plasma gun is used as an additional electron source in order to counterbalance the charge separation effects and supply the required current. Two drive methods are used in the device. First, an oscillating current through two parallel conductors drive the reconnection. Second, a stationary X-point topology is formed by the parallel conductors, and the drive is achieved by an oscillating current through a third conductor. In the first setup, the magnetic field of the axial plasma current dominates the field topology near the X-point throughout most of the drive. The second setup allows for the amplitude of the plasma current as well as the motion of the flux to be set independently of the X-point topology of the parallel conductors.
    The Review of scientific instruments 02/2014; 85(2):023501. · 1.58 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The Earth's magnetosphere provides an excellent laboratory for magnetic reconnection research. In particular, the magnetotail current sheet that is formed between the interface of the similar Northern and Southern Hemispheres of the magnetotail provides a relatively stable symmetric reconnection configuration that can be used to study basic aspects of the reconnection process. Of particular importance is the manner in which electrons are processed by the reconnection. Simulations and satellite data analyses of the ion diffusion region have suggested that the fluxes of electrons in the inflow regions of reconnection are greater in the directions parallel and anti-parallel to the magnetic field (field-aligned) whereas the electron flux in the outflow region is distributed more isotropically. However, this has only been studied experimentally on a case-by-case basis. In this paper, we investigate this claim by analyzing the degree of bulk electron field alignment in the outflow and inflow regions during encounters of the magnetic reconnection ion diffusion region by the Cluster spacecraft in the years 2001-2006. We demonstrate that while the median electron flux in the inflow region is indeed more field aligned than in the outflow region during some ion diffusion region encounters, the variation of the signature across events is so large that it cannot be said to be a general feature of magnetic reconnection in the Earth's magnetotail.
    Annales Geophysicae 01/2012; 30(1):109-117. · 1.68 Impact Factor

Full-text

Download
0 Downloads
Available from