ICS-13 Abstract View


Kinetic Effects in Magnetotail Dynamics: Emergence of Finite-Width Reconnection Exhausts and the Occurrence of Localized Reconnection Onset
Pritchett, P.L., pritchet@physics.ucla.edu (1)
3-D particle-in-cell simulations are used to investigate two aspects of magnetic reconnection in a magnetotail configuration: (1) the emergence of finite-width exhaust structures from a X line and (2) the localized onset of reconnection in response to the imposition of a near-Earth, high-latitude electric field. Both issues are addressed using an initial plasma sheet configuration that consists of a 2-D dipole field joined to an asymptotic tail equilibrium with finite Bz.

An earthward-moving reconnection jet from a finite X line is found to separate initially into two segments of width ~10-15 di each. The dawnward segment moves ahead of the duskward one due to a finite ion-gyroradius effect and continues to intensity, while the latter is buffeted by the return flows generated by the former and stagnates. The leading front develops the characteristic structure of a sharp Bz increase and density drop on a 1-2 di scale. The current associated with this Bz increase is carried mainly by the electrons, and the jet front is the site of a Region 1 sense field-aligned current system. Close to the dipole region the ion velocity distribution behind the front contains a drop out in parallel phase space at speeds of ~1.5-2.5 VA.

It has been difficult to understand the emergence of localized (in y) reconnection flow channels in the tail. Unlike the case of a localized driving Ey applied in the lobes, which spreads rapidly across the tail due to the high Alfven speed, a localized Ey imposed at high latitude on the earthward boundary remains localized and creates a Bz reduction region that stretches duskward. This leads to a dawn-dusk asymmetry with a thinner current sheet and weakened Bz on the duskward edge of the flow channel. Eventually, this leads to localized reconnection with a tilted X line and the emergence of strong earthward and tailward flows. The significance for substorm onset will be discussed.
(1) Department of Physics & Astronomy, UCLA, Los Angeles, CA 90095-1547, USA