JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. A7, PAGES 11,555-11,567, JULY 1, 1991 Forced Magnetic Reconnection in a Plasma Sheet With Localized Resistivity Profile Excited by Lower Hybrid Drift Type Instability M. HOSHINO 1 Institute of Geophysics and Planetary Physics, University of California Lawrence Livermore National Laboratory, Livermore A forced magnetic reconnection process with a temporal evolution of resistivity is studied for a plasma sheet with a nonuniform resistivity profile based on the nonlocal mode structure of the lower hybrid drift type instability. The growth rate of the mode found is almost independentof the resistivity at the neutral sheet, but depends on the resistivity of the region of maximum density gradient away from the neutral sheet. We study this by usingboth a nonlinear numerical MHD simulation and a linear theory. The mode may be relevant to the prevalent theoretical concept of MHD reconnection and the localized anomalousresistivity profile based on the lower hybrid drift instability. 1. INTRODUCTION Magnetic reconnection processes are thought to be crucial to understanding fundamental plasma phenomena in both astrophysical plasmas (e.g., the Earth's magnetotail, solar flares, accretion disks) and laboratory plasmas (e.g., re- versed-field theta pinch). The essential feature of magnetic reconnection is that it violates the concept of the frozen-in condition around the neutral sheet and that magnetic field energy can be rapidly converted into particle energy. Observations in space plasmas reveal that the magnetic field energy stored in the magnetotail can be released by the magnetic reconnection, and many theoretical magnetic re- connection processes have been proposed to explain ade- quately the observational phenomena [e.g., Hones, 1984]. The solar wind energy which is transferred into the magneto- sphere is considered to be stored in the magnetotail as the magnetic field energy. As the magnetic flux in the magneto- tail lobe is increased, the plasma sheet is compressed by the increased magnetic pressure and becomes thin (i.e., growth phase). At the same time the electric current which flows in the plasma sheet is intensified. In this stage the magnetic reconnection in the plasma sheet would be stimulated by the stored magnetotail energy and results in the explosive re- lease of the stored magnetic energy (i.e., sudden onset of substorm). The reconnection hypothesisfor substormseems to be successful. There exists, however, a significant unre- solved problem of the MHD magnetic reconnection process to date, because the magnetic diffusion process around the neutral sheet has not yet been self-consistently understood from both microscopic and macroscopic pointsof view. Since the classical resistivity (i.e., Coulomb collision) in many applications is very small, an anomalous resistivity excited by kinetic plasma instabilities should be assumed for the MHD magnetic reconnection process. The anomalous transport associated with the evolution of microinstabilities is expected to dominate classical transport, because the 1Now at Institute of Physical and Chemical Research (RIKEN), Wako, Saitama, Japan. Copyright 1991 by the American Geophysical Union. Paper number 91JA00984. 0148-0227/91/91 JA-00984505.00 scale length of the plasma sheet is not much larger than the ion Larmor radii. The most reasonable candidate instability of the anomalous resistivity in collisionless plasmas is the lower hybrid drift (LHD) instability [Huba et al., 1978; Papadopoulos, 1979]. This instability is driven by diamag- netic currents associated with the pressure gradient. The LHD wave, however, cannot propagate into the neutral sheet, and the anomalous resistivity caused by LHD turbu- lence is expected to be found only away from the neutral sheet because the LHD wave is strongly absorbed by electronsin high-/3 region of the neutral sheet [Huba et al., 1980]. Therefore because of the localization of LHD turbu- lence far from the neutral sheet, it remains a significant unresolved problem in combining the resistive MHD recon- nection with the concept of anomalous resistivity. One of the approaches to bridge a gap between the resistive MHD reconnection and the concept of microscopic anomalousresistivity is to find some microscopic anomalous resistivity to limit the electric current near the neutral sheet. One might find a new fascinating form of anomalous resis- tivity, and then the resistivity is incorporated into the macroscopic MHD magnetic reconnection. This approach is reasonable when the saturation mechanism of the micro- scopic process is not strongly altered by the macroscopic evolution [Davidson and Krall, 1977]. However, the trans- port coefficients in many casesare strongly modified by the plasma state of the macroscopic evolution, and this ap- proach is no longer valid. The collisionless tearing instability [Laval et al., 1966; Coppi et al., 1966] may be one of possible models of magnetic reconnection in a plasma sheet without a signifi- cant anomalousresistivity in the vicinity of the neutral sheet. The energy dissipation in the collisionless reconnection is provided by a Cherenkov interaction between an inductive electric field and unmagnetized electrons near the neutral sheet. In the Earth's magnetotail with the normal magnetic field (namely, the north-south component), however, the electrons are magnetized in the normal magnetic field, and the energy dissipation due to the electron Cherenkov inter- action is prohibited [Galeev and Zeleny, 1976]. The ion tearing mode due to the ion Cherenkov interaction was suggested as an alternative possible mechanism of magnetic 11,555