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Near Earth Current Meander (Necm) Model of Substorms
Authors:W.J. Heikkila  T. Chen  Z.X. Liu  Z.Y. Pu  R.J. Pellinen  T.I. Pulkkinen
Affiliation:(1) University of Texas at Dallas, Richardson, U.S.A;(2) Laboratory of Solar-Terrestrial Physics, Chinese Academy of Sciences, Beijing, China;(3) Dept of Geophysics, Peking University, Beijing, China;(4) Finnish Meteorological Institute, Helsinki, Finland
Abstract:We propose that the appropriate instability to trigger a substorm is a tailward meander (in the equatorial plane) of the strong current filament that develops during the growth phase. From this single assumption follows the entire sequence of events for a substorm. The main particle acceleration mechanism in the plasma sheet is curvature drift with a dawn-dusk electric field, leading to the production of auroral arcs. Eventually the curvature becomes so high that the ions cannot negotiate the sharp turn at the field-reversal region, locally, at a certain time. The particle motion becomes chaotic, causing a local outward meander of the cross-tail current. An induction electric field is produced by Lenz's law, Eind=–partA/partt. An outward meander with Bz>0 will cause E×B flow everywhere out from the disturbance; this reaction is a macroscopic instability which we designate the electromotive instability. The response of the plasma is through charge separation and a scalar potential, Ees=–nablaphgr. Both types of electric fields have components parallel to B in a realistic magnetic field. For MHD theory to hold the net Epar must be small; this usually seems to happen (because MHD often does hold), but not always. Part of the response is the formation of field-aligned currents producing the well-known substorm current diversion. This is a direct result of a strong Eparind (the cause) needed to overcome the mirror force of the current carriers; this enables charge separation to produce an opposing electrostatic field Epares (the effect). Satellite data confirm the reality of a strong Epar in the plasma sheet by counter-streaming of electrons and ions, and by the inverse ion time dispersion, up to several 100 keV. The electron precipitation is associated with the westward traveling surge (WTS) and the ion with omega (OHgr) bands, respectively. However, with zero curl, Ees cannot modify the emf epsi=conintEsdotdl=–dPHgrM/dt of the inductive electric field Eind (a property of vector fields); the charge separation that produces a reduction of Epar must enhance the transverse component Ebottom. The new plasma flow becomes a switch for access to the free energy of the stressed magnetotail. On the tailward side the dusk-dawn electric field with EsdotJ<0 will cause tailward motion of the plasma and a plasmoid may be created; it will move in the direction of least magnetic pressure, tailward. On the earthward side the enhanced dawn-dusk induction electric field with EsdotJ>0 will cause injection into the inner plasma sheet, repeatedly observed at moderate energies of 1–50 keV. This same electric field near the emerging X-line will accelerate particles non-adiabatically to moderate energies. With high magnetic moments in a weak magnetic field, electrons (ions) can benefit from gradient and curvature drift to attain high energies (by the ratio of the magnetic field magnitude) in seconds (minutes).
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