HR: 16:45h
AN: SM34A-06    [Abstracts]
TI: Electron Demagnetization and Collisionless Magnetic Reconnection in β_e ≪ 1 Plasmas: Theory and Observations
AU: * Scudder, J D
EM: jack-scudder@uiowa.edu
AF: University of Iowa, Van Allen Hall Jefferson at Dubuque St., Iowa. City, Ia 52240 United States
AU: Mozer, F S
EM: fmozer@aol.com
AF: University California Berkeley, SSL Grizzly Peak Blvd, Berkeley, Ca United States
AB: Abrupt, intense bipolar and unipolar electric spikes with E > 100mV/m surveyed over 3 years of Polar data (Mozer et al 2005) have been organized to answer the limited question: can they be involved in the local demagnetization of thermal electrons? We determine a lower bound on the electric strength sufficient to cause non-gyrotropic effects on the electron pressure tensor of the form E>E^*=B{w_e}/{8cℑ}, where B is the ambient magnetic field strength, w_e=√2kT_e/m_e, c is the speed of light, and ℑ is an electron velocity space weighted average displacement along the electric field while transiting the layer (assumed localized with a scale Δ x=a ρ_e, where ρ_e is the electron thermal gyroradius). The variation of ℑ as a function of a for equal mean energy Maxwellian and more typical κ distributions seen in the Earth's magnetosphere provides strong evidence that the surveyed electric field spikes are generally smaller than E^* (assuming ≈ 1), although 23% (n=57) exceed E^* . Only 11% (n=6) of the bipolar class exceed E^* ; the frequency of occurrence distribution for the bipolar class of spikes is peaked at 0.1E^*. The unipolar occurrence is flat below E^*, but has a significant 26% subgroup (n=51) that exceed E^* . While E^* does not depend on the ambient density, the occurrence distribution of all demagnetizing events is well organized by the ratio ℜ=λ_De/ρ_e=&Omega_ce/ω_pe, residing almost exclusively in the regime ℜ <1. Spikes with E < E^* generally occur with ℜ >1 . All the electrostatic spike events surveyed occur in the regime 10^-8≤β_e≤3×10^-2. The demagnetizing events of either class occupy the more restricted low beta regime 10^-3≤β_e≤3×10^-2. Because these demagnetizing events occur in β_e ≪ 1 they would not, however, be considered unmagnetized at current channels as thin as the electron skin depth, d_e, since for such current channels ρ_e ≡ β_e^-1/2d_e ≪ d_e. As a group the subset of unipolar events with E > E^* are consistently understood as sites where the electron pressure tensor could become deformed from cylindrical symmetry by electric field enhancement in layers with scale sizes up to the local thermal electron's gyroradius. Such a deformation is critical for a viable mechanism that supports collisionless reconnection. After selecting events as demagnetizing based on the size of the relevant forces and work done, the geophysical locale of their detection has been investigated. Previously, all E spikes in this survey were found near the invariant latitudes Λ of the earth's magnetic cusps but at all magnetic local times. The demagnetizing events identified here via E^* are strongly organized at magnetic local noon (with a secondary, much shallower maximum at local magnetic midnight), occur preferentially at orbit apogee, and without significant preference for the magnetic latitude of the spacecraft. These geophysical organizations are consistent with the demagnetizing E spikes as indices of ongoing, collisionless reconnection in low β_e regimes at the earth's subsolar magnetopause. The identification of this sub-class of electric spikes at low β_e with E>E^* widens the observed venues in the E and B fields where topology changing departures from ideal MHD should be anticipated in collisionless astrophysical plasmas.
DE: 7519 Flares
DE: 7807 Charged particle motion and acceleration
DE: 7815 Electrostatic structures
DE: 7827 Kinetic and MHD theory
DE: 7835 Magnetic reconnection
SC: SPA-Magnetospheric Physics [SM]
MN: 2005 Joint Assembly