HR: 1400h
AN: SM53B-02    [Abstracts]
TI: A New Scenario for the Production of Weak Bipolar Fields in Space: "Notch" Instabilities Resulting From Electron Velocity Dispersion*
AU: Newman, D L
EM: David.Newman@colorado.edu
AF: University of Colorado at Boulder, Center for Integrated Plasma Studies, 390 UCB, Boulder, CO 80309-0390, United States
AU: * Goldman, M V
EM: goldman@stripe.colorado.edu
AF: University of Colorado at Boulder, Center for Integrated Plasma Studies, 390 UCB, Boulder, CO 80309-0390, United States
AB: The bipolar signatures of weak (eφ_max/T_e ≪ 1) electron phase-space holes have now been observed in numerous near-Earth space-plasma environments such as the polar cusp region1 and the solar wind² at 1 AU. While families of stationary solutions of the Vlasov-Poisson equations consistent with these observations have been found,³ the question of how shallow phase-space density depressions supporting these bipolar fields form remains an open one. While strong bipolar fields associated with deep phase-space holes in Earth auroral downward-current region are consistent with saturated two-stream instabilities resulting from double-layer electron acceleration,⁴ the weak bipolar fields observed in other space environments may require an alternative generation mechanism. One such mechanism involves the formation of narrow minima in the electron velocity-space distribution resulting from stretching due to velocity dispersion of phase-space density minima that are initially localized in physical space (e.g., constant-density regions with temperatures greater than their surroundings). These velocity-space minima, which become narrower as they are dispersively stretched, eventually cross the threshold condition for a "notch" instability, which saturates by forming an expanding series of shallow phase-space holes and their associated weak bipolar fields. 1-D Vlasov-Poisson simulations show that this process can be a robust mechanism for generating a large ensemble of shallow holes. Simulations with different background electron distributions show that the properties of the holes that form depend sensitively on the characteristics of the embedding plasma environment. * Research supported by NSF, NASA, and DOE.
¹ J. R. Franz, et al., JGR, 110, doi:10.1029/2005JA011095 (2005).
² A. Mangeney, private communication.
³ M. V. Goldman, et al., this meeting.
⁴ R. E. Ergun, et al., PRL, 87, 045003 (2001); D. L. Newman, et al. PRL, 87, 255001 (2001).
DE: 2712 Electric fields (2411)
DE: 2772 Plasma waves and instabilities (2471)
DE: 7815 Electrostatic structures
DE: 7839 Nonlinear phenomena (4400, 6944)
SC: SPA-Magnetospheric Physics [SM]
MN: 2007 Joint Assembly