A New Scenario for the Production of Weak Bipolar Fields in Space: "Notch" Instabilities Resulting From Electron Velocity Dispersion*

Details A New Scenario for the Production of Weak Bipolar Fields in Space: "Notch" Instabilities Resulting From Electron Velocity Dispersion* A New Scenario for the Production of Weak Bipolar Fields in Space: "Notch" Instabilities Resulting From Electron Velocity Dispersion*

May 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agusmsm53b..02n&link_type=abstract

American Geophysical Union, Spring Meeting 2007, abstract #SM53B-02

Other

2712 Electric Fields (2411), 2772 Plasma Waves And Instabilities (2471), 7815 Electrostatic Structures, 7839 Nonlinear Phenomena (4400, 6944)

Scientific paper

The bipolar signatures of weak (eφmax/Te ≪ 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).

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