Solitary Structure Formation on Auroral Field Lines

Details Solitary Structure Formation on Auroral Field Lines Solitary Structure Formation on Auroral Field Lines

Jan 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........15m&link_type=abstract

Thesis (PH.D.)--DARTMOUTH COLLEGE, 1995.Source: Dissertation Abstracts International, Volume: 56-08, Section: B, page: 4386.

Physics

Ion Beams

Scientific paper

The formation of ion acoustic solitary structures driven by electron and ion beams in the auroral acceleration region is studied using two-dimensional electrostatic particle simulations. The beams are consistently present in regions of moderate potential drop (<=1 keV) where weak double layers have been observed on both the S3-3 and Viking spacecraft. The presence of more than one ion species introduces the ion two-stream besides the ion acoustic instability into the system and modifies previous analysis and simulation results of solitary wave formation. Solitary structures form as a result of the microinstability development. The numerical simulation results show that positively peaked (phi > 0) localized structures are formed in the system driven by a dense (n_ {ib}~ nic~ ne/2) ion beam. The solitary waves move in the direction of the ion beam velocity. By contrast, negative potential solitary structures form when the ion beam density is reduced to 10% (n_ {ib}~ 0.1ne) and electron drift relative to background ions is sustained by an applied electric field. In this case, solitary waves drift downward at subsonic speeds relative to the background ions, which may carry the localized pulses upward. Evolving solitary waves do not carry any significant net potential drop and therefore cannot contribute much to the auroral particle acceleration. They are found to be a consequence of the larger scale V-shaped potential distribution in the auroral region. A reduced low-frequency magnetohydrodynamic description is developed for kinetic Alfven waves with finite ion Larmor radius. These waves play an important role in coupling between the equatorial magnetosphere, where they are generated, and the auroral acceleration region, where the solitary waves studied in the thesis are found to occur. The finite Larmor radius effects are usually disregarded. However, under magnetospheric conditions, the ion temperature cannot be considered negligible, since T_i >= Te. The finite ion gyroradius is accounted for by the ion stress tensor, calculated in the thesis by taking moments of the Vlasov equation and subsequent expansion into an omega /Omegai power series. Both one- and two-fluid nonlinear MHD systems of equations are shown to adequately describe kinetic Alfven waves in a uniform low-beta plasma, beta << 1. The one-fluid set of equations is shown to conserve the energy of the plasma system. The two-fluid system of magnetohydrodynamic equations allows one to construct exact nonlinear analytical solutions.

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