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Quasi-perpendicular Shock Structure and Processes

Imperial College London Now at Space and Atmospheric Physics, The Blackett Laboratory London UK
Space Science Reviews (Impact Factor: 5.87). 118(1):161-203. DOI: 10.1007/s11214-005-3827-0
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    ABSTRACT: A study is presented for the oblique propagation of low-frequency ion-acoustic (IA) shock waves in a magnetized plasma having cold viscous ion fluid and nonextensively distributed electrons. A weakly nonlinear analysis is carried out to derive a Korteweg de-Vries-Burger like equation. Dependence of the shock wave characteristics (height, width and nature) on plasma parameters is then traced and studied in details. We hope that our results will aid to explain and interpret the nonlinear oscillations occurring in magnetized space plasmas.
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    ABSTRACT: The free energy provided by the ion temperature anisotropy is considered to be the source of ion cyclotron waves in the downstream of a quasi-perpendicular shocks. Besides the proton cyclotron waves excited by the proton temperature anisotropy, He2 + is decelerated differentially from the protons by the shock due to its different charge-to-mass ratio, and forms a bunched ring-like distribution in the immediate downstream of the quasi-perpendicular shock. However, how the helium cyclotron waves associated with the anisotropic distribution of He2 + are excited, is still in debate. In this paper, with two-dimensional (2-D) hybrid simulations, we investigate He2 + dynamics and its role in the ion cyclotron waves downstream of quasi-perpendicular shocks (the proton plasma beta in the upstream is 0.4). A bunched ring-like distribution of He2 +is formed in the immediate downstream of the quasi-perpendicular shocks, then it evolve into a shell-like distribution. At last, a bi-Maxwellian distribution of He2 + is generated in the far downstream. In the medium and low Mach number shocks, besides the proton cyclotron waves excited near the shock front, there is another enhancement of the magnetic fluctuations in the downstream. The results show that the helium cyclotron waves can be driven directly by the bunched ring-like distribution of He2 + in a low or medium Mach number quasi-perpendicular shock. The relevance of our simulation results to the satellite observations is also discussed in this paper.
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