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    ABSTRACT: [1] The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft at Mercury and the Cassini spacecraft at Saturn provide us with orbiters around planets at more diverse heliocentric distances than ever before. The dramatically different solar wind conditions at these two planets should mean that Mercury's bow shock is considerably weaker (lower Mach numbers) than Saturn's bow shock. This is expected to produce different magnetic overshoot amplitudes at each bow shock, because the Relative Overshoot Amplitude (ROA) has been shown to increase with both fast magnetosonic Mach number and upstream plasma β. We qualitatively compare the parameter regimes of Mercury's and Saturn's bow shock by determining ROAs. We analyze 133 MESSENGER encounters with Mercury's bow shock and 90 Cassini encounters with Saturn's bow shock, all with a clear shock ramp. At five of the 133 Mercury bow shock encounters, there is no resolvable magnetic overshoot, whereas all Saturn bow shock encounters have a clear overshoot. We find that the ROA of Mercury's bow shock ranges from ~0 (no overshoot) to ~0.6, with a typical value of ~0.2. We find that the ROA of Saturn's bow shock ranges from ~0.2 to ~5, with a typical value of ~2. This clear ROA difference is consistent with the expected lower fast magnetosonic Mach number and lower upstream plasma β at Mercury's bow shock, and we suggest that it is very likely to be primarily caused by the different Mach numbers. This confirmed variation in bow shock parameter regime may produce a different solar wind-magnetosphere interaction at these two planets.
    Journal of Geophysical Research: Space Physics 07/2013; 118(7). · 3.44 Impact Factor
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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.
    Journal of Geophysical Research: Space Physics 04/2014; · 3.44 Impact Factor
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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.
    Astrophysics and Space Science 05/2014; 351(1). · 2.40 Impact Factor

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May 16, 2014