Other
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmsm13a1116x&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #SM13A-1116
Other
2730 Magnetosphere: Inner, 2744 Magnetotail, 2764 Plasma Sheet, 3215 Instability Analysis
Scientific paper
The interchange instability analysis of Xing and Wolf [2007] has been generalized to the case of arbitrary plasma β, where β is the ratio of plasma thermal pressure to magnetic pressure, averaged within a magnetic flux tube. The theoretical model focuses on a boundary layer between two plasma regions with uniform entropy parameters PV5/3 on each side (where P is the plasma thermal pressure and V=\int ds/B is the flux tube volume); the gradients of PV5/3 and V are at an angle α to each other in the boundary layer. The development of a small perturbation was investigated numerically and the results suggest that shorter wavelength (wave length λ compared with boundary layer thickness 2Δ) perturbations have a wider rage of stability in the plasma boundary layer than the longer wavelength perturbations do. For example when λ=100Δ, the plasma with β<<1 is unstable when α>π/2, but when λ=10Δ, the unstable angles are α>1.73. When λ=Δ, the unstable range goes to α>2.38, and the threshold angle keep getting closer to π as the wavelength decreases. The results for higher magnetic β values indicate more instability. The test calculation for λ=100Δ with β=10 shows that the plasma is unstable when α>1.03; for λ=10Δ the unstable range is α>1.45; and when λ=Δ, the unstable range becomes α>2.20. These critical angles are all reduced compare to the β<<1 cases. The overall conclusion is that increasing either β or wavelength increases the region of instability. Combined with the characteristic of the Earth's plasma sheet calculated from empirical models, our criterion shows that the statistical-average plasma sheet is interchange stable.
Wolf Robert
Xing Xiangjun
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