Physics
Scientific paper
Sep 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994jgr....9917351c&link_type=abstract
Journal of Geophysical Research (ISSN 0148-0227), vol. 99, no. A9, p. 17,351-17,366
Physics
45
Anisotropy, Ballooning Modes, Earth Magnetosphere, Magnetohydrodynamic Waves, Wave-Particle Interactions, Distribution Functions, Magnetosonic Resonance, Perturbation, Plasma Pressure
Scientific paper
We have carried out a theoretical analysis of the stability and parallel structure of coupled shear Alfven and slow magnetosonic waves in Earth's inner magnetopause (i.e., at equatorial distances between about five and ten Earth radii) including effects of finite anisotropic Grad-Shafranov equation yields an approximate self-consistent magnetohydrodynamic (MHD) equilibrium. This MHD equilibrium is used in the numerical solution of a set of eigenmode equations which describe the field line eigenfrequency, linear stability, and parallel eigenmode structure. We call these modes anisotropic Alfven-ballooning modes. The main results are: (1) The field line eigenfrequency can be significantly lowered by finite pressure effects. (2) The parallel mode structure of the transverse wave components is fairly insensitive to changes in the plasma pressure, but the compressional magnetic component can become highly peaked near the magnetic equator as a result of increased pressure, especially when P(sub perpendicular to) is greater than P(sub parallel) (here P(sub perpendicular to) and P(sub parallel) are the perpendicular and parallel plasma pressure). (3) For the isotropic (P(sub parallel) = P(sub perpendicular to) = P) case ballooning instability can occur when the ratio of the plasma presure to the magnetic pressure, β = P/(B squared/8 pi), exceeds a critical value βB0 is approximately equal to 3.5 at the equator. (4) Compared to the isotropic case the critical beta value is lowered by anisotropy, either due to decreased field line bending stabilization when P(sub parallel) is greater than P(sub perpendicular to) or due to increased ballooning-mirror destabilization when P(sub perpendicular to) is greater than P(sub parallel). (5) We use a β-δ stability diagram to display the regions of instability with respect to the equatorial values of the parameters bar β and δ, where bar β = (1/3)(βparallel) + 2 βperpendicular to) is an average β value and δ = 1 - Pparallel)/ Pperpendicular to) is a measure of the plasma anisotropy. The diagram is divided into regions corresponding to the firehose, mirror and ballooning instabilities. It appears that observed values of the plasma pressure are below the critical value for the isotropic ballooning instability but it may be possible to approach a ballooning-mirror instability when Pperpendicular to/Pparallel is greater than or approximately 2.
Chan Anthony A.
Chen Liu
Xia Mengfen
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