Anisotropic Alfven-ballooning modes in Earth`s magnetosphere

Journal Article · Thu Sep 01 04:00:00 EDT 1994 · Journal of Geophysical Research

DOI:https://doi.org/10.1029/93JA03353· OSTI ID:166263

Chan, A A ^[1]; Xia, M ^[2]; Chen, L ^[3]

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1. Rice Univ., Houston, TX (United States)
2. Peking Univ., Beijing (China)
3. Univ. of California, Irving, CA (United States)

The authors have carried out a theoretical analysis of the stability and parallel structure of coupled shear Alfven and slow magnetosonic waves in Earth`s inner magnetosphere (i.e., at equatorial distances between about five and ten Earth radii) including effects of finite anisotropic plasma pressure. Multiscale perturbation analysis of the 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. The authors 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 in 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{perpendicular}>P{parallel} (here P{perpendicular} and P{parallel} are the perpendicular and parallel plasma pressure). (3) For the isotropic (P{parallel}=P{perpendicular}=P) case ballooning instability can occur when the ratio of the plasma pressure to the magnetic pressure exceeds a critical value {beta}{sub 0}{sup B}{approx} 3.5 at the equator. (4) Compared to the isotropic case the critical beta value is lowered by anisotropy. (5) The authors use a {beta}-{delta} stability diagram to display the regions of instability with respect to the equatorial values of the parameters {beta} and {delta}, where {beta} = (1/3)({beta}{parallel}+2{beta}{perpendicular}) is an average beta value and {delta}=1-P{parallel}/P{perpendicular} is a measure of the plasma anisotropy. The diagram is divided into regions corresponding to the firehose, mirror and ballooning instabilities. 31 refs., 8 figs.

• OSTI ID: 166263
• Journal Information: Journal of Geophysical Research, Journal Name: Journal of Geophysical Research Journal Issue: A9 Vol. 99; ISSN JGREA2; ISSN 0148-0227
• Country of Publication: United States
• Language: English

Related Subjects

66 PHYSICS
ALFVEN WAVES
ANISOTROPY
BALLOONING INSTABILITY
EARTH MAGNETOSPHERE
MAGNETOACOUSTIC WAVES
MHD EQUILIBRIUM
STABILITY