Beat, modulational, and decay instabilities of a circularly polarized Alfven wave

Journal Article · Thu Dec 01 04:00:00 EST 1994 · Journal of Geophysical Research

DOI:https://doi.org/10.1029/94JA02185· OSTI ID:91323

Hollweg, J V ^[1]

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1. Univ. of New Hampshire, Durham, NH (United States)

A circularly polarized low-frequency electromagnetic pump wave propagating along an ambient magnetic field is known to be unstable to the growth of several parallel-propagating parametric instabilities. If ion-cyclotron effects are retained in a two-fluid description, the dispersion relation is a sixth-order polynomial. The author presents a series of new analytical approximations to this dispersion relation. He emphasizes new results for the beat instability that occurs as an interaction of the forward propagating upper sideband with the backward propagating lower sideband. The nature of the beat instability depends on {beta} = (v{sub sound}/v{sub A}){sup 2} and on the sense of polarization of the pump wave. The beat and decay instabilities can occur together if the pump is left-handed (i.e., ion resonant) and if {beta} {approx_lt} 1, but they cannot occur together if the pump is right-handed. For a left-handed pump the beat mode is the only instability if {beta}{approx_lt}1. If the pump is right-handed and {beta} {approx_gt} 1, then the beat instability exists only when the pump amplitude exceeds a threshold value, and the beat will be the only instability if the pump amplitude is large enough to stabilize the modulational instability. If the pump is left-handed and {beta} {approx_lt} 1, then the beat mode is stabilized when the pump amplitude becomes sufficiently large. The beat instability primarily produces a forward propagating transverse wave in the upper sideband. Thus if {beta} {approx_lt} 1, the instabilities considered here do not produce the backward propagating waves which are thought to affect turbulence and the evolution of cross helicity in the solar wind. New analytical results are presented also for the decay and modulational instabilities when {beta} {approx} 1. 40 refs., 18 figs.

• OSTI ID: 91323
• Journal Information: Journal of Geophysical Research, Journal Name: Journal of Geophysical Research Journal Issue: A12 Vol. 99; ISSN JGREA2; ISSN 0148-0227
• Country of Publication: United States
• Language: English

Related Subjects

66 PHYSICS
ALFVEN WAVES
DECAY INSTABILITY
PARAMETRIC INSTABILITIES
POLARIZATION