Theory of magnetohydrodynamic waves: The WKB approximation revisited
- NASA Ames Research Center, Moffett Field, CA (United States)
Past treatments of the eikonal or WKB theory of the propagation of magnetohydrodynamics waves have assumed a strictly isentropic background. IF in fact there is a gradient in the background entropy, then in second order in the WKB ordering, adiabatic fluctuations (in the Lagrangian sense) are not strictly isentropic in the Eulerian sense. This means that in the second order of the WKB expansion, which determines the variation of wave amplitude along rays, the violation of isentropy must be accounted for. The present paper revisits the derivation of the WKB approximation for small-amplitude magnetohydrodynamic waves, allowing for possible spatial variation of the background entropy. The equation of variation of wave amplitude is rederived; it is a bilinear equation which, it turns out, can be recast in the action conservation form. It is shown that this action conservation equation is in fact equivalent to the action conservation law obtained from Lagrangian treatments.
- OSTI ID:
- 6991576
- Journal Information:
- Journal of Geophysical Research; (United States), Journal Name: Journal of Geophysical Research; (United States) Vol. 97:A8; ISSN JGREA; ISSN 0148-0227
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
Ionospheric
& Magnetospheric Phenomena-- (1992-)
665510 -- Magnetohydrodynamics-- (1992-)
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
ACCELERATION
ADIABATIC PROCESSES
CONSERVATION LAWS
ENTROPY
HYDROMAGNETIC WAVES
PHYSICAL PROPERTIES
SOLAR ACTIVITY
SOLAR WIND
THERMODYNAMIC PROPERTIES
VARIATIONS
WAVE PROPAGATION
WKB APPROXIMATION