Physics – Plasma Physics
Scientific paper
Jan 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992phdt........12v&link_type=abstract
Ph.D. Thesis Maryland Univ., College Park.
Physics
Plasma Physics
Acoustic Velocity, Discontinuity, Dispersing, Magnetohydrodynamic Waves, Nonlinear Equations, Nonlinearity, Plasma Physics, Plasmas (Physics), Wave Packets, Wave Propagation, Gyrofrequency, Low Frequencies, Magnetic Fields, Upstream
Scientific paper
This dissertation examines low frequency (approximately less than ion gyrofrequency) wave packets and rotational discontinuities (RD's) in a dispersive two-fluid plasma. A one dimensional numerical hybrid code with ion simulation particles and a massless electron fluid is used. The evolution, speed, and structure of strongly modulated wave packets, which propagate parallel to an external magnetic field, are investigated as a function of helicity, amplitude, beta(defined equal to 8 pi P/B(exp 2)), and beta(sub i)/beta(sub e) (= T(sub i)/T(sub e)). Gradients of the wave envelope are found to have a substantial influence on wave speeds and other properties. Weakly nonlinear packets show wave coupling with structuring that qualitatively resembles wave solutions of the derivative nonlinear Schrodinger (DNLS) equation for all beta and beta(sub i)/beta(sub e). Nonlinearity has its greatest impact on evolution when the initial packet speed and the linear sound speed of the plasma are equal. Strongly nonlinear Alfven (ion sense of helicity) wave packets do not undergo wave collapse, but do steepen for beta approximately less than 1 and form substantial fast (electron sense of helicity) wave regions on their leading sides. This evolution is shown to differ qualitatively from DNLS wave solutions. An extensive parameter variation on the evolution of RDs is perfromed with particular emphasis on beta, beta(sub i)/beta(sub e), theta(sub B) (the angle between the normal and total magnetic field), and the helicity of the RD. The RD structure is shown to have features in common with the evolution of both strongly modulated, nonlinear wave packets and linear dispersive wave propagation in oblique magnetic fields. For moderate theta(sub B) (approximately less than 45 deg), nonlinearity is important and strong coupling to a compressional (sonic) component can occur in the main current layer. When beta(sub i) = 0, there is a critical value of beta(sub e)(=beta(sub *)) when the intermediate wavetrain moves from the downstream side of the RD to the upstream, and is replaced on the downstream side by slow modes. This is reflected in the hodograms by a change of the wave polarization on both sides. As beta(sub i)/beta(sub e) increases, the spreading rate of the current layer increases for moderate theta(sub B). For large theta(sub B) (approximately greater than 45 deg), RDs with electron (ion) sense of rotation show increased (decreased) spreading with increasing beta(sub i)/beta(sub e). These results are applied to RDs observed in the solar wind and at the magnetopause.
Vasquez Bernard John
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