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It's more math help than physics help I need here though. I know that I'll have to start with the dispersion relations for a right and left circularly polarized waves:

Note that capital omegas [tex]\Omega[/tex] represent cyclotron frequencies, small omegas [tex]\omega[/tex] represent plasma frequencies, and the i/e subscripts refer to electrons or ions. The [tex]\omega[/tex] with no subscript is the frequency of the propagating wave.

[tex] \frac{k_R^2 c^2}{\omega^2}=1-\frac{\omega_{pe}^2}{\omega (\omega - \Omega_e)}-\frac{\omega_{pi}^2}{\omega (\omega + \Omega_i)} [/tex]

[tex] \frac{k_L^2 c^2}{\omega^2}=1-\frac{\omega_{pe}^2}{\omega (\omega + \Omega_e)}-\frac{\omega_{pi}^2}{\omega (\omega - \Omega_i)} [/tex]

Note that I'm dealing with an Alfven wave, so when manipulating the above formulae, the following assumption can be made: [tex] \omega << \Omega_i << \Omega_e [/tex]

The angle that the wave's plane of polarization rotates through is

[tex] \Theta = \frac{(k_R-k_L)\Delta z}{2} [/tex]

The problem lies in finding the [tex] k_R-k_L [/tex] term from the first two equations listed.

Using the relationship [tex] \frac{\omega_{pi}^2}{\Omega_i} = \frac{\omega_{pe}^2}{\Omega_e} [/tex] I can simplify the term for the R-wave to get

[tex] \frac{k_R^2 c^2}{\omega^2} \approx 1-\frac{\omega}{\Omega_i}+\frac{\Omega_i^2}{\omega_{pi}^2} [/tex]

I know this is correct, but repeating the same process for the L-wave doesn't yield a term that can easily be combined with this one to get the [tex] k_R-k_L [/tex] expression I'm looking for.

If anyone could guide me through this, it would be much appreciated!