# Analyzing Azimuthal Distortions

We will adopt the notation of J. E. Tohline & I. Hachisu (1988, ApJ, 361, 394). Specifically, drawing on their equation (2) but ignoring variations in the vertical coordinate, the mass density is given by the expression,

 $~\rho$ $~=$ $~\rho_0 \biggl[ 1 + f(\varpi)e^{-i(\omega t - m\phi)} \biggr] \, ,$

where it is understood that $~\rho_0$, which defines the structure of the initial axisymmetric equilibrium configuration, is generally a function of the cylindrical radial coordinate, $~\varpi$.

Using the subscript, $~m$, to identify the time-invariant coefficients and functions that characterize the intrinsic eigenvector of each azimuthal eigen-mode, and acknowledging that the associated eigenfrequency will in general be imaginary, that is,

 $~\omega_m$ $~=$ $~\omega_R + i\omega_I \, ,$

we expect each unstable mode to display the following behavior:

 $~\biggl[ \frac{\rho}{\rho_0} - 1 \biggr]$ $~=$ $~f_m(\varpi)e^{-i[\omega_R t + i \omega_I t - m\phi_m(\varpi)]}$ $~=$ $~\biggl\{ f_m(\varpi)e^{-im\phi_m(\varpi)}\biggr\} e^{-i\omega_R t } \cdot e^{\omega_I t}$ $~=$ $~\biggl\{ f_m(\varpi)e^{-i[\omega_R t + m\phi_m(\varpi)]} \biggr\} e^{\omega_I t} \, .$

Adopting Kojima's (1986) notation, that is, defining,

 $~y_1 \equiv \frac{\omega_R}{\Omega_0} - m$ and $~y_2 \equiv \frac{\omega_I}{\Omega_0} \, ,$

the eigenvector's behavior can furthermore be described by the expression,

 $~\biggl[ \frac{\rho}{\rho_0} - 1 \biggr]$ $~=$ $~\biggl\{ f_m(\varpi)e^{-i[(y_1+m) (\Omega_0 t) + m\phi_m(\varpi)]} \biggr\} e^{y_2 (\Omega_0 t)}$ $~=$ $~\biggl\{ f_m(\varpi)e^{-im[(y_1/m+1) (\Omega_0 t) + \phi_m(\varpi)]} \biggr\} e^{y_2 (\Omega_0 t)} \, .$

Note that, as viewed from a frame of reference that is rotating with the mode pattern speed,

$\Omega_p \equiv \frac{\omega_R}{m} = \Omega_0\biggl(\frac{y_1}{m}+1\biggr) \, ,$

we should find an eigenvector of the form,

 $~\biggl[ \frac{\rho}{\rho_0} - 1 \biggr]_\mathrm{rot} \equiv \biggl[ \frac{\rho}{\rho_0} - 1 \biggr]e^{im\Omega_p t}$ $~=$ $~\biggl\{ f_m(\varpi)e^{-im[\phi_m(\varpi)]} \biggr\} e^{y_2 (\Omega_0 t)} \, ,$

which is unchanging (inside the curly braces) except for a uniform exponential amplitude growth.