  We discuss three modes of oscillation of accretion disks around rotating
magnetized neutron stars which may explain the separations of the kilo-Hertz
quasi periodic oscillations (QPO) seen in low mass X-ray binaries. The
existence of these compressible, non-barotropic magnetohydrodynamic (MHD) modes
requires that there be a maximum in the angular velocity $\Omega_\phi(r)$ of
the accreting material larger than the angular velocity of the star $\Omega_*$,
and that the fluid is in approximately circular motion near this maximum rather
than moving rapidly towards the star or out of the disk plane into funnel
flows. Our MHD simulations show this type of flow and $\Omega_\phi(r)$ profile.
The first mode is a Rossby wave instability (RWI) mode which is radially
trapped in the vicinity of the maximum of a key function $g(r){\cal F}(r)$ at
$r_{R}$. The real part of the angular frequency of the mode is
$\omega_r=m\Omega_\phi(r_{R})$, where $m=1,2...$ is the azimuthal mode number.
The second mode, is a mode driven by the rotating, non-axisymmetric component
of the star's magnetic field. It has an angular frequency equal to the star's
angular rotation rate $\Omega_*$. This mode is strongly excited near the radius
of the Lindblad resonance which is slightly outside of $r_R$. The third mode
arises naturally from the interaction of flow perturbation with the rotating
non-axisymmetric component of the star's magnetic field. It has an angular
frequency $\Omega_*/2$. We suggest that the first mode with $m=1$ is associated
with the upper QPO frequency, $\nu_u$; that the nonlinear interaction of the
first and second modes gives the lower QPO frequency, $\nu_\ell =\nu_u-\nu_*$;
and that the nonlinear interaction of the first and third modes gives the lower
QPO frequency $\nu_\ell=\nu_u-\nu_*/2$, where $\nu_*=\Omega_*/2\pi$.
