Title: Absolute versus convective helical magnetorotational instabilities in Taylor-Couette flows,
Abstract: We study magnetic Taylor-Couette flow in a system having nondimensional radii
$r_i=1$ and $r_o=2$, and periodic in the axial direction with wavelengths
$h\ge100$. The rotation ratio of the inner and outer cylinders is adjusted to
be slightly in the Rayleigh-stable regime, where magnetic fields are required
to destabilize the flow, in this case triggering the axisymmetric helical
magnetorotational instability (HMRI). Two choices of imposed magnetic field are
considered, both having the same azimuthal component $B_\phi=r^{-1}$, but
differing axial components. The first choice has $B_z=0.1$, and yields the
familiar HMRI, consisting of unidirectionally traveling waves. The second
choice has $B_z\approx0.1\sin(2\pi z/h)$, and yields HMRI waves that travel in
opposite directions depending on the sign of $B_z$. The first configuration
corresponds to a convective instability, the second to an absolute instability.
The two variants behave very similarly regarding both linear onset as well as
nonlinear equilibration.