  We analyze the quasinormal mode spectrum for Kerr black holes surrounded by
an asymptotically uniform magnetic field, modeled with the Ernst-Wild geometry.
A perturbative expansion in both the rotation parameter $a$ and the magnetic
field $B$ allows separation of the perturbation equations, and we obtain the
spectrum for a variety of scalar quasinormal modes over a range of parameters
using the continued fraction method. We then interpolate the low-lying mode
spectrum to construct an Ernst-Wild template for the ringdown, and use the
LIGO-Virgo-KAGRA analysis tool pyRing to assess the impact of the magnetosphere
on the extraction of ringdown signatures from several observed binary black
hole mergers.
