  Deep observations of nearby galaxy clusters with Chandra have revealed
concave 'bay' structures in a number of systems (Perseus, Centaurus and Abell
1795), which have similar X-ray and radio properties. These bays have all the
properties of cold fronts, where the temperature rises and density falls
sharply, but are concave rather than convex. By comparing to simulations of gas
sloshing, we find that the bay in the Perseus cluster bears a striking
resemblance in its size, location and thermal structure, to a giant
($\approx$50 kpc) roll resulting from Kelvin-Helmholtz instabilities. If true,
the morphology of this structure can be compared to simulations to put
constraints on the initial average ratio of the thermal and magnetic pressure,
$\beta= p_{\rm th} / p_{\rm B}$, throughout the overall cluster before the
sloshing occurs, for which we find $\beta=200$ to best match the observations.
Simulations with a stronger magnetic field ($\beta=100$) are disfavoured, as in
these the large Kelvin-Helmholtz rolls do not form, while in simulations with a
lower magnetic field ($\beta=500$) the level of instabilities is much larger
than is observed. We find that the bay structures in Centaurus and Abell 1795
may also be explained by such features of gas sloshing.
