Turbulent magnetic fields in merging clusters: A case study of Abell
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  Kelvin-Helmholtz Instabilities (KHI) along contact discontinuities in galaxy
clusters have been used to constrain the strength of magnetic fields in galaxy
clusters, following the assumption that, as magnetic field lines drape around
the interface between the cold and hot phases, their magnetic tension resists
the growth of perturbations. This has been observed in simulations of rigid
objects moving through magnetised media and sloshing galaxy clusters, and then
applied in interpreting observations of merger cold fronts. Using a suite of
MHD simulations of binary cluster mergers, we show that even magnetic field
strengths stronger than yet observed ($\beta = P_{\rm th}/P_B = 50$) show
visible KHI features. This is because our initial magnetic field is tangled,
producing Alfven waves and associated velocity fluctuations in the ICM;
stronger initial fields therefore seed larger fluctuations, so that even a
reduced growth rate due to magnetic tension produces a significant KHI. The net
result is that a stronger initial magnetic field produces more dramatic
fluctuations in surface brightness and temperature, not the other way around.
We show that this is hard to distinguish from the evolution of turbulent
perturbations of the same initial magnitude. Therefore, in order to use
observations of KHI in the ICM to infer magnetic field strengths by comparing
to idealized simulations, the perturbations which seed the KHI must be
well-understood and (if possible) carefully controlled.
