Abstract:
We explore the decay of turbulence and magnetic fields generated by fluctuation dynamo
action in the context of galaxy clusters where such a decaying phase can occur in the aftermath
of a major merger event. Using idealized numerical simulations that start from a kinetically
dominated regime we focus on the decay of the steady state rms velocity and the magnetic
field for a wide range of conditions that include varying the compressibility of the flow, the
forcing wavenumber, and the magnetic Prandtl number. Irrespective of the compressibility of
the flow, both the rms velocity and the rms magnetic field decay as a power law in time. In the
subsonic case we find that the exponent of the power law is consistent with the
−
3/5 scaling
reported in previous studies. However, in the transonic regime both the rms velocity and the
magnetic field initially undergo rapid decay with an
≈
t
−
1.1
scaling with time. This is followed
by a phase of slow decay where the decay of the rms velocity exhibits an
≈−
3/5 scaling in
time, while the rms magnetic field scales as
≈−
5/7. Furthermore, analysis of the Faraday
rotation measure (RM) reveals that the Faraday RM also decays as a power law in time
≈
t
−
5/7
;
steeper than the
∼
t
−
2/5
scaling obtained in previous simulations of magnetic field decay in
subsonic turbulence. Apart from galaxy clusters, our work can have potential implications in
the study of magnetic fields in elliptical galaxies.
