Fast growth of magnetic fields in galaxy clusters: a self‐accelerating dynamo

Abstract

AbstractWe propose a model of magnetic‐field growth in galaxy clusters whereby the field is ampli.fied by a factor of about 108 over a cosmologically short time of ∼ 108 yr. Our model is based on the idea that the viscosity of the intracluster medium during the field‐amplification epoch is determined not by particle collisions but by plasma microinstabilities: these give rise to small‐scale .uctuations, which scatter particles, increasing their effective collision rate and, therefore, the effective Reynolds number. This gives rise to a bootstrap effect as the growth of the field triggers the instabilities which increase the Reynolds number which, in turn, accelerates the growth of the field. The growth is explosive and the result is that the observed field strength is reached over a fraction of the cluster lifetime independent of the exact strength of the seed field (which only needs to be above ∼ 10–15 G to trigger the explosive growth). (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)