Browsing by Author "Shukurov, A."

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Evolving turbulence and magnetic fields in galaxy clusters
(2006-01-10) Subramanian, Kandaswamy; Shukurov, A.; Haugen, N. E. L.
We discuss, using simple analytical models and MHD simulations, the origin and parameters of turbulence and magnetic ﬁelds in galaxy clusters. Any pre-existing tangled magnetic ﬁeld must decay in a few hundred million years by generating gas motions even if the electric conductivity of the intracluster gas is high. We argue that tur- bulent motions can be maintained in the intracluster gas and its dynamo action can prevent such a decay and amplify a random seed magnetic ﬁeld by a net factor typically 10⁴ in 5Gyr. Three physically distinct regimes can be identiﬁed in the evolution of turbulence and magnetic ﬁeld in galaxy clusters. Firstly, the ﬂuctuation dynamo will produce microgauss-strong, random magnetic ﬁelds during the epoch of cluster formation and major mergers. At this stage pervasive turbulent ﬂows with r.m.s. velocity of about 300 kms−ᶥ can be maintained at scales 100–200 kpc. The magnetic ﬁeld is intermittent, has a smaller scale of 20–30 kpc and average strength of 2 G. Secondly, turbulence will decay after the end of the major merger epoch; we discuss the dynamics of the decaying turbulence and the behavior of magnetic ﬁeld in it. Magnetic ﬁeld and turbulent speed undergo a power-law decay, decreasing by a factor of two during this stage, whereas their scales increase by about the same factor. Thirdly, smaller-mass subclusters and cluster galaxies will produce turbulent wakes where magnetic ﬁelds will be generated as well. Although the wakes plausibly occupy only a small fraction of the cluster volume, we show that their area covering factor can be close to unity, and thus they can produce some of the signatures of turbulence along virtually all lines of sight. The latter could potentially allow one to reconcile the possibility of turbulence with ordered ﬁlamentary gas structures, as in the Perseus cluster. The turbulent speeds and magnetic ﬁelds in the wakes are estimated to be of order 300 kms−ᶥ and 2 G, respectively, whereas the turbulent scales are of order 200 kpc for wakes behind subclusters of a mass 3 × 10ᶥᶟM⊙ and about 10 kpc in the galactic wakes. Magnetic ﬁeld in the wakes is intermittent and has the scale of about 30 kpc and 1 kpc in the subcluster and galactic wakes, respectively. Random Faraday rotation measure is estimated to be typically 100–200 radm−², in agreement with observations. We predict detectable polarization of synchrotron emission from cluster radio halos at wavelengths 3–6 cm, if observed at suﬃciently high resolution.
Galactic dynamo and helicity losses through fountain flow
(2006-01-19) Shukurov, A.; Sokoloff, Dmitry; Subramanian, Kandaswamy
Nonlinear behaviour of galactic dynamos is studied, allowing for magnetic helicity removal by the galactic fountain ﬂow. Methods. A suitable advection speed is estimated, and a one-dimensional mean-ﬁeld dynamo model with dynamic α-effect is explored. Results. It is shown that the galactic fountain ﬂow is eﬃcient in removing magnetic helicity from galactic discs. This alleviates the constraint on the galactic mean-ﬁeld dynamo resulting from magnetic helicity conservation and thereby allows the mean magnetic ﬁeld to saturate at a strength comparable to equipartition with the turbulent kinetic energy.
Galactic dynamos supported by magnetic helicity fluxes
(2007-03-08) Sur, Sharanya; Shukurov, A.; Subramanian, Kandaswamy
We present a simple semi-analytical model of nonlinear, mean-ﬁeld galactic dynamos and use it to study the effects of various magnetic helicity ﬂuxes. The dynamo equations are reduced using the ‘no-z’ approximation to a nonlinear system of ordinary differential equations in time; we demonstrate that the model reproduces accurately earlier results, including those where nonlinear behaviour is driven by a magnetic helicity ﬂux. We discuss the implications and interplay of two types of magnetic helicity ﬂux, one produced by advection (e.g., due to the galactic fountain or wind) and the other, arising fromanisotropy of turbulence as suggested by Vishniac & Cho (2001). We argue that the latter is signiﬁcant if the galactic differential rotation is strong enough: in ourmodel, forRω . −10 in terms of the corresponding turbulent magnetic Reynolds number. We conﬁrm that the intensity of gas outﬂow from the galactic disc optimal for the dynamo action is close to that expected for normal spiral galaxies. The steady-state strength of the large-scale magnetic ﬁeld supported by the helicity advection is still weaker than that corresponding to equipartition with the turbulent energy. However, the Vishniac-Cho helicity ﬂux can boost magnetic ﬁeld further to achieve energy equipartition with turbulence. For stronger outﬂows that may occur in starburst galaxies, the Vishniac-Cho ﬂux can be essential for the dynamo action. However, this mechanism requires a large-scale magnetic ﬁeld of at least≃ 1 Gto be launched, so that it has to be preceded by a conventional dynamo assisted by the advection of magnetic helicity by the fountain or wind.
Origin and evolution of cluster magnetism
(2006-04-02) Shukurov, A.; Subramanian, Kandaswamy; Haugen, N. E. L.
Random motions can occur in the intergalactic gas of galaxy clusters at all stages of their evolution. Depending on the poorly known value of the Reynolds number, these motions can or cannot become turbulent, but in any case they can generate random magnetic ﬁelds via dynamo action. We argue that magnetic ﬁelds inferred observationally for the intracluster medium require dynamo action, and then estimate parameters of random ﬂows and magnetic ﬁelds at various stages of the cluster evolution. Polarization in cluster radio halos predicted by the model would be detectable with the SKA.