Annu. Rev. Astron. Astrophys. 2002. 40: 319-348 Copyright © 2002 by Annual Reviews. All rights reserved

8. SYNTHESIS

In Table 1 we summarize the cluster magnetic field measurements. Given the limitations of the current instrumentation, the limited number of sources studied thus far, and the myriad physical assumptions involved with each method, we are encouraged by the order-of-magnitude agreement between cluster field strengths derived from these different methods. Overall, the data are consistent with cluster atmospheres containing ~ µG fields, with perhaps an order of magnitude scatter in field strength between clusters, or within a given cluster.

Table 1. Cluster Magnetic Fields

Method	Strength µG	Model Parameters
Synchrotron Halos	0.4 - 1	minimum energy, k = = 1,
		low = 10MHz, high = 10 GHz
Faraday rotation (embedded)	3 - 40	cell size = 10 kpc
Faraday rotation (background)	1 - 10	cell size = 10 kpc
Inverse Compton	0.2 - 1	= -1, radio ~ 18000, xray ~ 5000
Cold Fronts	1 - 10	amplification factor ~ 3
GZK	> 0.3	AGN = site of origin for EeV CRs

The rotation measure observations of background radio sources, and in particular the observations of a complete X-ray selected sample of clusters by Clarke, Kronberg, & Böhringer 2001 dictate that µG magnetic fields with high areal filling factors are a standard feature in clusters, and that the fields extend to large radii (0.5 Mpc or more). The rotation measure observations of extended radio galaxies embedded in clusters impose order on the fields, with coherence scales of order 10 kpc, although larger scale coherence in overall RM sign can be seen in some sources. Observations of inverse Compton emission from a few clusters with radio halos provide evidence against much stronger, pervasive and highly tangled fields.

In most clusters the fields are not dynamically important, with magnetic pressures one to two orders of magnitude below thermal gas pressures. But the fields play a fundamental role in the energy budget of the ICM through their effect on heat conduction, as is dramatically evident in high resolution X-ray observations of cluster cold fronts.

If most clusters contain µG magnetic fields, then why don't most clusters have radio halos? The answer may be the short lifetimes of the relativistic electrons responsible for the synchrotron radio emission (see Section 4.1). Without re-acceleration or injection of relativistic electrons, a synchrotron halo emitting at 1.4 GHz will fade in about 10⁸ years due to synchrotron and inverse Compton losses. This may explain the correlation between radio halos and cluster mergers, and the anti-correlation between radio halos and clusters with relaxed X-ray morphologies. In this case, the fraction of clusters with radio halos should increase with decreasing survey frequency.

The existence of µG-level fields in cluster atmospheres appears well established. The challenge for observers now becomes one of determining the detailed properties of the fields, and how they relate to other cluster properties. Are the fields filamentary, and to what extent do the thermal and non-thermal plasma mix in cluster atmospheres? What is the radial dependence of the field strength? How do the fields depend on cluster atmospheric parameters, such as gas temperature, metalicity, mass, substructure, or density profile? How do the fields evolve with cosmic time? And do the fields extend to even larger radii, perhaps filling the IGM? The challenge to the theorists is simpler: how were these field generated? This topic is considered briefly in the next section.