# Talk:Galactic magnetic fields

The article is a nice and succinct introduction to the field. The main improvements would be to add a small number of references (see point 5) and more informative figure captions (point 6).

1. Introduction. The meaning of 'total', 'regular' and 'turbulent'/'random' magnetic field should be defined more precisely, along with their relation to the different observable quantities and the resolution of the telescope. I think it is preferable to refer to random rather than turbulent magnetic fields: tangling of field lines does not necessarily have to be caused by a classical turbulent flow. It could still be mentioned that compression or shear can produce anisotropy in a turbulent magnetic fluid.

'B-vectors' should be defined; my preference would be to not use the term B-vectors and just mention that the angle of the plane polarization is orthogonal to the magnetic field direction in the absence of Faraday rotation (short wavelength limit).

2. The Origin of Galactic Magnetic Fields. An additional problem for the primordial theory is finite pitch angle of the field: the field lines are not toroidal but spiral (as mentioned later in the article). The problems of the primordial theory call for a "mechanism to sustain and organise the magnetic field" (this does not HAVE to be a dynamo). Second last sentence: "...axisymmetric symmetry..." is a bit too symmetric. Last sentence: I suggest to reword as "These modes can be identified from the pattern of polarization angles and Faraday rotation in multi-wavelength observations."

3. Magnetic Field Strengths in Galaxies. The equipartition assumption may not be well founded at local scales in a galactic disc, but there is support for the field strength estimates from Faraday rotation estimates in the Milky Way and a few nearby galaxies. I do not think that "massive" is the right term for galaxies like M51 and NGC6946 in comparison to M31 --- how about "gas rich" or "galaxies with higher star forming rates".

4. Magnetic field structures. "M31 hosts a largely axisymmetric field" (not dominating) --- is it reallly true that axisymmetry is known to r=25 kpc? (Or just the presence of a magnetic field?) The lack of clear patterns in Faraday rotation in many galaxies may also be due to inadequate search techniques --- it is very difficult to separate by eye the superposition of 2 or 3 Fourier modes in noisy data!

5. A small number of references at the end would be very helpful: this is a vast topic and the article is heavily weighted towards the investigation of galactic magnetism via radio astronomy. Some review papers from the author's website could be linked as well as a 1996 article in Annual Reviews (available online via NED at http://nedwww.ipac.caltech.edu/level5/araa.html).

6. Some description in the figure captions about what data is being shown and a brief note of some key features that can be recognised would be very helpful; for example in Fig. 3 explain that M31 is highly inclined; that the radio emission is concentrated in a ring centered on a radius of 10 kpc; that the regular magnetic field can be seen to be highly ordered on scales of several kpc. There should also be full refernces or credits as the images could be easily copied for use elsewhere.

Review of " Galactic Magnetic Fields" by R. Beck (second reviewer)

The text is well written and informative. I have only a few minor comments:

Usage of "bold face" in the beginning of introduction looks strange.

against gravitation --> against gravity.

spiralling around interstellar magnetic fields -->
spiralling around interstellar magnetic field lines.

over a large radio wavelength range -->
over a large range of radio wavelengths

"Faraday rotation" in italics could well be a reference to another web page

I'd remove the plus/minus in connection with the 180 degrees ambiguity.
This is an ambiguity between 0 and 180, not between -180 and +180.

I believe the following suggested replacement represents better what is meant:
Such a "primordial" field is hard to maintain because -->
Explaining this as a "primordial" field is difficult because ....

I suggest to replace electromagnetic energy by magnetic energy, because
electric fields do not play an active role in the non-relativistic case.

non-homogeneous --> non-uniform

The expressions "regular magnetic field" and "total magnetic field" should
be explained and synonyms such as large scale or mean magnetic fields
could be introduced. Likewise for the fluctuating magnetic field.
This involves the definition of an average which may not be very precise.
It may be sufficient to say that it is customary to separate the magnetic
field into a mean or regular component and fluctuating or turbulent one.
Their sum is referred to as the total field.

Some 10 or more references should be given, just like in regular reviews.