# Extragalactic Magnetic Fields - General Information That I'm Missing

I have started studying Extragalactic Magnetic Fields, and mainly focusing on Biermann Battery Theory. I have gathered much information on other Dynamo Theories, as well as about our current Observational and Theoretical status on Magnetic Field Genesis.

What I'm looking for though, is a general "picture" on Extragalactic Magnetic Fields, as a whole. Faraday Rotation, AGN Jets, are basically the topics I have tracked, but I'm pretty sure there are a lot of others.

So I'm asking, if someone has studied in-depth extragalactic structures and magnetic field, possibly could guide me on some of the main and hot topics around them. This way I would have more knowledge on the topic which would be great for me!

I mean just Topic-Titles, would be great!

• What do you mean by "Magnetic Field Genesis"? – George Nov 25 '17 at 10:12
• @George The mechanism which supplies your system with a "Seed" Magnetic Field, from zero initial currents. – user174411 Nov 25 '17 at 16:35
• So something like magnetic field formation. – George Nov 25 '17 at 16:39
• @George Exactly – user174411 Nov 25 '17 at 17:05

On this question, the Widrow (2002) review is still pretty up-to-date. You can also have a look on the introduction of Beck (2013), a recent PhD thesis on the subject with a straightfoward introduction to the subject.

The main questions remain the same for decades now:

• What is the origin of the magnetic seed fields?
• How to amplify magnetic field up to observed values in the local Universe?
• How to get large scale magnetic field, in particular if the magnetic seed fields are produced locally?

# Origin of the magnetic seed fields

Where do we start? That is no easy question, with a diversity of answers. One of the most standard ones is the Biermann battery you mentioned (one significant problem with Biermann batteries is the timescale needed to amplify such small fields; see below) in the early Universe. Another one is that the magnetic field can be generated at small scales by first generations of AGN and supernovæ and then distributed at larger scales (dynamo can help but distribution at larger scale can still be an issue; see below). Lastly, you can have some exotic processes to generate seed fields in the first instants after the Big Bang, typically linked to inflation processes and such.

# Magnetic field amplification

If there is an initial magnetic field — which is not so easy to demonstrate — there are few mechanisms that could amplify magnetic fields efficently enough to explain the observed magnetic field. The most standard one is the $$\alpha-\omega$$ dynamo, combining a twist of the magnetic field lines in a differentially rotating disk with turbulent motions in the ISM that allows a regeneration of the magnetic fields (supernovæ explosions, stellar winds and other instabilities could do the work). Another possible mechanism is the magneto-rotational instability (MRI), that can easily generate a significant, large-scale field from almost any given seed field. There are other candidates such as Parker instability, or other flavors of dynamos.

A central question then is of timescale: if these mechanisms can generate magnetic fields, the needed timescale to amplify a seed field to the current value may be far larger than the age of the Universe.

# Distribution

Magnetic field is everywhere, not only in the ISM and at larger scale in galaxies, but also in galaxy clusters, in the IGM and so on. If the origin of the magnetic seed fields is of cosmic origin (exotic processes in the early Universe), you can explain the observed distribution; if not, things get tricky again. Turbulence, galaxy collisions and mergers may be of help, but the question is still largely open.

• Really appreciated dude ^_^ – user174411 Nov 28 '17 at 9:12
• Could you possible propose me any $\alpha - \omega$ dynamo paper? It's the only one I have not understand, and the review paper doesn't really explain it ( All the other subjects are very much explained and it really REALLY helped me :) ) – user174411 Jan 1 '18 at 15:03
• A good starting point may be a review by Parker himself, or one of his papers focusing specifically on dynamo: Parker 1970. It focuses on stellar magnetic fields but that is where the idea came from in his seminal paper of 1955 (Parker 1955). – MBR Jan 2 '18 at 11:06
• holy, really thank you, seems very nice. Honestly, if it was a publication I would legit acknowledge you. You have really helped me understanding these topics :) – user174411 Jan 2 '18 at 16:34