http://arxiv.org/abs/2211.03476
Spiral galaxies, including the Milky Way, have large-scale magnetic fields with significant energy densities. The dominant theory attributes these magnetic fields to a large-scale dynamo. We review the current status of dynamo theory and discuss various numerical simulations designed to explain either particular aspects of the problem or to reproduce galactic magnetic fields globally. Our main conclusions can be summarized as follows. (i) Idealized direct numerical simulations produce mean magnetic fields, whose saturation energy density tends to decline with increasing magnetic Reynolds number. This could imply that the observed large-scale galactic magnetic fields might not entirely originate from a mean-field dynamo. Much of the current numerical effort is focused on this unsolved problem. (ii) Small-scale dynamos are important throughout a galaxy’s life, and probably provide strong seed fields at early stages. (iii) Large-scale galactic magnetic fields of microGauss strengths can probably only be explained if helical magnetic fields of small or moderate length scales can rapidly be ejected or destroyed. (iv) The circumgalactic medium (CGM) may play an important role in driving dynamo action at small and large length scales. These interactions between the galactic disk and the CGM may be a key aspect in understanding galactic dynamos. We expect future research in galactic dynamos to focus on the cosmological history of galaxies and the interaction with the CGM as means of replacing the idealized boundary conditions used in earlier work.
A. Brandenburg and E. Ntormousi
Tue, 8 Nov 22
65/79
Comments: 46 pages, 15 figures, 4 tables, comments welcome!