Signature of Field Reversals

— and its detectability in observations of other galaxies —

Coordinator: Marijke Haverkorn

Project Outline
For decades, it has been known observationally that the large-scale magnetic field of our Milky Way exhibits a reversal in direction just inside the Solar circle. This has been confirmed in numerous observational studies using diverse data and methods. This is not unexpected theoretically, since dynamo models show that meso-scale or large-scale reversals in magnetic field direction can be common features in disk dynamos. However, in external spiral galaxies, there is no convincing evidence that large-scale reversals exist at all. Naively, one would think that magnetic field reversals in galaxy disks would be visible as large-scale rotation measure (RM) structures in inclined galaxies. However, there is no firm evidence for this in any published RM map of a spiral galaxy.
The central question in this project is: is it possible that magnetic field reversals are indeed present in external spirals but observational effects preclude us to see them? To answer that question, we use the Hamurabi code to model various magnetic field configurations in galaxy disks to see if they can be rendered invisible due to certain physical conditions in the galaxy or observational selection effects. Reversals in the magnetic field direction might be hidden by e.g. a large turbulent component of the field, (vertical) halo fields, the ‘classical’ way of determining RM (eg from 3cm and 6cm in M51) instead of probing Faraday depths, or a combination of these. These are situations that we can model and test. The plan suggested at the ISSI meeting was to use the version of Hammurabi modified by Xiaohui Sun for external galaxies, and input a magnetic field distribution which exhibits a large-scale reversal. The magnetic field could come from several sources:

  • the Sun et al (2008) model of the Milky Way magnetic field, with a reversal along a spiral arm;
  • the MHD simulations of galaxies with a reversal performed by the Krakow group (e.g., Kulesza-Zydzik et al. 2010)
  • dynamo models with global reversals (Poezd et al 1993) or localized reversals (Bykov et al 1997), for a review see Shukurov (2005).