Publication Abstracts

Howard and Kulsrud 1997

Howard, A.M., and R.M. Kulsrud, 1997: The evolution of a primordial galactic magnetic field. Astrophys. J., 483, 648-655, doi:10.1086/304262.

We consider the hypothesis that galactic magnetic fields are primordial. We also discuss the various objections to this hypothesis. To test this hypothesis properly we assume that there was a magnetic field present in the galactic plasma before the galaxy formed and explore how such a field would evolve assuming a specific model for the interstellar medium in the galactic disk. After the galactic disk formed, the lines of force thread through it and remain connected to the external cosmic medium. They enter through one side of the disk, proceed horizontally a distance l in the disk, and then leave through the other side. We find that the lines of force are stretched by the differential rotation of the galactic disk, amplifying the toroidal component of the field and increasing l. When the magnetic field is strong enough, it produces ambipolar velocities that try to lift the line out of the galactic disk but in opposite directions on different parts of the line. The result is that, instead of the line being expelled from the disk, its horizontal length l is shortened, both in the radial and in the toroidal direction. This leads to a reduction of the rate of horizontal stretching and finally a reduction in the magnetic field strength. After a sufficient time, the magnetic field at all points goes through this reduction and the field strength approaches a universal function of time. . This function is slowly decreasing and only depends on the ambipolar properties of the interstellar medium. At any given time the magnetic field is toroidal and has the same strength everywhere. On the other hand, it turns out that its direction varies rapidly with radius, changing sign every 100 parsecs or so. However, if the initial cosmic magnetic field is not uniform, the areas of one sign of the toroidal field dominate over the other. The resulting field has a net Faraday rotation. If such a field were observed with low resolution in an external galaxy, then the field would appear toroidal in between the spiral arms. The spiral density wave would turn it so that the lines appear to trace out the spiral arm, although the apparent lines really are the sum of pieces of magnetic lines as they cross the disk. They do not necessarily extend very far along the arms. We contend that this model of the magnetic field, which arises naturally from a primordial origin, can fit the observations as well as other models for the magnetic field, such as those arising from the mean field dynamo theory. Finally, because the field lines are topologically threaded through the disk, they cannot be expelled from the disk. This counters the objection against the primordial origin, namely that such a field could not survive very long in the galaxy.

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BibTeX Citation

  author={Howard, A. M. and Kulsrud, R. M.},
  title={The evolution of a primordial galactic magnetic field},
  journal={Astrophys. J.},

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RIS Citation

ID  - ho01300e
AU  - Howard, A. M.
AU  - Kulsrud, R. M.
PY  - 1997
TI  - The evolution of a primordial galactic magnetic field
JA  - Astrophys. J.
VL  - 483
SP  - 648
EP  - 655
DO  - 10.1086/304262
ER  -

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