The Evolution Of A Primordial Galactic Magnetic Field5

We consider the hypothesis that galactic magnetic elds are primordial. We also discuss the various objections to this hypothesis. We assume that there was a magnetic eld present in the galactic plasma before the galaxy formed. 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 nd that the lines of force are stretched rotation of the galactic disk, which ampliies the toroidal component of the eld and increases l. When the magnetic eld is strong enough, it produces ambipolar velocities that try to lift the line out of the galactic disk but in opposite directions on diierent parts of the line. The result is, 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, nally, a reduction in the magnetic eld strength. After a suucient time, the magnetic eld at all points goes through this stretching and reduction, and the eld 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 eld is toroidal, and has the same strength almost everywhere. On the other hand, it turns out that its direction varies rapidly with radius, changing sign every one hundred parsecs eld dominate over that of the other. The resulting eld has a net Faraday rotation. If such a eld were observed with low resolution in an external galaxy, then the eld 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 eld, which arises naturally from a primordial origin, can t the observations as well as other models for the magnetic eld, such as those arising { 3 { from the mean eld dynamo theory. Finally, because the eld lines are topologically threaded through the disk they …