GALACTIC DYNAMOS AND DENSITY WAVE THEORY .2. AN ALTERNATIVE TREATMENTFOR STRONG NON-AXISYMMETRY

A steady density wave in a disc-like galaxy could lead to a spiral for m for the crucial alpha-effect term in standard dynamo theory. We had earlier studied this problem under the thin-disc approximation and by effecting an approximate separation of variables between the z-depende nce and the r(c) phi-dependences of the magnetic field. We return to t his problem by making an alternative approximation of our equations, w hich allows a better treatment of the case of strong departures from a xisymmetry, albeit with a cruder treatment of the z-dependence. The nu merical solution of the equations reveals, as in our earlier work, the existence of rapidly growing, global, bisymmetric magnetic modes, cor otating with the density wave. The modes extend several kiloparsecs ar ound the radius r(c) where the wave corotates with the gas. The magnet ic spiral is closely aligned with the density wave in regions where it has maximum strength; leading within r(c) and trailing outside. These results are remarkably similar to that obtained in our earlier work, under a very different approximation scheme, encouraging belief in the robustness of the results obtained in both papers.