THE INTRINSIC MAGNETIC-FIELD ORIENTATION IN NGC-4631

We present radio continuum measurements of the edge-on spiral galaxy N GC 4631 at 4.86 and 8.44 GHz. We discuss the morphology of the radio h alo in total and polarized emission and its wavelength dependence. The rotation measure and intrinsic magnetic field orientation are determi ned using the two frequency pairs 4.86/8.44 GHz and 1.49/4.86 GHz, the latter with the help of published 1.49 GHz data. The results can be s ummarized as follows: i) The rotation measure in the radio halo is bet ween -15 and +20 rad m-2. The intrinsic large-scale B-field in the hal o is quite symmetric with respect to the major and minor axes of NGC 4 631. It generally points to, or away from the central 5 kpc. ii) The p rominent radio spurs of NGC 4631 are found to be an essential constitu ent of the radio halo. In all the regions where the B-field structure can be determined, the field orientation follows the spurs. iii) The s ources of the cosmic rays in the radio halo are most probably located in the disk of NGC 4631 and associated with star-forming regions. In p articular the NE-spur seems to have several tributaries, each originat ing at an HII region complex. iv) The magnetic field structure is esse ntial in explaining the unusual size and structure of the radio halo. The field in the halo probably results from an outflow of gas from the disk into the halo, which is caused or promoted by the gravitational interaction with NGC 4656. Based on the available observations and dyn amo models, the existence of an alpha2- or alphaomega-dynamo as the so urce of, or as a contributor to, the halo magnetic field can neither b e proven nor disproven. v) The depolarization in the plane of NGC 4631 has a clear wavelength dependence in the sense that there is less dep olarization at higher frequencies. Faraday depolarization is therefore an essential process in NGC 4631.