MAGNETIC-FIELD REVERSALS IN THE MILKY-WAY - CHERCHEZ LE CHAMP MAGNETIQUE

Radio observations of nearby spiral galaxies have tremendously enhance d our knowledge of their global magnetic field distributions. Recent t heoretical developments in the area of dynamos have also helped in the interpretation of magnetic field data in spiral galaxies. When it com es to the magnetic field in the Milky Way galaxy, our position in the Milky Way's galactic disk hinders our attempts at interpreting the obs ervational data. This makes the proposition of ''cherchez le champ mag netique'' a difficult one to follow. Some recent papers have attempted to fit magnetic field models to spiral galaxies, and in particular to the Milky Way galaxy. Magnetic field reversals in the Milky Way are c rucial to all interpretations, be they axisymmetric spiral (ASS) or bi symmetric spiral (BSS) global magnetic field models. Magnetic field re versals can be found in both ASS and BSS magnetic field models, not ju st BSS ones. The axisymmetric spiral (ASS) magnetic field models produ ced by the dynamo theory already predict magnetic field reversals, and they are of the type observed in the Milky Way. The small number of m agnetic field reversals observed in the Milky Way is compatible with t he ASS magnetic field models. The bisymmetric spiral (BSS) magnetic fi eld models as applied to the pulsar RM data and to the QSO and galaxie s data have many problems, due to the many pitfalls in model fitting t he magnetic field reversals observed in the Milky Way. Many pitfalls a re discussed here, including the incomplete comparisons of BSS versus ASS models, the number of spiral arms to be used in modelling, and the proper distance to pulsars via the more accurate distribution of ther mal electrons within spiral arms. The two magnetic field reversals in our Milky Way are clearly located in the interarm regions. Predicted m agnetic field reversals are periodic, while observed ones are not peri odic. Magnetic field reversals cannot be masked effectively by local i nterstellar magnetised shells. The strength and direction of the magne tic field with galactic radius show that the BSS magnetic field models are less suitable to explain the RM data in the Milky Way. The predic tion by the BSS magnetic held models of a large number of magnetic fie ld reversals differs from the available observations.