Radial and azimuthal components of the heliospheric magnetic field: Ulysses observations

E. J. Smith, A. Balogh, M. E. Burton, R. Forsyth, and R. P. Lepping

Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA


Two invariants associated with the Parker model of the solar wind involve the radial, BR, and azimuthal, BT, components of the heliospheric magnetic field. These invariants have been investigated using Ulysses data obtained at high latitudes in both the north and south polar hemispheres. The magnetic flux invariant, r2BR, is essentially independent of latitude in both hemispheres. However, comparisons with in-ecliptic IMP-8 and WIND observations reveal a small decrease with time of ~6%. Average values of the second invariant, rVRBT, which is related to the heliospheric electric field, differ systematically from the Parker theory, the difference being a function of latitude that cannot be accounted for simply by a changing solar rotation period. No asymmetry in the discrepancy is found between the two hemispheres. The difference vanishes at the solar equator as shown by a comparison with the in-ecliptic measurements. These results are consistent with the average spiral angle being more radial (underwound) at high latitude than is predicted by the Parker angle and an interval has been identified in which the observed angle is approximately zero on average. This behavior contrasts with the most probable value of the invariant or the spiral angle which does agree with theory. The difference between the average and the most probable value is caused by an asymmetry in the probability distribution. This apparent reduction in average BT may bear on the long-standing issue of a "flux deficit" in the distant heliosphere.

Adv. Space Res., 20, 47-53, 1997