RADIAL ALIGNMENT SIMULATION OF SOLAR-WIND STREAMS OBSERVED BY PIONEER-10 AND PIONEER-11 IN 1974

A particularly favorable radial lineup between spacecraft in Earth orb it and Pioneers 10 and 11 (near the ecliptic plane at similar to 5.5 A U and similar to 4.5 AU, respectively) occurred in mid-1974, when the solar corona was in a stable and well-defined warped-dipole configurat ion. The radial alignment study reported here differs from previous ap plications of the technique in two respects: (1) It is the first time a two-dimensional (2-D) MHD model has been tested over such a lengthy propagation interval; the 2-D capability is crucial for treatment of t he nonradial shearing motions occurring across the stream interface. ( 2) The three-dimensional (3-D) structure observed in the white light c orona at that time is related to the systematic patterns of nonradial flow deflections appearing at the Pioneer corotating interaction regio n (CIR) fronts. Comparison of predicted and observed flows for pairs o f streams in two successive rotations reveals that when the parent cor onal hole projects far across the solar equator in a predominantly nor th-south orientation (i.e., nearest the 2-D idealization), the mapping is accurate down to details of the flow structures. But where the spa cecraft tracks along a latitudinal boundary of a hole or the associate d stream front is inclined at a shallow angle to the equator, the nume rical projections deviate systematically from the observations. Among the sources of error are 3-D dynamical interactions neglected in the m odel, differential rotation effects, and slow temporal evolution of th e coronal structures. A characteristic pattern of north-south and east -west deflections indicative of the 3-D geometry appears across the CI Rs, but these patterns evidently reflect primarily the local, as oppos ed to global, orientation of the 3-D stream fronts. Such patterns appe ar common in CIRs observed by Pioneer during this period. These findin gs thus hold special relevance for the analysis of Ulysses observation s, since the present coronal configuration is similar to that of 1974.