Latitude dependence of solar wind velocity observed ≳1 AU

Abstract

The large‐scale solar wind velocity structure in the outer heliosphere has been systematically analyzed for Carrington rotations 1587–1541 (March 1972 to April 1976). Spacecraft data were taken from Imp 7/8 at earth, Pioneer 6, 8, and 9 near 1 AU, and Pioneer 10 and 11 between 1.6 and 5 AU. Using the constant radial velocity solar wind approximation to map all of the velocity data to its high coronal emission heliolongitude, we examined the velocity structure observed at different spacecraft for latitudinal dependence and compared it with coronal structure in soft X rays and Hα absorption features. The constant radial velocity approximation usually remains self‐consistent in decreasing or constant velocity solar wind out to 5 AU, enabling us to separate radial from latitudinal propagation effects. We found several examples of sharp nonmeridional stream boundaries in interplanetary space (∼5° latitude in width), often directly associated with features in coronal X rays and Hα. In one structure there is evidence for significant (up to 40°) nonradial flow of the plasma in the corona below the altitude of transition to super‐Alfvénic flow.