A large‐scale survey of corotating interaction regions and their successors in the outer heliosphere

Abstract

Corotating interaction regions (CIRs) and their successors play an important role in the structure and dynamics of the outer heliosphere. CIRs, consisting of a forward and reverse shock pair in association with a region of enhanced solar wind density and temperature, are a dominant structure in the solar wind at low heliographic latitudes at heliocentric distances between 2 and 8 AU. At larger heliocentric distances, CIRs spread, merge, and interact to form merged interaction regions (MIRs). MIRs are a dominant structure at low latitudes between 8 and 12 AU. At even larger heliocentric distances, considerable uncertainty remains regarding the small‐scale structure of the solar wind. Solar wind and interplanetary magnetic field observations from the Pioneer 10 and Voyager 2 spacecraft were used to conduct a large‐scale survey of CIRs and their successors over solar cycles 21 and 22 between heliocentric distances of 1 and 50 AU to categorize changes that occur as CIRs were convected outward from the Sun. At heliocentric distances less than 37 AU, similar changes were observed at similar heliocentric distances at different spacecraft during different portions of the solar cycle. These changes may reflect qualitative changes in the dynamics of the solar wind as CIRs spread and merge and the flux of kinetic energy available in solar wind streams declines with respect to the flux of thermal energy. At heliocentric distances greater than 37 AU, Pioneer 10 and Voyager 2 observed different types of structure. Interpretation of these differences is complicated by differences in the trajectories of the two spacecraft, and it remains to be determined whether they are associated with latitudinal gradients, solar cycle variations or the effects of interstellar pickup ions at Voyager 2.