Long-term enhancements in solar wind speed

Long-term enhancements in solar wind speed over timescales of the order of a year appear to be a common feature throughout the heliosphere over heliocentric distances that range from less than 0.72 AU to greater than 60 AU. The origin of these events remains to be determined, but they are almost certainly associated with long-term variations st the solar wind source, in contrast with smaller-scale structures such as corotating interaction regions (CIRs), merged interaction regions (MIRs), and global merged interaction regions (GMIRs), which are dynamical in origin. We present a survey of the long-term speed enhancements observed at the Pioneer Venus Orbiter (PVO), IMP 8, Voyager 2, and Pioneer 10 between 1974 and 1994 and compare this with published reports of smaller-scale events such as MIRs. We examine several of these long-term speed enhancements in detail to identify and characterize aspects of their structure, and then describe how that structure evolves with heliocentric distance. Finally, we discuss some of the implications of these events.