Evolution of Solar Wind Structures in the Inner Solar System

Pioneer Venus Orbiter (PVO) solar wind measurements at 0.7 AU from 1979-1987 and Wind and ACE measurements at 1.0 AU from 1995-2004 have been used to study the structures of stream interaction regions (SIRs) and interplanetary coronal mass ejections (ICMEs). The occurrence rate of SIRs changes only slightly during the solar cycle and is similar at the two radial distances. From 0.7 to 1 AU, average scale size of SIRs decreases, and the overall transition in the parameters across the SIR steepens, while the shock association rate increases significantly (from very seldom to about 25%). In contrast to SIRs, the occurrence rate of ICMEs follows solar activity. The average scale size of ICMEs increases with distance, while the fraction of magnetic clouds decreases. These suggest that the ICMEs may disturb a greater angular extent at 1 AU than at 0.7 AU. Shock association rate also increases from 0.7 to 1.0 AU. We note that some of the differences may be due to the use of two different solar cycles at the two heliocentric distances.