We are indebted to two excellent anonymous referees who provided much useful input on the review. We
are grateful to the Space Physics Data Facility (SPDF) and National Space Science Data Center (NSSDC)
for OMNI data, as well as magnetic field data from Pioneer 6 and 7 (PI N. Ness), Pioneer 10 and 11 (PI
E. Smith), Pioneer Venus Orbiter (PI C. Russell), Helios (PI N. Ness), Voyager (PI L. Burlaga)
ICE/ISEE3 (PI E. Smith), Ulysses (PI A. Balogh) and STEREO (PI M. Acuña). We have also benefitted
from the availability of WSO magnetograms (T. Hoeksema and Standford University) Ulysses SWOOPS
data (PI: D. McComas). The potential-field source-surface model used in this paper was based on code
provided by J. Luhmann.