... we still don't have the answer in regards to the Pionners and Voyagers (and Ulysses!):

The answer is probably in here somewhere or it may well be a combination of things.

These in particular, mho: (sigh, not much time for this 'star stuff' for me these days).

[Lu, J.Y., Y.C. Whang and L.F. Burlaga, "Interaction of a Strong Interplanetary Shock with the Termination Shock," J. Geophys. Res.," Vol. 104, p. 28,249-28,254, 1999. ]

[Rice, W.K.M. and G.P. Zank, ""The Effect of Cosmic Rays on the Propagation of Shock Waves in the Outer Heliosphere," Proc. 26th ICRC, ed. By D. Kieda, M. Salanon and B. Dingus, (Salt Lake City, UT), Vol. 6, p. 532, 1999.]

[Smith, C.W., "Solar Cycle, Radial and Latitudinal Variations of Magnetic Helicity: IMF Observations, Magnetic Helicity in Space and Laboratory Plasmas," Geophysical Monograph 111, Proceedings of the Chapman Conference on Magnetic Helicity in Space and Laboratory Plasma, eds. A.A. Pevtsov, R.C. Canfield, and M.R. Brown, American Geophysical Union,Washington, p. 239, 1999.]

-yum-