The Interplanetary Magnetic Field (IMF) as of solar origin was endorsed mainly due to the final report of the Pioneer V experiment on March 30-31, 1960. After reading point 3 in the poster you can decide for yourself (3. Weak Points in Pioneer-V Results Interpretation).
http://www.exmfpropulsions.com/Propulsion/Yousif-GC51D-1005.pdf