Change in interplanetary shock acceleration preceding STIP Interval 17

The intensity and frequency of shock acceleration events in the interplanetary medium decreased dramatically in early 1985. Low energy ions were observed by IMP 8 at 1 AU and Voyagers 1 and 2 at 22 and 16 AU, respectively. Voyager 1 was at 25 deg heliographic latitude while IMP 8 and Voyager 2 were near the solar equatorial plane. The decrease in low energy shock events led to a drop in the average ion flux by a factor of 20 to 50. It started about day 10 of 1985 in the approximately .5 MeV channel on IMP8 and took approximately 75 days to reach the new, lower, background level. The decrease at the Voyagers started approximately 50 days later. The time delay between the start of the decrease at IMP and at Voyager 2 implies that decrease was convected outward with a velocity of approximately 535 km/sec. The intensity and frequency of interplanetary shock events remained at the lower level for at least 1.5 years.