F. B. McDonald1, L. F. Burlaga2, R. E. McGuire2, and N. F. Ness3 1 Institute for Physical Science and Technology, University of Maryland, College Park, MD 2 NASA/Goddard Space Flight Center, Greenbelt, MD 3 Bartol Research Institute, The University of Delaware, Newark, DE
The onset of solar activity at 1 AU began in November 1997 with several modest events and was followed by an enhanced period of solar activity in April and May 1998. This latter series of events produced a well-defined step decrease in the cosmic ray intensity at 1 AU as observed by the Goddard MED experiment on IMP 8. Over the ensuing 18 months there are further decreases in the IMP-8 data with the cumulative effect being in reasonable agreement with the net decrease in cycle 21 at a similar time in the solar cycle. At V-1 (72AU, 34 degrees N) and V-2 (56 AU, 24 degrees S) there are transient increases in the intensity of 1.8-3 MeV H that mark the passage of a large interplanetary disturbance around 1998.75 that appears to be produced by the April-May activity. However there was no significant decrease observed in the galactic or anomalous cosmic rays. What is unique about this event in the outer heliosphere is the large precursor increase in the anomalous cosmic ray intensity that appears to be associated with this interplanetary disturbance as defined by the voyager magnetic field observations. The onset of a well-defined decrease in the V-2 data occurs around 1999.5possibly in association with the disturbances that produced the 2nd step-like decrease at 1 AU.