The onset of long-term cosmic ray modulation in cycle 23 coupled with a transient increase of anomalous cosmic rays in the distant heliosphere

The onset of significant solar activity in cycle 23 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 wel l-defined step decrease in the cosmic ray intensity at 1 AU as observed by the Goddard Cosmic Ray Experiment on IMP 8. Over the ensuing 2 years there are further decreases in the IMP 8 data with the cumulative effect being in good agreement with the net decrease in cycle 21 at a similar time in the solar cycle. At Voyager 1 (72 AU, 34 degrees N) and Voyager 2 (56 AU, 24 de grees S) there are transient increases in the intensity of 1.8-2.8 MeV H th at mark the passage of a large interplanetary disturbance around 1998.75 th at was most probably associated with the April-May activity. However, there was no durable decrease observed at that time in the galactic or anomalous cosmic rays. What is unique about this event at Voyager 2 (V 2) in the out er heliosphere is the precursor increase in the anomalous cosmic ray intens ity associated with this interplanetary disturbance as defined by the V 2 m agnetic field observations. This enhancement event establishes that reaccel eration of anomalous cosmic rays can occur in the outer heliosphere. The on set of durable modulation at V 2 begins around 1999.5, probably in associat ion with the disturbances that produced the second step-like decrease at 1 AU. At Voyager 1 there is preliminary evidence that the corresponding onset time is 1999.5.