Large cosmic ray transient decreases observed in the heliosphere at the Earth, Voyagers 1 and 2 and Pioneer 10 from 1987 to 1991

Using the >70-MeV cosmic ray rate data from the IMP, Voyager, and Pioneer spacecraft, we have examined the role of large transient decreases in the overall cosmic ray decrease from 1987 to 1990. At least 12 separate transient decreases with magnitudes >5% were observed at Voyager 2, which moved from a heliocentric radius of ~23 to 35 AU during the time period. Many, but not all, of these decreases were observed at IMP 8 near the Earth and at Pioneer 10 between 42 and 54 AU near the ecliptic plane. However, only five of these decreases were observed at Voyager 1 at north heliographic latitude ~30° and radial distance between 32 and 46 AU. The period of decreasing cosmic ray intensity from 1987 to 1990 can be divided into two phases. During phase 1, between mid-1987 and early 1989, the average heliospheric current sheet tilt rapidly increased from ~8° to 65° as the solar activity was also increasing rapidly and the intensity at the Earth decreased by about 35%. Four large transient decreases were observed at the Earth during this time; however, transient decreases beyond the Earth were observed only near the ecliptic plane. In phase 2, from early 1989 to mid-1990 when the intensity at the Earth decreased by an additional ~60%, five large transient decreases were observed at all latitudes. During this time the current sheet tilt was always greater than 60°. Using a procedure to separate these large transient decreases from any possible slower long-term variations, we argue that these decreases appear to be responsible for much of the overall intensity decrease of 20-35% observed at all spacecraft near the equatorial plane in phase 1 as well as most of the larger intensity decrease of about 45-60% observed at all spacecraft during phase 2.

The recovery of the cosmic ray intensity that started at the Earth in early 1990 moved outward in the heliosphere reaching the Voyager and Pioneer spacecraft about 6 months later. This is in contrast to what happened as the intensity began to recover in the previous cycle in 1981, when the recovery was observed to begin almost simultaneously at all radii <25 AU. This difference suggests that during the current cycle, the mechanism responsible for the recovery must have propagated outward from the Sun. This could have been related to the reversal of the solar polar magnetic field which occurred in early 1990. The effects of large transients propagating outward in the heliosphere and becoming ineffective when they reach the heliospheric boundary could not have played a major role in the onset of the recovery in 1990, since in this case the recovery should have been observed first in the outer heliosphere.