The relatively abrupt onset of cosmic ray modulation, observed in May 1987 on Earth and in September 1987 by the Pioneer 10 and 11 spacecraft, enables further investigation of the possible effect of the heliospheric current sheet (sector boundary) on modulation. According to the gradient drift theory of cosmic ray transport, modulation is associated with solar cycle changes in the current sheet, whose inclination increases systematically from solar minimum to solar maximum. The measure used in testing this hypothesis is the difference in the maximum latitudinal extent of the current sheet in the northern and southern solar hemispheres. This difference is obtained from contours of the current sheet produced by extrapolating photospheric magnetic field measurements to a solar wind source surface. When the latitude difference, divided by 2 and called the pseudoinclination or pseudotilt angle, is compared with cosmic ray count rates measured on Earth, an excellent correlation is found, as in a previous study of modulation in the years 1976--1986.

The abruptness of the onset can be attributed to the greater sensitivity of the cosmic ray intensity to changes in the current sheet when the solar-heliospheric magnetic field is inward in the northern hemisphere (above the current sheet) as predicted by the model. Thus the observations are found to be consistent with the predictions of the gradient drift model. A preliminary analysis is also carried out to test an alternative, but not necessarily incompatible, hypothesis that large-scale transient solar wind structures (coronal mass ejections, shocks, etc.) form barriers to the inward transport of cosmic rays. Available spacecraft magnetic field measurements (ICE, Pioneer 11), geomagnetic activity, including storm sudden commencements, and Forbush decreases do not reveal an obvious change in solar wind stream structure within several months of the modulation onset. It appears that in this instance, promoted by the abruptness of the onset, it may be possible to discriminate between the effect on modulation of the heliospheric current sheet and the possible effect of solar wind shocks and/or ejecta. ¿ American Geophysical Union 1990