THE PHYSICS OF COSMIC-RAY MODULATION - HELIOSPHERIC PROPAGATION DURING THE 1987 MINIMUM

The period in and near the 1987 solar minimum of modulation is a unique interval in the 11-year solar cycle modulation for determining whether both the heliospheric current sheet and charged particle gradient drift contribute significantly to the modulation of galactic cosmic rays and the anomalous components. Our investigation of this period is based on measurements from neutron monitors and the IMP 8 satellite at 1 AU and the Pioneer 10, Pioneer 11, Voyager 1, and Voyager 2 spacecraft distributed throughout the heliosphere to approximately 46 AU. Three observations essential for the analysis are as follows: (1) the brief time-intensity profile for cosmic ray minimum could be traced relatively unchanged from 1 AU to approximately 41 AU at the average solar wind velocity, (2) the onset of enhanced modulation in the new solar cycle at 1 AU began at the highest magnetic rigidity and progressed in time over a period of months to particles of low magnetic rigidity, and (3) the average tilt angle of the neutral current sheet was available from Hoeksema (1989) as a function of time for the period centered on the 1987 solar minimum at 1 AU. To account for the observations, we propose a model for the onset of the new modulation cycle which includes a change in the average tilt angle of the current sheet as it propagates outward through the heliosphere at the solar wind velocity. Calculations based on our model with an inwardly directed north solar polar magnetic field (which prevailed in the 1985-1988 period) include diffusion, convection, adiabatic deceleration, and drift for the inward propagation of cosmic ray protons in the energy range from 20 to 1500 MeV and reproduced the rigidity phase lag effect observed in the 1987 data. On the other hand, for the reversed solar magnetic field polarity (e.g., 1954, 1977) the model calculations show that an enhanced onset for these solar cycles has a negligible rigidity phase lag at 1 AU, in agreement with the observed 1977 onset of modulation. We point out that transient phenomena (e.g., Forbush decreases and interaction regions) appear to dominate the higher levels of solar modulation following the 1987 period. A consequence of our analysis is that at the time of solar minimum there is a minimum in the amplitude of the 11-year variation of the radial gradient of integral cosmic ray intensity which propagates outward at the solar wind velocity. Therefore the time dependence of the amplitude of the radial gradient is not the result of the inward-outward motion of a modulation boundary.