COSMIC-RAY MODULATION DURING SOLAR-CYCLE-22 - SOLAR MAXIMUM LOOPS AT EARTH AND GLOBAL TRANSIENTS IN THE HELIOSPHERE

Neutron monitor data from two pairs of cosmic ray stations, Kiel/Tsume b and Climax/Huancayo, are used to study the rigidity dependence of so lar modulation during the solar activity cycle 22. The long-term decre ase of cosmic ray intensity during the ascending phase of cycle 22 is characterized by the same rigidity dependence as for the long-term rec overy during the descending phase of cycle 21. This is equivalent to a lack of any significant spectral hysteresis in cosmic ray modulation for both half cycles surrounding the 1987 cosmic ray maximum. The abov e is true at least for 27-day averages of cosmic ray intensity and at neutron monitor rigidities. In December 1988 the first large Forbush d ecrease of cycle 22 marked the beginning of increased solar activity w hich resulted in several prominent interplanetary shocks in 1989-1991. These strong heliospheric disturbances produced global merged interac tion regions (GMIRs) which were responsible for large step decreases i n cosmic ray intensity seen at all spacecraft throughout the heliosphe re, including Voyager 2, Pioneer 10 and 11, as well as Voyager 1. It w as found that in the neutron monitor rigidity range, such deceases wer e followed by rigidity-dependent recoveries. The rigidity dependent re covery manifests itself by formation of hysteresis loops on correlatio n plots for low- versus high-rigidity cosmic ray intensity changes. Th e loops, if completed, close approximately at the modulation level fro m which the corresponding step decrease started. On the other hand, th e current long-term cosmic ray recovery (starting from Bartels solar r otation 2162) follows the ''normal'' rigidity dependence characteristi c of both the A>O recoveries and A<O decreases/recoveries (where A<O a nd A>O are two different phases of the solar magnetic cycle). All this together confirms the bimodal character of cosmic ray modulation for 1985-1992, the same as found previously for cycles 19-21; the transien t modulation (T congruent to 0.5 to similar to 1 year) related to GMIR s is accompanied on recovery by the phenomenon of hysteresis, while th e long-term modulation (T>1 year) is hysteresis free. The two explanat ions of the phenomenon of spectral hysteresis proposed in the literatu re, i.e., a time-dependent modulation via convection, diffusion, and d rift effects and an acceleration of a normal cosmic ray spectrum in mu ltiple shocks in the outer heliosphere (as proposed by Stoker and Mora al (1986)), are discussed in the context of the presented cosmic ray d ata.