2. Evidence of a 22-year cosmic-ray variation
Neutron Monitor records available for more than forty years show how cosmic radiation is excluded from the solar system at times of maximum solar activity. Monthly cosmic-ray values corrected for pressure at Climax Neutron Monitor Energies (2.96 GV) from 1953 to the end of 1995 with monthly values of the sunspot number from 1946 to 1995 (Solar Geophysical Data Reports, 1995) are presented in Fig. 1. The epochs of the solar polar magnetic field reversals are indicated, and the notations and indicate the magnetic moment parallel and antiparallel to the angular velocity axis of rotation of the Sun respectively (Otaola et al., 1985; Page, 1995).
There are differences in solar activity from cycle to cycle. There are series of cycles with very high activity level (19th, 21st) as well as quite low activity (20th, 22nd). A different behaviour between even and odd solar cycles is presented in solar activity (Dodson and Hedeman, 1975), where even sunspot cycles are characterised by two well-defined "stillstands" in the level of activity during the declining phases of such cycles. When the rises and the declines are compared, most cycles are very asymmetrical, with fast increase and much slower decrease. There are also symmetrical cycles. Generally the rise is faster and the decline longer for higher cycles, although that is not always true. Thus, there are essential differences in the behaviour of the Sun during the declining phases of different solar cycles (Storini, 1995).
Looking at the current solar cycle (22nd), we observe that this cycle reached its maximum rapidly, about one year before expectations and stayed very long from half of 1989 through to the end of 1991, in a fast maximum phase with a secondary maximum in 1991, only slightly lower than the primary one. Thereafter, the cycle declined steeply, but the decay became slower since late 1992. An explanation of this behaviour according to Svetska (1995) is that the most powerful events can occur only before or after the maximum, when activity is high enough to produce large energy storage in interplanetary space and Earth's magnetosphere, but not so high that the storage is disrupted too early in consequence of another activity nearby. This explanation is not necessarily be the correct one.
As concerns the behaviour of the cosmic-ray flux as measured by Neutron Monitor near the Earth, cycles No 20 and 22 differed from cycles 19 and 21. In cycle No 20, the cosmic-ray flux became high shortly after the cycle maximum and stayed high for seven years (1972-1978). In cycle No 22, the flux stayed high for about three years (1992-95) with a giant secondary minimum in 1991. This transient decrease originating in June 1991 reduced the cosmic-ray intensity back to nearly the same level as that at the 11-year intensity minimum in 1990 (Webber and Lockwood, 1993). In cycles 19 and 21, the flux rose slowly and peaked early, close to the cycle minimum for only one year (1965, 1986 respectively). So we have cycles characterised by a "saddle-like" shape and others characterised by a "peak-like" shape.
We underline that the cosmic-ray recovery of the 20th and 22nd solar cycles is rather rapid, whereas the recovery of cycles 19 and 21 were completed over a long period (about 4-5 years). Ahluwalia (1995) has shown that the recovery of cosmic-ray intensity follows two distinct patterns. During odd solar activity cycles, when magnetic polarity is negative in the northern hemisphere (), recovery is completed in 5 to 8 years, while the recovery period is less than half as much for even cycles (when ). The rapid recovery seems to set in following the reversal of the polar magnetic field unless interrupted by solar activity (as for cycle 22). Gnevyshev (1967) suggested that the 11-year solar cycle consists of two parts, one peaks at solar activity maximum and the other 2-3 years later, and the most energetic events appear during the second maximum.
The differences between solar cycles, or at least some of them,seem to be due to different behaviours of odd and even solar cycles, i.e. of the two parts of the basic 22-year solar periodicity. Otaola et al. (1985) have shown that the different behaviours of the cosmic-ray intensity during even and odd solar cycles is due to the parallel and antiparallel states of polarity of the polar magnetic field of the Sun relative to the galactic magnetic field. Their analysis shows a tendency towards a regular alternation of cosmic-ray intensity cycles with double and single maxima.
The polarity of the solar field reverses sign about every 11 years near the time of maximum solar activity or minimum cosmic-ray intensity. Thus, successive activity minima are characterised by a different solar field polarity. Table 1 shows the times of magnetic field polarity change, completing the work of Webber and Lockwood (1988) along with some of the features of the modulation that we have noted for the various cycles.These features can certainly be related to the 22-year magnetic field cycle. To understand how cosmic-rays move in the heliosphere under the influence of drifts, one needs to recognise the importance of the current sheet that divides the heliosphere into two hemispheres containing oppositely directed magnetic fields (Kota and Jokipii, 1991).
Table 1. Solar magnetic field polarity
© European Southern Observatory (ESO) 1998
Online publication: January 16, 1998