I. A. Mironova and M. I. Pudovkin
Physical Institute, St. Petersburg University, St. Petersburg, Russia
Received 5 April 2001, published online 26 January 2002
The problem of the influence of solar activity on the atmosphere is still questioned. It is very important to find out the possible relationship between the long-term variation of the solar activity and the atmospheric phenomena and to determine the possible mechanisms of that influence. According to the present-day ideas, the solar activity affects the state of the lower atmosphere by means of modulation of the cosmic ray flux intensity (of both the solar and the galactic origin), which in turn changes the cloudiness and atmosphere's transmittance and thereby the solar radiation input into the lower atmosphere. The variation of the solar radiation intensity in the lower atmosphere results in noticeable changes of the altitudinal profiles of the temperature and pressure in the troposphere and stratosphere (a direct or primary effect of the cosmic ray flux variation). At the same time, it is quite evident that the change of the altitudinal pressure profile has to produce corresponding changes in the dynamics of the lower atmosphere, which has to result in an additional variation of the temperature and pressure profiles (a secondary effect). These secondary effects are especially important at middle and low latitudes where the direct effect of the cosmic ray variation decreases due to the geomagnetic field cutoff effect. Indeed, cosmic rays, first of all of Galactic origin, have an energy that is sufficient to penetrate down to the lower atmosphere, to ionize air molecules in the lower stratosphere, and therewith to effect significantly the velocity of physical-chemical processes in clouds and haze layers. The troposphere and stratosphere at high and low latitudes respond differently to the change of cosmic ray fluxes. This difference can be explained by the influence of the Earth`s magnetic field (cutoff effect of the geomagnetic field). Only high-energy particles can penetrate to the lower stratosphere on middle and low latitudes. Thus the direct effect of cosmic rays seems to be implausible at low latitudes, and the mechanism of the influence of galactic cosmic rays and solar activity (SA) on the low atmosphere of middle and low latitudes have to be studied separately.
Some investigators have already considered the problem of solar activity influence on cloudiness. Indeed, as was shown by Veretenenko and Pudovkin [1997, 1999], the total solar radiation intensity ( I ) in the course of the 11-year solar cycle at latitudes 60o-80o anticorrelates with the intensity of the galactic cosmic rays (GCR) flux; correspondingly, it is maximum at the epoch of the minimum of the solar activity and minimum at the epoch of the solar activity maximum; the amplitude of these variations amount to the value of about 5%. Besides, it was shown that the solar cycle effect disappears at a latitude of about 55o, and at the latitude of about 50o, the sign of the correlation between GCR and I values inverts, so the atmosphere's transmittance is maximum at the solar activity minimum. This result seems to contradict the observations by Svensmark and Friis-Christensen ; according to their data, the increase of the GCR intensity is accompanied by the increase of the cloudiness within the latitudinal belt -40o<j<40o. Thus the sign of correlation of the solar activity level with the cloudiness at low latitudes, according to Svensmark and Friis-Christensen , proves to be the same as that at high latitudes. One possible explanation of this contradiction may be associated with local peculiarities of atmospheric processes. Indeed, the relationship between the solar radiation and the GCR flux intensity was obtained by Veretenenko and Pudovkin on the basis of the data obtained at Russian observatories only, while the Svensmark and Friis-Christensen  results are based on the global-scale data obtained onboard geostationary satellites. Of course, the territory of Russia is relatively large; however, some local effects still may take place. In this connection, in this paper the cyclic variation of the atmosphere transmittance is investigated with the use of independent data from observatories in the U.S. territory.
As the solar radiation in the lower atmosphere is controlled by the state of the cloudiness, one can judge on the state of the latter on the account of the variations of the former. Correspondingly, in this analysis, we considered yearly data of sunshine (percentage of maximum possible sunshine) at the surface of the Earth. The term "sunshine" is used for the solar radiation intensity at the ground measured by a spectral device (a kind of a glass sphere). The sunshine was measured at several stations in the United States distributed at latitudes from 30o N to 50o N for the years 1891-1987. Sunspot numbers were used for the same period of the time as a measure of solar activity. The intensity of GCR was measured by a neutron monitor at Climax (39o N, 106o W), Colorado State. Availability of data for a rather long period of time allows us to study the variation of cloudiness at low latitudes within the cycle of solar activity.
As was said above, cycling variations of cloudiness and hence sunshine intensity are supposed to be caused by the corresponding variations of the intensity of the cosmic rays flux. Indeed, analysis of the cosmic ray intensity variations shows that contrary to the Svensmark and Friis-Christensen  results, the maxima of S (correspondingly, minima of cloudiness) at the low-latitudinal observatories Kansas City and Dodge City are observed during the years of the maxima of the GCR flux intensity, which agrees with the results of Pudovkin and Veretenenko  and Veretenenko and Pudovkin .
To estimate quantitatively the observed relationship between the variations of the sunshine duration, GCR flux intensity and solar activity presents, in Table 1, coefficients of correlation between the corresponding values ( r1, for annual-mean values; r2, for mean values averaged by superposed epoch method).
The data listed in Table 1 show that the increase of the intensity of GCR is really accompanied by an increase of sunshine intensity (correspondingly by a decrease of cloud cover) at low latitudes ( j<45o ).
Thus the data presented above show that at continental low-latitudinal observatories, the sunshine intensity increases (correspondingly the cloud cover decreases) with the increase of the cosmic ray flux intensity. At higher latitude (Portland, j = 45o ), the sign of correlation between the sunshine and the cosmic factors changes, and as might be expected, the increase of the cosmic rays flux causes an increase of the cloudiness. These observations are in a good agreement with the results of Pudovkin and Veretenenko  and Veretenenko and Pudovkin [1997, 1998, 1999]. At the same time, these results raise at least two questions:
1. What is the physical mechanism responsible for the increase of the cloudiness at low latitudes during years of high solar activity;
2. What may be the cause of the obvious disagreement of the Veretenenko and Pudovkin  and Svensmark and Friis-Christensen  data concerning the cyclic variations of the cloudiness at the lower latitudes. This problem needs a special and more extensive investigation.
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