Variations in the Intensity of Cosmic Radiation Associated with Solar Activity

The authors of the paper under review*) measured the intensity of cosmic-radiation neutrons arising from small stars, born in lead. The measurements were carried out during the course of a year simultaneously at three stations located at different latitudes \(\lambda\) and altitudes \(h\): 1) \(h=0\); \(\lambda=52^\circ\) N; 2) \(h=3.6\) km; \(\lambda=50^\circ\) N and 3) \(h=3.1\) km; \(\lambda=41^\circ\) N. The neutrons were recorded continuously, and their intensity was noted at 12-hour intervals. Analysis of the curve of neutron intensity versus time shows that some maxima on this curve are repeated with a period of 27 days. As is known, this period is the period of the so-called synodic (as observed from the Earth) rotation of the equatorial regions of the Sun. It is therefore natural to connect these maxima with the moment at which definite regions of the Sun pass through the meridian of the point of observation. The authors show that the increase in the intensity of the recorded neutrons is associated with the passage through the meridian of regions of enhanced solar activity. As an example, we consider the curve of neutron intensity versus time (see figure), obtained as the result of superposing five curves, cor-

) J. Simpson, W. Fonger, Phys. Rev. 85*, 366 (1952).

corresponding to five successive 27-day periods: the ordinate for any \(n\)-th day of this curve is equal to the sum of the ordinates for the \(n\)-th, \(n+27\)-th, \(n+54\)-th, \(n+81\)-st, and \(n+108\)-th days. On the curve four regions of increased intensity are visible for \(n=n-5\), \(n\), \(n+10\), and \(n+15\). To within \(\pm 1\) day these regions coincide with the times at which four regions of increased solar activity that existed during the observations (1950–1951) passed through the central meridian. The paper gives data on the radio emission of these regions (at a frequency of 2800 Mc/s), from which it follows that the times of increase in neutron intensity coincide with the times of increase in the intensity of radio noise. It should be noted that the amplitude of the maxima on the curve lies far beyond the limits of possible statistical errors.

The authors consider two mechanisms by which solar activity may affect the intensity of the recorded neutrons:

1. Solar activity changes the energy of primary cosmic rays. For example, one may suppose that charged particles of cosmic radiation are accelerated by colliding with beams of atoms ejected from active regions of the Sun.

2. The Sun is a direct source of a small fraction of cosmic rays.

They point out that both assumptions are equally difficult to accept. The greatest difficulty is encountered by the second assumption, since it has not yet been possible to establish the existence of a day–night variation in the intensity of cosmic rays.

A. V.