Cosmic-Radiation Intensity-Time Variations and Their Origin. III. The Origin of 27-Day Variations

Abstract

It is shown that (a) the 27-day recurring cosmic-radiation intensity variations are not produced by geomagnetic field disturbances, and (b) this 27-day variation is a primary intensity variation produced by a charged particle accelerating mechanism.These results were obtained without invoking special models for the magnetic field disturbances or requiring detailed time correspondence between cosmic-radiation intensity and magnetic field intensity variations. The conclusions were based upon experimental observations covering a 19-month period in 1951 and 1952. The results are derived from neutron-intensity variations measured as a function of time in aircraft and neutron monitor piles. The data reveal that an indirect association exists between the 27-day intensity variation and geomagnetic disturbances: i.e., geomagnetic disturbances are most likely to occur approximately 2 days after the 27-day maxima of cosmic-ray intensity. It is also shown that intensity changes of \ensuremath{\sim}3-6 percent are sometimes not followed by any geomagnetic disturbance.An example is given of a nonrecurring sharp intensity decrease of >6 percent, and it is shown that even this event is not produced by geomagnetic field variations.The results suggest that there is a common mechanism which produces both the accelerating process for cosmic-radiation particles and, indirectly, the geomagnetic disturbances. A search was made for the probable location of such a mechanism. Varying electrical fields of terrestrial origin were considered whereby the incoming primary radiation would undergo an acceleration or deceleration either (1) before entering the geomagnetic field, (2) within the magnetic field region, or (3) after passing through the magnetic field. None of these possibilities, nor a combined geomagnetic and geoelectric field storm, accounts for all the established experimental facts. In view of these results it is concluded that the accelerating mechanism probably is not of terrestrial origin. The 27-day recurrence corresponds in time to the proper rotation of the solar equatorial latitudes and, since it has been shown that active solar regions at these latitudes are associated with the 27-day cosmic-radiation intensity variations, the required accelerating mechanism is probably controlled by solar processes and may be located near the sun.From the dependence of the 27-day intensity variation upon particle rigidity, the experimental results show that primary protons undergo the variation, but it is still not proved whether or not particles of $Z\ensuremath{\ge}2$ also display this variation.The experimental data also exclude the production of this variation by the influence of a solar dipole magnetic field.