11-year Modulation Research Articles

Rigidity dependence for Forbush decreases in 1968 compared with that for the 11-year variation

Comparisons have been made of the cosmic ray intensities measured by the Pioneer 8 detector and by ground-based neutron monitors located at different vertical cutoff rigidities for three Forbush decreases in 1968 and for the 11-year solar modulation. It is observed that the rigidity dependence of the three large Forbush decreases was substantially different than the corresponding 11-year variation at rigidities from <1 to >50 Gv.

Solar activity and the 11-year modulation of cosmic rays

The cosmic-ray intensity during the 18th and 19th solar cycles is examined in the light of Gnevyshev's suggestion of the presence of two maxima in each solar cycle. The 18th solar cycle (1944–54) has two prominent and widely separated cosmic-ray minima corresponding in phase with the two maxima in Bartel's Ap index. For the 19th solar cycle the existence of two minima is less prominent than for the 18th solar cycle. The maximum at higher solar latitudes is more effective in reducing cosmic-ray intensity than the maximum at the lower latitudes. Ap, however, has a larger maximum during the lower latitude solar maximum. A relation between Ap and cosmic-ray intensity is obtained. This relationship is shown to be consistent with Parker's solar-wind theory of the modulation of cosmic rays.

Some Astrophysical Implications of Recent Measurements on the Intensity of Cosmic-Ray Electrons

We have extended our recent measurements on the extraterrestrial cosmic ray electron spectrum, this spectrum now being determined over the energy range from ~15 MeV to 6 GeV. The extraterrestrial electron intensity between 15 MeV and 200 MeV can be determined unambiguously by studying the diurnal variation of these particles. We have also measured the effects of the 11-year solar modulation on the electrons, thus enabling the electron spectrum observed near the Earth to be extrapolated to the local region of the spiral arm. It is the purpose of this paper to relate these measurements to: (i) calculations of ‘secondary’ electrons produced by cosmic ray nuclei moving in the Galaxy; and(ii) the observations of non-thermal radio emission from disk components of the Galaxy.

The 11-year solar modulation of cosmic rays as deduced from neutron monitor variations and direct measurements at low energies

The variation in the cosmic-ray intensity recorded by neutron monitors from 1958 to 1965 has been investigated to deduce the form of the solar modulation of the cosmic radiation. The neutron monitor data were carefully selected and examined. The observed quarter-year average changes in the intensity at the neutron-monitor stations were then compared with the calculated changes using different modulation functions. Finally, the observed variations in the intensities recorded by neutron monitors were compared with the variations of the radiation observed at the top of the atmosphere and in space. The change in both the neutron monitor intensity and the primary cosmic-radiation can be described by a modulation of the form exp(−K/P) in the rigidity range 0.5 < P < 50 Gv.

Cosmic rays and large distance features of the interplanetary space

The effect on cosmic rays of a static barrier possibly surrounding the region of the steadily expanding solar corona is studied at high rigidities. The barrier could be formed at any distance where the solar wind is substantially decelerated and some turbulence is set in; its transparency might trace the solar activity with some delay, in agreement with the observations on the 11-year modulation of cosmic rays as recorded by the ground stations.