New data from underground mu‐meson telescopes, at depths of 40 MWE, in the northern and southern hemispheres are presented. Analysis in sidereal time of three years of data from the northern hemisphere telescopes and two years of data from the southern hemisphere telescopes confirm that a maximum intensity is observed near 18 hours sidereal time for telescopes pointing into the northern hemisphere and near 06 hours sidereal time for telescopes pointing into the southern hemisphere, in agreement with previous observations by others. A model is proposed in which just such a ‘sidereal’ variation can be produced in the vicinity of the earth as a result of the radial cosmic‐ray density gradient and the interplanetary magnetic field; the model is tested by dividing the cosmic‐ray data according to the direction of the interplanetary field and performing the same sidereal time analysis on these data. The results confirm the principal features of the model and demonstrate that the ‘sidereal’ variation can be produced in the inner solar system, correctly predicting the observed times of maximum in the northern and southern hemispheres, without recourse to assumptions about anisotropies of the particle flux in galactic space.

We present a new set of data on relative sunspot number (total, northern hemisphere, and southern hemisphere), taken for the 37-yr period 1947 to 1983; this constitutes a particularly coherent and consistent set of data, taken by the same observer (Hisako Koyama) using the same observing instrument. These data are combined with earlier data (White and Trotter, 1977) on the variation of sunspot areas for both solar hemispheres from 1874 to 1971. The combined data, covering 110 years and 10 solar cycles, are examined for periodicity in solar activity north-south asymmetry. We show that, in general, northern hemisphere activity, displayed as either An/(An + As) or Rn/(Rn + Rs), peaks about two years after sunspot minimum. This peak is greater during even cycles, pointing to a 22-yr periodicity in north-south asymmetry in solar activity, suggesting that the asymmetry is related to the 22-yr solar magnetic cycle. We demonstrate that the largest and most protracted period of northern-hemisphere activity excess in the last 110 years has occurred from 1959 to 1970; we show that there is a strong correlation between northern activity excess and a cosmic-ray density gradient perpendicular to the ecliptic plane, pointing southward, which is evident in cosmic-ray diurnal variation data from the Embudo underground cosmic-ray telescope.

On the basis of earlier work that suggested that the sidereal diurnal variation in cosmic ray intensity could be due to the cooperative effects of the interplanetary magnetic field and a radial heliocentric cosmic ray density gradient, a more comprehensive data analysis has been made to test the model. Data from underground μ meson telescopes in the northern and southern hemispheres have been analyzed in sidereal time as a function of the direction of the interplanetary magnetic field. The observed anisotropy is shown to be dependent on the magnetic field direction. Sidereal cosmic ray data from several underground detectors are displayed as a function of the threshold rigidity of the detectors, and the results appear to indicate that the mechanism responsible for the ‘sidereal’ anisotropy has an upper cutoff rigidity below 100 Gv. Other observed sidereal cosmic ray anisotropies are reviewed in the light of these results, and the likelihood of observing a true sidereal anisotropy above and below 100 Gv is discussed.

Ground‐level solar cosmic ray events are usually observed by high latitude neutron monitors at relatively low rigidities (> 1 GV) and are not usually observed by underground cosmic ray detectors because of their much higher threshold rigidity. However, the September 29, 1989 ground‐level event was sufficiently large and unusual that it was recorded by the Embudo, New Mexico underground muon telescope which has a threshold rigidity of 19 GV. The observed increase was 2.2 ± 0.2% in the hour interval from 1100 to 1200 UT. There was no statistically significant increase in the data from the Socorro underground muon telescope which has a threshold rigidity of 45 GV. This is the first detection of a ground‐level solar cosmic ray event by any underground cosmic ray muon telescope where the event has been clearly distinguishable above the background cosmic ray intensity.

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