Derek B. Swinson

Long-term variations in North-South asymmetry of solar activity

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.

Corrected sidereal anisotropy for underground muons

Abstract Data from underground muon telescopes in New Mexico and Bolivia are analyzed in sidereal time and anti-sidereal time to study anisotropies in the rigidity range 20 GV to a few 100s of GV. Using both vertical and North- and South-pointing telescopes in both hemispheres, a latitude range of 70°N–50°S is covered. The distribution of cosmic rays in the inner heliosphere gives rise to a diurnal variation in anti-sidereal time, and also produces a spurious contribution to the anisotropy in sidereal time. It is shown that the anti-sidereal variation is of the P 2 1 type, having opposite phase in the Northern and Southern Hemispheres, and maximum amplitude at mid latitudes. The anti-sidereal data are used to correct the sidereal data, using the Nagashima method (Nagashima, Sakakibara, Fenton and Humble, 1985); the resulting corrected sidereal vectors for Northern Hemisphere telescopes have their sidereal maxima close to 3 h sidereal time, in reasonable agreement with sidereal data at higher energies from small air showers. The Nagashima correction also appears to eliminate effects due to the reversal of the Suns polar magnetic field which show up in the unconnected sidereal data, and which also remain in corrected data using an alternative correction.

Correlation of cosmic ray diurnal anisotropies with the interplanetary magnetic field over 21 years

Abstract Twenty-one years of data from the Embudo underground cosmic ray muon telescope and 18 years of data from the Socorro underground cosmic ray muon telescope are used to establish numerical correlations of the cosmic ray solar and sidereal diurnal variations with the interplanetary magnetic field (IMF) as measured by spacecraft. Using these correlations we examine the influence of the IMF on cosmic ray anisotropies both in the ecliptic plane and perpendicular to the ecliptic plane. The correlations are examined for their variability with the solar rotation period, looking in particular for recurrences within a period of 27 days (the solar rotation period). The long-term variability of these correlations is examined as a function of the level of solar activity and as a function of the magnetic polarity of the heliosphere.

Waves in the cosmic ray North-South anisotropy with periods of 27 days, 1 year, and 11 years

We use data from 1971 to 1989 from the Nagoya cosmic ray muon telescope to look at waves in the cosmic ray north-south anisotropy. The GG component of the Nagoya data is used because it is particularly sensitive to the north-south anisotropy. The presence of waves with a period of 27 days is demonstrated through most of the period. For each 27-day solar rotation the 27 daily values for the GG component have been harmonically analyzed to determine the amplitude of the best fit sine wave to the data. When these GG 27-day amplitudes are plotted for each solar rotation during the 19-year period, it becomes apparent that there is a very marked annual modulation of these amplitudes; the annual variation is greatest at solar maximum, and is also an 11-year modulation of the effect. The cosmic ray north-south anisotropy arises from the By × ▽NR, anisotropy resulting from the interaction of the y component of the interplanetary magnetic field (IMF), By and ▽NR, the radial heliocentric cosmic ray density gradient. We have also determined the daily average values for By from 1971 to 1987, and for each 27-day solar rotation these daily values have also been harmonically analyzed to obtain the best fit sine wave of period 27 days. When the amplitudes of the 27-day By waves are plotted for each solar rotation, an annual wave and an 11-year wave are also found; the behavior is very similar in nature to the behavior of the waves in the Nagoya GG data. These two sets of data have been compared with the values of α, the tilt of the heliospheric neutral sheet, which is available for most of the solar rotations between 1971 and 1989. The largest annual waves in the amplitude of the 27-day GG waves appear to coincide with relatively rapid excursions in the tilt of the neutral sheet. The annual mean of the amplitudes as well as the range (highest to lowest) in the amplitudes for the 27-day waves in the GG component and in the By component have been determined for each year; in each case there is a periodicity of about 11 years comparable to the solar activity periodicity.

North/South Asymmetry in Solar Activity and its Effects on the High Energy Cosmic Ray Diurnal Variation

Using a data base extending from 1955 through 1980 we find that the northern hemisphere of the sun was decidedly more active than the southern hemisphere for the period 1959 through 1970. From 1971 through 1980 there was no systematic asymmetry. Extending the time period through 1984, but using a different data base, we find an apparent change around October 1982, after which there are decidedly more flares occurring in the southern hemisphere of the sun than in the northern hemisphere. Examination of the diurnal variation amplitude present in the extremely high energy (approximately 100 GeV) cosmic radiation measurements indicates a consistent north/south asymmetry until about 1971 which we suggest is the result of the asymmetric solar activity.