Waves in the cosmic ray north‐south anisotropy with periods of 27 days, 1 year, and 11 years

Abstract

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.