Hale cycle effects in cosmic ray east‐west anisotropy and interplanetary magnetic field

Abstract

We have reanalyzed diurnal anisotropy data obtained with the shielded ion chamber (IC) at Cheltenham/Fredericksburg and the neutron monitor (NM) at Swarthmore/Newark. IC data are for the 1936‐1977 period and NM data are for the 1965‐1988 period. These data have been studied previously by Bieber and Chen [1991a]. However, we have corrected IC data for the diurnal temperature effect. Application of this correction results in a better agreement between IC and other data sets, thereby making it possible to study the long‐term changes in the diurnal anisotropy using IC data. The behavior of the annual mean east‐west anisotropy is studied for 53 years of observations. The period encompasses more than two solar magnetic (Hale) cycles. Its amplitude undergoes the expected 11 and 22 year variations [Ahluwalia et al., 1965], with the largest changes occurring near solar activity minima. Moreover, the data indicate the presence of the subsidiary maxima [Ahluwalia, 1992b] for the entire 53‐year period, following the solar polar field reversals, during the declining phases of activity cycles when high‐speed solar wind streams are present in the heliosphere [Ahluwalia et al., 1990]. The data suggest that the amplitude of the subsidiary maximum is large when the solar polar magnetic field points toward the Sun in the northern hemisphere, and radial anisotropy is absent. These could also be the epochs when the annual mean magnitude of interplanetary magnetic field (IMF) is the largest in the heliosphere, indicating a possible Hale cycle variation of IMF flux. We have calculated the values of the important modulation parameter λ∥Gr for the 1968‐1970 period, over a wide range of primary rigidities. The time period covers a part of the negative solar magnetic polarity epoch. The computed mean value is (1.085 ± 0.015)%.