Short-period fluctuations of cosmic ray intensity at the geomagnetic equator and their solar and terrestrial relationship

Abstract

By using power spectrum analysis on the μ meson intensity records of a high counting rate instrument (10⁶ counts/min) operated at Chacaltaya, Bolivia, it has been possible to identify the presence of cosmic ray fluctuations of 15 to 19 and 25 to 28 cycles per hour (cph). These frequencies were found at a 99% level of significance in the average of 1034 three-hour samples extending from November 1965 to June 1966. Comparison with Pioneer 6 measurements (Ness et al., 1966) indicates that the magnetosheath field, at a distance of about 10 earth radii (R_E), and the interplanetary magnetic field show peaks in spectral density that correspond closely (though not identically) with peaks in muon intensity. It is most plausible that these cosmic ray fluctuations are caused by the fluctuations of geomagnetic field corresponding to an amplitude of about 20 γ in the dipole magnetic field. Magnetic field measurements conducted in the magnetosphere by Explorer 6 (Judge and Coleman, 1962) and Explorer 12 (Patel and Cahill, 1964) show similar oscillations. Recent observations by ATS 1 confirm the presence of magnetic oscillations of similar frequencies at about 6 R_E . These frequencies are also found in the geomagnetic micropulsations observed at the surface of the earth. The integrated power of cosmic ray oscillations in the frequency range of 6 to 30 cph has been studied at various periods, and its solar-terrestrial relationship is examined. The association of observed frequencies in cosmic rays with frequencies in the solar photosphere and the interplanetary magnetic field, as well as the resonance frequencies of the magnetosphere, are discussed.