Diurnal variation of cosmic ray intensity I. Two approaches to the study
Abstract
The investigation of the daily variation of cosmic ray intensity has been carried out over the past two decades by either of two approaches: the traditional Fourier series method and the more recently introduced power spectral method. A comparison of the two approaches is essential to the proper understanding of the results derived from them. The present study, for the first time, adopts both approaches for investigating the dta from the Sulfur Mountain super neutron monitor for the period of midDecember 1965 to April 1966 (extending over five solar rotations), when interplanetary magnetic field data from Pioneer 6 were also available. Problems relating to the analyses of both data sets on a daytoday basis and on a statistical basis over a number of days are discussed. The power spectral analysis method cannot provide information on the phase of the diurnal variation or information on the diurnal amplitude on a daytoday basis. This method, however, provides excellent estimates of the diurnal anisotropy amplitude independent of any daily phase shifts in the anisotropy. In addition, this method provides a measure of the ambient anisotropy amplitude which, for the entire period studied, has a mean value of ~0.1%. The Fourier series method can yield reliable measures of the amplitude and phase on a daytoday basis, provided the time series is reasonably stationary. This method cannot estimate the ambient anisotropy amplitude which, for small amplitudes, contributes to large uncertainties in the Fourier coefficients. We find that there is a general agreement between the observed diurnal variation over each period considered and that predicted theoretically from parameters derived from the interplanetary data. However, for most of the periods examined the ratio of the perpendicular diffusion coefficient to the parallel diffusion coefficient is rather small (K_{1}/K <~0.1). As such the diurnal variation amplitude is generally most sensitive to the interplanetary field direction and the solar wind velocity and not to the calculated values of the diffusion coefficients. Further tests of the theory will require analyses of other periods when perhaps the perpendicular diffusion is larger and/or measurements of cosmic rays outside the atmosphere are available in order to obtain better energy resolution on the detected particles.
 Publication:

Journal of Geophysical Research
 Pub Date:
 November 1978
 DOI:
 10.1029/JA083iA11p05139
 Bibcode:
 1978JGR....83.5139S
 Keywords:

 Cosmic Rays;
 Diurnal Variations;
 Particle Flux Density;
 Anisotropy;
 Data Reduction;
 Diffusion Coefficient;
 Fourier Series;
 Interplanetary Magnetic Fields;
 Magnetic Effects;
 Power Spectra;
 Solar Wind Velocity;
 Space Radiation