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Measurements of the interplanetary magnetic field in relation to the modulation of cosmic rays

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Abstract

Field strength distributions and low frequency power spectra are derived from interplanetary field measurements made by the HEOS-1 and HEOS-2 satellites during the years 1969–1973. The spectral analysis involved the use of a technique which is shown to allow correctly for missing data. Comparison spectra, derived by the same technique, are presented for the years 1963–1968. The use of mear-field-aligned co-ordinates enabled the easy separation of the transverse and longitudinal fluctuation spectra. A power law function involving a ‘break point’-frequency was fitted to each spectrum by a least squares technique.

The total power level, the power spectral density at zero frequency and the correlation length are found to vary significantly and in a similar way over the solar cycle. The magnitude and phase of these variations are compared with measurements of the cosmic ray neutron monitor rate and the coronal green line intensity and the influence of mid-latitude solar phenomena on the character of the interplanetary field in the ecliptic is demonstrated. The correlation length and zero frequency power density are found to be considerably larger than previously estimated and, contrary to the usual assumption in modulation theory, the rms amplitude of the perturbation field is comparable to the mean field experienced by the high rigidity particles. Although the mean interplanetary field strength is found to be independent of the level of solar activity, during higher activity the most probable vector average decreases by approximately 0.5 γ due to the enhanced directional fluctuation in the field. Power anisotropy measurements suggest that Alfvénic disturbances in the solar wind have fluctuation spectra confined mainly to frequencies larger than 10−3 Hz.

The data are interpreted as indicating that the cosmic ray intensity in the Galaxy is some 75% larger than the intensity recorded by neutron monitors on Earth. Previous failure to find a correlation between neutron monitor intensity and interplanetary field parameters is attributed to a lack of statistical accuracy in the field data. The measured power spectra are used to estimate the magnitude of the parallel diffusion coefficient using the relationships derived by Klimas and Sandri, Jokipii, and Quenby et al.

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  1. Imperial College, S.W.7., London, UK

    P. C. Hedgecock

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Hedgecock, P.C. Measurements of the interplanetary magnetic field in relation to the modulation of cosmic rays. Sol Phys 42, 497–527 (1975). https://doi.org/10.1007/BF00149929

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