Magnetic field strength distributions and spectra in the heliosphere and their significance for cosmic ray modulation: Voyager 1, 1980–1994

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Abstract
Annual variations of the hourly averages of the heliospheric magnetic field strengthBobserved by Voyager 1 (V1) have been analyzed for the 15‐year interval 1980–1994 inclusive. During this period, V1 moved from a radial distance of 6.9–58.1 AU and from a heliolatitude of −5.4 to 32.7°. The annual distribution function ofBis nearly lognormal. The width of the distribution, w, does not change systematically with time, distance, latitude or solar activity: 〈w〉 = 0.61 ± 0.02. The standard deviation ofB, SD(B), normalized by the average,Bav, is also invariant: 〈SD(B)/Bav〉 = 0.67 ± 0.03. There is a decadal frequency range for which the power spectral density (PSD) ofB(t) is well described by a power law with an exponent s that ranges from −2.0 near solar maximum (1980, 1990) when GMIRs were present to −1.5 near solar minimum (1987–1988). A Global Merged Interaction Region (GMIR) is a shell‐like region that encircles the Sun, propagates away from the Sun, has a radial thickness of the order of a few AU and contains intense, disturbed magnetic fields. The lower limit of the scaling range corresponds to a period of ≈ 5 days and is invariant in the 15 years studied. The relative amplitude of the PSD at the lower limit of this range is also invariant. However, the relative amplitude of the fluctuations at the solar rotation period of 26 days decreases with increasing distance from the sun asR−(0–9 ± 0.2). These derived observational results in time and frequency domains require theoretical models of the solar wind expansion into the distant heliosphere that include both physical and statistical features. Such models could be based on (1) energy and/or entropy principles or (2) conventional MHD along with temporally complex but statistically simple input functions. Our results suggest an anticorrelation between the annual magnetic field strength and cosmic ray intensity, as is observed during the 15‐year period considered here. Statistical models of the magnetic field might provide a basis for new models of cosmic ray modulation.