Spectral Properties of the N Component of the Heliospheric Magnetic Field from IMP and ACE Observations for 1973–2020

Abstract

Abstract We analyze the normal (N) component of the heliospheric magnetic field observed by the Interplanetary Monitoring Platform and the Advanced Composition Explorer spacecraft for the period 1973–2020. Parameters characterizing the frequency spectrum are calculated with a novel technique, which is based on calculating variances at incremental lags to yield the integral of a turbulence spectrum. We compare this technique with the standard second-order structure function to show their similarity in the inertial range, and use the latter to calculate correlation functions. We find that the yearly average for magnetic field magnitude and the variance attained their lowest values since spacecraft observations began for the period that includes the 2020 solar minimum, 4.2 nT and 3.3 nT2, respectively. The ratio of the magnitude of fluctuations of the N component to the field magnitude shows little variation, with an average value of 0.43 ± 0.04. The average value of the spectral index of the energy range for the whole data set is −1.0 ± 0.1, and shows some solar-cycle dependence. The average value for the inertial range is an almost constant −1.69 ± 0.04. While the break between the energy and the inertial range is difficult to determine accurately to search for a solar-cycle dependence, an indirect indication of such a dependence follows when the ratio of spectral levels in the energy and in the inertial range is calculated. The e-folding correlation length has an average value of 1.1 ± 0.3 Mkm, with a clear solar-cycle dependence.