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Abstract

Spatial gradients of galactic cosmic-ray (GCR) fluxes are important for studying charged particle transport in the heliosphere. Little is known so far about how these gradients evolve with time. Here we present how the radial gradient (G _r ) evolves between 2006 January and 2017 September using Cassini measurements of >300 MeV protons, which we combine with proton spectra obtained by advanced observatories at Earth (BESS, BESS-Polar, PAMELA, and AMS-02). All gradient calculations were performed for a nearly constant heliocentric distance of Cassini from Earth's orbit and near the ecliptic, thus revealing only how G _r changes with time. The variability patterns of G _r are well established as they rely on a single data set for ~9.5 au and accurately cross-calibrated GCR spectra for Earth at 1 au. We show that over solar-cycle timescales, G _r is regulated by both the polarity of the solar magnetic field and the solar-cycle phase. During the negative-polarity phase (A < 0, 2006–2014), gradients are stronger and more stable with an average of G _r = 3.5 ± 0.3%/au and with evidence of a minimum around the 2009 solar minimum. The gradient peaks at ~4%/au around the solar maximum and subsequently experiences a gradual drop to ~2%/au in the A > 0 phase (2014–2017). Regular G _r enhancements over yearly or biennial timescales are also observed, in phase with quasi-biennial oscillations in GCR intensity. While all aforementioned results are based on Cassini measurements that are integral in energy, they are most representative for GCR protons in the low gigaelectronvolt range.