Spatial gradients of GCR protons in the inner heliosphere derived from Ulysses COSPIN/KET and PAMELA measurements

TL;DR

This study measures the spatial gradients of galactic cosmic ray protons in the inner heliosphere during an unusual solar minimum, revealing energy-dependent variations that inform cosmic ray transport models.

Contribution

It provides the first detailed measurements of radial and latitudinal GCR proton gradients during a unique solar minimum using combined Ulysses and PAMELA data.

Findings

Latitudinal gradient magnitude varies with rigidity.

Energy dependence of gradients differs from previous solar magnetic epochs.

Results challenge existing cosmic ray transport models.

Abstract

During the transition from solar cycle 23 to 24 from 2006 to 2009, the Sun was in an unusual solar minimum with very low activity over a long period. These exceptional conditions included a very low interplanetary magnetic field (IMF) strength and a high tilt angle, which both play an important role in the modulation of galactic cosmic rays (GCR) in the heliosphere. Thus, the radial and latitudinal gradients of GCRs are very much expected to depend not only on the solar magnetic epoch, but also on the overall modulation level. We determine the non-local radial and the latitudinal gradients of protons in the rigidity range from ~0.45 to 2 GV. This was accomplished by using data from the satellite-borne experiment Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) at Earth and the Kiel Electron Telescope (KET) onboard Ulysses on its highly inclined Keplerian orbit around the Sun with the aphelion at Jupiter's orbit. In comparison to the previous A>0 solar magnetic epoch, we find that the absolute value of the latitudinal gradient is lower at higher and higher at lower rigidities. This energy dependence is therefore a crucial test for models that describe the cosmic ray transport in the inner heliosphere.