Variations of the Galactic Cosmic Rays in the Recent Solar Cycles

TL;DR

This study analyzes galactic cosmic ray variations during solar cycles 23 and 24, revealing record-high GCR intensities during the 2019-2020 solar minimum and highlighting differences in solar modulation compared to previous minima.

Contribution

It provides new insights into GCR variations and solar modulation differences during recent solar minima, supported by measurements from ACE/CRIS and neutron monitors.

Findings

Record-breaking GCR intensities in 2019-2020 surpass previous cycles.

Significant differences in solar minimum conditions affecting GCR modulation.

Lower neutron monitor counts despite higher GCR intensities in space.

Abstract

In this paper, we study the galactic cosmic ray (GCR) variations over the solar cycles 23 and 24, with measurements from the NASA's ACE/CRIS instrument and the ground-based neutron monitors (NMs). The results show that the maximum GCR intensities of heavy nuclei (nuclear charge 5-28, 50-500 MeV/nuc) at 1 AU during the solar minimum in 2019-2020 break their previous records, exceeding those recorded in 1997 and 2009 by ~25% and ~6%, respectively, and are at the highest levels since the space age. However, the peak NM count rates are lower than those in late 2009. The difference between GCR intensities and NM count rates still remains to be explained. Furthermore, we find that the GCR modulation environment during the solar minimum P24/25 are significantly different from previous solar minima in several aspects, including remarkably low sunspot numbers, extremely low inclination of the heliospheric current sheet, rare coronal mass ejections, weak interplanetary magnetic field and turbulence. These changes are conducive to reduce the level of solar modulation, providing a plausible explanation for the record-breaking GCR intensities in interplanetary space.