Cosmic Ray Intensity Variation Lags Sunspot number: Implications of Late Opening of Solar Magnetic Field

TL;DR

This paper reveals that the lag in galactic cosmic ray intensity relative to sunspot numbers is mainly due to delayed opening of the solar magnetic field, with implications for space radiation risk assessment.

Contribution

It introduces the concept that late opening of the solar magnetic field, especially from low latitudes, explains the cosmic ray lag better than previous models.

Findings

GCR lag is linked to delayed opening of solar magnetic flux.

Open flux from low latitudes significantly influences the lag.

Odd-even cycle differences affect the magnetic field opening delay.

Abstract

Galactic cosmic rays (GCRs), the highly energetic particles that may raise critical health issues for astronauts in space, are modulated by solar activity with their intensity lagging behind the sunspot number (SSN) variation by about one year. Previously, this lag has been attributed to a combined effect of outward convecting solar wind and inward propagating GCRs. However, the lag's amplitude and its solar-cycle dependence are still not fully understood (Ross & Chaplin, 2019). By investigating the solar surface magnetic field, we find that the source of heliospheric magnetic field -- the open magnetic flux on the Sun, already lags behind SSN before it convects into heliosphere along with the solar wind. The delay during odd cycles is longer than that during sequential even cycles. Thus, we propose that the GCR lag is primarily due to the greatly late opening of the solar magnetic field with respect to SSN, though solar wind convection and particle transport in the heliosphere also matter. We further investigate the origin of the open flux from different latitudes of the Sun and found that the total open flux is significantly contributed by that from low latitudes where coronal mass ejections frequently occur and also show an odd-even cyclic pattern. Our findings challenge existing theories, and may serve as the physical basis of long-term forecasts radiation dose estimates for manned deep-space exploration missions.