Cosmic Ray Modulation During Extremely Weak Solar Cycle

The Sun is exhibiting low solar activity levels since the descending phase of the last solar cycle, and it is likely to be continued as well as in the case of the past grand solar minima. In the current and next weak solar cycles, it is expected that the flux of galactic cosmic ray (GCR) becomes higher than that in the previous solar cycles, leading to the increase in the radiation exposure [1] and single event upset rates in space and the atmosphere. However, quantitative estimates of the GCR flux and resultant effects on the geospace environment such as the radiation exposure during such a recent weak solar cycle is still one of the greatest challenges in space weather research. Grand Minimum 7 (Gm7) is a new project for the study of such a weak solar wind conditions and cosmic-ray modulation including extremely weak solar cycle such as the past ground minima. Gm7 aims to reveal the following issues: fundamental structures of the solar wind and the heliospheric magnetic field during extremely weak solar cycle such as the past ground minima; a mechanism of unique cosmic-ray modulation during the past ground minima, such as anomalous spikes of GCRs at the qA negative solar minima [2]; extreme space weather conditions on the resultant radiation exposure, single event upset rates, and the altitude distribution of the Earth's atmospheric ionization. In order to achieve these objectives, we have developed a cosmic-ray modulation model based on the stochastic differential equation (SDE) [1] and a dynamical MHD model of the heliosphere with the adaptive mesh refinement (AMR) technique [3]. By coupling these models, we develop a hybrid simulation code of the cosmic-ray modulation. Our model has a potential for helping understand the effects of the GCRs on the geospace environment in the recent weak solar cycle. The overview and our latest results made by Gm7 project will be presented at the meeting.

[1] Miyake, S., R. Kataoka, and T. Sato, Space Weather, 15(4), 589-605, 2017.

[2] Kataoka, R., H. Miyahara, and F. Steinhilber, Space Weather, 10, S11001, 2012.

[3] T. Matsumoto, PASJ, 59, 905-927, 2007.