A Comparative Study of Two Contrasting Cosmic-Ray Events Caused by Solar Eruptions from NOAA AR 12673 in 2017 September

Two major solar eruptions on AR 12673 produced a Forbush decrease (FD) event (reduction of cosmic rays) on 2017 September 8 and ground-level enhancement (GLE; enhancement of cosmic rays) on 2017 September 10. The occurrence of two contrasting cosmic-ray events within 2 days that are associated with two similar X-class solar flares (X9.3 and X8.2) and share the same active region on the Sun provides us a rare opportunity to understand the dominant factors in determining the properties of transient cosmic-ray events. Using a suite of modern-day instruments continuously tracking solar eruptions from the Sun to the Earth with ground-based cosmic-ray detectors, we reveal the complete cause-effect chain of activities for these two events. We conclude that the different consequences on the ground arise from two effects of the eruptions near the Sun: (1) the geometric effect of CMEs and (2) the intensity effect of CME-driven shocks. The first eruption, which originated at the heliographic longitude of W34° on 2017 September 6, had its CME ejecta and CME-driven shock intercept the Earth, leading to the FD event. The second eruption, which occurred on September 10 at W88°, only had its far flank reach the Earth. The peak shock speed of 3344 km s^-1 of the second eruption, much faster than the 2175 km s^-1 of the first eruption, is the dominant factor producing the GLE event, even though the first eruption is better connected magnetically to the Earth and has a similar flare. The results indicate that the production of GLE particles can be dominated by fast-enough CME-driven shocks.