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 degrees 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 degrees, 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.