10Be Signature of the Cosmic Ray Event in the 10th Century CE in Both Hemispheres, as Confirmed by Quasi‐Annual 10Be Data From the Antarctic Dome Fuji Ice Core.

Cosmogenic nuclides are good indicators of past cosmic ray events and variations. To verify such phenomena, it is important to evaluate them using multiple nuclides from different archives. The cosmic ray event in 993–994 Common Era (CE) has already been confirmed with 14C and 10Be data, which show rapid increases in the concentrations. However, the 10Be data were obtained from the Greenland ice cores in the Northern Hemisphere. To investigate the extent of the 10Be increase in the Southern Hemisphere, we measured quasi‐annual 10Be concentrations between 980 and 1,011 CE in the Antarctic Dome Fuji ice core. We observed a ~50% increase in 10Be concentration around 994 CE, consistent with the Greenland data. Increases in 10Be concentrations in both hemispheres support a solar origin of the 994‐CE event. In addition, we propose a method of evaluating the so‐called "system effect" for 10Be deposition by extracting common components from 10Be and Na+ data. Plain Language Summary: New quasi‐annual beryllium‐10 measurements were made with the Dome Fuji ice core from Antarctica over the period in which the 994 cosmic ray event would be expected. We observed an approximately 50% increase in beryllium‐10 concentrations, which is consistent with the beryllium‐10 increases observed in the Greenland ice cores. This lends support to a solar origin of the 994 event. We propose a phenomenological method for evaluating a common deposition component between quasi‐annual beryllium‐10 and sodium ion data. Key Points: New quasi‐annual 10Be data were taken with Antarctic Dome Fuji ice core over the period when the 994‐CE cosmic ray event took placeWe observed a ~50% increase in 10Be concentrations, which is consistent with the 10Be increases observed in the Greenland ice coresWe propose a phenomenological method for evaluating a common deposition component between quasi‐annual beryllium‐10 data and Na+ data [ABSTRACT FROM AUTHOR]