Previously, we have observed that the atmospheric (14)C content measured in tree rings showed a strong increase from AD 774 to 775. Although the cause of this event can be explained by a large solar proton event or a short gamma-ray burst, a more detailed discussion of the cause is difficult because the rate of occurrence of such rapid (14)C events remains unknown. Here we report new (14)C measurements from AD 822 to 1020, and the discovery of a second rapid increase of (14)C content from AD 992 to 993. The (10)Be flux in the Antarctic ice core shows peaks corresponding to these two (14)C events. The proportions of flux increase ((14)C/(10)Be) of the two events are consistent with each other. Therefore, it is highly possible that these events have the same origin. Considering the occurrence rate of (14)C increase events, solar activity is a plausible cause of the (14)C increase events.
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