  Ice cores are known to be rich in information regarding past climates, and
the possibility that they record astronomical phenomena has also been
discussed. Rood et al. were the first to suggest, in 1979, that nitrate ion
(NO3-) concentration spikes observed in the depth profile of a South Pole ice
core might correlate with the known historical supernovae (SNe), Tycho (AD
1572), Kepler (AD 1604), and SN 1181 (AD 1181). Their findings, however, were
not supported by subsequent examinations by different groups using different
ice cores, and the results have remained controversial and confusing. Here we
present a precision analysis of an ice core drilled in 2001 at Dome Fuji
station in Antarctica. It revealed highly significant three NO3- spikes dating
from the 10th to the 11th century. Two of them are coincident with SN 1006 (AD
1006) and the Crab Nebula SN (AD 1054), within the uncertainty of our absolute
dating based on known volcanic signals. Moreover, by applying time-series
analyses to the measured NO3- concentration variations, we discovered very
clear evidence of an 11-year periodicity that can be explained by solar
modulation. This is one of the first times that a distinct 11-year solar cycle
has been observed for a period before the landmark studies of sunspots by
Galileo Galilei with his telescope. These findings have significant
consequences for the dating of ice cores and galactic SN and solar activity
histories.
