Past solar activity is studied based on analysis of data on the nitrate con
tent of Greenland ice in the period from 1576-1991. Hundred-year lover the
entire period) and quasi-five-year tin the middle of the 18th century) vari
ations in the nitrate content are detected. These reflect the secular solar
-activity cycle and cyclicity in the flare activity of the Sun. (C) 2000 MA
IK "Nauka/Interperiodica".