Solar activity during the last millennium, estimated from cosmogenic in-situ C14 in South Pole and GISP2 ice cores

The principal aim of this research is to determine the precise manner in which solar activity has varied in the past 1000 years. During this period, four periods of very low solar activity have been identified: Wolf (1305-1345 AD), Spoerer (1418-1540 AD), Maunder (1645-1715), and one period of high solar activity (1100-1250 A.D.) have been deduced based on available historical records of sunspot numbers and aurora. Our proposal aims to study the solar activity during the past 1000 years in detail using a new method, based on studies of polar ice, as developed earlier (Earth and Planetary Science Letters, 234, 335-349, 2005). The method is based on the fact that greater solar activity leads to production of greater magnetic fields in the heliosphere, which reduces the primary cosmic ray flux in the near Earth environment, and vice-versa. Consequently if one can measure the primary cosmic ray flux in the near Earth space, it becomes a direct measure of the solar activity. Lal et al. (Earth and Planetary Science Letters, 234, 335-349, 2005) concluded that the best way of measuring the primary cosmic ray flux would be to measure the concentration of cosmogenic in-situ produced 14C in polar ice sheets, which was discovered by Lal et al. (Nature 346, 350-352, 1990). Following this idea Lal et al. (op. cit.) measured cosmogenic in-situ produced in 19 samples from the GISP 2 core covering time range of 375-31,250 yrs B.P. Their studies showed that there were two periods of very low solar activity in this time bracket (during 8500-9500 B.P and 27,000-32,000 B.P.), and one high solar activity period during 12,000-16,000 yrs B.P. In order to provide an independent check on the veracity of the new method, we decided to apply it to the historical period, < 1000 yrs B.P. The inferred Solar activities based on the study of cosmogenic in-situ produced 14C in South Pole ice samples clearly establish that there was a period of high Solar activity during 1100-1250 A.D., and a period of very low solar activity during 1416-1534 A.D, designated as the Spoerer Minimum. These results however do not confirm the proposed dates for the Dalton and the Maunder Minimum periods, predicted to be 1795-1825 A.D. and 1654-1714 A.D. respectively. Instead, our studies show that there was a long duration period of low solar activity during 1750-1860 A.D. These results make it quite clear that we should carry out more studies to fully establish the temporal behavior of the Solar activity in the past 1000 yrs.