Solar cosmic ray events for the period 15611994 2. The Gleissberg periodicity

A total of 125 large fluence solar proton events identified from the nitrate deposition in ice core from Greenland for the period 1561-1950 are examined in an exploratory study of the geophysical information that will be available from such data in the future. These data have been augmented with ionospheric and satellite data for the period 1950-1994. There were five periods in the vicinity of 1610, 1710, 1790, 1870, and 1950, when large >30 MeV proton events with fluence greater than 2 x 109 cm -2 were up to 8 times more frequent han in the era of satellite observation. There is a welldefined Gleissberg (approximately 80 year) periodicity in the large fluence proton events, with six well-defined minima, two in close association with the Maunder and Dalton minima in sunspot number. The present "satellite" era is recognized as a recurrence of this series of minima. Comparison of the total solar proton production for the five Gleissberg cycles since 1580 shows that the cycle 1820-1910 was the most active followed by the cycle 1580-1660. The present Gleissberg cycle is one of the least effective i n the production of solar proton events at Earth. It is shown that the solar and solar proton event data both indicate that the Maunder Minimum ended about 1700, 16 years before the commonly accepted date of 1716. It is proposed that the delayed "switch on" of aurorae after the Maunder Minimum is due to the changing nature of the solar corona from "Maunder Minimum" conditions to the more active conditions of the Gleissberg cycle, and a physical mechanism is proposed in which variations in the coronal densities modulate the efficiency of solar proton event production throughout he Gleissberg cycle. The "streaming limited fluence" for >30 MeV protons i estimated tobe 6-8 x 10 9 cm -2, and the rapid decrease inthe probability of occurrence ofsolar proton events observed in the vicinity of this fluence is proposed to be due to this effect.