Changes in the cosmic ray and heliomagnetic components of space climate, 1428-2005, including the variable occurrence of solar energetic particle events

The cosmic-ray record has been used to study the variations in the space climate, 1428-2005. Inversion of the data shows that the heliomagnetic field (HMF) near Earth increased steadily over the past 580 years, exhibiting the strongest fields during the last 50 years. The estimates indicate that the HMF at sunspot minimum has exhibited steadily increasing "floors" between the several Grand Minima, similar to the one evident in satellite data since 1965. The cosmogenic data for the past 10,000 years contain an similar to 2300 year periodicity, and it is proposed that the increasing HMF strength since the 15th century represents the first quarter cycle of a similar to 2300 year quasi-periodicity. It is concluded that the 11-year average total magnetic flux of the Sun has increased by a factor of similar to 4.5 over the past 580 years. It is speculated that this could indicate a factor of similar to 9 variation over the similar to 2300 year cycle. The cosmic ray data and theoretical considerations show that the 22-year periodicity in the cosmic radiation flux at Earth was more dominant at times of low solar activity, compared to the present epoch. Comparison of the occurrence of solar energetic particle events, and the estimated HMF, shows that a substantial decrease in the size and frequency of occurrence of GLE (ground level enhancements) after 1958 coincides with a substantial increase in the HMF. This is consistent with the conclusion of [McCracken, K.G., Dreschhoff, G.A.M., Smart, D.F., et al. A study of the occurrence of large-fluence solar proton events and the strength of the interplanetary magnetic field. Solar Phys., 224, 359-372, 2004.] that lower values of the solar magnetic fields result in increased values of the Alfven Mach number, and thence to more efficient acceleration of solar cosmic rays prior to 1958. It is suggested that the variability of the solar and heliospheric magnetic fields may have introduced long term changes into the nature of geomagnetic phenomena; particle acceleration throughout the heliosphere and heliosheath; and possibly to the luminosity of the facular network of the Sun. (C) 2007 COSPAR. Published by Elsevier Ltd. All rights reserved.