The 2300 year Modulation in the Galactic Cosmic Radiation

This paper examines the properties of the ~2300-year periodicity in the galactic cosmic radiation, previously recognized in power spectra of cosmogenic Be and C. It shows that the periodicity consists of short episodes (50-100 year) of high cosmic ray intensity, such as accompanied the Spoerer and Maunder Minima, separated by long intervals (>1000 years) of low intensities similar to, or lower than those observed by cosmic ray instruments since 1936. The cosmic ray data are used to investigate the long-term variation in the strength of the heliospheric magnetic field (HMF). The terrestrial and solar implications of the ~2300 yr periodicity are discussed. The Cosmic Ray Intensity Variations in the Past The worldwide neutron monitor network has shown that the cosmic ray intensity at sunspot minimum was essentially invariant from 19542007, with 11-year modulation cycles depressing the intensity below that level in anti-phase with solar activity. More recently the instrumental ionization chamber measurements (1933-1969), and the cosmogenic Be and C data have shown that the cosmic ray intensity since 1954 has been one of the lowest in the past 1000 years [1,2,3]. This is illustrated by Figure 1, which shows that the 22-year average “modulation potential” was ~100 MV when the Sun was inactive, increasing to ~650 MV in the present epoch. Figure 1 shows that there were six periods between 850 and 2000AD during which the cosmic ray intensity was substantially above the adjacent values, and above the intensities observed since 1954. The enhancements near 1700, 1810, and 1895 correspond to the Maunder, Dalton, and the1895 minima evident in the sunspot record. The other three enhancements correspond to the Oort (~ 1050); Wolf (1325-1375); and Spoerer (1420-1540) minima in auroral activity, implying low solar activity. The Spoerer enhancement in Figure 1: The temporal variation of the cosmogenic Be for the interval 850-1958 AD. The periods of low solar activity, and their counterparts in the Be records are identified as follows; O=Oort Minimum; W=Wolf; S= Spoerer; M= Maunder; D= Dalton; and G= the Gleissberg Minimum of ~1895. The Be concentrations are given in atoms per gram. The right hand scale and the horizontal lines give the modulation potential. The bottom panels present the annual group sunspot number (1600-2000), and the normalized frequency of occurrence of the mid-latitude aurora between 1000-1900 AD. Other details of this figure are given in reference 1. IC R C 2007 P rocedings P re-C onrence E dtion THE 2300 YEAR MODULATION the cosmic ray intensity appears to have been the most profoundthe cosmogenic data indicate that it lasted for about a century, and that the cosmic ray modulation was very low, suggesting a very low level of solar activity. The six minima in solar activity mentioned above are sometimes called the “Grand Minima” Inspection of Figure 1 shows that the cosmic ray enhancements associated with the Grand Minima had the following characteristics. (a) Their rise and fall times were short, typically ≤50 years. (b) The overall duration is short, typically 50-100 years. (c) The cosmogenic data increased by ~80% above the adjacent values and the modulation potential decreased by ~500 MV to the vicinity of 100-150 MV. 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5