Climatic Data - Climate Change: Identification and Projections - page 21

(Maunder minimum) and sporadic. The Wolf index is well reconstituted
from 1700, and even more so from 1750. Wolf started numbering the cycles
in 1750. The peak of cycle 24 was reached in 2012-2013.
Figure 2.7 6 shows the evolution of the Wolf number since 1700, and its
moving average over 11 years. This clearly shows that average solar activity
reached its maximum values during the last half-century and is now in
decline.
Isotopic proxies
A third effect of activity is the solar magnetic field. Direct measurements
are limited to the recent period. However, an alternative measurement over a
millennium is calculated with levels of isotopes known as cosmogenic , or
cosmonuclides : beryllium-10 ( 10 Be), or carbon-14 ( 14 C). These isotopes are
generated in the atmosphere by cosmic rays, which are themselves regulated
by the solar magnetic field.
Two databases were used (Figure 2.8), based on 10 Be (changed sign in
order to respect TSI direction of variation).
Figure 2.8. Proxies for TSI
6 Source: ROYAL OBSERVATORY OF BELGIUM. Av. Circulaire, 3- B-1180 BRUSSELS
http://sidc.oma.be/html/sunspot.html.