Editing Solar cycle (section)

==== Galactic cosmic ray flux ====
The outward expansion of solar ejecta into interplanetary space provides overdensities of plasma that are efficient at scattering high-energy [[cosmic rays]] entering the [[Solar System]] from elsewhere in the galaxy. The frequency of solar eruptive events is modulated by the cycle, changing the degree of cosmic ray scattering in the outer Solar System accordingly. As a consequence, the cosmic ray flux in the inner Solar System is anticorrelated with the overall level of solar activity.<ref>{{cite journal|last1=Potgeiter|first1=M.|title=Solar Modulation of Cosmic Rays|journal=Living Reviews in Solar Physics|volume=10|issue=1|page=3|doi=10.12942/lrsp-2013-3|arxiv = 1306.4421 |bibcode = 2013LRSP...10....3P |year=2013|doi-access=free |s2cid=56546254}}</ref> This anticorrelation is clearly detected in cosmic ray flux measurements at the Earth's surface.

Some high-energy cosmic rays entering Earth's atmosphere collide hard enough with molecular atmospheric constituents that they occasionally cause nuclear [[Cosmic ray spallation|spallation reactions]]. Fission products include radionuclides such as [[Carbon-14|<sup>14</sup>C]] and [[Beryllium-10|<sup>10</sup>Be]] that settle on the Earth's surface. Their concentration can be measured in tree trunks or ice cores, allowing a reconstruction of solar activity levels into the distant past.<ref>{{Cite journal
 | first1=Sami K.| last1=Solanki
 | author-link=Sami Solanki
 | first2=Ilya G.| last2=Usoskin
 | first3=Bernd  | last3=Kromer
 | first4=Manfred| last4=Schüssler
 | first5=Jürg   | last5=Beer
 | title=Unusual activity of the Sun during recent decades compared to the previous 11,000 years
 | journal=Nature
 | volume=431 | date=2004 | pages=1084–7
 | url=http://cc.oulu.fi/%7Eusoskin/personal/nature02995.pdf
 | doi=10.1038/nature02995
 | pmid=15510145
 | issue=7012
 |bibcode = 2004Natur.431.1084S | s2cid=4373732
 }}</ref> Such reconstructions indicate that the overall level of solar activity since the middle of the twentieth century stands amongst the highest of the past 10,000 years, and that epochs of suppressed activity, of varying durations have occurred repeatedly over that time span.{{Citation needed|date=February 2024}}