How does the 11-year solar cycle alter the cosmic ray flux?

Looking at a flux vs. time graph of secondary cosmic rays (e.g. from a neutron station: www.nmdb.eu/nest/search.php can plot you some. I just tried for example Moscow, from 14 Oct 1951 to today, scaling options $\to$ Specify your own Y axis scale -20,20, left untouched all other options), it's easy to see a periodicity that fits well with the solar 11 - 22 year cycles.

So, why do solar cycles influence the rate at which cosmic rays hit the Earth?

I suspect the reason is so well known among experts that is not mentioned in the introduction of the first random article about this I looked for. Maybe you know a nice, simple book/review article?

For our purposes, there are two types of cosmic rays: Particles coming from the Sun ("Solar energetic particles", or SEPs) and cosmic rays from other sources outside the Solar System.

During periods of intense solar activity - not just during the maxima of the solar cycles - SEPs may be released in large numbers. However, energetic particles from the Sun, as well as its magnetic field, can scatter galactic cosmic rays, leading to a much smaller number reaching Earth. This is known as a Forbush decrease.

Here's an example of this, during two coronal mass ejections (CMEs), unrelated to the solar maxima:

A and C are when the Sun undergoes CMEs, and B and D are when particles from those CMEs reach Earth. Both lead to Forbush decreases, although the second is more dramatic, as is the spike preceding it (likely from SEPs).

On a larger scale, of course, there is indeed a correlation between the solar cycle and cosmic ray detection:

This happens for much the same reasons as the Forbush decreases. The above chart, by the way, matches very well the observations recorded in the graph you specified:

• Also looking at the link, it's "... and B and D are when particles from those CMEs reach Earth." – Effervescenza Naturale Oct 14 '16 at 18:12
• @EffervescenzaNaturale That's a typo on my part. Thanks. – HDE 226868 Oct 14 '16 at 18:13
• Thank you for the nice and prompt answer. The fact that the charts match it's not a coincidence, is it? Once you correct for pressure etc, it's only about the Sun and the rest of the universe, so different stations should at most shift by some hours due to Earth rotation. – Effervescenza Naturale Oct 14 '16 at 18:40
• @EffervescenzaNaturale Correct, it's not a coincidence. And on a graph with a scale as large as this one, the differences should be extremely hard to detect, although they should still be there. – HDE 226868 Oct 14 '16 at 18:43
• @EffervescenzaNaturale - No, it's not a coincidence. The large (and twisted) magnetic fields of CMEs act like giant magnetic mirrors (or scattering centers) off of which cosmic rays scatter and/or reflect. Since CMEs expand nearly as fast as they propagate, they quickly become extremely large (several AU in size by the time they reach Earth) and can act like giant shields. However, the Forbush decrease only applies to particles below roughly TeV energies. Particles at higher energies can "plow through" even CMEs so their fluxes are not affected as much. – honeste_vivere Oct 16 '16 at 16:46