In their way to the Earth, the cosmic rays produced in the galaxy by many different sources (called Galactic Cosmic Rays, GCR), find themselves deflected by magnetic fields.
The main effect is produced by the solar magnetic field. When it is intense (ie. when the sun is active), GCR are deflected and the flux at Earth is therefore reduced. When it is lower (ie. when the sun is calm), GCR find their way more easily to the Earth and the flux observed is higher.
By measuring the GCR flux, one can therefore determine indirectly the solar activity. This allows CR experiments to observe the 11 (and 22) years cycles of the sun, as well as violent transients called Forbush decreases.
When a coronal mass ejection (CME) is produced at the sun, a huge mass of magnetised plasma is sent through the interplanetary medium. Upon reaching Earth, this plasma acts as a shield to GCR, and a rapid disminution of GCR flux can be observed (a few percents in a few hours). Then, once the CME goes on his way, the GCR flux slowly comes back to its original value, on a time scale of days. This GCR flux decrease is called Forbush decrease and can be nicely observed by any cosmic ray detector on Earth.
LAGO WCD are being used to study the solar activity. As they can measure different cosmic ray events (electromagnetic part, muons, high energy cosmic ray extended air shower), they can provide a unique information on transient events of solar origin.