Galactic cosmic ray short-term variations are observed in association with the passage of recurrent and non-recurrent large-scale interplanetary structures. Long term precise monitoring of such variations has been provided by neutron monitor data, that are currently available alongside relatively more recent data from space-based missions, measuring the cosmic ray differential energy fluxes.

A combination of such precise GCR measurements and state of the art modelling is employed in order to study events of particular relevance for SW, as Forbush decreases and SEP. Among the main objectives of this WP: studying in detail Forbush decrease events, studying the relationship between interplanetary magnetic structures, Forbush decreases and geomagnetic storms, investigating the effect of the magnetic cloud configuration in modulating the GCR intensity, and modelling the SEP/GLE spectrum at the top of the atmosphere for selected transient events.