Speaker
Description
Galactic cosmic rays (GCRs) constitute a continuous flux of high-energy particles, mostly protons and helium nuclei, flowing through the Heliosphere and interacting with solar wind and interplanetary magnetic field. The result of this interaction on long periods of time is the well-known modulation of the low end of the GCR energy spectrum (below a few GeVs), following the 11-year activity cycle of the Sun and the 22-year cycle of global solar magnetic field polarity reversals. On smaller time scales, GCRs become powerful diagnostics of interplanetary transient structures such as interaction regions between fast and slow solar wind streams and interplanetary coronal mass ejections (ICMEs). These structures are characterized by enhanced turbulence, shocks, large-scale plasma compressions, magnetic flux-ropes, and other phenomena that can inhibit the particle transport and induce a decrease in the GCR flux. Such depressions have been routinely observed from ground- and space-based instruments over several decades, and are known as Forbush decreases. In this talk, I review the main observational properties of the short-term GCR modulation driven by interplanetary transients. By combining ground-based observations from the worldwide neutron monitor network with multipoint measurements from spacecraft at different locations in the heliosphere, it is possible to investigate how the main properties of GCRs (e.g., energy spectrum) vary in time during a transient event. In addition, the analysis of directional responses from neutron monitors allows us to study how interplanetary magnetic field structures induce anisotropies in the GCR flux, especially during strong disturbances such as fast and magnetically complex ICMEs. These observations provide important constraints on particle transport processes (such as diffusion, drift, and cross-field transport) and place GCR short-term modulation within the broader context of space weather, as the same interplanetary structures that modulate GCRs can drive solar energetic particle events and geomagnetic disturbances.