The significance of CME-driven sheath regions as drivers of magntetospheric activity

Sheath regions ahead of Coronal Mass Ejections (CMEs) are large-scale heliospheric structures that form gradually from the inhomogeneous plasma and field the CME encounters as it travels through the solar wind. As a consequence, sheaths are compressed regions that embed large-amplitude magnetic field fluctuations over various timescales. These features make sheaths to couple particularly strongly with the Earths magnetosphere/ionosphere and they often lead to geomagnetic storms, strong substorms, intense wave activity in the inner magnetosphere and dramatic changes in outer radiation belt electron fluxes. In this presentation, we give an overview of the current understanding of the internal structure of sheaths, the spatial extent of geoeffective fluctuations and present examples of their consequences in near-Earth space.