The continuous outflow of plasma from the Sun fills the heliosphere with tenuous gas of charged particles carrying the Solar magnetic field. Largely shielded by the internal geomagnetic field, only a small fraction of the protons and electrons make it to the near-Earth space - but those that do, have a strong impact on our electromagnetic and radiation environment. As the society increasingly relies on space-based assets, it has increasingly important to develop preditions of high-energy (keV to MeV) particle fluxes in the near-Earth space. This talk discusses the effects of Coronal Mass Ejections from the Sun as they reach the geospace: The bright auroral displays, ground currents harmful to power networks and other infrastructure, and the radiation belt electrons in regions where the navigation and communication satellites reside. Our methodologies include satellite observations of the Sun and the fields and plasmas in near-Earth space, and large-scale numerical models to model the complex interactions between the solar wind and the geospace plasma environments.