The Sun-Earth interaction is a complex system of multi-scale processes. The spatial scales of interest vary from the mega-meter size of solar corona structures to the few hundred kilometers of the terrestrial magnetopause and even less when kinetic effects need to be considered. The temporal variations also span a wide range of scales, from thousands of years for the hydrological ocean cycles driven by the total solar radiation to scales of minutes and below for particle acceleration in magnetic reconnection. In this chapter we introduce the building blocks of the Sun-Earth system and briefly describe its important components. Solar disturbances such as solar flares and coronal mass ejection (CME) have the largest impact on geomagnetic activity, especially magnetic storms. Magnetic storms are responsible for large depressions in the horizontal (H) component of the Earth’ surface magnetic field. The strength of a storm is quantified by the Dst index, which is a local time average of the depression measured along the magnetic equator. The depression during a storm is caused by a ring current around the Earth with additional contributions from the magnetopause and tail currents. We review recent developments of empirical prediction algorithms for the Dst index using observations made upstream of the Earth, and alternative procedures based on the same concept including neural networks and the NARMAX method. Future improvements in empirical prediction will require more data from extreme events, additional physical insight to identify the role of other processes, and better measurements of the inputs to the system.

In the context of space weather effects, magnetosphere-ionosphere coupling is one of the fundamental processes controlling energy transfer and dissipation in geospace. Alfvén waves appear to play a key role in this coupling, specifically in coupling the dynamics of magnetospheric convection to the ionosphere and in generating the region 1 and region 2 global field-aligned current systems. The momentum transport from the magnetosphere to the ionosphere can be described as the result of the generation and propagation of Alfvén waves, for example as arising along newly reconnected magnetic field-lines, and in general in terms of their incidence on and reflection from the ionosphere. The thermosphere experiences dramatic changes in density and composition during magnetic storms. Intense Joule heating and particle precipitation at auroral latitudes cause intense thermal expansion, air upwelling and strong wind circulations. The Joule heating at E-layer altitudes can cause both density enhancements and depletions at higher altitudes, and complicate the interpretation of mass density anomalies at high latitudes. The thermospheric response to storms at middle and low latitudes is less complicated, where the averaged density enhancement is linearly proportional to the solar wind input. Magnetic substorms during active periods also cause mass density perturbations. Magnetic storms and substorms can cause disturbances up to thousands of nT at the Earth’s surface. The time derivative of the magnetic field provides a proxy for the associated geoelectric field, which can drive geomagnetically induced currents in Earthed conductors. The geoelectric field is thus a key quantity for space weather effects on technological systems such as power grids, and it can be obtained by modelling the magnetic field using ionospheric currents and model ground conductivity as inputs.