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.

Over the past decade, policy-makers around the world have recognised that space weather is a natural hazard that poses a significant risk to modern societies. This chapter outlines how this risk has gradually developed over nearly 200 years through impacts on technological systems, and now affects a diverse set of technologies, including systems vital to provision of power, transport and communications. It illustrates the importance of these impacts through detailed examination of some key impact areas, looking at the physics that creates these impacts as well their societal consequences. The areas examined include (a) the impact of geomagnetically induced currents on electrically-grounded infrastructures such as electric power grids and railway track circuits, (b) the disruption of satellite navigation by space-weather-induced changes in the upper atmosphere, and (c) the impact of space radiation on satellites, on aircraft and on ground systems. A final section reviews our wider knowledge of space weather impacts and discusses how this can be applied, and improved, to meet future challenges. It stresses the need for improved socio-economic assessments of space weather impacts to aid decision-making and, in particular, to assess the value of investment in improved forecasting and other systems for mitigation.