This review presents the physical basics for understanding why and how solar activity affects technological systems and humans in space and on ground. The final goal of space weather research is to be able to produce reliable forecasts and nowcasts of the space environment as well as assess the risks that are associated with a wide variety of technological applications. While tremendous progress has been made during the recent years, the field still faces several major challenges.
Observations of the near-Earth space environment are sparse and information of the global properties are often not available, as the plasmas for the most part are too tenuous for imaging. Beginning from the solar wind observations, a single point measurement in the solar wind, sometimes far away from the Sun-Earth line, is not always sufficient to determine what impinges on the magnetopause an hour later. In the magnetosphere, the lack of global observations of the mass and energy circulation as well as the dynamics of the electromagnetic fields still limit our capabilities to evaluate the variety of physical processes that can be associated with the observed phenomena. On the ground, the largest and most harmful currents are highly localized, which makes their detection as well as prediction challenging. In many instances, the magnetospheric activity can be quantitatively classified only based on a variety of magnetic indices and proxies derived using the solar wind input and the magnetic indices. It is clear that such proxies include both systematic and statistical errors, which limits our capability to establish causal correlations.
Large-scale or global models of the coupled solar wind-magnetosphere-ionosphere system have advanced greatly in the recent past, both due to better understanding of the critical processes and due to the increased computational capabilities. Global MHD simulations can be run in near-real time, and in many cases reproduce the observed topological changes and even in-situ measurements to quite good accuracy. On the other hand, their use still has serious limitations, both due to missing physics especially in the ionosphere and in the inner magnetosphere, and due to the limitations of the MHD approach in describing the collisionless, multicomponent plasmas in the magnetosphere. While hybrid codes describing the full ion motion while treating the electrons as a fluid already are available for our sister planets, the strong magnetic field and the large system size still limits that approach to localized problems in the terrestrial magnetosphere.
For prediction of the hazards, it is important to note that the effects are dependent both on the space environment and on the engineering and operations of the technological system, be that a telecommunication satellite or a power transmission line on ground. Warnings and predictions can be effectively used to schedule non-standard or maintenance operations during periods of low solar activity, while better shielding and other design improvements are key for successful operations and long lifetime of the systems. To reach these goals requires close contacts between space physics and engineering sciences.
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