= Space weather
<Space weather> describes changing conditions in the <Sun>, <solar wind>, <magnetosphere> and upper atmosphere that affect space-based or terrestrial systems. Its effects depend on the physical disturbance. <Solar flares> can increase ionization and disrupt radio propagation, energetic particles can damage spacecraft electronics, and <coronal mass ejections> can trigger <geomagnetic storms>. These mechanisms need different observations and forecasts.<Space Weather Impacts by NOAA Space Weather Prediction Center>[Phenomena and effects on radio, satellites and electricity transmission]{ref}
A useful measurement chain starts with spacecraft upstream of Earth, often near the Sun-Earth <inner Lagrange point>. <Magnetometers> measure the interplanetary <magnetic field>, while plasma instruments estimate particle density, speed and temperature. NOAA combines these measurements with models to monitor approaching disturbances; the magnetic-field direction matters as well as the flow speed.<Solar Wind by NOAA Space Weather Prediction Center>[Usage, in situ magnetic-field and plasma measurements; displayed data quantities]{ref}
At the ground, changing magnetic fields induce <electric fields>. <Faraday's law of induction> gives the loop relation $\mathcal E=-d\Phi_B/dt$. In an extended conducting system, the resulting current also depends on electrical resistance and the spatially varying fields. A large magnetic disturbance can therefore affect a power grid even though its conductors are far below the space plasma. Measurements and a physical coupling model connect the solar disturbance to the terrestrial effect.
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