Space weather

Unlike terrestrial weather, space weather concerns the flow of radiation, charged particles, and magnetic fields through space and their interactions with planets and other bodies. The Sun is its principal source near Earth. The continuous solar wind carries plasma and magnetic field outward, while episodic solar flares emit intense radiation and coronal mass ejections (CMEs) expel large clouds of magnetized plasma. Solar energetic particle events can also accelerate charged particles to high energies. The term covers both the conditions themselves and the resulting effects in near-Earth space and on technology.

Different solar emissions have different travel times and effects. Radiation from a flare travels at the speed of light and can affect the sunlit side of Earth's ionosphere within minutes. Energetic particles can arrive soon after an eruption, while a CME commonly takes many hours to several days to reach Earth, depending on its speed and direction. Not every solar eruption is Earth-directed, and the strength of any resulting disturbance depends on factors including the magnetic field carried by the solar material and how it couples with Earth's magnetic environment. In particular, sustained solar-wind conditions and a southward-directed magnetic field can transfer energy efficiently into the magnetosphere.

Radio blackout events occur when flare-generated X-rays disturb the sunlit ionosphere, degrading or blocking some high-frequency radio communications. Solar radiation storms involve energetic particles that can pose radiation hazards to astronauts and, in some circumstances, people on high-altitude polar flights; they can also affect spacecraft electronics and polar radio links. Geomagnetic storms arise when energy from the solar wind—often associated with a CME or a high-speed solar-wind stream—disturbs Earth's magnetosphere. These categories describe different processes and impacts, although a single solar eruption can produce more than one type of event.

Earth's atmosphere and magnetic field protect people at the surface from many direct effects of solar radiation and particles, but they do not make infrastructure immune. Ionospheric changes can degrade radio communications and the accuracy of satellite navigation signals. Energetic particles can affect spacecraft instruments and electronics. During geomagnetic storms, heating of the upper atmosphere can increase drag on satellites in low Earth orbit. Changing magnetic fields can also drive geomagnetically induced currents in long conductors, creating operational risks for electric power grids and pipelines. Auroras are a visible result of charged particles exciting gases in the upper atmosphere; their appearance is not by itself a measure of damage to infrastructure.

Space-weather forecasts draw on ground-based instruments and spacecraft that observe the Sun, solar wind, and Earth's magnetic environment. Forecast centres issue alerts and severity scales to help sectors such as satellite operations, aviation, communications, and power systems prepare. Forecast lead time varies: flare-related radio effects can begin too quickly for long warning, whereas an observed CME travelling toward Earth may allow notice before arrival. Forecasts estimate risks and likely impacts; they cannot remove uncertainty about an eruption's direction, speed, or magnetic properties. Agencies including NOAA and the European Space Agency provide monitoring or user-focused services.

Solar activity waxes and wanes over an approximately 11-year cycle, tracked in part through sunspot activity. Eruptions and related disturbances are generally more frequent around periods of higher solar activity, but significant space-weather events are not confined to a single phase. Continuous monitoring remains important throughout the cycle.