What Is the Solar Wind? The Sun Is Continuously Blowing Away

The sun's outer atmosphere, the corona, is at a temperature of one to three million degrees, which is far hotter than the visible surface below it at around five and a half thousand, and that inversion is one of the long-standing puzzles in solar physics with heating by magnetic wave activity and small-scale reconnection events the leading explanations. At those temperatures the particles move fast enough that the sun's gravity cannot retain the outer regions, and the corona expands continuously outward. That expansion was predicted theoretically by Eugene Parker in 1958, on the reasoning that a static corona was not a stable solution, and the prediction was ridiculed by referees before being confirmed by spacecraft measurements within a few years. The flow becomes supersonic within a few solar radii and continues outward, carrying the sun's magnetic field with it, since the plasma is a good conductor and the field is frozen into it and dragged along. The sun's rotation twists that field into a spiral pattern extending across the solar system.

The flow interacts with everything it meets and the consequences are observable:

• •Comet ion tails, which are swept directly away from the sun by the flow regardless of the comet's motion and which show knots and disconnections when the wind changes
• •The Earth's magnetosphere, which the flow compresses on the sunward side and stretches into a long tail on the night side
• •Aurorae, produced when particles are funnelled down magnetic field lines into the upper atmosphere near the poles and excite atmospheric gases
• •Stripping of unprotected atmospheres, which is a leading explanation for the loss of Mars's atmosphere over billions of years, since Mars has no global magnetic field
• •The heliosphere, a vast bubble carved out of interstellar gas by the outward pressure of the flow, whose boundary was crossed by the Voyager spacecraft in 2012 and 2018
• •Two speeds, with a slow stream around four hundred kilometres per second from the equatorial regions and a fast stream around twice that from coronal holes where field lines open into space

Variations in the flow have practical consequences that have grown as infrastructure has become more electrical. Coronal mass ejections, enormous eruptions of plasma and magnetic field, travel outward and can strike the Earth within one to three days, compressing the magnetosphere and inducing currents in long conductors on the ground. Those induced currents can saturate transformers in electricity grids, and a storm in 1989 collapsed the Quebec grid within ninety seconds, leaving millions without power. The Carrington event of 1859 remains the benchmark for severity, having set telegraph offices on fire and produced aurorae visible near the equator, and a comparable event today is a recognised risk requiring grid operators to plan for it. Satellites suffer charging, component damage and increased atmospheric drag as the upper atmosphere expands during storms, which has caused losses including a batch of satellites lost shortly after launch in 2022. Radiation exposure rises for high-altitude flights on polar routes and is a serious constraint on human spaceflight beyond Earth's magnetic shelter.

Measurement requires instruments in space, and the field has advanced through progressively closer missions. Early probes established the basic properties. Ulysses flew over the sun's poles and mapped the difference between fast and slow streams. The Solar and Heliospheric Observatory has monitored eruptions continuously since 1995 from a position between the Earth and the sun, providing the warning time that space weather forecasting depends on. The Parker Solar Probe, launched in 2018 and named after the man who predicted the wind, has flown repeatedly through the corona itself, using a heat shield and a series of Venus flybys to lower its orbit, and has passed inside the point where the flow becomes supersonic, sampling the region where it is accelerated. Solar Orbiter observes from a tilted orbit giving views of the poles. Forecasting remains difficult, since the arrival time and the magnetic orientation of an ejection determine its effect and the orientation is not known until it arrives, which limits useful warning to tens of minutes for the most important parameter.

A corona at millions of degrees cannot be held by gravity, so it expands continuously and carries the sun's magnetic field outward with it, which Parker predicted in 1958 to general ridicule. The flow sweeps comet tails away from the sun, drives aurorae and stripped Mars of its atmosphere. Eruptions induce currents that collapsed the Quebec grid in ninety seconds, and the magnetic orientation that determines severity is unknown until arrival.