In brief
The solar wind is a plasma mostly of protons and electrons, continuously emitted by the solar corona at an average 400 km/s. It carries the Sun's magnetic field throughout the solar system and interacts permanently with Earth's magnetosphere.
A wind that never stops
Despite the name, the solar wind is not atmospheric wind: it is a plasma, meaning gas so hot that its atoms are ionised. It is continuously emitted by the solar corona and fills the entire solar system. At Earth, 150 million km from the Sun, we measure on average 5 particles per cm³ racing at 400 km/s.
Its existence was inferred in the 1950s by Eugene Parker, then confirmed by Luna (Soviet, 1959) and Mariner 2 (American, 1962) probes. The NASA Parker Solar Probe (2018) has since directly sampled coronal plasma a few solar radii from the surface.
Two regimes
The solar wind is not uniform. We distinguish:
| Regime | Speed | Density | Origin |
|---|---|---|---|
| Slow wind | 300-500 km/s | 5-10 cm⁻³ | Equatorial coronal belt |
| Fast wind | 500-800 km/s | 2-5 cm⁻³ | Polar coronal holes |
A coronal hole is a region where the Sun's magnetic field opens out to space, letting plasma escape freely. When a coronal hole faces Earth, we receive days of fast wind that alone can trigger moderate auroras without a flare.
How it is measured
Instruments onboard ACE and DSCOVR (at L1, 1.5 million km sunward of Earth) continuously measure:
- Solar-wind speed (km/s)
- Density (particles/cm³)
- Temperature (~10⁵ K typical)
- Interplanetary magnetic field (IMF), decomposed into Bx, By, Bz (nT)
These parameters are relayed in near-real-time to NOAA SWPC, which redistributes them freely via APIs. Pulsar aggregates and displays them on each city page, with a particular focus on Bz, the parameter most correlated with auroral activity.
Impact on Earth
Without the solar wind, no auroras, no deformed magnetopause, no magnetotail. The wind compresses the dayside magnetosphere to about 10 Earth radii and stretches the nightside to 200 Earth radii, forming the sock-like magnetic envelope you see on every space-weather diagram.
When solar-wind dynamic pressure suddenly rises (e.g. during a CME passage), the magnetopause contracts to 6-7 Earth radii, sometimes below geostationary orbit. Some satellites suddenly find themselves exposed directly to the solar wind, sometimes with fatal consequences.
Beyond Earth
The solar wind travels far beyond Earth's orbit, gradually slowing until it meets the interstellar medium at about 120 astronomical units. This boundary, the heliopause, was crossed by Voyager 1 in 2012 and Voyager 2 in 2018: to date the only human-made objects out of the Sun's direct reach.
The large-scale geometry the flow takes, a spiral, is called the Parker spiral, after the physicist who theorised it in 1958. At Earth this translates into an interplanetary magnetic field whose direction changes slowly, alternating positive and negative sectors with the 27-day solar rotation.