The Solar Wind: What It Is, How Fast It Blows, and How We Measure It

A theory before it was a measurement

The existence of a continuous outflow of particles from the Sun was proposed on theoretical grounds by physicist Eugene Parker in 1958, based on the puzzle of why the Sun's corona - its outer atmosphere - is heated to over a million degrees Celsius, far hotter than the visible surface beneath it. At those temperatures, Parker argued, the corona's own thermal pressure would be enough to overcome the Sun's gravity and drive a continuous outward flow of plasma into space, rather than the static "solar atmosphere" most physicists of the time assumed. The idea was controversial until NASA's Mariner 2 spacecraft directly measured this outflow during its 1962 journey to Venus, confirming Parker's prediction and giving the phenomenon its name: the solar wind.

What it is made of, and how fast it moves

The solar wind is a stream of plasma - ionised gas - dominated by protons (roughly 95% by number) and helium nuclei (around 4%), with a small residue of heavier ions such as carbon, oxygen and iron, plus enough electrons to keep the whole thing electrically neutral overall. It moves at speeds typically between 300 and 800 kilometres per second, arriving in two broad flavours: a slower stream, around 300-400 km/s, that originates from the Sun's equatorial regions, and a faster stream, 500-800 km/s or more, that flows from coronal holes - darker, cooler patches of the corona where the Sun's magnetic field opens directly into space rather than looping back to the surface, giving plasma a relatively unobstructed path outward.

Despite temperatures that can exceed 100,000 Kelvin, the solar wind is so tenuous - typically just a handful of particles per cubic centimetre by the time it reaches Earth - that it carries very little heat energy in absolute terms; "hot" here describes the extreme speed of individual particles, not something that would feel warm.

The Parker spiral and the magnetic field it carries

Because the Sun rotates roughly every 27 days while continuously emitting solar wind, the outflowing plasma traces out a spiral pattern through the solar system, much as water from a rotating garden sprinkler forms a curved rather than straight stream. This is known as the Parker spiral, and it means the interplanetary magnetic field embedded in the solar wind - carried along because the plasma is an excellent electrical conductor - arrives at Earth at an angle rather than pointing straight out from the Sun.

The single most important property of that embedded field, for space weather purposes, is its north-south component, denoted Bz. When Bz points southward, opposite to Earth's own field at the point of contact, the two fields can link up in a process called magnetic reconnection, opening a channel through which solar wind energy pours into Earth's magnetosphere - the essential trigger for a geomagnetic storm. A northward Bz tends to deflect around Earth's field with comparatively little effect, which is why forecasters watching for a potential storm pay close attention to Bz above almost any other single number.

How solar wind conditions turn into storms

Fast, dense solar wind carrying a persistently southward Bz for several hours is the recipe for a significant geomagnetic storm; any one of those ingredients alone is usually not enough. Two different solar phenomena supply the raw material: coronal mass ejections, sudden eruptions of billions of tonnes of magnetised plasma that can travel anywhere from a few hundred to a few thousand kilometres per second and are responsible for the most severe storms, and high-speed streams from coronal holes, which recur with the Sun's 27-day rotation and tend to produce milder, if more frequent, disturbances.

Watching it in real time

Continuous monitoring is possible because spacecraft sit at the L1 Lagrange point, roughly 1.5 million kilometres from Earth toward the Sun, where the gravitational pull of the Sun and Earth balance in a way that lets a satellite hold a stable position. NASA's ACE, launched in 1997, and NOAA's DSCOVR, launched in 2016, both measure solar wind speed, density and magnetic field there, feeding data back to forecasters roughly 15 to 60 minutes before that plasma reaches Earth - enough time to issue a short-notice storm warning, though not enough to change the outcome, only to prepare for it.