What Is Solar Wind?


This artist’s rendering shows a solar storm hitting Mars and stripping ions from the planet's upper atmosphere. NASA/GSFC

Solar wind is a continuous stream of charged, subatomic particles emitted by the sun. To humans, the flow is kind of a mixed blessing. The GPS signals we now depend on can get disrupted by solar wind. But solar wind is also a driving mechanism behind those stunning northern lights — and their equally gorgeous southern counterparts.

Earth isn't the only place that's affected by the streaming particles. Newly gathered data indicates that solar wind may have visibly changed the moon's iconic face. Plus it helps form a cosmic bubble that envelops our whole planetary neighborhood.

Plasma Extravaganza

Hydrogen and helium are solar wind's two major ingredients. It's no coincidence that those two elements also represent about 98 percent of the sun's chemical make-up. The extremely high temperatures associated with this star break down large quantities of both hydrogen and helium atoms, as well as those from other assorted elements like oxygen.

Energized by the intense heat, electrons start to drift away from the atomic nucleuses they once orbited. That creates plasma, a phase of matter that includes a mixture of free-ranging electrons and the nuclei they've left behind. Both carry charges: The roaming electrons are negatively charged while those abandoned nucleuses have positive charges.

Solar wind is made of plasma — and so is the corona. A faint layer of the sun's atmosphere, the corona starts roughly 1,300 miles (2,100 kilometers) above the solar surface and protrudes far into space. Even by solar standards, it's blisteringly hot. Temperatures within the corona can far exceed 2 million degrees Fahrenheit (1.1 million degrees Celsius), making this layer hundreds of times hotter than the actual surface of the sun beneath it.

About 20 million miles (32 million kilometers) away from that surface, portions of the corona transition into solar wind. Here, the sun's magnetic field weakens its grip on the fast-moving subatomic particles that comprise the corona.

As a result, the particles start to change their behavior. Inside the corona, electrons and nuclei move around in a somewhat orderly fashion. But those who pass that transition spot behave more erratically after doing so, like the flurries in a winter storm. Upon ditching the corona, the particles go forth into space as solar wind.

Starting Points

Individual solar wind streams travel at different speeds. The slow ones cover roughly 186 to 310 miles (300 to 500 kilometers) per second. Their faster counterparts put those numbers to shame, flying by at 373 to 497 miles (600 to 800 kilometers) per second.

The quickest winds come whizzing out of coronal holes, temporary patches of cool, low-density plasma that appear in the corona. These serve as great outlets for solar wind particles because open magnetic field lines run through the holes.

Basically, the open lines are highways that shoot charged particles out of the corona and into the heavens beyond. (Don't confuse them with closed magnetic field lines, looping channels along which plasma bursts out of the sun's surface and then plunges right back down into it.)

Less is known about how the slow winds form. However, their point of origin at any given time seems to be affected by the sunspot population. When these things are scarce, astronomers observe slow winds coming out of the sun's equatorial region and fast ones streaking out of the poles. But when sunspots become more common, the two kinds of solar wind appear in closer proximity to each other all across the glowing spheroid.

Welcome to the Heliosphere

No matter how fast a gust of solar wind is moving as it bids the corona "farewell," it will eventually slow down. Solar winds exit the sun in all directions. By doing so, they maintain a capsule of space that houses the sun, the moon and every other body in our solar system. It's what scientists call the heliosphere.

The seemingly vacant spaces between the stars in our galaxy are actually full of interstellar medium (ISM), a cocktail that includes hydrogen, helium and amazingly small dust particles. Essentially, the heliosphere is a giant cavity surrounded by this stuff.

Rather like a super-sized onion, the heliosphere is a layered construct. The termination shock is a buffer zone far beyond Pluto and the Kuiper Belt where solar wind rapidly declines in speed. Past that point lies the heliosphere's outer boundary, a place in which the interstellar medium and solar winds become evenly matched in terms of strength.

Auroras, Satellites and Lunar Geology

Closer to home, the particles in solar winds are responsible for the aurora borealis ("northern lights") and aurora australis ("southern lights"). Earth has a magnetic field whose twin poles are located above the Arctic and Antarctic regions. When solar wind contacts this field, its charged particles get pushed towards those two areas. Atoms in our atmosphere become energized after they contact the winds. Said energy triggers mesmerizing light shows.

While other planets — like Venus and Saturn — also witness auroras, Earth's moon does not. And yet, solar winds might explain the existence of "lunar swirls," portions of our moon that tend to be darker or lighter in complexion than the surrounding turf.

Their origins are a mystery, but evidence collected by an ongoing NASA space mission suggests that the discolored splotches are — in effect — giant sunburn marks. Parts of the lunar surface are shielded from solar wind by small, isolated magnetic fields. But other areas are exposed. So in theory, when the winds hit those spots, they might be setting off chemical reactions that alter the hues of certain rocks.

Man-made devices are vulnerable to the traveling plasma, too. The electrical components on artificial satellites have been known to malfunction after getting bombarded by charged, subatomic particles of solar origin.