Solar Wind
You might think that space is empty, but here in the Solar System, the wind is always blowing; the solar wind. There's a constant stream of particles ejected from the Sun's upper atmosphere, moving outwards through the Solar System. These particles are boosted up to high energies in the Sun's atmosphere, but the exact process of how they escape the Sun's gravity at such a high speed isn't understood so well.
We can see evidence of solar winds all around us. Powerful geomagnetic storms can knock out satellites and electrical systems here on Earth; beautiful auroras, like the Northern and Southern lights. Perhaps the best evidence is the long tails that stream back from comets as they pass near the Sun. Particles of dust shed from comets are picked up by the solar wind and carried away from the Sun. This is why comet tails always point away from the Sun.
The solar wind emanates from a region of the Sun's upper atmosphere called the corona. In this region, temperatures rise to more than 1 million Kelvin, and particles can get energized to more than 1 keV. The vast majority of these particles aren't going fast enough to escape the Sun, but a few reach velocities where they can join the solar wind and stream out from the Sun.
There are actually two parts of the solar wind, the slow wind, and the fast wind. This is an effect that can be seen in comets. If you look at images of comets carefully, you'll see that they often have two tails. One is straighter, and the other is longer and curves back.
The fast solar wind is traveling at 750 km/s, and astronomers think they originate from coronal holes - the regions where magnetic field lines pierce the surface of the Sun. The slow solar wind is only going about 400 km/s, and seems to come from the Sun's equatorial belt. The slow wind is thicker and denser than the fast wind, which creates the longer, brighter cometary tail.
If it weren't for the Earth's magnetic field, the solar wind would be dangerous to life on Earth. Instead, though, the magnetic field channels the particles around the planet, protecting us from radiation. In fact, the shape and size of the magnetic field is defined by the strength and speed of the solar wind.
We have written many articles about the solar wind on Universe Today. Here's a video that helps describe what astronomers think is the source of the slow solar wind, and a story about how scientists are working to better predict the solar wind.
Here's more information on the solar wind from NASA's Solar Physics site, and another article from NASA's Cosmicopia site.
We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.
Filed under: Astronomy
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