Solar Wind

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Solar wind

The solar wind is a continuous stream of particles that flows outward from the Sun through the solar system . The particles escape from the Sun because its outer atmosphere is very hot, and the atoms there move too rapidly for the Sun's gravity to hold onto them. The solar wind , which is made mainly of ionized hydrogen (free protons and electrons), flows away from the Sun at a velocity of several hundred kilometers per second. The solar wind continues past the outermost planet , Pluto , to the point where it becomes indistinguishable from the interstellar gases; this marks the end of the Sun's domain and is called the heliopause. Little of the solar wind reaches Earth's atmosphere, because the charged particles are deflected by our planet's magnetic field.

Origin and nature of the solar wind

One of the mysteries of the Sun is that its atmosphere becomes hotter at larger heights from its visible surface, or photosphere. While the photosphere has a temperature of 9,981°F (5,527°C), the chromosphere, only a few thousand kilometers higher, is more than twice as hot. Further out is the corona, with gas heated to one or two million degrees Kelvin.

Although the reasons for this temperature rise are not well understood, the effects on the particles comprising the gas are known. The hotter a gas is, the faster its particles move. In the corona, the free protons and electrons move so rapidly that the Sun's gravity cannot hold them, and they escape entirely, flowing into the solar system. This stream of particles is called the solar wind.

The solar wind is made mainly of free protons and electrons. These particles are much lighter than the atoms (such as iron ) in the solar corona, so the Sun has a weaker hold on them than on their heavier counterparts. When the solar wind reaches Earth , the protons and electrons are flowing along at speeds up to 621 mi/s (1,000 km/s). By comparison, a commercial jet might fly 621 MPH (1,000 km/hr), and only if it has a good tailwind pushing it along. The solar wind could flow from New York to Los Angeles in less than ten seconds.

There is, therefore, gas from the Sun literally filling the solar system. We cannot see it, however, because there is not much of it—only a few protons and electrons per cubic centimeter. The solar wind therefore represents an insignificant source of mass loss for the Sun, not nearly enough to have any impact on its structure or evolution. (Some very massive stars do have strong winds that affect how they evolve.)

The solar wind and the earth

Beautiful aurorae are caused when charged particles, like protons and electrons, stream into the earth's atmosphere and excite the nitrogen and oxygen atoms in the upper atmosphere. When these atoms return to their normal, nonexcited state, they emit the shimmering, green or red curtains of light so familiar to individuals living in parts of Canada or the northern United States.

If the solar wind is continuous, why don't we see aurorae all the time? Earth is surrounded by a magnetic field, generated by its rotation and the presence of molten, conducting iron deep in its interior. This magnetic field extends far into space and deflects most particles that encounter it. Most of the solar wind therefore streams around the earth before continuing on its way into space. Some particles get through, however, and they eventually find their way into two great rings of charged particles that surround the entire Earth. These are called the Van Allen belts, and they lie well outside the atmosphere, several thousand kilometers up.

Besides the gentle, continuous generation of the solar wind, however, the Sun also periodically injects large quantities of protons and electrons into the solar wind. This happens after a flare, a violent eruption in the Sun's atmosphere. When the burst of particles reaches the earth, the magnetic field is not sufficient to deflect all the particles, and the Van Allen belts are not sufficient to trap them all above the atmosphere. Like water overflowing a bucket, the excess particles stream along the earth's magnetic field lines and flow into the upper atmosphere near the poles. This is why aurorae typically appear in extreme northern or southern latitudes, though after particularly intense solar flares, aurorae may be seen in middle latitudes as well.

The solar wind and the heliopause

Six billion kilometers from the Sun is the planet Pluto. At this distance, the Sun is only a brilliant point of light, and gives no warmth to heat the dead and icy surface of its most distant planet.

The solar wind still flows by, however. As it gets farther from the Sun, it becomes increasingly diffuse, until it finally merges with the interstellar medium, the gas between the stars that permeates the Galaxy . This is the heliopause, the distance at which the Sun's neighborhood formally ends. Scientists believe the heliopause lies between two and three times as far from the Sun as Pluto. Determining exact location is the final mission of the Pioneer and Voyager spacecraft, now out past Pluto, their flybys of the planets complete. Someday, perhaps in twenty years, perhaps not for fifty, they will reach the heliopause. They will fly right through it: there is no wall there, nothing to reveal the subtle end of the Sun's domain. And at that point, these little machines of man will have become machines of the stars.