Solar Wind
Solar Wind
Origin and nature of the solar wind
The solar wind and Earth
The solar wind and the heliopause
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 dwarf planets Pluto, Ceres, and Eris, 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 the 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 5,800 Kelvin (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 the Earth, the protons and electrons are flowing along at speeds up to 621 miles per second (1,000 km/s). By comparison, a commercial jet might fly 621 miles per hour (1,000 km/h), and only if it has a good tailwind pushing it along. The solar wind could flow from New York City to Los Angeles, California, in less than ten seconds.
There is, therefore, gas from the sun literally filling the solar system. Humans 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, however, have strong winds that affect how they evolve.)
The solar wind and Earth
Beautiful aurorae are caused when charged particles, like protons and electrons, stream into Earth’s atmosphere and excite the nitrogen and oxygen atoms in the upper atmosphere. When these atoms return to their normal, non-excited state, they emit the shimmering, green or red curtains of light (the Northern Lights, or aurora borealis) familiar to individuals living parts of Canada or the northern United States.
If the solar wind is continuous, why do humans not 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 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 out.
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 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 dwarf 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 one of its most distantly held celestial bodies.
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 Milky Way 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. In May 2005, NASA stated that Voyager 1 was in the heliosheath, and that it should reach the heliopause in 2015. As of August 2005, it was about 100 AU from the Sun, the furthest human made object from Earth. According to NASA, Voyager 2 left the heliosheath in December 2004 and is about 80.5 AU away from Earth, as of September 2006. On March 4, 2006, NASA lost contact with Pioneer 10. Three months earlier, it was about 89.7 AU away from Earth. NASA lost contact with Pioneer 11 in November 1995.
Someday, perhaps in ten or more years, they will reach the heliopause—with or without them contacting their human creators. They will fly right through the heliopause: there is no wall there, nothing to reveal the subtle end of the sun’s domain. And at that point, these little machines of humans will have become machines of the stars.
Resources
BOOKS
Arny, Thomas. Explorations: An Introduction to Astronomy. Boston, MA: McGraw-Hill, 2006.
Bhatnager, Arvind. Fundamentals of Solar Astronomy. Hackensack, NJ: World Scientific, 2005.
Kundt, Wolfgang. Astrophysics: A New Approach. Berlin and New York: Springer, 2005.
Laureta, D.S., H.Y. McSween, Jr., eds. Meteorites and the Early Solar System. Tucson, AR: University of Arizona Press, 2006.
Lewis, John S. Physics and Chemistry of the Solar System. Amsterdam and Boston, MA: Elsevier Academic Press, 2004.
Morbidelli, Alessandro. Modern Celestial Mechanics: Aspects of Solar System Dynamics. London and New York: Taylor and Francis, 2002.
Jeffrey C. Hall
Solar Wind
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.
Resources
books
Beatty, J., and Chaikin, A., The New Solar System. Cambridge: Cambridge, University Press, 1990.
Introduction to Astronomy and Astrophysics. 4th ed. New York: Harcourt Brace, 1997.
Kaufmann, W., Discovering the Universe. 2nd ed. San Francisco: Freeman, 1991.
Jeffrey C. Hall
Solar Wind
Solar Wind
The area between the Sun and the planets, the interplanetary medium, is a turbulent area dominated by a constant stream of hot plasma that billows out from the Sun's corona. This hot plasma is called the solar wind.
The first indication that the Sun might be emitting a "wind" came in the seventeenth century from observations of comet tails. The tails were always seen to point away from the Sun, regardless of whether the comet was approaching the Sun or moving away from it.
Basic Characteristics
The solar wind is composed mostly of protons and electrons but also contains ions of almost every element in the periodic table. The temperature of the corona is so great that the Sun's gravity is unable to hold on to these accelerated and charged particles and they are ejected in a stream of coronal gases at speeds of about 400 kilometers per second (1 million miles per hour). Although the composition of the solar wind is known, the exact mechanism of formation is not known at this time.
The solar wind is not ejected uniformly from the Sun's corona but escapes primarily through holes in the honeycomb-like solar magnetic field. These gaps, located at the Sun's poles, are called coronal holes. In addition, massive disturbances associated with sunspots , called solar flares , can dramatically increase the strength and speed of the solar wind. These events occur during the peak of the Sun's eleven-year sunspot cycle.
The solar wind affects the magnetic fields of all planets in the solar system. The interaction of the solar wind, Earth's magnetic field, and Earth's upper atmosphere causes geomagnetic storms that produce the awe-inspiring Aurora Borealis (northern lights) and Aurora Australis (southern lights).
Undesirable Consequences
Although the solar wind produces beautiful auroras , it can also cause a variety of undesirable consequences. Electrical current surges in power lines; interference in broadcast of satellite radio, television, and telephone signals; and problems with defense communications are all associated with geomagnetic storms. Odd behavior in air and marine navigation instruments have also been observed, and geomagnetic storms are known to alter the atmospheric ozone layer and even increase the speed of pipeline corrosion in Alaska. For this reason, the U.S. government uses satellite measurements of the solar wind and observations of the Sun to predict space weather.
Major solar wind activity is also a very serious concern during spaceflight. Communications can be seriously disrupted. Large solar disturbances heat Earth's upper atmosphere, causing it to expand. This creates increased atmospheric drag on spacecraft in low orbits, shortening their orbital lifetime. Intense solar flare events contain very high levels of radiation. On Earth humans are protected by Earth's magnetosphere , but beyond it astronauts could be subjected to lethal doses of radiation.
There have been a number of scientific missions that have enabled scientists to learn more about the Sun and the solar wind. Such missions have included Voyager, Ulysses, SOHO, Wind, and POLAR. The latest mission, Genesis, was launched in August 2001 and during its two years in orbit it will unfold its collectors and "sunbathe" before returning to Earth with its samples of solar wind particles. Scientists will study these solar wind samples for years to come.
see also Solar Particle Radiation (volume 2); Space Environment, Nature of the (volume 2); Sun (volume 2).
Alison Cridland Schutt
Bibliography
Kaler, James B. Extreme Stars. Cambridge, UK: Cambridge University Press, 2001.