Solar Wind

The Sun emits a vast cloud of particles that stream past and through all the planets and out of the solar system.This continuous emission is called the solar wind, or as it is sometimes called, corpuscular radiation. The solar wind varies both in the density of particles that issue from different regions of the surface of the Sun, and it varies in time.

Venus is imaged almost exclusively in radar because of its dense, complete, permanent cloud cover. Radar images of Venus have been taken by several spacecraft and can also be taken from Arecibo Observatory on Earth. The image below makes a comparison between the resolution possible from Earth using Arecibo (left), and the resolution from the Magellan spacecraft (right). Arecibo's image is 560 miles (900 km) across and has a resolution of 1.9 miles (3 km). The Magellan image corresponds to the small white rectangle in the Arecibo image, 12 X 94 miles (20 X 120 km) in area. Magellan's resolution is a mere 400 feet (120 m) per pixel.

The far greater resolution obtained by the Magellan craft (right) shows the relative disadvantage of taking images of Venus from the Earth (left) using the Arecibo Observatory. (NASA/Magellan/JPL)

On average, the solar wind consists of 5 million electrons and 5 million protons per cubic meter. There are two components of the solar wind: a fast, hot, and uniform wind, moving at about 460 miles per second (750 km/s), and a slower, highly variable wind, moving at about 250 miles per second (400 km/s).The slow wind is formed by coronal loops and streamers, while the magnetic network radiating through coronal holes is thought to create the fast wind. Coronal

Temperature of Sun v. Radius il

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500,000 (800,000)

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at 9,900'F /

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The photosphere, visual surface of the Sun, followed by

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the thin transition zone to the corona

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This graph of temperature in the Sun versus radius shows the peaks of temperature in the corona.

loops are lines of streaming plasma trapped within tubes of magnetic flux. Close images of coronal loops are shown in the upper color insert on page C-7 in an infrared image from the SOHO orbiter.

In addition to the solar wind, the Sun projects its magnetic field into space, and this effects the solar wind. The solar wind is divided roughly along the ecliptic by a wave interface where the Sun's magnetic field changes from north to south. This interface is called the current sheet because it carries electric currents that are created by the opposing magnetic fields. The current sheet acts as a barrier to cosmic rays, since when they strike the sheet they flow along it rather than through it. Because the shape of the sheet is irregular and changes with time, the Earth passes through it, moving from above it to below it and back, at irregular times during its orbit. When the Earth is on the south magnetic side of the sheet, the Sun's magnetic field tends to cancel the effects of the Earth's magnetic field, making us more susceptible to geomagnetic storms from solar emissions.

While hydrogen is ejected from the Sun preferentially over other larger, heavier atoms and particles, the solar wind also contains helium and neon. The composition of the solar wind is impossible to measure from Earth because the charged particles of the solar wind interact with the Earth's magnetic field and do not reach the Earth's surface. The Moon, however, has no atmosphere and a negligible magnetic field, and so its surface is bombarded with the solar wind continuously. All the materials on the surface of the Moon have elements of the solar wind embedded into their surfaces. The mineral ilmenite (titanium iron oxide), for example, does not naturally contain hydrogen, helium, or neon, so samples of ilmenite returned to Earth by the Apollo or Luna missions can be analyzed to determine the composition of the solar wind:The solar wind constituents are literally embedded in the crystal. The solar wind components are extracted from the crystal by gradually heating the crystal under a vacuum. As the heat adds energy to the solar wind gases, they escape the crystal into the surrounding vacuum because they are not bound into the framework of the crystal with atomic bonds.The gases released are then fed into a mass spectrometer, an instrument that separates the elements and even isotopes according to their weights using a series of large magnets.The different elements or isotopes are accelerated through a series of tubes by the magnets, and strike detectors that count them one by one. In this way the exact abundances of the constituents of the solar wind can be measured. In fact, using these techniques, any change in composition of the solar wind over time can also be measured.

While the hydrogen to helium ratio on the surface of the Sun can be measured using spectroscopy and is found to be about 10 to one, the hydrogen to helium ratio found on the lunar surface is 20 to one. This confirms the idea that hydrogen is ejected preferentially from the Sun. Any change in the isotopic composition of the solar wind over time can also be measured; members of the Ulysses mission team have determined that the neon and helium isotopic compositions have remained constant over the lifetime of the Sun.

The solar wind radiates between and among the planets, expanding and becoming cooler as it travels. The solar wind fills all the space in the solar system except those volumes that are protected by planetary magnetic fields.Though solar wind flows outward constantly from the

Sun, it does have an outside edge, where it meets the interstellar wind.This edge is known as the heliopause.

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