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Simulation of Random Walk

Light From the Sun
The Sun may be described as a giant ball of incandescent gas. As such, it emits the whole spectrum of electromagnetic waves. It emits visible light, of course, but also infrared and ultraviolet "light"as well as radio waves, x-rays, and gamma rays. All of these forms of energy are related to what we know of as "light"—only the wavelength is different. We are unable to detect wavelengths other than visible light with our eyes, though we can "sense" infrared waves from the Sun with our skin—it feels warm. We can also sense ultraviolet with our skin when it gets burned. Microwaves at high intensity can be sensed as warmth as well, since like food, our skin can be heated by microwaves. However, the Sun gives off far too few radio waves in the microwave region to be sensed as heat. X-ray, and gamma rays from the Sun are thankfully blocked by the atmosphere but may pose a hazard to space travelers unprotected by Earth's layer of air.

All of the light and waves (properly called electromagnetic waves) emitted by the Sun travel at the speed of light, 300,000 kilometers per second, or 186,000 miles-per second. With this incredibly fast speed, you might suppose that sunlight reaches us almost instantaneously. This is not true. Because of the large distance from the Sun to the Earth, it actually takes light 8 minutes to travel from the surface of the Sun to the Earth. If a dramatic event takes place on the Sun, light (and information about the event) reaches the Earth 8 minutes later.

Outer Layers of the Sun
Nothing could be more obvious than the edge of the Sun. When we watch a sunset or sunrise it is apparent that the Sun is well defined in its shape and size. However, this is really not true. Most of the yellowish-white light we see comes from the photosphere of the Sun, a thin layer of the Sun that is about 300 km thick. The photosphere is efficient at emitting light, but in reality the photosphere has little material in it and is very diffuse. The density of the photosphere is less than that of the most perfect vacuums that scientists can set up on Earth. This photosphere is a shell, a thin layer representing about 5/100 of one percent of the radius of the Sun. There are other layers of the Sun beyond the photosphere. The chromosphere is above the photosphere and was discovered during a total solar eclipse and named for its red color. Above the chromosphere is the corona (crown!), which extends outwards into the solar system. This hot, diffuse material from the outermost layer of the Sun's atmosphere expands continuously into space and may extend halfway to the nearest star. The material in the corona and solar wind is very rarefied. For example, the electrons in the corona are so widely spaced apart that a million cubic kilometers of the corona would weigh only about 10 grams. This material is also very hot—millions of degrees, even though it is farthest from the center of the Sun.

The corona is dim compared to the rest of the Sun, and is only visible by eye during a total solar eclipse, with a brightness of about the full Moon. The shape of the corona is not constant and varies with the number of sunspots and solar activity. Using a special telescope that creates an artificial eclipse, the corona can be seen and studied regularly.

The Solar Wind
The Sun is always in the process of boiling off the outer layer of its atmosphere. There is a constant flow of electrified matter moving away from the Sun. This solar wind represents the expansion of the Sun's corona into the space between the planets. The solar wind is very different from a wind on Earth. The moving matter is an electrified stream of charged particles, such as protons and electrons. Although the charged particles are small, being parts of atoms, there is a large amount of matter being blown off the Sun. It is estimated that a million tons of matter is blown off the Sun every second, which is still small compared to the mass of the Sun.

The existence of the solar wind was first inferred from the directions of comet tails, which point away from the Sun instead of trailing behind the comet along its orbital path. The space between the planets is not empty but filled with the solar wind. The solar wind has been measured by instruments on spacecraft traveling to observe Venus, Mars, and Jupiter.

The solar wind can be gusty, with normal speed of 300-700 kilometers per second and gusts of twice that amount. At a speed of 400 kilometers per second, the solar wind would take 2-4 days to travel the Sun-Earth distance, versus only 8 minutes for light.

Constant Solar Effects on Earth
The Constant Solar Effects on Earth. (Click for a larger image.)
© UC Regents

The solar wind is now understood to be a plasma, a mixture of protons and electrons that moves away from the Sun at supersonic speeds. Plasma is a very hot state of matter where atoms have been ripped apart into their smaller parts, protons and electrons. Since plasmas are charged, and affected by magnetic fields, it is very important to understand how the solar wind plasma is affected by the magnetic fields of the planets it encounters. For example, the solar wind has a very different effect on the Earth, which has a strong magnetic field, than it has on Mars, which has a much weaker magnetic field.

Phases of Matter
The Phases of Matter. (Click for a larger image.) © UC Regents

Energetic Particles
In addition to the solar wind particles, occasionally the Sun produces bursts of more energetic (faster) particles. Some of these are produced in solar flares, which show themselves as intense brightening of a small patch of the photosphere—also giving off blasts of x-rays, gamma rays, and radio waves. Flares produce a beam of energetic particles in space that can reach us at nearly the speed of light if we are in its path.

Dynamic Solar Effects on Earth
Dynamic and Constant Solar Effects on Earth. (Click for a larger image.) © UC Regents

The Sun also has a second way of producing such energetic particle storms. Sometimes large loops of coronal material erupt into the solar wind in what is called a coronal mass ejection or CME. The CME can produce a shock wave in the solar wind as it travels outward, and the shock wave itself makes a broad source of energetic particles. Because the CME shock wave source is much wider than the flare beam, CMEs are thought to be the primary cause of energetic particle events detected near the Earth. The CME disturbance in the solar wind, including the shock wave ahead of it, reaches Earth in 2-3 days, faster than most solar wind. It is this disturbance that causes geomagnetic storms on the Earth. The auroras that occur with these storms are a result of particles in the magnetosphere reacting to the passing disturbance.

Related to chapter 2 in the print guide.
Related Materials

For more on the constant effects of particles from the Sun see the Solar Wind. To learn more about other emissions of particles see Solar Flares and Coronal Mass Ejections .

Glossary Terms

Click for the definitions of the following words that are used on this page: (Definitions appear in a pop-up window.)

coronal mass ejection
geomagnetic field
geomagnetic storm
radiation belts
solar wind

View the full, printable version of the glossary.

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