The gas and radiation beneath the Sun's surface are so hot that electrons are being knocked free from the hydrogen and helium atoms constantly, by atomic collisions and by photons. As a result, at any given time about one percent of the hydrogen atoms are separated into free electrons and positively charged ions (atoms lacking electrons), and so the gas becomes a very good conductor of electricity. We call such an ionized gas plasma. The physical behavior of plasmas is a very interesting and complicated subject. One very important property of plasmas is that they can cause magnetic fields to increase when they flow. That certainly happens inside the Sun. There are two obvious kinds of motions that can increase the Sun's magnetic field. The first is the differential rotation seen at the Sun's surface: the gas at the equator rotates faster than that the gas at higher latitudes.

When the magnetic fields within the Sun's interior become strong enough, they become buoyant and tend to rise toward the photosphere. They eventually break through the photosphere, forming loops of magnetism with relatively cool sunspots at their footprints. Some sunspots are larger than the Earth.

Galileo was the first to observe sunspots with a small telescope in 1610 (he must have put very dark glass in front of the telescope; otherwise he would have been blinded). Since then, people have been keeping records of the number of sunspots at any given time. The number varies with cyclically with time, reaching a solar maximum every 11 years and a solar minimum of almost no sunspots between maxima. Click here for historical records of this sunspot cycle, reaching back to Galileo's time. They show that there was a period, from about 1645 to 1715, called the Maunder minimum, when there were almost no sunspots. Click here for modern records of this sunspot cycle, which shows that the last solar maximum occurred in about 1991, and that the Sun's activity is now just about at the maximum of its cycle. The average latitude on the Sun's surface where sunspots appear tends to move from about +/- 30^o at solar maximum toward the equator at solar minimum. We can see this behavior in a plot called a butterfly diagram.

We do, however, know a great deal about what the magnetic field does after it emerges from the photosphere, because then we can observe it. Magnetic fields can make some spectral lines split into two, by a mechanism called the Zeeman effect. By measuring the amount of splitting, we can map the magnetic field strength. Here's a solar magnetogram, a map of the magnetic field strength at the Sun's photosphere as measured by the Zeeman effect. Note that the regions of strongest magnetic field are associated with sunspots.

Violent activity occurs above the Sun's photosphere. It is caused both by solar convection and by instabilities in the solar magnetic field. The convection, which is a relatively steady rolling motion below the photosphere, turns into a violent splashing motion above the photosphere, just as relatively smooth waves in the open sea become violent breakers when they reach the shore. We can see the splashing motion above the Sun's photosphere as spicules. Moreover, the magnetic field often becomes unstable above the Sun's photosphere and erupts outward, causing solar flares, which can be seen with radio telescopes as well as with optical telescopes.

When solar flares occur, the explosive release of magnetic pressure can cause waves to propagate through the Sun. This movie shows circular waves propagating out through the solar photosphere after such an event.

The violent release of magnetic energy in sunspots and flares heats the very tenuous gas above the photosphere to temperatures of millions of degrees, creating the corona, which extends far beyond the optical disk of the Sun. We can see the relatively faint optical radiation from the corona during solar eclipses, when the Moon blocks the much greater optical light from the Sun's photosphere.

(Don't miss an opportunity to see a total eclipse of the Sun. They are wonderful! See Eclipse Home Page to make your travel plans.)

Ultraviolet and X-ray images of the solar corona give us very detailed information about the distribution and of temperature and density in the solar corona. The best ultraviolet images of the corona come from the SOHO satellite, while the best X-ray images come from a Japanese satellite called Yohkoh.

A few solar radii above the photosphere, the Sun's gravity is no longer strong enough to hold in the hot gas of the corona, and the corona turns into the solar wind, which flows outward through the solar system. Moving at velocities of 400 - 500 km/s, the solar wind takes about four days to travel from the Sun to the Earth.

Disturbances in the corona resulting from solar flares propagate out through the solar wind as coronal mass ejections, which may reach the Earth a couple of days after the flare. Click here for a spectacular movie of such an event, taken with the optical coronagraph on SOHO. The size of the Sun's photosphere is represented by the circle at the center of the disk, which blocks the light of the inner corona so that the instrument can see the outer corona. Notice the two little comets that enter from the lower right early in the movie. They will both crash into the Sun. Most important, notice the two coronal mass ejections. The first, a moderate one on the lower left, occurs at about 5 AM on June 2, 1998. The second, a really huge one on the lower right, begins at about noon on June 2.

Although the fraction of solar power in the corona and wind is relatively small (about 10^-5) compared to the power radiated by the photosphere, these disturbances can have noticeable effects on Earth. In fact, a few days after the coronal mass ejection that you just saw in the movie above, the cloud of high energy particles reached the Earth's orbit and destroyed the electronics on at least one telecommunications satellite that cost hundreds of millions. Such events also cause high-energy particles to enter the Earth's atmosphere in rings surrounding the North and South Poles. (They are channeled into these rings by the Earth's magnetic field.) When these particles hit oxygen and nitrogen molecules in the Earth's atmosphere, people at northern latitudes can see the spectacular optical displays called the northern lights, or aurora borealis (or near the South Pole, the aurora australis).

In April 1998, NASA launched the TRACE satellite (built by Lockheed-Martin Missiles and Space Laboratory) to observe the Sun at ultraviolet wavelengths. TRACE is produced spectacular images that tell us much about the Sun's magnetism and corona. You can also find a nice educational summary of the Sun there.

Because the disturbances in the solar wind can cause trouble with satellites and radio communications, especially military communications, the US Air Force and the National Oceanic and Atmospheric Administration (NOAA) constantly monitor the weather on the Sun and interplanetary space with instruments on the ground and in space. For today's weather on the Sun and interplanetary space, see the Stanford Solar Center and the Space Weather Report from the NOAA Space Environment Center in Boulder

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Last modified January 24, 2002
Copyright by Richard McCray