The Sun

The Sun gives us light and heat, sustaining life on Earth. Its energy comes from nuclear fusion deep in its interior, and its heat constantly churns up its outer layers, observable by telescopes on Earth and aboard spacecraft. Like the Earth, the Sun also rotates around its axis, once in about 27 days, but unlike Earth, its rotation is not uniform, the equator goes around faster than regions near the poles.

This uneven rotation, coupled with the churning of the upper layers, might well be what produces (by a "dynamo mechanism", described in a later section) regions of intense magnetic field, seen by observers on Earth as dark sunspots. A German amateur astronomer, Heinrich Schwabe, noticed around 1850 that the number of such spots rose and fell in an irregular cycle of about 11 years (see figure below, tracking the "sunspot number" for over a century).

Sunspot Cycle (1850 - 1975)

Near the peak of the cycle violent energy releases occur near sunspots, emitting x-rays, radio bursts and rapidly moving plasma clouds, which can produce magnetic storms when they reach Earth. On occasion, especially in large events, the releases also produce high energy ions which spread out through interplanetary space, to the Earth's orbit and beyond. Such events are often associated with "flares," sudden brightenings in the chromosphere, a high layer in the Sun's atmosphere. The chromosphere is best seen through filters which only pass the red light of hydrogen, and it is through such filters that flares are usually seen--though the very first flare observation, by Richard Carrington in 1859, was made through a regular telescope.

More about the Sun

Click here to read Richard Christopher Carrington's account of observing a flare on September 1, 1859.

The Sun's Corona

When the moon covers the bright face of the Sun during a total eclipse, the fainter outer layers become visible: the reddish chromosphere, and above it, the long streamers of the corona. Near sunspots those streamers seem to be shaped by the Sun's magnetic field lines, and above the Sun's poles they suggest field lines rising from twin magnetic poles like those of the Earth.

Since the Sun's heat comes from its deep core, one would expect the temperature of its layers to drop with increasing distance from the central furnace. In fact, this does not happen. While the visible face of the Sun (the photosphere layer) has a temperature around 6000 deg. C, the corona which begins only a few thousand kilometers higher reaches a million degrees (1.8 million deg. F). No satisfactory explanation has ever been given--somehow, apparently, energy is transmitted to the outer layers of the Sun in ways that go beyond the ordinary flow of heat.

More about the Sun's corona

The Solar Wind

The plasma of the corona is so hot that the Sun's gravity cannot hold it down. Instead, the upper fringes flow away in all directions, in a constant stream of particles known as the solar wind. Moving at about 400 km/sec (about 250 miles/sec), the wind needs about 4-5 days to reach Earth, and as many months to attain the outermost planets: its outer limits, the boundary between the space dominated by the Sun and the interstellar medium, is probably more distant still. The space probes Voyager 1 and 2, launched in 1977, are expected to reach that boundary early in the 21st century, and NASA is hopeful that the nuclear batteries which power those spacecraft will last long enough to observe the transition.

As the solar wind leaves the corona, it picks up the local magnetic field--contributed by sunspots and by the Sun's magnetic poles--and drags its field lines into space, forming the interplanetary magnetic field (IMF). The IMF is quite weak--at the Earth's orbit, only 1/10,000 of the field at the Earth's surface--but as shown in a later section, it exerts an extraordinary influence on the Earth's magnetosphere.

As already noted, field lines in a plasma act like wires on which ions and electrons are strung. If the field is strong, these particles are forced to go to wherever the lines guide them. On the other hand, when the particles are numerous and energetic, as is the case with the solar wind, they can push the magnetic field around. When their flow is deflected, for instance, the lines will change shape, so as to always thread the same particles. Because of this effect, the structure of the IMF even at the greatest distances tends to "remember" the Sun's rotation at its region of origin.

More about the solar wind