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
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