We all know that the Sun is overwhelmingly
important to life on Earth, but few of us have
been given a good description of our star and its
variations.
The Sun is an average star, similar to millions of others in the Universe. It is a prodigious energy machine, manufacturing about 4.0E023 kilowatts of energy per second. In other words, if the total output of the Sun was gathered for one second it would provide the U.S. with enough energy, at its current usage rate, for the next 9,000,000 years. The basic energy source for the Sun is nuclear fusion, which uses the high temperatures and densities within the core to fuse hydrogen, creating energy and producing helium as a by-product. The core is so dense and the size of the Sun so great that energy released at the center of the Sun takes about 50,000,000 years to make its way to the surface, undergoing countless absorptions and reemissions in the process. (If the Sun were to stop producing energy today, it would take 50,000,000 years for the effects to be felt at Earth!)
The Sun has been producing its radiant and thermal energies for the past four or five billion years. It has enough hydrogen to continue producing for another hundred billion years. However, in about ten to twenty billion years the surface of the Sun will begin to expand, enveloping the inner planets (including Earth). At that time, our Sun will be known as a red giant star. If the Sun were more massive, it would collapse and re-ignite as a helium-burning star. Due to its average size, however, the Sun is expected to merely contract into a relatively small, cool star known as a white dwarf.
It has long been known that the Sun is neither featureless nor steady. (Theophrastus first identified sunspots in the year 325 B.C.) Some of the more important solar features are explained in the following sections.
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Sunspots, dark areas on the solar surface, are transient, concentrated magnetic fields. They are the most prominent visible features on the Sun; a moderate-sized sunspot is about as large as Earth. Sunspots form and dissipate over periods of days or weeks. They occur when strong magnetic fields emerge through the solar surface and allow the area to cool slightly, from a background value of 6000 degrees C down to about 4200 degrees C; this area appears as a dark spot in contrast with the Sun. The darkest area at the center of a sunspot is called the umbra; it is here that the magnetic field strengths are the highest. The less-dark, striated area around the umbra is called the penumbra. Sunspots rotate with the solar surface, taking about 27 days to make a complete rotation as seen from Earth. Sunspots near the Sun's equator rotate at a faster rate than those near the solar poles. Groups of sunspots, especially those with complex magnetic field configurations, are often the sites of flares.
Over the last 300 years, the average number of sunspots has regularly waxed and waned in an 11-year sunspot cycle. The Sun, like Earth, has its seasons but its year equals 11 of ours. The last solar maximum was in 1989, and the next minimum is expected in 1995-1996.
Coronal holes are variable solar features that can last for months to years. They are seen as large, dark holes when the Sun is viewed in x-ray wavelengths. These holes are rooted in large cells of unipolar magnetic fields on the Sun's surface; their field lines extend far out into the solar system. These open field lines allow a continuous outflow of high-velocity solar wind. Coronal holes have a long-term cycle, but it doesn't correspond exactly to the sunspot cycle; they holes tend to be most numerous in the years following sunspot maximum. At some stages of the solar cycle, these holes are continuously visible at the solar north and south poles.
Solar prominences (seen as dark filaments on the disk) are usually quiescent clouds of solar material held above the solar surface by magnetic fields. Most prominences erupt at some point in their lifetime, releasing large amounts of solar material into space.
The outer solar atmosphere, the corona, is
structured by strong magnetic fields. Where
these fields are closed, often above sunspot
groups, the confined solar atmosphere can suddenly
and violently release bubbles or tongues
of gas and magnetic fields called coronal mass
ejections. A large CME can
contain
10.0E16 grams (a billion tons) of matter that can be
accelerated to several million miles per hour in
a spectacular explosion. Solar material streaks
out through the interplanetary medium, impacting
any planets or spacecraft in its path.
CMEs are sometimes associated with flares but
usually occur independently.
The area between the Sun and the planets has been termed the interplanetary medium. Although sometimes considered a perfect vacuum, this is actually a turbulent area dominated by the solar wind, which flows at velocities of approximately 250-1000 km/s (about 600,000 to 2,000,000 miles per hour). Other characteristics of the solar wind (density, composition, and magnetic field strength, among others) vary with changing conditions on the Sun. The effect of the solar wind can be seen in the tails of comets (which always point away from the Sun).
The solar wind flows around obstacles such as planets, but those planets with their own magnetic fields respond in specific ways. Earth's magnetic field is very similar to the pattern formed when iron filings align around a bar magnet. Under the influence of the solar wind, these magnetic field lines are compressed in the Sunward direction and stretched out in the downwind direction. This creates the magnetosphere, a complex, teardrop-shaped cavity around Earth. The Van Allen radiation belts are within this cavity, as is the ionosphere, a layer of Earth's upper atmosphere where photo ionization by solar x-rays and extreme ultraviolet rays creates free electrons. Earth's magnetic field senses the solar wind its speed, density, and magnetic field. Because the solar wind varies over time scales as short as seconds, the interface that separates interplanetary space from the magnetosphere is very dynamic. Normally this interface called the magnetopause lies at a distance equivalent to about 10 Earth radii in the direction of the Sun. However, during episodes of elevated solar wind density or velocity, the magnetopause can be pushed inward to within 6.6 Earth radii (the altitude of geosynchronous satellites). As the magneto sphere extracts energy from the solar wind, in ternal processes produce geomagnetic storms.
The aurora is a dynamic and delicate visual manifestation of solar-induced geomagnetic storms. The solar wind energizes electrons and ions in the magnetosphere. These particles usually enter Earth's upper atmosphere near the polar regions. When the particles strike the molecules and atoms of the thin, high atmosphere, some of them start to glow in different colors.
Aurorae begin between 60 and 80 degrees latitude. As a storm intensifies, the aurorae spread to ward the equator. During an unusually large storm in 1909, an aurora was visible at Singapore, on the geomagnetic equator. The aurorae provide pretty displays, but they are just a visible sign of atmospheric changes that wreak havoc on technological systems.
Energetic protons can reach Earth within 30 minutes of a major flare's peak. During such an event, Earth is showered with highly energetic solar particles (primarily protons) released from the flare site. Some of these particles spiral down Earth's magnetic field lines, reaching the upper layers of our atmosphere.
One to four days after a flare or eruptive
prominence occurs, a slower cloud of solar material
and magnetic fields reaches Earth, buffet
ing the magnetosphere and resulting in a geomagnetic storm.
These storms are
extraordinary variations in Earth's surface
magnetic field. During a geomagnetic storm,
portions of the solar wind's energy is transferred
to the magnetosphere, causing Earth's
magnetic field to change rapidly in direction
and intensity.
Some military detection or early-warning
systems are also affected by solar activity. The
Over-the-Horizon Radar bounces signals off
the ionosphere in order to monitor the launch of
aircraft and missiles from long distances. During
geomagnetic storms, this system can be severely
hampered by radio clutter. Some submarine
detection systems use the magnetic
signatures of submarines as one input to their
locating schemes. Geomagnetic storms can
mask and distort these signals.
The Federal Aviation
Administration routinely receives alerts
of solar radio bursts so that
they can recognize communication problems
and forego unnecessary maintenance. When an
aircraft and a ground station are aligned with
the Sun, jamming of air-control radio frequencies
can occur. This can also happen when
an Earth station, a satellite, and the Sun are in
alignment.
Systems such as LORAN and OMEGA are
adversely affected when solar activity disrupts
their radio wavelengths. The OMEGA system
consists of eight transmitters located through
out the world. Airplanes and ships use the very
low frequency signals from these transmitters
to determine their positions. During solar
events and geomagnetic storms, the system can
give navigators information that is inaccurate
by as much as several miles. If navigators are
alerted that a proton event or geomagnetic
storm is in progress, they can switch to a backup
system. GPS signals are affected when solar activity
causes sudden variations in the density of
the ionosphere.
Geomagnetic storms and increased solar ultraviolet emission
heat Earth's upper atmosphere, causing it to expand. The heated air
rises, and the density at the orbit of satellites up
to about 1000 km increases significantly. This
results in increased drag on satellites in space,
causing them to slow and change orbit slightly.
Unless low-Earth-orbit satellites are routinely
boosted to higher orbits, they slowly fall, and
eventually burn up in Earth's atmosphere.
Skylab is an example of a spacecraft re-entering
Earth's atmosphere prematurely as a result
of higher-than-expected solar activity.
During the great geomagnetic storm of March
1989, four of the Navy's navigational satellites
had to be taken out of service for up to a week.
As technology has allowed spacecraft components
to become smaller, their miniaturized
systems have become increasingly vulnerable
to the more energetic solar particles. These particles
can cause physical damage to microchips
and can change software commands in satellite-
borne computers.
Differential Charging. Another problem for
satellite operators is differential charging. During
geomagnetic storms, the number and ener
gy of electrons and ions increase. When a satellite
travels through this energized environment,
the charged particles striking the spacecraft
cause different portions of the spacecraft to be
differentially charged. Eventually, electrical
discharges can arc across spacecraft components,
harming and possibly disabling them.
Bulk Charging.
Intense solar flares release very-high-energy
particles that can be as injurious to humans as
the low-energy radiation from nuclear blasts.
Earth's atmosphere and magnetosphere allow
adequate protection for us on the ground, but
astronauts in space are subject to potentially lethal
dosages of radiation. The penetration of
high-energy particles into living cells, measured
as radiation dose, leads to chromosome
damage and, potentially, cancer. Large doses
can be fatal immediately. Solar protons with
energies greater than 30 MeV are particularly
hazardous. In October 1989, the Sun produced
enough energetic particles that an astronaut on
the Moon, wearing only a space suit and caught
out in the brunt of the storm, would probably
have died. (Astronauts who had time to gain
safety in a shelter beneath moon soil would
have absorbed only slight amounts of radiation.)
Solar proton events can also produce elevated
radiation aboard supersonic aircraft fly
ing at high altitudes over the polar caps. To
minimize this risk, routine forecasts and alerts
are sent through the FAA so that a flight in potential
danger can alter its course and reduce altitude
to minimize radiation exposure.
When magnetic fields move about in the vicinity
of a conductor such as a wire, an electric
current is induced into the conductor. This happens
on a grand scale during geomagnetic
storms. Power companies transmit alternating
current to their customers via long transmission
lines. The nearly direct currents induced in
these lines from geomagnetic storms are harm
ful to electrical transmission equipment. On
March 13, 1989, in Montreal, Quebec, 6 million
people were without commercial electric
power for 9 hours as a result of a huge geomagnetic
storm. Some areas in the northeastern
U.S. and in Sweden also lost power. By receiving
geomagnetic storm alerts and warnings,
power companies can minimize damage and
power outages.
Rapidly fluctuating geomagnetic fields can
induce currents into pipelines. During these
times, several problems can arise for pipeline
engineers. Flow meters in the pipeline can
transmit erroneous flow information, and the
corrosion rate of the pipeline is dramatically in
creased. If engineers unwittingly attempt to
balance the current during a geomagnetic
storm, corrosion rates may increase even more.
Pipeline managers routinely receive alerts and
warnings to help them provide an efficient and
long-lived system.
While the solar cycle has been nearly regular
during the last 300 years, there was a period of
70 years during the 17th and 18th centuries
when very few sunspots were seen (even
though telescopes were widely used). This drop
in sunspot number coincided with the timing of
the little ice age in Europe, implying a Sun-
to-climate connection. Recently, a more direct
link between climate and solar variability has
been speculated. Stratospheric winds near the
equator blow in different directions, depending
on the time in the solar cycle. Studies are under
way to determine how this wind reversal affects
global circulation patterns and weather.
During proton events, many more energetic
particles reach Earth's middle atmosphere.
There they cause molecular ionization, creating
chemicals that destroy atmospheric ozone
and allow increased amounts of harmful solar
ultraviolet radiation to reach Earth's surface. A
solar proton event in 1982 resulted in a temporary
70% decrease in ozone densities.
Possibly the most closely studied of the variable
Sun's biological effects has been the degradation
of homing pigeons' navigational abilities during
geomagnetic storms. Pigeons and
other migratory animals, such as dolphins and
whales, have internal biological compasses
composed of the mineral magnetite wrapped in
bundles of nerve cells. While this probably is
not their primarily method of navigation, there
have been many pigeon race smashes, a term
used when only a small percentage of birds return
home from a release site. Because these
losses have occurred during geomagnetic
storms, pigeon handlers have learned to ask for
geomagnetic alerts and warnings as an aid to
scheduling races.
It has been realized and appreciated only in
the last few decades that solar flares, CMEs,
and magnetic storms affect people and their activities.
The list of consequences grows in proportion
to our dependence on technological
systems. The subtleties of the interactions between
Sun and Earth, and between solar particles
and delicate instruments, have become
factors that affect our well being. Thus there
will be continued and intensified need for space
environment services to address health, safety,
and commercial needs.
Disrupted Systems
Conclusion
Suggested Reading
Photo Credits
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