Our Star, the Sun
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 3.8 x 1023
kiloWatts (or kiloJoules/sec). 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, producing energy and creating helium as a byproduct.
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 re-emissions in the process. If
the Sun were to stop producing energy today, it would take 50,000,000 years for
significant 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.
(MPEG movie 24 kbytes, 38 frames, 300x300 pixels)
Sunspots, dark areas on the solar surface, contain 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.
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.
Solar flares are intense, temporary releases of energy. They are seen at
ground-based observatories as bright areas on the Sun in optical wavelengths and
as bursts of noise at radio wavelengths; they can last from minutes to hours.
Flares are our solar system's largest explosive events which can be equivalent
to approximately 40 billion Hiroshima-size atomic bombs. The primary energy
source for flares appears to be the tearing and reconnection of strong magnetic
fields. They radiate throughout the electromagnetic spectrum, from gamma rays to
x-rays, through visible light out to kilometer-long radio waves.
Coronal Mass Ejections
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.
Between Sun and Earth
The region between the Sun and the planets has
been termed the interplanetary medium. Although once considered a perfect vacuum,
this is actually a turbulent region 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 magnetosphere extracts energy
from the solar wind, internal processes produce geomagnetic storms.
Solar Effects at Earth
Some major terrestrial results of solar variations are the aurora, proton
events, and geomagnetic storms.
The aurora is a
dynamic and visually delicate 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 toward 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
may wreak havoc on technological systems.
Aurora in El Paso County, Texas, August 12, 2000.
Courtesy of Christopher Grohusko.
Energetic protons can reach Earth within 30 minutes of a major flare's peak.
During such an event, Earth is showered energetic solar particles (primarily
protons) released from the flare site. Some of these particles spiral down
Earth's magnetic field lines, penetrating the upper layers of our atmosphere
where they produce additional ionization and may produce a significant increase
in the radiation environment.
One to four days after a flare or eruptive prominence occurs, a slower cloud
of solar material and magnetic fields reaches Earth, buffeting 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 and energize the
particle populations within it.
Many communication systems utilize the ionosphere to reflect radio signals
over long distances. Ionospheric storms can affect radio communication at all
latitudes. Some radio frequencies are absorbed and others are reflected,
leading to rapidly fluctuating signals and unexpected propagation paths. TV and
commercial radio stations are little affected by solar activity, but ground-to-air,
ship-to-shore, Voice of America, Radio Free Europe, and amateur radio are
frequently disrupted. Radio operators using high frequencies rely upon solar
and geomagnetic alerts to keep their communication circuits up and
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.
- Navigation Systems
Systems such as LORAN and OMEGA are adversely affected when solar
activity disrupts their signal propagation. 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
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 energy 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.
Bulk charging (also called deep charging) occurs when energetic
particles, primarily electrons, penetrate the outer covering of a satellite and
deposit their charge in its internal parts. If sufficient charge accumulates in
any one component, it may attempt to neutralize by discharging to other
components. This discharge is potentially hazardous to the satellite's
- Radiation Hazards to Humans
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 aircraft
flying at high altitudes. Although these risks are small, monitoring of solar
proton events by satellite instrumentation allows the occassional exposure to
be monitored and evaluated.
- Geologic Exploration
Earth's magnetic field is used by geologists to determine subterranean rock
structures. For the most part, these geodetic surveyors are searching for oil,
gas, or mineral deposits. They can accomplish this only when Earth's field is
quiet, so that true magnetic signatures can be detected. Other surveyors prefer
to work during geomagnetic storms, when the variations to Earth's normal
subsurface electric currents help them to see subsurface oil or mineral
structures. For these reasons, many surveyors use geomagnetic alerts and
predictions to schedule their mapping activities.
- Electric Power
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 harmful 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 increased. 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.
The Sun is the heat engine that drives the circulation of our atmosphere.
Although it has long been assumed to be a constant source of energy, recent
measurements of this solar constant have shown that the base output of the Sun
can vary by up to two tenths of a percent over the 11-year solar cycle.
Temporary decreases of up to one-half percent have been observed. Atmospheric
scientists say that this variation is significant and that it can modify
climate over time. Plant growth has been shown to vary over the 11-year sunspot
and 22-year magnetic cycles of the Sun, as evidenced in tree-ring records.
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.
There is a growing body of evidence that changes in the geomagnetic field
affect biological systems. Studies indicate that physically stressed human
biological systems may respond to fluctuations in the geomagnetic field.
Interest and concern in this subject have led the Union of Radio Science
International to create a new commission entitled Electromagnetics in
Biology and Medicine.
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
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.
- Davies, K., 1990, Ionospheric Radio. Peter Peregrinus, London.
- Eather, R. H., 1980, Majestic Lights. AGU, Washington, D.C.
- Garrett, H. B., and C. P. Pike, eds., 1980, Space Systems and Their
Interactions with Earth's Space Environment. New York: American Institute
of Aeronautics and Astronautics.
- Gauthreaux, S., Jr., 1980, Animal Migration: Orientation and Navigation.,
Chapter 5. Academic Press, New York.
- Harding, R., 1989, Survival in Space. Routledge, New York.
- Joselyn, J.A., 1992, The impact of solar flares and magnetic storms on
humans. EOS, 73(7): 81, 84-85.
- Johnson, N. L., and D. S. McKnight, 1987, Artificial Space Debris.
Orbit Book Co., Malabar, Florida.
- Lanzerotti, L. J., 1979, Impacts of ionospheric / magnetospheric process on
terrestrial science and technology. In Solar System Plasma Physics, III,
L. J. Lanzerotti, C. F. Kennel, and E.N. Parker, eds. North Holland Publishing
Co., New York.
- Campbell, W.H., 2001, Earth Magnetism: A Guided Tour Through Magnetic
Fields, Harcourt Sci. and Tech. Co., New York
H-alpha image of the Sun courtesy U.S. Air Force Solar Optical Observing
White light image of the sun from Japanese Yohkoh satellite, courtesy
X-ray image of the sun from Japanese Yohkoh satellite, courtesy Hiraiso
Coronal mass ejection from Holloman Airforce Base, SOON system.
All other images courtesy the Space Environment Center,