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.
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.
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, 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.
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.
Some major terrestrial results of solar variations are the aurora, proton events, and 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.
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 running.
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. Bulk charging (also called deep charging) occurs when energetic particles, primarily electrons, penetrate the outer cover ing 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 electronic systems.
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.
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.
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.
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 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.
H-alpha image of the Sun courtesy U.S. Air Force Solar Optical Observing Network.
White light image of the sun from Japanese Yohkoh satellite, courtesy Hiraiso Observatory.
X-ray image of the sun from Japanese Yohkoh satellite, courtesy Hiraiso Observatory.
Coronal mass ejection from Holloman Airforce Base, SOON system.
All other images courtesy the Space Environment Center, NOAA.
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