Space Weather

ttere is danger blowing in the Sun's wind, which contains powerful gusts and squalls, ttey are the cosmic equivalent of terrestrial blizzards or hurricanes. Down here on the ground, we are shielded from this space weather by the Earth's atmosphere and mag netic fields, keeping us from bodily harm. But out in deep space there is no place to hide, and both humans and satellites are vulnerable.

tte storms in space can even kill an unprotected astronaut repairing a space station or walking on the Moon or Mars, tte Sun storms can disrupt global radio signals, and disable satellites used for cell phones or pagers, navigation, military reconnaissance and surveillance. Or they might overload electrical grids on the ground, causing massive power blackouts.

Violent explosions on the Sun, known as solar flares and coronal mass ejections, produce space weather, and the most intense storms, producing the greatest damage at Earth, are legendary, ttere were the record-breaking Halloween storms of October and November 2003, disrupting radio communications across the sunlit side of the Earth; several satellites were also damaged and a power blackout hit Sweden. A couple of years earlier, in March and April 2001, the space weather spawned by solar flares and coronal mass ejections also cut off radio communications, and disrupted or damaged several military and commercial satellites. And damage from the Bastille Day flare (Fig. 8.12), on 14 July 2000, was mitigated by alerts and warnings to industry, the military, and space agencies.

High-Flying Humans At Risk

Although most airplanes fly very low in the atmosphere, well below any satellite, transcontinental flights taking the polar route pass through regions of Earth's magnetic field

FIG. 8.12 Solar flare produces energetic particle storm Apowerful solar flare (left:), occurring at 10 hours 24 minutes Universal Time on Bastille day 14 July 2000, unleashed high-energy protons that began striking the SOHO spacecraft near Earth about 8 minutes later, continuing for many hours, as shown in the image taken on 22 hours 43 minutes Universal Time on the same day (right). Both images were taken at a wavelength of 19.5 nanometers, emitted at the Sun by eleven times ionized iron, denoted Fe XII, at a temperature of about 1.5 million kelvin, using the Extreme Ultraviolet Imaging Telescope, abbreviated EIT, on the SOlar and Heliospheric Observatory, or SOHO for short. (Courtesy of the SOHO EIT consortium. SOHO is a project of international cooperation between ESA and NASA.)

where solar particles become concentrated, tte high-energy particles created by solar explosions can be channeled along the magnetic field and penetrate to low altitudes in the polar regions, exposing airline crews and passengers to elevated levels of particles and radiation from space, tte higher the plane is flying and the closer to the poles, the higher the radiation dose, tte health risk is small, but highest for frequent fliers, pilots and flight attendants who travel polar routes often. Pregnant women are advised to not take an airplane flying a polar route during a storm on the Sun, to avoid risk of birth defects, but the problem is some women at risk might not even know they are pregnant.

ttere are even greater hazards aboard spacecraft at higher altitudes. According to an expert at the United States Defense Nuclear Agency, military pilots can be provided with drugs that will make them temporarily survive a lethal dose of solar particles or radiation. It's a matter of patriotism and cost, ttey are going to die anyway, so why not take the injection, save the spacecraft, and come on home to die. ttere is no sense in being the first corpse in space.

Go far enough into space and the chemical bonds in your molecules will be broken apart by storms from the Sun, increasing the risk of cancer and errors in genetic information. Space agencies therefore set limits to the exposure to solar energetic particles and radiation an astronaut can have while traveling or working unprotected in space. Because of the potential genetic damage, it is said that astronauts are supposed to have had all their children before flying in outer space; otherwise they might have some very weird offspring. Probably because of hormones, men are more radiation-resistant than women, and the resistance peaks between the ages of forty-five and fifty. So, if genetic harm and other health risks are the dominant factor, most astronauts will be middle-aged men, and they usually are.

Solar energetic particle events can endanger the health and even the lives of astronauts when they are in outer space, unprotected by the Earth's magnetic field, tte shielding of a typical spacecraft is not then enough to protect a human from cataracts or skin cancer during a major solar energetic particle event, and high-energy protons from a solar flare or coronal mass ejection can easily pierce a space suit, causing damage to human cells and tissues, ttey can even kill unprotected astronauts that venture into space (Figs. 8.13, 8.14) to unload spacecraft cargo, construct a space station or walk on the Moon or Mars.

Solar astronomers therefore keep careful watch over the Sun during space missions, to warn of possible activity occurring at just the wrong place or time. Flight controllers can then postpone space walks during solar storms, keeping astronauts within the heavily shielded recesses of a satellite or space station, ttey would also be told to curtail any strolls on the Moon or Mars, instead moving inside underground storm shelters.

A disaster has so far been avoided because previous stays on the Moon were of short duration (a few days) when no major solar eruption occurred, tte manned Apollo 16 and 17 missions to the Moon in April and December 1972, respectively occurred before and after a large solar flare, on 7 August 1972, which would have caused disabling radiation sickness and possible death to astronauts walking on the Moon.

A longer, future trip to Mars will involve considerable risks. Astronauts would spend six months or more in transit each way, and stay on the Martian surface for as long as a year-and-a-half, until the red planet again moved closest to the Earth. Some

FIG. 8.13 Man in space Astronaut Donald H. Peterson (1933- ), on a 50-foot (15-meter) tether line during his 4-hour, 3-orbit space walk, moving toward the tail of the Space Shuttle Challenger as it glides around the Earth. Hundreds of kilometers above the Earth, there is no air and astronauts must wear a spacesuit. It supplies the oxygen they need and insulates their body from extreme heat or cold. However, a spacesuit cannot protect an astronaut from energetic particles hurled out from explosions on the Sun. He or she must then be within the protective shielding of a spacecraft or other shelter to avoid the danger. (Courtesy of NASA.)

FIG. 8.13 Man in space Astronaut Donald H. Peterson (1933- ), on a 50-foot (15-meter) tether line during his 4-hour, 3-orbit space walk, moving toward the tail of the Space Shuttle Challenger as it glides around the Earth. Hundreds of kilometers above the Earth, there is no air and astronauts must wear a spacesuit. It supplies the oxygen they need and insulates their body from extreme heat or cold. However, a spacesuit cannot protect an astronaut from energetic particles hurled out from explosions on the Sun. He or she must then be within the protective shielding of a spacecraft or other shelter to avoid the danger. (Courtesy of NASA.)

estimate that every third human cell would be damaged by solar energetic particles during the flight, and others worry about how to keep the astronauts from being irradiated to death. Long exposures to cosmic rays in space also increase the risk of getting cancer, apparently to a forty-percent lifetime chance after a voyage to Mars and far above acceptable thresholds of government agencies. A future return trip to the Moon, with an extended stay, or to explore Mars, must include methods of protection of the crew from the harmful effects of Sun-driven space weather and cosmic rays.

FIG. 8.14 Unprotected from space weather The first untethered walk in space, on 7 February 1984, where there is no place to hide from inclement Sun-driven storms. Astronaut Bruce McCandless II (1937- ), a mission specialist, wears a 300-pound (136-kilogram) Manned Maneuvering Unit (MMU) with 24 nitrogen gas thrusters and a 35 mm camera. The MMU permits motion in space where the sensation of gravity has vanished, but it does not protect the astronaut from solar flares or coronal mass ejections. High-energy particles resulting from these explosions on the Sun could kill the unprotected astronaut. (Courtesy ofNASA.)

FIG. 8.14 Unprotected from space weather The first untethered walk in space, on 7 February 1984, where there is no place to hide from inclement Sun-driven storms. Astronaut Bruce McCandless II (1937- ), a mission specialist, wears a 300-pound (136-kilogram) Manned Maneuvering Unit (MMU) with 24 nitrogen gas thrusters and a 35 mm camera. The MMU permits motion in space where the sensation of gravity has vanished, but it does not protect the astronaut from solar flares or coronal mass ejections. High-energy particles resulting from these explosions on the Sun could kill the unprotected astronaut. (Courtesy ofNASA.)

Failing to Communicate

Eight minutes after an energetic solar flare, a strong blast of X-rays and extreme ultraviolet radiation reaches the Earth and radically alters the structure of the planet's upper atmosphere, known as the ionosphere, by producing an increase in the amount of free electrons that are no longer attached to atoms. Ms can cause the ionosphere to absorb the radio signals it usually reflects, resulting in faded signals and sometimes radio blackouts. During moderately intense flares, long-distance radio communications can be temporarily silenced over the Earth's entire sunlit hemisphere.

tte radio blackouts are particularly troublesome for the commercial airline industry, which uses radio transmissions for weather, air traffic and location information, and the United States Air Force and Navy are also concerned about this solar threat to radio communications. A solar flare once blacked out contact with a jet carrying President Ronald Reagan to China; for several hours the country's military Commander in Chief was unable to send or receive messages, tte Air Force operates a global system of ground-based radio and optical telescopes and taps into the output of national, space-borne X-ray telescopes and particle detectors in order to continuously monitor the Sun for intense flares that might severely disrupt military communications and satellite surveillance.

Space weather interference with radio communication can be avoided by using short-wavelength signals that pass right through the ionosphere, relaying the transmitted signals by satellite, tte communication-satellite industry is nevertheless also threatened by the tempestuous Sun. Solar eruptions apparently incapacitated Pan Am Sat's Galaxy IV satellite in May 1998, halting pager service to 45 million customers in North America, including doctors, nurses and irate business people. In addition, radio and television stations, including national public radio, could not distribute their programs. Several new Motorola Iridium satellites were disabled at nearly the same time, even before they were put into operation.

And communication satellites are not the only ones whose failure can disrupt our lives. We are children of the Space Age, increasingly dependent on many different kinds ofEarth-orbiting satellites.

Satellites in Danger

About 1,000 commercial, military and scientific satellites are now in operation, affecting the lives of millions of people, and the performances and lifetimes of all these satellites are affected by Sun-driven space weather. Geosynchronous satellites, which orbit the Earth at the same rate that the planet spins, stay above the same place on Earth to relay and beam down signals used for cellular phones, global positioning systems and internet commerce and data transmission, ttey can guide missiles or automobiles to their destinations, enable aviation and marine navigation, aid in search and rescue missions, and permit nearly instantaneous money exchange or investment choices. Other satellites revolve around our planet in closer, low-Earth orbits, scanning air, land and sea for environmental change, weather forecasting and military reconnaissance.

Space weather can noticeably increase the atmospheric friction exerted on satellites in low Earth orbit, at altitudes of 300 to 500 kilometers, causing the satellite orbits to decay more quickly than expected, tte enhanced extreme ultraviolet and X-ray radiation from solar flares heats the atmosphere and causes it to expand, and similar or greater effects are caused by coronal mass ejections, tte expansion of the terrestrial atmosphere brings higher gas densities to a given altitude, increasing the friction and drag exerted on a satellite, pulling it to a lower altitude, and sometimes causing ground controllers to lose contact with them.

Increased atmospheric friction caused by rising solar activity has sent several satellites to a premature, uncontrollable and fatal spiral toward the Earth, including Skylab and the Solar Maximum Mission. Both spacecraft were ungratefully destroyed by the very phenomenon they were designed to study - solar flares and coronal mass ejections. Space stations have to be periodically boosted in altitude to higher orbit to avoid a similar fate.

Precise monitoring of all orbiting objects depends on accurate knowledge of atmospheric change caused by storms from the Sun. tte U.S. Space Command, for example, often has to recompute the orbits of many hundreds of low-Earth-orbit objects affected by the increase in atmospheric friction.

At higher altitudes, above low-Earth orbit, geosynchronous satellites are endangered by the coronal mass ejections that cause intense geomagnetic storms, ttese satellites orbit our planet at about 6.6 Earth radii, or about 40,000 kilometers, moving around the Earth once every 24 hours. A coronal mass ejection can compress the magnetosphere from its usual location at about 10 Earth radii to below the satellites' synchronous orbits, exposing them to the full brunt of the gusty solar wind and its charged, energized ingredients.

tte Van Allen radiation belts provide a persistent, ever-present threat to highflying satellites, tte energetic electrons trapped in the radiation belts can move right through the thin metallic skin of a spacecraft, damaging the delicate microchip electronics inside. Moreover, when intense solar storms buffet the magnetosphere, they can accelerate the particles trapped in the radiation belts, greatly increasing the amounts of dangerous high-energy electrons. Metal shielding and radiation-hardened computer chips are used to guard against this recurrent hazard, and satellite orbits can be designed to minimize time in the radiation belts, or to avoid them altogether.

Nothing can be done to shield the solar cells used to power nearly all Earth-orbiting satellites; the photovoltaic cells convert sunlight to electricity and therefore have to be exposed to space, tte danger was first realized back in 1962, when the United States exploded a 1.4-megaton nuclear bomb, called Starfish, about 500 kilometers up in the atmosphere, tte explosion increased the energy of the particles in the radiation belts and created new ones, ttey wiped out the solar arrays of several satellites at the time. As satellites repeatedly pass through the Earth's natural radiation belts, exposure to its energetic particles slowly deteriorates and shortens the useful lives of their solar cells.

tte recurrent threat of moving within the radiation belts is particularly acute for satellites in low orbits that pass through the South Atlantic Anomaly, which is caused by a displacement of the Earth's magnetic center by about 500 kilometers from the planet's center. As a result, particles trapped in the inner radiation belt can approach closer to the Earth's surface above the South Atlantic than elsewhere, so this region is anomalous because of its proximity. To avoid malfunctions, the instruments aboard the Hubble Space Telescope and some other NASA satellites have had to be shut down each time they repeatedly pass through the South Atlantic Anomaly.

Infrequent, exceptionally large eruptions on the Sun can hurl very energetic protons toward the Earth and elsewhere in space, tte solar protons can enter a spacecraft like ghosts, producing erroneous commands and crippling their microelectronics. Such single event upsets have already destroyed at least one weather satellite and disabled several communications satellites. Space weapons can produce a similar effect; so if you didn't know the Sun was at fault, you might think someone was trying to shoot down our satellites.

To put the space-weather threat in perspective, just a few satellites have been lost to storms from the Sun out of thousands deployed. And the U.S. military is more concerned with disruption of radio signals, since they build satellites that can withstand the effects of a nuclear bomb exploded in space, tte commercial satellite industry, which constructs satellites that are less expensive and more vulnerable, may not want to recognize the problem, since natural disasters, including those from the Sun, are not covered by insurance policies, but engineering failures are.

A Wired World tte whole Earth has become wired together, first with telegraph wires, then by telephone lines and electrical-power grids. And disabling electrical currents and voltages can be produced in the wires when solar storms produce changes in the Earth's magnetism, ttis threat is greatest in high-latitude regions where the currents are strongest, such as Canada, the northern United States and Scandinavia.

Even back in the 1840s, when telegraph lines were first deployed, operators noticed extra current whenever overhead auroras signaled the presence of an intense geomagnetic storm. And about a century and a half later, on 13 March 1989, a particularly severe geomagnetic storm, produced by a coronal mass ejection, plunged virtually all of the Canadian province of Quebec into complete darkness without warning and within a few seconds. Six million customers were without electricity for over nine hours, in the middle of a frigid winter night, costing around 500 million dollars counting losses only from unserved demand, tte disturbed magnetic fields induced electric currents in the Earth's surface, which in turn created voltage surges on the long-distance power lines, blowing circuit breakers, overheating or melting the windings of transformers, and causing the massive electrical failure.

As demand for electricity increases, utility companies rely more and more on large, interconnected grids of power transmission lines that can span continents, providing rapid response to the diverse energy demands of users scattered throughout the world. In the United States alone, nearly a million kilometers of electrical transmission lines connect more than ten thousand power stations. Such power distribution systems are becoming increasingly vulnerable to severe geomagnetic storms, initiated when a coronal mass ejection with the right magnetic orientation plugs into the magnetosphere. ttey can plunge major urban centers, like New York City or Montreal, into complete darkness, causing social chaos and threatening safety, tte threat doesn't occur very often, perhaps once a year, but the potential consequences are serious enough to employ early warning systems.

Here Comes the Sun

Space weather is here to stay, and the dangers blowing in the Sun's wind are not going away. Humans are spending more and more time in space, while those on the ground become increasingly dependent on satellites that whiz over their heads. In tens of minutes, intense explosions hurl out energetic particles that can endanger astronauts, and their survival may depend on how well one can predict space weather. Forceful solar flares or mass ejections can damage or destroy Earth-orbiting satellites, increase cancer risk for people using commercial airlines over polar routes, and create power surges that can blackout entire cities.

Recognizing our vulnerability, astronomers use telescopes on the ground and in situ particle detectors or remote-sensing telescopes on satellites to carefully monitor the

Sun, and government agencies post forecasts that warn of threatening solar activity. Ms enables evasive action that can reduce disruption or damage to communications, defense and weather satellites, as well as electrical power systems on the ground. Once we know a Sun storm is on its way, the launch of manned space flight missions can be postponed, and walks outside spacecraft or on the Moon or Mars might be delayed. Airplane pilots can be warned of potential radio communication failures. Operators can power down sensitive electronics on communication and navigation satellites, putting them to sleep until the danger passes. Utility companies can reduce load in anticipation of trouble on power lines, in that way trading a temporary "brown out" for a potentially disastrous "black out."

What everyone wants to know is how strong the storm is and when it is going to hit us. Like winter storms on Earth, some of the effects can be predicted days in advance. A coronal mass ejection, for example, arrives at the Earth one to four days after leaving the Sun, and solar astronomers can watch it leave the Sun. tte STEREO mission will additionally track coronal mass ejections headed for Earth.

Solar flares are another matter. A soon as you see a flare on the Sun, its radiation and fastest particle have already reached us, taking just 8 minutes to travel from the Sun to Earth. One promising technique is to watch to see when the solar magnetism has become twisted into a stressed situation, for it may then be about to release a solar flare. Another one is to look through the Sun and see active regions develop before they rotate to face the Earth. Both of these methods of predicting explosions on the Sun were discussed in Section 7.7.

So we now turn to our life-sustaining atmosphere, which provides the oxygen we breathe and the warmth that keeps the oceans from freezing. Our dynamic atmosphere is being transformed by the Sun above and by humans below.

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