4. Sun-Earth Connection
Danger blowing in the wind
Sun-driven space weather endangers humans whenever they venture into space. Down here on the ground, we are shielded from the direct onslaught of the raging solar wind by the Earth’s atmosphere and magnetic fields, but out in deep space there is no place to hide. Harmful high-energy particles, carried by gusts and squalls in the solar wind, can wipe out unprotected astronauts and destroy satellite electronics. They are of serious concern to future astronauts who might construct the International Space Station and explore the Moon or Mars.
Satellites can also be disabled by stormy weather in space. Powerful blasts from coronal mass ejections can compress the Earth’s magnetic field and send energetic particles into the magnetosphere, providing threats to Earth-orbiting satellites. Intense radiation from solar flares can change the electrical properties of our atmosphere, disrupting radio navigation or communication systems, and making the atmosphere expand farther into space than usual. Friction can develop between the expanded atmosphere and satellites traveling in it, slowing down the satellites, altering their orbits, and bringing them to a premature end. Forceful coronal mass ejections can also generate strong currents in out atmosphere, overloading transmission lines on the ground and producing power surges that can blackout entire cities.
Our technological society has become so vulnerable to the potential devastation of these storms in space that national centers employ space weather forecasters, and continuously monitor the Sun from ground and space to warn of threatening solar activity.
The hazards of space travel
The ultimate vacation, a trip into deep space, is fraught with danger, primarily from energetic particles. Even in the comparative safety of low-Earth orbits, beneath the protection of Earth’s magnetic field, astronauts have reported flashing lights inside their eyes. Energetic protons, perhaps trapped in the Van Allen radiation belts, pass through the satellite walls and the astronaut’s eyelids, striking their retinas and making their eyeballs glow inside.
Once outside the Earth’s magnetosphere, astronauts are exposed to the full blast of the ever-flowing solar wind. They could then suffer serious consequences from solar energetic particles even within their spacecraft, resulting in cataracts, skin cancer or even lethal radiation poisoning.
Energetic protons hurled out from intense solar explosions are especially hazardous. The largest events could inflict serious radiation damage on any astronaut caught in space without adequate shielding (Fig. 8.10, Fig. 8.11). Several of these proton events, each lasting 1 to 3 days, occur each year on the average. The high-speed solar protons could even kill an unprotected astronaut that ventures into space. Astronauts walking on the lunar surface in 1972 had at least one close call involving potentially deadly solar-flare events.
Fast coronal mass ejections plow into the slower-moving solar wind and act like a piston that drives shock waves ahead of them, accelerating electrons and protons as they go, much as ocean waves propel surfers. The mass ejections move straight out of the Sun and flatten everything in their path, like a gigantic falling tree or a car out of control. They energize particles on a grand scale that covers large regions in interplanetary space.
The crucial information is how strong the storm is and if and when it is going to hit us. The exact warning time will depend on the type of solar hazard, since they travel with different velocities and on various trajectories in space (Fig. 8.12). Intense radiation from powerful solar flares moves from the Sun to the Earth in just 8 minutes, traveling at the speed of light. Energetic particles, accelerated during the flare process or by the shock waves of coronal mass ejections, can reach the Earth within an hour or less (for energies above 10 MeV). A coronal mass ejection arrives at the Earth as a dense cloud of magnetic fields, electrons and protons one to four days after leaving the Sun.
Satelites in danger
Energetic charged particles from solar explosions can seriously damage satellites. When an energetic flaring proton, above 10 MeV in energy, strikes a spacecraft, it can destroy its electronic components. Metal shielding and radiation-hardened computer chips are used to guard against this persistent, ever-present threat to satellites, but nothing can be done to shield solar cells. Since they use sunlight to power spacecraft, solar cells must be exposed to space. Energetic solar protons scour their surface and shorten their lives. They have destroyed the solar cells on at a least one weather satellite.
Increases in the dynamic pressure of the Sun’s winds during solar activity compresses the magnetosphere and puts high-flying satellites at risk. When a coronal mass ejection slams into the Earth, the force of impact can push the bow shock, at the day side of the magnetosphere, down to half its usual distance of about 10 times Earth’s radius. Geostationary spacecraft, that stay over the same spot on Earth, orbit our planet at about 6.6 Earth radii, moving around it once every 24 hours or at the same rate that the planet spins. When the magnetosphere is compressed below their geosynchronous orbits, these satellites are exposed to the full brunt of the gusty solar wind and its charged, energized ingredients.
Turning off the lights
During an intense geomagnetic storm, associated with a colliding coronal mass ejection, strong electric currents flow in the auroral ionosphere. They induce potential differences in the ground beneath and produce strong currents in any long conductor such as a power line (Fig. 8.13). Up to 100 Amperes of Direct Current, or DC, surge through long-distance power lines designed to carry Alternating Current, or AC, blowing circuit breakers, overheating and melting the windings of transformers, and causing massive failures of electrical distribution systems.
A coronal mass ejection can thereby plunge major urban centers, like New York City or Montreal, into complete darkness, causing social chaos and threatening safety. It is capable of permanently damaging multi-million dollar equipment in power generation plants, and producing hundreds of millions of dollars in losses from unserved power demand or disruption of factories. The threat is greatest in high-latitude regions where the auroral currents are strongest, such as Canada, the northern United States and Scandinavia. In fact, one great magnetic storm in March 1989 put the entire Quebec electric power system out of operation, turning off the lights in a large part of the area for 9 hours.
Forecasting space weather
Space weather is here to stay, and the dangers blowing in the Sun’s winds are not going away. In tens of minutes, intense explosions hurl out energetic particles that can endanger humans in space and destroy satellites. Forceful solar mass ejections can also damage or destroy Earth-orbiting satellites, and create power surges that can blackout entire cities. Recognizing our vulnerability, government agencies post forecasts that warn of threatening solar activity.
The Space Environment Center (SEC) of the National Oceanic and Atmospheric Administration collects and distributes the relevant data, using satellites and ground-based telescopes to monitor the Sun and relay information about conditions in interplanetary space. Its Geostationary Operational Environmental Satellites, or GOES for short, monitor threatening activity as it nears the Earth, including the powerful X-ray emission of solar flares (Fig. 8.14), and high-speed electrons and protons.
Predictions about space weather events, based on data from the SEC and NASA satellites are given at www.spaceweather.com. For instance, the Solar and Heliospheric Observatory, or SOHO, watches solar flares explode on the surface of the Sun at least 8 minutes before their radiation or particles strike Earth, and it also provides a few days warning coronal mass ejections that might hit our planet (Fig. 8.15).
Solar astronomers are now looking back at the source of it all, developing methods of predicting solar explosions based on the Sun’s magnetic contortions or the growth of active regions on the invisible back side of the Earth. Space scientists are extending this effort, studying the vital links and dynamic interplay between the Sun and the Earth and viewing them as an interconnected whole. A variety of spacecraft are making coordinated, simultaneous measurements of the Sun, the solar wind, and the Earth’s magnetosphere, providing a new global perspective of the intricate coupling between the Sun and Earth under the auspices of an International Solar Terrestrial Physics (ISTP) program. For the first time ever, we can now track every move of possibly destructive events from their beginning on the Sun, to their passage through space, and their ending impact on Earth (Fig. 8.16).
(page 2 of 4)
Copyright 2010, Professor Kenneth R. Lang, Tufts University