4. Third rock from the Sun - restless Earth

Space weather

Fig. ..c  Fig. .. 

There is danger blowing in the Sun's winds. Energetic charged particles and magnetic fields are being hurled into interplanetary space by explosions on the Sun, producing gusts and squalls in the solar wind that can wipe out unprotected astronauts and destroy satellites. Down here on the ground, we are shielded from their direct onslaught by the Earth’s atmosphere and magnetic fields, but out in deep space there is no place to hide.

Powerful explosions on the violent Sun come in two main varieties, known as solar flares and coronal mass ejections, or CMEs for short. Both kinds of solar activity are powered by the Sun's magnetic energy, and they both vary in step with the Sun's 11-year cycle of magnetic activity. Solar flares and CMEs are more frequent and tend to be more powerful during the maximum in the activity cycle.

Solar flares are brief, catastrophic outbursts that flood the solar system with intense radiation and high-speed electrons and protons. In just a few minutes they can release an explosive energy of up to 1025 Joule, equivalent to 20 million 100 megaton terrestrial nuclear bombs, raising the temperature of Earth-sized regions on the Sun to tens of millions of degrees. The other type of solar explosive activity, the CMEs, expand away from the Sun at speeds of hundreds of thousands of meters per second, becoming larger than the Sun and removing up to fifty billion tons, or 5 x 1013 kilograms, of the Sun's atmosphere.

Energetic protons hurled out from intense solar flares are especially hazardous. They endanger any astronaut caught in space without adequate protection. The high-speed solar protons could even kill an unprotected astronaut that ventures into space. The electrically-charged particles follow a narrow, curved path once they leave the Sun, guided by the spiral structure of the interplanetary magnetic field. Solar astronomers therefore keep careful watch over the Sun during space missions, to warn of possible activity occurring at just the right place on the Sun, at one end of a spiral magnetic field line that connects the flaring region to wherever astronauts happen to be. They can then avoid making repairs to their space stations, and curtail any strolls on the Moon or Mars, instead moving inside storm shelters.

Our technological society has become increasingly vulnerable to explosions on the Sun. They emit energetic particles, intense radiation, powerful magnetic fields and strong shocks that can have enormous practical implications when directed toward Earth. The solar emissions can disrupt navigation and communication systems, pose significant hazards to humans in space, destroy Earth-orbiting satellites, and create power surges that can black out entire cities. Recognizing our vulnerability, national centers and defense agencies continuously monitor the Sun from ground and space to forecast threatening activity. An example is the Space Environment Center, abbreviated SEC, of the United States National Oceanic and Atmospheric Administration. It collects and distributes space weather data, using satellites and ground-based telescopes to monitor the Sun and interplanetary space.

Fig. .. 

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 CME arrives at the Earth one to four days after leaving the Sun, and solar astronomers can watch solar explosions happen. Solar flares are another matter. As soon as you can see a solar flare on the Sun, its radiation and fastest particles have already reached us, taking just 8 minutes to travel from the Sun to Earth. Dangerous, but less energetic particles, might take an hour to get here. The ultimate goal of space-weather forecasters is to predict when the Sun is about to unleash its pent-up energy, before a solar flare or CME occurs. One promising technique is to watch to see when the magnetism has become twisted into a stressed situation, for it may then be about to explode.

Summary Diagram

Fig. ..summary 

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Copyright 2010, Professor Kenneth R. Lang, Tufts University