BLASTS of particles spat out by the Sun are one of the main threats to communications satellites. But it鈥檚 a mystery why, because objects within the Earth鈥檚 magnetic field should be protected from the onslaught. Now a British physicist says he has the answer. While the solar particles themselves are deflected by the field, some of their energy is transferred to even more destructive particles in the Earth鈥檚 radiation belts.
The Earth is continuously blasted by the solar wind 鈥� charged particles ejected from the Sun. Our planet鈥檚 magnetic field deflects them, but if there is a large increase in these particles, such as in a coronal mass ejection, the field is disturbed.
The link between CMEs and satellite damage was confirmed in 1997, when the SOHO satellite spotted one coming our way. During the resulting magnetic storm, a $200 million telecoms satellite called Telstar 401 stopped working. It is thought that Telstar 401 and other satellites that fail during such storms are blasted by particles and electric discharges that charge them to hundreds or even thousands of volts.
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But the charged particles of the CME cannot directly penetrate the Earth鈥檚 magnetic field. And compared with the strength of magnetic storms, the particles emitted by the Sun are of quite low energy.
Richard Horne of the British Antarctic Survey in Cambridge suspected a phenomenon called whistler waves. These are complex waves formed when charged particles move through space. A strange mix of high and low frequencies, they are commonly produced during lightning storms and are heard as a whistle falling in pitch when picked up by headphones.
Horne wondered whether whistler waves generated by a CME could affect the Van Allen belts 鈥� two regions of highly energetic charged particles partly surrounding the Earth, trapped at heights of 19,000 kilometres by the planet鈥檚 magnetic field. Perhaps the CME drives the formation of whistler waves in a Van Allen belt from a distance, by a process similar to induction.
Horne created a computer simulation of the process, and demonstrated that whistler waves could cause the high-energy particles in the Van Allen belt to build up, until they collectively have enough energy to escape the constraints of the belts. The particles then travel along the magnetic field lines, sometimes coming to Earth at the poles.
It is these particles that damage satellites, Horne told the UK/Ireland National Astronomy meeting in Dublin earlier this month. To test the idea, he and colleagues at the British Antarctic Survey tracked whistler waves at their research base in Antarctica. Sure enough, they detected large numbers of the waves coming to Earth during magnetic storms. And satellites monitoring the storms detected corresponding build-ups in the solar wind and in magnetic activity just before the waves arrived on Earth.
鈥淲e are always looking for mechanisms to explain this process,鈥� says Andre Balogh, professor of space physics at Imperial College, London. 鈥淭he Van Allen belts are normally very stable, but a cascade of whistler waves could cause sufficient disturbance to spread problems elsewhere. Even if you avoid the centre of the belt, where the strongest particles normally are, they can get knocked out and hit your spacecraft elsewhere.鈥�
The findings are bad news for the owners and insurers of the thousands of communications satellites in orbit around the Earth. When Horne matched his data to satellite failures he concluded that up to 37 per cent were caused by magnetic storms. But aluminium shields should help protect any sensitive electronic equipment. 鈥淚f we know what can cause [the damage], we can work out the voltage we need to guard against,鈥� says Balogh.
The designers of Galileo, the European equivalent of the GPS navigation system, have consulted with Horne, and are considering whether to include such shielding in their fleet of satellites. But they face a delicate balancing act between risk and cost. The aluminium means extra weight, which jacks up the launch costs dramatically.