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From Physics Web, July 19, 2006. This is acdtually a service for broadcasters - the BSN wrote and edited the story and then local newscasters can add their own narration based on the script provided. British Satellite News.

Solar explosions in 3D

By Edwin Cartlidge
July 19, 2006

Huge eruptions on the Sun known as coronal mass ejections can generate violent magnetic storms in the Earth's atmosphere. Edwin Cartlidge describes a new space mission that will provide unprecedented views of these explosions

In March 1989 residents of the Canadian city of Quebec got a taste of the Sun's disruptive potential. A magnetic storm in the Earth's atmosphere produced a small power surge in part of the city's electrical grid, which flipped a circuit breaker. This, in turn, caused another breaker to flip, and so on until the whole grid went down. As a result, local people went without power and heat for nine hours, the underground railway system and airport shut down, and schools and businesses were forced to close.

The phenomenon that is believed to be responsible for this and other geomagnetic storms is a coronal mass ejection (CME). These enormous explosions on the surface of the Sun can eject billions of tonnes of plasma into space at millions of kilometres per hour and send electrical currents and high-energy particles coursing through the Earth's protective magnetic field. Existing Sun-monitoring satellites can provide some warning of approaching storms, for example allowing power companies to isolate certain parts of their networks or satellite operators to put their devices into "safe mode". But these warnings are often too late or inaccurate - shortcomings that can prove extremely costly. Indeed, scientists predict that a particularly powerful geomagnetic storm could cause up to $70bn worth of damage to satellites when the associated loss of service is taken into account.

But help will soon be at hand in the shape of NASA's Solar Terrestrial Relations Observatory (STEREO). Due to be launched this summer, the $540m mission will consist of two almost identical satellites - one positioned ahead of the Earth in its orbit around the Sun, the other behind. These satellites will provide a 3D view of CMEs, which should not only transform our understanding of these events but also provide much better forecasting of the consequent geomagnetic storms.

The Sun, like the Earth, can be thought of as containing a huge bar magnet, or dipole. The field strength of this magnet is unexceptional - about 50 G (5 × 10-3 T), roughly equal to that of a fridge magnet. But local distortions of the field can be much stronger. These distortions are thought to be caused by the Sun's differential rotation, whereby regions near the poles take about 33 or 34 days to rotate while the equator zips round in about 25 days. As a result, the magnetic field created in the centre of the Sun becomes twisted and entangled, leading to regions of dense field lines that protrude out from the centre. These regions, which have field strengths of thousands of gauss, are thought to limit the upward convection of heat from the centre and generate the dark patches on the solar surface known as sunspots.

Eventually the fields become so entangled that they reach breaking point and suddenly disentangle, releasing huge amounts of energy as they do so. These energy releases are believed to cause solar flares - the enormous explosions in the Sun's atmosphere that accelerate charged particles emitted by the Sun to great speeds. They are also believed to cause CMEs.

A CME expels huge quantities of plasma in an expanding U-shaped bubble that is threaded by a magnetic field partially rooted at the Sun (see figure 1). A bubble ejected towards the Earth generally takes two or three days to arrive. On its journey it pushes against the slower moving solar wind, a stream of charged particles continuously emitted by the Sun. The bubble acts like a snow plough, piling up the solar wind into a high-density region that has a well-defined leading edge known as a shock wave. This shock wave in turn accelerates a small fraction of solar-wind particles to higher energies.

The arrival of the CME bubble at the Earth has two main effects. First, the shock wave compresses the "nose" of the magnetosphere - the region of space filled by the Earth's magnetic field. This can be problematic for geosynchronous satellites if the nose, which is normally located at over 60,000 km from the Earth's surface, is pushed below the orbit of these satellites at about 40,000 km. If this happens, high-energy particles from solar flares or the solar wind can penetrate the casing of these satellites, causing their computer memories to switch from on to off.

Second, the magnetic field lines from within the bubble connect with those in the Earth's magnetosphere, releasing huge amounts of energy. This energy accelerates particles that already exist within the magnetosphere, and possibly some from the solar wind. It is the interaction of these particles with gas molecules in the upper atmosphere that is responsible for the Northern and Southern lights. However, by setting up strong electric currents within the magnetosphere, these accelerated particles can also play havoc with the microelectronics in satellites, as well as damaging power supplies on the Earth's surface, which is what caused the problems in Quebec.

Geomagnetic storms also have the potential to harm astronauts. STEREO project scientist Michael Kaiser points out that a big storm in January last year would have delivered very dangerous doses of radiation to any Mars-bound astronauts who happened to be space-walking at the time. There is also a small chance that anyone outside the relatively low-altitude International Space Station would have been affected. "You would want to be inside," Kaiser says.

The power of two

Existing spacecraft can warn of impending solar storms. For example, NASA's Advanced Composition Explorer (ACE) can give a fairly accurate warning of a storm about an hour before it strikes by measuring energetic particles from the solar wind just before they reach the Earth. The NASA/European Space Agency Solar and Heliospheric Observatory (SOHO), on the other hand, observes material much nearer the Sun, which means it can provide several days' notice of a potential storm. But any error in SOHO's measurements of the direction of the discharge will be magnified as the material travels to Earth, making its predictions far less reliable.

By making measurements in all three dimensions, STEREO will be able to make far more accurate predictions of the evolution of material from a CME. Its two satellites, launched using a single rocket, will work just like a pair of eyes - judging the direction and speed of matter heading towards Earth by virtue of the slight offset between the two observatories (see figure 2). A suite of visible and ultraviolet imagers aboard the spacecraft will track the 3D evolution of CMEs from their origin at the solar surface, through the Sun's atmosphere (known as the corona) and via the interplanetary medium to their eventual intersection with the Earth. These measurements will include an estimate of the volume of the CME bubble, which provides an indication of the size of the initial explosion and therefore how much energy it will dump in the Earth's magnetosphere.

In addition, the spacecraft will contain radio receivers to detect emissions given off by the solar-wind particles that are accelerated by the shock wave. These emissions will also generate a 3D picture of the CME bubble, providing an independent and complementary view to that obtained at visible and ultraviolet wavelengths. Another set of instruments on board the satellites will detect the dangerous high-energy particles and measure the orientation of the magnetic field in the CME bubble relative to the direction of the Earth's magnetic field. These two fields generate far more severe storms when they oppose one another than when they lie in the same direction. "STEREO will combine the capabilities of ACE and SOHO, and do so from two vantage points," says Kaiser. "We will not really be measuring anything new but we will be making these measurements from a different perspective."

Storm warning

STEREO will send a stream of data in real time to a group of specialized tracking stations around the world organized by the National Oceanic and Atmospheric Administration - the organization that provides weather forecasts, including space weather, to the US. According to Kaiser, the STEREO data - which will consist of a compressed picture of the Sun and measurements of energetic particles - could improve the storm notice from one hour with ACE to a couple of days.

However, for some purposes a two-day warning is not enough. Airlines, for example, would like a week's notice in order to reroute aircraft away from the poles - the regions most affected by geomagnetic storms. But since the material ejected by a CME only takes two to three days to reach Earth, this would involve being able to predict the occurrence of a CME and its potential severity by studying conditions on the Sun. STEREO will not be sensitive enough to provide such predictions, but the Japanese Solar-B mission, due to be launched later this year, and NASA's Solar Dynamics Observatory, which is scheduled to blast off in a couple of years' time, should take us some of the way there. Both will make very accurate measurements of the changing magnetic fields on the Sun.

Kaiser compares the progress made on understanding space weather in the last 10 years with our much-improved ability to predict weather on the Earth's surface. "If you look back at the 1950s, predicting hurricanes was really black magic," he says. "But these days it is possible to predict where they are going to hit within about 10 miles and when they are going to form. We are moving in that same direction."

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