THERE are three realms of the Earth that we know well: the sphere of rock, the sphere of water and the sphere of air. But there is a fourth realm that is almost unknown. A hundred thousand kilometres into space, the Earth鈥檚 magnetic field traps ionised gases at temperatures of more than 10 million degrees. This realm of fire, called the magnetosphere, protects us from cosmic radiation. But it is an unsteady shield.
The magnetosphere is occasionally rocked by an explosive convulsion that flings some of its energies at the Earth, switching on spectacular auroras, damaging satellites, and knocking out electric power grids. No one knows what triggers these violent events, but now an expedition is being put together to find out. THEMIS, a flotilla of five spacecraft, has been given the go-ahead by NASA to enter the sphere of fire and discover what makes it so unstable. THEMIS should tell us what is behind the dancing lights of the aurora, and solve the mystery of how our cosmic protector works.
The magnetosphere is a kind of elastic fire. It forms where the Earth鈥檚 magnetic field meets the hot plasma 鈥� the ionised gases 鈥� at the edge of the planet鈥檚 atmosphere. The magnetic field exerts a force on the electrically charged particles within the plasma, and as they move within the field, they in turn generate their own magnetic field. The net result is that the field and the plasma become 鈥済lued鈥� together into a single substance that is threaded by strong electric currents: one flows sideways across the tail, for example, and a 鈥渞ing current鈥� in the inner magnetosphere circles the Earth.
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Left to itself, the magnetosphere would be as spherical as its name suggests. However, the solar wind, a stream of particles emanating from the sun, pulls the outer layer of this elastic fire along with it like wind-blown hair. So, while the inner part remains spherical, the outer part is distorted into a comet-like shape, with a round head about 10 times the planet鈥檚 diameter on the side facing the sun, and a long tail stretched out on the opposite side, away from the sun (see Diagram).
The frequent convulsions that shake up this settled arrangement are caused by the solar wind鈥檚 magnetic field. It is extremely weak 鈥� at only a few nanoteslas, it is about a millionth of the field generated by a typical fridge magnet 鈥� but the chaotic flow of the solar wind means that this field is constantly shifting around. Much of the time it lines up with Earth鈥檚 field, and simply flows around the Earth like a stream flowing around a boulder. But when the magnetic field turns, so that it runs contrary to the Earth鈥檚 field, things get nasty.
As the opposing fields are pushed together, they can join up in a process called magnetic reconnection. This reconfigured field peels off and moves around to the tail, intensifying the field behind the Earth. This can鈥檛 go on for ever; eventually something snaps, and then all hell breaks loose. Giant gobbets of plasma are hurled at high speed along the tail. A powerful electric current that normally flows across the tail instead cascades down to Earth, and 100 kilometres above the Earth鈥檚 surface the writhing curtains of the aurora flare up and migrate towards the poles.
鈥淭hese phenomena happen within a couple of minutes, like an explosion of energy鈥�, says Vassilis Angelopoulos of the Space Sciences Laboratory at the University of California, Berkeley. They take place so fast, in fact, that no one has yet been able to work out for sure what kicks the whole thing off.
According to one theory, the trigger is in the tail. In the core of the tail two magnetic fields point in opposite directions: one is heading outwards from the Earth鈥檚 south pole; the other runs inwards towards the north pole. As the tail is squeezed and elongated by the solar wind, its field stores energy like a stretched piece of elastic. Eventually, the tail snaps: in a small region around 25 Earth radii (Re) out, the opposing fields on opposite sides of the tail link up and the magnetic tension is suddenly released (see Diagram). This catapults one blob of plasma down the tail away from us, and another earthwards. Each plasma bullet is bigger than our planet and hotter than the sun.
Fortunately, the one heading our way doesn鈥檛 get to ground level unimpeded. 鈥淎t about 8 to 10 Re it piles up against the rigid magnetic field of the inner magnetosphere,鈥� says Robert McPherron of the University of California, Los Angeles. This impact disrupts the electric current flowing across the tail, he thinks, diverting some of it to flow parallel to the magnetic field, towards the Earth. The energetic electrons that form the current hit molecules of the outer atmosphere, making them glow and so creating the aurora.
Although most of this sequence of events is uncontroversial, many physicists don鈥檛 think that magnetic reconnection is the trigger. Anthony Lui of Johns Hopkins University in Baltimore, Maryland, is one of them. He believes that the plasma and magnetic field are glued together so tightly that reconnection is not likely to happen spontaneously: something else has to break that glue. Lui believes it all begins with the current in the magnetosphere鈥檚 tail.
Whereas the plasma as a whole is glued to the magnetic field, individual electrons and ions can spiral around magnetic field lines. The diameter of the spiral depends on the energy and mass of each particle: ions travel in much fatter spirals than electrons, and go in the opposite direction. Throughout most of the magnetosphere these gyrations cancel out, so there is no net movement of charge. But right in the middle of the tail, at the point where the direction of the magnetic field switches over, the motions of charges don鈥檛 cancel, and that leaves a net electric current.
When the solar wind pushes some magnetic field from the head round to the tail, the resulting compression of the tail makes the magnetic discontinuity at its core sharper, and so increases the cross-tail current. And currents flowing inside plasmas are notoriously unstable. Eventually, Lui thinks, the current becomes too strong to be stable and starts to thrash around like a loose garden hose. Some of this loose current arcs down to Earth to generate the aurora, while waves of turbulence generated by this instability spread out along the tail. This breaks the glue that normally maintains the tail鈥檚 twin magnetic fields, and allows them to reconnect.
Judgement day
In principle it should be simple to tell which theory is right. If current disruption is the root cause of these 鈥渟ubstorms鈥�, it will happen first and be followed by reconnection; if reconnection is the trigger it will happen first and be followed by a change in the flow of current. The obvious way to decide is to station sentries at the crucial points in the magnetosphere to time exactly when these events take place 鈥� and that is exactly what THEMIS is designed to do.
The name stands for 鈥渢ime history of events and macroscale interactions during substorms鈥�. Themis is also the ancient Greek goddess of impartial justice, and the hope is that she will provide the necessary objectivity to put an end to the 30 years of heated arguments about the true trigger of magnetic substorms.
According to the THEMIS team, four satellites ought to be enough to do the job, but they are playing safe and launching five. The spare craft will act as a back-up in case one fails. If all goes well, three craft will pass through the region where the cross-tail current is disrupted. The fourth will go about twice as far away, looping out to 20 Re, while the fifth will go way out to 30 Re, which is about halfway to the moon. These outer two will straddle the region where the magnetic field in the tail is thought to reconnect.
Each probe will carry an array of detectors for measuring electric and magnetic fields, and the trajectories and energies of electrons and ions in the plasma. This information should reveal when something violent is happening nearby: the magnetic field will jump around, and there will be a sudden jump in the number of high-energy particles hitting the spacecraft.
The crucial thing is to be in the right place at the right time. The five orbits have been chosen so that every four days all the spacecraft come into conjunction in a long line stretching outwards along the magnetosphere鈥檚 tail. The inner three craft will have orbits with a period of exactly one day, the fourth will orbit in two days, and the fifth in four days. When they are aligned, they should be ideally placed to fix the time and the place of the key stages of a substorm, telling us whether reconnection or current disruption happens first.
In addition to the spacecraft and the tracking stations needed to receive and process data, there will be a network of more than 50 ground stations, mostly in Canada and Alaska, that will time the onset of auroral displays. Taken together, this will tell THEMIS researchers about the relative timing of the reconnection, current disruption and the brightening and expansion of the auroral emissions that define a substorm.
The whole project comes with a modest price tag of $160 million 鈥� far less than many single space probes. 鈥淢any people still don鈥檛 believe we can do it鈥�, says Angelopoulos, the mission鈥檚 lead scientist. If all goes to plan, THEMIS should launch in 2007. Angelopoulos calculates that in the first year of operation, it should catch some 35 substorms in the act and solve the mystery of reconnections.
Apart from its interest to physicists, will the data THEMIS gathers be of any practical use? Angelopoulos is confident it will help us predict the onset of severe space weather caused by a series of substorms. But far more destructive than substorms are the full-blown magnetic storms that occur when a particularly fast or dense stream in the solar wind hits Earth. The crucial sign of a storm is that the ring current circling the Earth becomes much stronger. Acting like an electromagnet, this current can generate a magnetic field in opposition to Earth鈥檚. This weakens the protection provided by the magnetosphere, exposing satellites and astronauts to intense radiation from space. In 1994, for example, two Canadian communications satellites, Anik E-1 and Anik E-2, failed. Electrical charges are thought to have built up within them during a storm, causing damage to their circuitry when they discharged. One of the satellites was out of control for months.
The enhanced ring current may also induce currents on the Earth鈥檚 surface that can overload electrical power grids. In 1989, for instance, such an event brought down Quebec鈥檚 entire power supply system. Six million people were cut off for 9 hours, and the cost ran into hundreds of millions of dollars.
The best hope of avoiding such disasters is to predict them, but before we can do that we need to understand their cause. Some researchers think that substorms can stack up to create full-blown storms. Since substorms inject power into the ring current, it perhaps seems natural to suppose that a rapid sequence of violent substorms could contribute to a storm. 鈥淢ost strong storms are associated with intense substorms,鈥� says Lui. 鈥淭here are a few storms without substorms,鈥� he admits, 鈥渂ut these are usually weaker ones.鈥�
Others insist that storms are entirely separate phenomena. McPherron, for instance, says that in a storm, the magnetic field of the solar wind can peel and push Earth鈥檚 field in a continuous motion and that this is wholly responsible for boosting the ring current. Substorms, he says, are irrelevant.
THEMIS should resolve this issue, too. If it catches a series of substorms occurring during a storm, its instruments should be able to show whether they are powerful enough to be causing the storm, or are just squalls on the surface of a larger weather pattern. And getting a picture of how storms develop should make it easier to predict whether a building storm is going to wreak havoc.
At present, our knowledge of the sphere of fire amounts to little more than a dead map of the magnetosphere, because we lack a real grasp of how this protective shield moves and works. THEMIS will bring the picture to life. And as well as showing us what causes auroras on Earth, it will also indicate how they form elsewhere. The magnetospheres of Mercury and Jupiter probably behave in the same way as Earth鈥檚, as would any magnetic field buffeted by a magnetic wind on any planet in the universe. So when THEMIS pronounces judgement, we will understand what drives the dancing lights on countless alien worlds.