Aurora from Space

Auroras: What powers the greatest light show on Earth?
See New Scientist's gallery of auroras

A few times a day, a gigantic explosion shakes the Earth's magnetic shield, triggering a chain of events that lights up the polar skies with dazzling auroras. These explosions are substorms, and how they happen has long been a mystery. Until now, no one has been able to explain how they gather the energy to create such spectacular displays, or what happens to trigger them.

Now a flotilla of NASA satellites is finally providing answers. They could help us understand not only one of nature's greatest spectacles, but also help predict more serious space weather, which can endanger satellites and astronauts, and even scramble electrical systems on Earth.

The northern and southern lights have fascinated people throughout human history, and there has been no shortage of attempts to explain them. Galileo described these auroras as sunlight reflected in vapours rising from the Earth, while Descartes proposed reflections from ice crystals instead. In the late 1600s, Edmond Halley was the first to correctly link the aurora to the Earth's magnetic field, though it wasn't until the 1950s that scientists confirmed that the display is created when electrons are funnelled by magnetic fields into the upper atmosphere.

Auroras, substorms and more hazardous kinds of space weather all begin with the solar wind - a thin, hot gas of charged particles ejected from the sun, laced with magnetic fields and threaded with electric currents. This magnetic hurricane is blowing over the planet at 1.6 million kilometres per hour, but we don't feel so much as a breeze. That's because most of it is deflected by the Earth's magnetic field, which maintains a zone of relatively calm weather around the planet, called the magnetosphere. As the solar wind blows past the Earth, it pushes and stretches this protective shield out on the night side of the planet, like hair blown in the wind.

Despite this protection, the solar wind buffets and stirs up the magnetosphere, sending high-energy particles showering into the Earth's upper atmosphere. There they light up the gases like a neon tube, creating an aurora that appears as a slowly shifting curtain of green light as the charged particles smash into oxygen atoms. These "quiet auroral arcs" are usually quite faint. "People will often not realise there's an aurora. The sky will look a bit weird maybe, with a diffuse glow," says Eric Donovan of the University of Calgary, Alberta, who monitors the aurora borealis in Canada.

When a substorm rips through the magnetosphere, though, unleashing the energy of a few megatons of TNT, the effects are unmistakable. Magnetic fields whip through space, the electrical currents that circle the magnetosphere thrash wildly, and the aurora is transformed into a much brighter and more dynamic display that sweeps across the sky for 10 to 15 minutes. "It is not uncommon to get a hundred or thousandfold increase in brightness," says Donovan. The aurora also becomes more colourful, as high-energy electrons smash into molecules of the air, exciting red and green light from oxygen and blue from nitrogen.

During a substorm, it is not uncommon to get a hundred or thousandfold increase in brightness

It was already known that what makes the difference between subtle auroral arcs and the dazzling light shows caused by substorms is the direction of magnetic field in the solar wind. Most of the time the field aligns with that of the Earth, which allows the solar wind to flow uninterrupted around the planet. When the two fields point in opposite directions, though, they can become connected, and that loads the magnetosphere with the energy needed to create a substorm. It was not clear exactly how this happened, however.

Satellites such as the Geotail mission, led by the Japan Aerospace Exploration Agency, have helped tell part of the story. Since 1992, Geotail has ranged around the long tail of the Earth's magnetic field, studying its interaction with the solar wind. But a single spacecraft can only tell what's happening at one point; it can't get a big picture of the rapid and complex changes that shake the whole magnetosphere. "In the past, we only had 'pinprick' observations," says David Sibeck of NASA's Goddard Space Flight Center in Greenbelt, Maryland.

To broaden this view, NASA launched a flotilla of satellites, collectively named THEMIS, in February 2007 to catch substorms as they happen. The five small spacecraft orbit the Earth like juggled balls, each following a different looping path, so when something interesting happens in the magnetosphere there's a good chance that they will be in a suitable arrangement to see it.

Three months after launch, THEMIS encountered the beginnings of a substorm. "We had five spacecraft lined up in a row perpendicular to the outer boundary of Earth's magnetic field, some just inside, some just outside," says Sibeck.

This position turned out to be the perfect spot to answer one of the mission's questions: how the solar wind pumps energy into the magnetosphere to power a substorm.

THEMIS's recordings revealed changes in the Earth's magnetic field as the solar wind connected with the magnetosphere. A bulge of twisted magnetic fields formed and slid along its boundary, towards the night side of the Earth. The team recognised this as a phenomenon called a flux rope, which some researchers had suggested would be linked to substorms.

Flux ropes connect the magnetic fields in the solar wind with those of the magnetosphere and the two become entwined, linking Earth's domain with that of the sun. This allows high-energy particles to stream in, loading the magnetosphere with pent-up energy (see diagram).

As the solar wind blows over the Earth, it pulls on its end of the flux rope, dragging the rope and its magnetic fields away from Earth's day side and out into the tail of the magnetosphere.

As more and more flux ropes form and are pulled into the tail, the day side of the Earth loses more and more of its magnetic field. That does not go on forever, of course. "It would completely deplete the day-side magnetic field", says Vassilis Angelopoulos of the University of California in Los Angeles, who heads the THEMIS mission. Earth's shield would disappear, leaving us exposed to carcinogenic cosmic rays. Over geological timescales, the atmosphere might even be stripped away by the solar wind.

Clearly, and luckily for us, that doesn't happen. Instead, after a few hours of building magnetic tension, a substorm strikes. Several things happen almost simultaneously: the tail snaps, hurling plasma towards the Earth, and the electric current that girdles the Earth is disrupted. But which of these triggers the substorm and the resulting aurora? To find out, the THEMIS researchers needed to know which happens first.

There are two competing theories. One school of thought has it that the impetus must come from the powerful electric current that flows around the magnetosphere about 60,000 kilometres up. The motion of magnetic fields drives this current, as in a dynamo, and it is known to be boosted when magnetic field is added to the tail. Does it get so strong that it becomes unstable and showers the atmosphere with high-energy electrons?

The other theory is that the trigger comes from the tail itself. As more magnetic field is added to it, the tail gets compressed tighter and tighter. Around the pinched core of the tail, these magnetic fields point in opposite directions, one running outwards from the north pole and the other running in towards the south pole. As these two field lines are stretched and squeezed by the solar wind, perhaps the two opposing fields spontaneously reconnect, cutting the tail in half and sparking a substorm (see diagram).

As luck would have it, on 26 February 2008, a substorm hit while the THEMIS flotilla was strung out on the night side of the Earth, straddling the region where the current would be disrupted and also where the tail would be expected to snap and reconnect. It was the perfect opportunity to settle the argument.

The first thing the satellites recorded was the tail of the magnetic field snapping off and reconnecting, suggesting that substorms do start with changes in this area. Case closed? Not quite. There was also a big surprise for the THEMIS team. Angelopoulos expected that the break in the tail would first destabilise the current encircling the planet, which in turn would spray electrons down to Earth to cause the aurora. Instead, the aurora began to intensify about a minute after reconnection in the tail, and, crucially, before the disruption of the ring current. "I was shocked," says Angelopoulos. "We never expected that within a minute you would see the aurora light up."

Not everyone is convinced that the team's findings, settle the matter, however. Indeed, Anthony Lui of Johns Hopkins University in Baltimore, Maryland, disputes the whole sequence of events. He thinks that Angelopoulos and his team have misinterpreted the THEMIS data and that reconnection in the tail happens later. "Then the sequence would be opposite to that stated in the Science article, implying that current disruption is the trigger of substorms instead," Lui says.

Although the THEMIS team have since recorded several more substorms, with the same results, Lui maintains that the spacecraft have never been in quite in the right positions to give definitive results. Angelopoulos has decided to alter their orbits to address this problem. Over the coming months, that may remove any remaining controversy about what sparks substorms and perhaps explain how their auroras appear so quickly.

The mission might also illuminate the link between substorms and full-blown geomagnetic storms, which can cause more than a pretty display. These storms are caused by violent outbursts from the sun and can play havoc with satellites, scramble GPS signals, endanger astronauts and even blow power lines on Earth. During a geomagnetic storm there are typically several substorms, but how the two are connected is unclear. So far during the mission, solar activity has been low, but it should increase over the coming year or so, giving THEMIS a chance to watch a much larger storm unfold.

Angelopoulos will be looking forward to it, and not just for the scientific opportunities. Perhaps surprisingly for someone who spends much of his time pondering substorms, Angelopoulos has seen very few with his own eyes. That is part of his motivation to understand them. "I want to go and watch them, so I'm working on predictive models," he says. "With a good model of how the trigger mechanism works, it should be possible to predict the onset of a substorm to within minutes, he says. "Then I can run outside."