NASA satellites and ground observatories will be able to study the solar substorms behind aurora borealis. (Bill Rockwell/Canadian Press)

Ian Mann has seen the dancing lights of aurora borealis countless times, but never quite so captivating as they were in the fall of 2003.

"I was in Edmonton and I have a powerful memory of the dancing colours that night," said the University of Alberta professor. "They were so bright they stood out among the street lights. It was really one of the most beautiful things to see in nature."

While the shimmering northern lights continue to draw skyward across northern Canada, the energy discharges behind them are what really fascinate scientists, in particular the causes of them: the interaction between charged particles expelled by the sun and the Earth's magnetic field.

To get a better understanding into the nature of these magnetic substorms, the Canadian Space Agency and NASA teamed on the THEMIS project, which sent five satellites into orbit on Feb. 16, 2007, to travel around the Earth and chart the storms from different positions in the Earth's magnetic field. While the satellites keep track of the substorms, 20 ground-based observatories — 16 in northern Canada and four in Alaska — track the activity of the northern lights.

Their first major findings, based on results from a Feb. 26, 2008, substorm and published in the journal Science, has shed new light on the process behind the storms.

The solar wind has an average speed of 400kilometres per second, streaming around Earth like water around a rock. (Canadian Space Agency)

A giant magnet

The process starts, as most things do in our solar system, with the sun, and a phenomenon known as the solar wind, a stream of ionized protons and electrons that escape from the star and flow out into space at between 200 and 900 kilometres per second near Earth.

Fortunately for us, we have a magnetic field around the planet extending thousands of kilometres into space to buffer us from most of particles of the solar wind. It's believed Earth owes its field — called the magnetosphere — to the movement of super-hot molten iron in the planet's core that combines with the planet's rotation to create the somewhat spherical field lines and two poles close to the geographical poles of the planet.

But while the magnetic field buffers much of the solar wind sent to us from the sun, it also helps trap radiation around the planet.

The most commonly known areas of radiation are the Van Allen belts, named for James Van Allen, the scientist in charge of the Explorer satellite missions that confirmed their existence in the 1950s.

The two main Van Allen belts come from very different sources. The inner belt is a product of cosmic radiation striking the Earth's atmosphere. Most cosmic rays are fast positively charged ions — atoms that have lost an electron — that are expelled from other stellar objects and fly through the universe. When they strike gases in the Earth's atmosphere they break them up into their elementary parts. Neutrons created during this process that decay into energetic protons become trapped by the magnetic field and make up the bulk of the inner belt.

The outer Van Allen belt is created through an entirely different process. It owes its existence to the magnetic storms and substorms created by the interaction between the magnetic field and the solar wind. These storms feed highly charged electrons and other high-energy ions into the belt.

Scientists believe regular magnetic storms occur because of increased activity in the sun — such as solar flares — sends more than the usual amount of ionized particles hurtling towards the Earth. This extra-powerful solar wind can add fuel to the outer Van Allen belt and can affect satellite communications. But it's also possible the storms could be the result of a series of smaller substorms.

From sphere to teardrop and back again

For the THEMIS scientists, it's the substorms beyond the outer belt that are of particular interest.

Though it's easy to assume the Earth's magnetosphere is spherical, it more accurately resembles a teardrop, with the solar wind pushing the side facing the Sun against the planet while stretching the field outward on the "dark side" of the planet.

"It's kind of a bubble inside the solar wind, compressed on one end and forming a tail at the other end," said Mann.

A substorm occurs when energy from the solar wind builds up at the tail and through a process not yet understood, sends some of the charged electrons close to the planet's atmosphere, where they interact with the gas molecules to create the spectacular displays of the northern lights.

Scientists have two theories on where this happens, said Mann.

One is that a portion of the magnetosphere about eight to 10 Earth radii away becomes unstable in response to the solar wind, ballooning out like a bubble forming on an inflated bicycle tire.

The other is that the magnetic field farther out — about 20 Earth radii away — snaps back in response to an energy build-up and pushes the energized particles towards the planet. While astronomers know this occurs during storms and substorms, they don't know if this is the cause or just a side effect of the real cause.

A satellite passing by Jupiter on its way to Pluto may also help in our understanding of magnetic storms.

The New Horizons probe is equipped with a special tool — the Solar Wind Around Pluto, or SWAP, instrument, designed to measure the interactions between the solar wind and Jupiter's magnetosphere. It's a potentially important development in research, both because it will provide scientists with another planet to compare to Earth and because Jupiter's enormous magnetosphere might create some spectacular storms.

But while New Horizons approaches the giant planet, Canadian observers will monitor the storms behind the northern lights from stations across Canada and from the five satellites launched by NASA in the unique U.S.-Canadian partnership. They hope understanding the "why" will lead to more practical questions like "when."

"If we discover what theory accounts for the substorms we see, we could be closer to accurately predicting space weather and know when these storms are coming," said Mann.

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