Why are the north and south lights different?

Dazzling green and red lights dance regularly in the night sky over the north and south poles of the Earth. For decades, scientists had assumed that when auroras shone simultaneously in both regions, flashing patterns were reflected. But in 2009, they found that this was not the case. They were surprised and disconcerted as to why. Now, a team of researchers from Norway, Germany and the United States has discovered the culprit: a turbulent sun.

The Earth generates a magnetic field that gives the impression that a magnetic bar goes from the South Pole to its North Pole. Field lines bend outward from both poles, far beyond the atmosphere, the outer arcs forming the boundary of a magnetic bubble around our planet. This magnetosphere repels the charged particles that reach us from space. Auroras occur when charged particles released by the sun pierce the magnetosphere. The particles accelerate along the magnetic field lines of the Earth towards the icy polar regions. When they reach the atmosphere, they collide with atoms and molecules, releasing colored photons that illuminate the sky.

When magnetic field lines curve symmetrically around the Earth, auroras should appear at identical locations in the northern and southern hemispheres. And, if you could simultaneously display the two bright screens, they would look pretty much alike. But such a scenario is actually "quite rare," says Aaron Ridley, a magnetospheric researcher at the University of Michigan, who did not participate in the new study.

This is because the sun also has a strong magnetic field. It modifies the path traced by the Earth's field lines, crushing the lines on the side of our planet facing the sun and lengthening the lines on the night side, creating a magnetic tail. As a result, the Earth's magnetic field seems to trace the outline of a housefly – the rounded head of the insect looking towards the sun, its elongated body and its tail pointing in the opposite direction.

On rare occasions, the poles of the sun's magnetic field align perfectly with those of the Earth. But most of the time, the sun and the poles of the Earth are inclined, creating a form of domestic fly with a twisted tail for the latter case. The fluctuating solar wind "stirs" the tail, breaking and reforming its field lines – events called reconnections. Scientists thought that reconnections were moving one aurora over the other. But Nikolai Østgaard, a scientist in space science at the University of Bergen in Norway, and his colleagues tested this idea and discovered that it was false. They discovered another effect responsible for auroral differences: the solar magnetic field compresses the Earth's magnetic field in a non-uniform way. They also showed that an explosion, or "sub-storm", of extra charged particles in the tail could cancel out the effects of uneven compression, thus eliminating the shift.

The team studied images captured by a spacecraft for 10 pairs of auroras found simultaneously in the northern and southern hemispheres between 2001 and 2005. Auroras began at asymmetric locations on the globe. For example, on November 15, 2002, aurora borealis (northern lights) blinked to the west of the aurora borealis (aurora borealis). But as the light displays unfolded, their positions changed, becoming more symmetrical. The changes coincided with the sub-storms.

Comparing these observations to the activity of the terrestrial magnetotail, Østgaard and his colleagues found that reconnection events coincided with a decrease in auroral asymmetries. "The reconnection has exactly the opposite effect of what people thought," says Østgaard. What matters instead, he continues, is how the sun's magnetic field squeezes the Earth. His team's modelizations and observations show an unequal compression in the northern and southern hemispheres that distorts the Earth's field lines and displaces the auroras. The breaking of the field lines – what they observe occurs when the sub-storms strike – releases the magnetic pressure created by the compression and eliminates the asymmetry.

Ridley and Ingo Mueller-Wodarg, a scientist in planetary sciences at Imperial College London, describe the sightings as "surprising" given their disagreement with previous models. The fact that the team can understand the physics behind the aurora while viewing images "is very cool," says Ridley.

The intense bursts of solar radiation that occur during auroras and sub-storms can harm astronauts in space and alter the trajectory of orbiting satellites. They can also interfere with GPS positioning, power grids and other technological systems. Scientists can not accurately predict where and when the weather will hit, said Mueller-Wodarg. But they at least solved a brilliant mystery in the night sky.