Why do the northern and southern lights differ?

Dazzling green and red light display regularly dance across the night sky above Earth’s northern and southern poles. For decades scientists had assumed that when aurorae shimmer simultaneously in both regions, the flashing patterns mirror each other. But in 2009 they found that was not the case. They were surprised and stumped as to why. Now a team of researchers from Norway, Germany and the U.S. has discovered the culprit: a boisterous sun.

Earth generates a magnetic field that looks as if a bar magnet runs from the South Pole through its core to the North Pole. The field lines curve outward from both poles, far beyond the atmosphere, with the outer arcs forming the boundary of a magnetic bubble around our planet. This magnetosphere fends off charged particles hurtling toward us from space. Aurorae occur when charged particles spewed out by the sun break through the magnetosphere. The particles accelerate along Earth’s magnetic field lines toward the icy polar regions. When they hit the atmosphere they collide with atoms and molecules, releasing colourful photons that light up the sky.

Aaron Ridley, a magnetosphere researcher at the University of Michigan who was not involved in the new study said, when the magnetic field lines curve symmetrically around Earth, aurorae should appear in identical places in the Northern and Southern hemispheres. And, if you could view both light displays simultaneously, they would look pretty much the same. But such a scenario is actually quite rare.

That’s because the sun also has a powerful magnetic field. It alters the path traced by Earth’s field lines, squashing the lines on our planet’s dayside facing the sun and elongating the lines on the nightside, creating a magnetic tail. As a result, Earth’s magnetic field appears to trace the outline of a housefly the insect’s rounded head looking toward the sun and its elongated body and tail pointing away.

At rare times the poles of the sun’s magnetic field align perfectly with those of Earth. But most of the time the sun and Earth’s poles are skewed, creating a housefly shape with a crooked tail for the latter case. The fluctuating solar wind “waggles” the tail, breaking and reforming its field lines—events termed reconnections. Scientists thought the reconnections displaced one aurora relative to the other. But Nikolai Østgaard, a space scientist at the University of Bergen in Norway, and his colleagues tested this idea and discovered it was wrong. They found another effect responsible for auroral differences: The solar magnetic field squeezes Earth’s magnetic field in nonuniform ways. They also showed a burst, or substorm of additional charged particles in the tail can undo the effects of the uneven squeezing, removing the mismatch.

The team studied images captured by spacecraft for 10 pairs of aurorae that occurred simultaneously in the Northern and Southern hemispheres between 2001 and 2005. The aurorae started out at asymmetric locations on the globe. For example, on November 15, 2002, the southern lights (aurora australis) flashed west of the northern lights (aurora borealis). But as the light displays proceeded, their positions shifted, becoming more symmetric. The shifts coincided with substorms.

Matching these observations to activity in Earth’s magnetotail, Østgaard and his colleagues found reconnection events coincide with a decrease in auroral asymmetries. Reconnection has exactly the opposite effect of what people thought. What matters instead, he continues, is how the sun’s magnetic field squeezes Earth’s. His team’s modelling and observations show uneven squeezing in the Northern and Southern hemispheres skews Earth’s field lines and relocates the aurorae. Breaking of the field lines which they observe happens when the substorms hit releases the magnetic pressure that built up from the squeezing and removes the skew.

Ridley and Ingo Mueller-Wodarg, a planetary scientist at Imperial College London, both call the observations surprising given the disagreement with previous models. That the team can understand the physics behind aurorae by looking at images is very cool.

The intense solar radiation bursts that occur during aurorae and substorms can harm astronauts in space and alter the paths of orbiting satellites. They can also interfere with GPS positioning as well as power grids and other technological systems. Scientists cannot accurately predict where and when space weather will hit, Mueller-Wodarg says. But they have at least solved one shining mystery in the night sky.

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