The Dancing Aurora Borealis

Space scientists at the University of California, Los Angeles (UCLA) believe they have solved some of the mystery behind the northern lights phenomenon.

For the past 30 years, there have been two competing theories to explain the onset of substorms, which are energy bursts in the Earth's magnetosphere that create the northern lights (aurora borealis), according to Vassilis Angelopoulos, a UCLA professor of Earth and space sciences and principal investigator of the NASA-funded mission known as THEMIS (Time History of Events and Macroscale Interactions during Substorms).

One theory is that the trigger happens relatively close to Earth, about one-sixth of the distance to the moon. According to this theory, large currents building up in the space environment, which is composed of charged ions and electrons, or "plasma," are suddenly released by an explosive instability. The plasma implodes toward Earth as the space currents are disrupted, which is the start of the substorm.

A second theory says the trigger is farther out, about one-third of the distance to the moon (80,000 miles), and involves a different process - when two magnetic field lines come close together due to the storage of energy from the sun, a critical limit is reached and the magnetic field lines reconnect, causing magnetic energy to be transformed into kinetic energy and heat. Energy is released, and the plasma is accelerated, producing accelerated electrons.

In other words, the Earth’s magnetic field lines are stretched like rubber bands by solar energy. Once they are stretched to a certain point they actually snap and are thrown back to Earth and reconnect, in effect creating a short circuit. This snapping, or flare-up occurs every two or three days on average.

Using five NASA Themis satellites and a network of 20 ground observatories located throughout Canada and Alaska, this group of scientists, led by Vassilis Angelopoulos, discovered that magnetic explosions about one-third of the way to the moon cause the northern lights to burst in spectacular shapes and colors, and dance across the sky.

Each of the 20 ground stations use a magnetometer and a camera pointed upward to determine where and when an auroral substorm will begin. Instruments measure the auroral light from particles flowing along Earth's magnetic field and the electrical currents these particles generate.

"We discovered what makes the Northern Lights dance," says Angelopoulos.

The findings should help scientists better understand the more powerful but less common geomagnetic storms that can knock out satellites, harm astronauts in orbit and disrupt power and communications on Earth, scientists said.

The Themis Satellites observed the beginning of a geomagnetic storm in February, while ground observatories in Canada and Alaska recorded the brightening of the northern lights. The southern lights, aurora australis, also brightened and darted across the sky at the same time.

At present, about 20 of these geomagnetic storms are being analyzed. Scientists hope to eventually learn, via this project, more about the bigger solar storms that occur about 10 times a year and can lead to far more expansive and prolonged northern and southern lights.

"We need to understand this environment and eventually be able to predict when these large energy releases will happen so astronauts can go inside their spacecraft and we can turn off critical systems on satellites so they will not be damaged," says Angelopoulos. "This has been exceedingly difficult in the past, because previous missions, which measured the plasma at one location, were unable to determine the origin of the large space storms. To resolve this question properly requires correlations and signal-timing at multiple locations. This is precisely what was missing until now."

"Armed with this knowledge, we are not only putting to rest age-old questions about the origin of the spectacular auroral eruptions but will also be able to provide statistics on substorm evolution and model its effects on space weather," says Angelopoulos.

Themis is the fifth medium-class mission under NASA's Explorer Program. The program, managed by the Explorers Program Office at Goddard provides frequent flight opportunities for world-class space investigations in heliophysics and astrophysics.

Themis was the blindfolded Greek goddess of order and justice.

For more information, visit: www.ucla.edu or www.nasa.gov