Courtesy Mark RyanMany years ago, three friends and I were heading home from a road trip to western Canada. It was about 2:30 in the morning, and I was driving while everyone else was sleeping. I was probably half-asleep myself. But as we chugged along Highway 2 near the outskirts of Bemidji, Minnesota, something in the upper corner of the windshield caught my eye. When I looked up, the sight was so spectacular I immediately pulled over and woke everyone up to see it. My friends were none too happy as I coaxed them out of the van into the cold northern Minnesota night.
Above us, the night sky was alive with the most incredible display of the Northern Lights I have ever witnessed. Bright, vibrant fingers of yellow, blue, red, and green light spread out from a point overhead, like a brilliant hand reaching down from the black sky. I’ve never seen colors like that since. The display was something I’ll never forget and it’s hard to convey how beautiful it actually was, but let me just say my friends soon discarded any thoughts of pummeling me with their fists.
Now, scientists have figured out the mystery behind the phenomenon. According to a new study published in the journal Science, the catalyst of the aurora borealis (and their southern counterpart aurora australis) takes place way out in space about 80,000 miles from Earth during an event called magnetic reconnection.
"Our data show clearly and for the first time that magnetic reconnection is the trigger," said Vassilis Angelopoulos, the project’s principal investigator. "Reconnection results in a slingshot acceleration of waves and plasma along magnetic field lines, lighting up the aurora underneath even before the near-Earth space has had a chance to respond."
The data was gathered by five strategically positioned satellites (a NASA mission known as THEMIS) and compared with that from ground-based detectors.
The process actually begins on the Sun. Turbulent activity on its surface sends out massive energy bursts via the solar wind that interact with the Earth’s magnetic fields and cause all sorts of havoc with our power grids and communication networks. They also create wonderful auroras. But these massive solar outbursts are only occasional, occurring maybe 10 times a year. More frequently – about every three or four hours - the geomagnetic fields are bombarded by substorms; smaller energy bursts that also create auroras. But don’t let the diminutive name deceive you. The energy generated by each substorm is huge, anywhere between one million to two million amps over one or two hours. The THEMIS project determined that, during substorms, the Earth’s magnetic field lines are stretched out like rubber bands building up tremendous amounts of energy before suddenly snapping and flinging charged particles back toward the Earth’s poles. The results are the dancing auroras seen in the northern and southern regions.
Magnetic reconnection is common throughout the known Universe and has been suspected by many as the trigger of auroras. For three decades, though, a competing theory argued the auroras were triggered much closer to Earth, by the disruption (or short-circuit) of charged ions interacting with the magnetic field.
But the new data seems to show otherwise. During a substorm studied in February, the satellites’ data showed the magnetic reconnection occurred first, followed soon after by an aurora display. Only after the display was the short-circuit observed.
Looks like the 30-year debate may be over.