Catching the Auroral Waves
The ghostly glows and the curtains of colours that are hallmarks of the Northern Lights have mesmerized untold generations of skywatchers. However, their inner clockwork has remained a mystery—until now. Scientists believe they may have uncovered one of the elusive triggers behind what causes the most spectacular visual outbursts of auroras known as substorms.
Development of a substorm from charged particles that flow from the Sun towards Earth, releasing energy rapidly along the Earth's magnetic field lines, and thereby making the auroras brighten. Credit: NASA/Goddard Space Flight Center- Conceptual Image Lab
Just before a dull, faint glow turns into a shimmering dance of colourful light that fills the night sky, bizarre waves of tiny charged particles seem to ripple through continent-wide bands of auroras in the upper atmosphere. This is the latest breakthrough discovery made by a team of University of Calgary scientists part of the THEMIS mission. Led by NASA with support from the Canadian Space Agency, THEMIS (short for "Time History of Events and Macroscale Interactions during Substorms") uses 5 satellites and a fleet of 20 ground-based, specialized cameras (16 of which are managed by the University of Calgary) scattered across northern Canada to seek out factors that spark the substorms that originate in the Earth's magnetic field near the North and South Poles. By simultaneously looking at the charged particles in space and the aurora, researchers are gaining a unique insight into Northern Lights in unprecedented detail.
Since this celestial light show takes place in the upper atmosphere, it may not appear to be space phenomena. But it turns out the enigmatic waves that cause auroras may actually represent a visible manifestation of complex interactions occurring between our Sun and the Earth's magnetic field.
"We are hoping these waves will directly start telling us what kind of things are happening 80 000 to 100 000 kilometres away in the magnetotail of the Earth," says Eric Donovan, Canadian Principal Investigator of THEMIS and astronomy professor at the University of Calgary. "Things we have no other way of measuring in this mysterious region of space."
Auroral displays originate from a flow of charged particles, called the solar wind, which streams off the surface of the Sun. When a cloud of these particles slams into Earth's protective bubble-like magnetosphere, it stretches out the field on the dark side of Earth, forming a region known as the magnetotail. This tear-drop shaped magnetotail acts like a rubber band: when it is stretched too far, "it eventually snaps and releases the energy," says postdoctoral fellow and THEMIS team member Emma Spanswick of the University of Calgary.
For the first time, researchers have observed this ramping up in the aurora in the form of waves before the snapping. "We are now able to look at the structures in the aurora and relate their evolution in time with processes that are happening further out in space," adds Donovan. The solar wind pushes on the Earth's magnetic field, pumping it up with energy and stretching it until it can't maintain its shape anymore. After about an hour or so, the field snaps back to its normal configuration in a matter of a few seconds, releasing a flood of energy. Donovan and his team believe this is the suspect process that actively energizes particles in near-Earth space, which then races down the magnetic field lines and causes waves to form. This is the moment when a dynamic change in the aurora happens—a magnetic substorm—as many as two or three times a day. "We think these waves may be the calling card of a plasma process that is triggering the snapping," says Spanswick.
These mysterious cosmic tsunami waves appear to span 100 kilometres across and travel at speeds of many kilometres per second, quietly undulating for hundreds—if not thousands—of kilometres across auroral arcs (or bands) that span Canada from east to west. In 2008, the 16 ground-based observatories created a landmark mosaic snapshot of one single auroral band sweeping across most of the country. "The discovery of these waves is testimony to the fact that the Canadian science community has created something that has better spatial coverage and better quality than anything done before in this field," adds Donovan.
Donovan is excited about what may be around the corner for his THEMIS team. Mining through gigabytes of stored and a flood of incoming images, he says, are paving the way to new science. "I expect this is only the tip of the iceberg. Just by looking through the data we have already accumulated, we are going to find lots of things we never expected."
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