The northern and southern lights, or auroras, are caused by a gigantic, magnetic wrestling match that unleashes not solar, but earthly particles, to create the brilliant night displays, scientists said Thursday.
Using data from NASA and Japanese spacecraft, scientists with the International Solar-Terrestrial Physics program (ISTP) say they are the first to find direct evidence to settle a 50-year-old controversy over the source of auroras.
The process relies, they say, on "reconnection," a union of solar and Earthly magnetic fields that lets the solar wind -- a flow of charged particles from the sun -- punch through sections of the Earth's magnetic shell.
"Reconnection is the fundamental process for transferring and exchangingenergy in the Sun-Earth system."
That process dices up magnetism into energy particles that become the space weather storms known as the northern and southern lights.
"Reconnection is the fundamental process for transferring and exchanging energy in the sun-Earth system," said Atsuhiro Nishida, a researcher with the Japan Society for the Promotion of Science. Nishida also is with the ISTP program.
"Reconnection on the dayside of Earth is critical for allowing solar-wind energy to come into the magnetosphere. Nightside reconnection is critical for the transfer of that energy down to the atmosphere."
Data settles debate
Until recently, scientists disagreed about the cause of auroras, with some promoting the theory of reconnection. But recently, NASA's Polar spacecraft flew through a region on the sunlit side of Earth to witness reconnection directly. Also, scientists looked at data from Japan's Geotail spacecraft, which passed through Earth's magnetic field and found the area on the nightside where reconnection occurs. That gave scientists the data they needed to correlate reconnection with auroras.
The finding has business and lifestyle implications, as solar weather can disrupt radio communications, spacecraft operations and power grids on Earth. Extreme solar weather can harm astronauts in space. Understanding how solar weather works could help us avoid scrambled communications and avert disaster.
Scientists think reconnection could be the key to understanding solar flares and other solar eruptions, as well as the source of galactic X-rays beyond our solar system and the process of nuclear fusion.
Torn cocoon
Before this finding, many believed auroras came about when the particles spewing from the sun to Earth charged through our planet's atmosphere near the North and South Poles.
In fact, the sun provides the energy, not the particles, Nishida said. The particles were already there, trapped in Earth's magnetosphere. And the process starts not at the poles but at "reconnective regions" elsewhere around the globe.
To understand that, think of Earth's magnetic shell, or magnetosphere, as a cocoon around the planet, said Jack Scudder, a University of Iowa physicist who studies data from the Polar spacecraft.
"There are often times when the solar wind creates tears in this cocoon, allowing charged particles and energy from the sun to enter the space around Earth," he said. "This tearing -- reconnection -- is what we directly observed with Polar."
Once the tears occur, the solar wind combines with the magnetosphere to overload the system of magnetism nearby. That energy excites nearby particles and causes another bounce and reconnection of magnetic fields that shoot energy to the auroral zones at the Earth's poles and into its radiation belts.
In the past eight years, Geotail has studied Earth's magnetosphere in search of colliding and combining solar and earthly magnetic fields -- reconnection. Geotail's success has helped scientists pinpoint the reconnection area -- about 85,000 to 96,000 miles (140,000 to 160,000 kilometers) downwind of Earth.
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