Revealed: magnetic force which creates the spectacle of the Northern Lights
By Steve Connor, Science Editor
Magnetic "ropes" tying the Earth to the Sun have been discovered by Nasa scientists exploring the mysterious energy that powers the spectacular displays of Northern Lights which illuminate the night sky over the Arctic region.
The giant ropes are made from twisted magnetic fields that act as invisible superhighways for solar particles to travel from the Sun to the outer atmosphere where they trigger immense displays of light for up to several hours at a time.
The discovery was made with the help of a fleet of Nasa satellites launched earlier this year which have been monitoring the Northern Lights, or the aurora borealis, from space at the same time as scientists on the ground take measurements from underneath the atmosphere.
David Sibeck, a Nasa project scientist from the Goddard Space Flight Centre in Maryland, said that the Themis satellites may have answered the question of where all the energy comes from that powers the amazing spectacle of the Northern Lights.
"The satellites have found evidence of magnetic ropes connecting the Earth's upper atmosphere directly to the Sun. We believe that solar wind particles flow in along these ropes, providing energy for geomagnetic storms and auroras," Dr Sibeck said.
The magnetic rope is formed from a twisted bundle of magnetic fields organised much like the twisted hemp of a mariner's rope. Although spacecraft have detected these invisible ropes before, it was the five micro satellites of Themis that allowed scientists to visualise their three-dimensional structure.
"Themis encountered its first magnetic rope on 20 May. It was very large, about as wide as the Earth, and located approximately 40,000 miles above Earth's surface in a region called the magnetopause," Dr Sibeck said.
The magnetopause is where the solar wind from the Sun meets the magnetic field of the Earth. At this point they push against each other like two cosmic Sumo wrestlers locked in combat, Dr Sibeck explained. It was here that the rope was seen to form and then unravel in the space of just a few minutes, providing a brief but significant conduit for the solar wind to conduct energy from the Sun to the Earth.
Analysing the nature of the aurora borealis began last March when a spectacular eruption of the Northern Lights, called a substorm, occurred over Alaska and Canada for about two hours. As the satellites monitored it from space, cameras on the ground photographed its development, said Vassilis Angelopoulos of the University of California at Los Angeles, the mission's principal investigator.
"The substorm behaved quite unexpectedly. The auroras surged westward twice as fast as anyone thought possible, crossing 15 degrees of longitude in less than a minute. The storm traversed the entire polar time zone, or 400 miles, in 60 seconds flat," Professor Angelopoulos said.
Calculations suggest that the total energy of the two-hour event amounted to some 500,000 billion Joules, which is equivalent to the total energy released during an earthquake of magnitude 5.5 on the Richter scale, he said.
It now appears that the energy that powers a show of Northern Lights comes to Earth from the Sun along the magnetic ropes that the satellites have detected.
How the light shows work
* The Northern Lights' alternative name – aurora borealis – derives from the Roman goddess of the dawn, Aurora, and the Greek goddess of the wind, Boreas. These light shows in the northern night sky occur most often in September-October and March-April, although not exclusively.
* They are generated by the interaction between the solar wind (a stream of charged particles emanating from the Sun), the Earth's magnetic field and atoms or molecules of gases that occur at very high altitudes. The types of gases involved determine the colour of the light show.
* When particles of the solar wind collide with the gases they impart energy which excites electrons in the atoms to a higher energy state. When the electrons cool and return to a lower energy state they emit packets of energy in the form of visible light.
* Most aurorae are emissions of green and red light which when solar wind particles collide with oxygen atoms. On the other hand, nitrogen in molecular form and charged nitrogen ions produce low-level red and very high blue or violet aurorae.
* The light shows often appear as a diffuse glow in the sky, or as shimmering curtains usually extending in an east-west direction. The orientation of the curtain and the many parallel rays of light are a function of the direction of Earth's magnetic field lines.
* A similar light show occurs in the sourthern hemisphere, called the austral borealis.
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