POLAR Scientists Discover the Energy Source Powering the Aurora

John Wygant , University of Minnesota
Physicists have long known that the aurora are created by intense electron beams which impact the upper atmosphere of the earth creating visible displays. These electron beams carry a major portion of the energy released in magnetospheric substorms. Until the Polar observations it has never been clear where the energy comes from to create these beams.

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The Polar spacecraft is instrumented with the first three dimensional electric field detector to fly in the outer magnetosphere of the Earth. This detector consists of spherical sensors of the spacecraft at the ends of 100m and 130 m booms in the spin plane of the spacecraft, as well as 14 meter booms along the spacecraft spin axis. The spacecraft also has imagers which the provide pictures of the aurora and estimates of the energy flux involved in the creation of the aurora.

The Polar spacecraft encounters a layer of intense Alfven surface waves propagating along magnetic field lines from the distant magnetic tail towards the Earth. These waves are revealed by fluctuations in the electric and magnetic field component shown in Figure 1.

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As a consequence of these waves, magnetic field lines connected the earth behave like a childıs "slinky" or a rubber band which moves because of sharp transverse (sideways) pulses and waves along its extent.

The movement of these pulses and waves carry a surprisingly large amount of energy towards the Earth. In fact, they carry enough energy to power all known auroral acceleration mechanisms in the auroral acceleration region. They appear to carry most of the energy released during magnetospheric substorms.

The waves become more and more intense as they approach the earth because magnetic field lines converge as the magnetic field gets closer to the Earth.

The waves are observed at the boundary between open and closed magnetic fields lines between the tail lobes and plasmasheet. This boundary is characterized by a density jump as shown in the Hydra data. It has been suspected that these region maps to the reconnection region associated with the release of magnetic energy stored in stressed magnetic field configuration of the geomagnetic tail.

Images from the Polar Ultra Violet Imager show that the waves are observed when the Polar spacecraft is on magnetic field lines which map to the vicinity of most intense aurora.

The Alfven waves are observed at altitudes of 30,000 to 40,000 km. At these altitudes they travel at a velocity of 10,000 km/s (6,000 miles/s). Analysis and observations from Polar and other spacecraft at lower altitudes suggest they speed up as they approach the Earth to near the speed of light.

The auroral acceleration process includes the ejection of ion beams from the ionosphere which stream away from the earth along magnetic fields. The data from the Polar Hydra instrument shows that these outflowing ion beams coincide exactly with the appearance of the intense Alfven waves. These beams are a major source of plasma in the plasmasheet during magnetospheric substorms and geomagnetic storms. This implies that the Alfven waves are a major link in establishing the basic structure of the plasmasheet and the outer magnetophere during active times.

Observations of the Alfven waves at higher time resolution reveals a startling feature. The Alfven waves appear to striate or fragment into smaller scale or higher frequency waves.

These smaller scale waves have several important properties that identify them as kinetic Alfven waves and suggest they are very efficient particle accelerators.

Observations from the Polar Hydra Experiment indicate that the large and small scale Alfven waves coincide with very strong electron beams supporting the idea that the small scale waves are responsible for particle acceleration important for the aurora. There is ample evidence from lower altitude spacecraft that a major portion of the conversion of wave energy to particle energy occurs at lower altitudes.

Similar processes involving Alfven waves and the acceleration of particles have been postulated to drive a number of important processes in space and astrophysical systems. They have been invoked to explain the heating of the solar corona), and the acceleration of the solar wind, energy release during solar flares. Alfven waves have been thought to generate the stellar winds from several classes of stars. Alfven waves are a candidate mechanisms for the acceleration of jets from galactic nuclei which are powered by accretion disks around rotating black holes.


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