Friday, February 09, 2007

Polar Lights

In an old television tube (not the recent LCD or plasma TV screens), accelerated electrons hit a phosphorescent screen and cause the phosphor to glow. The acceleration region generating the aurora works similarly.

Electrons in the atmosphere get accelerated in an 'acceleration region' between about 5000 and 8000 kilometres altitude, and rush down to the Earth's ionosphere – a region of the upper atmosphere. They finally crash into ionospheric atoms and molecules, transferring to them some of the energy and cause them to glow, creating aurorae.

Giant electrical circuits power the magical open-air light show of the auroras, forming arcs in high-latitude regions like Scandinavia. New results obtained thanks to ESA's Cluster satellites provide a new insight into the source of the difference between the two types of electrical circuits currently known to be associated to the auroral arcs.

Auroras form in high latitude regions of Earth, and appear in many different shapes.

The aurora in the early evening sky forms a green arc that stretches across the sky in an east-west direction. The longitudinal extent (length) of an auroral arc can be as large as several thousands kilometres, but its width can be as small as 100 metres.
The deep mechanisms that rule the creation of such beutiful natural light displays (also called polar lights), have been the subject of studies that have been keeping solar and plasma scientists busy for years, with more to come. While early rockets and ground-observations have already provided a few important clues for the understanding of these phenomena, the real break-throughs in our knowledge have started with dedicated auroral satellites, such as S3-3, Dynamics Explorer, Viking, Freja and FAST, and have now come to full fruition with ESA's multi-point mission Cluster.

Photo Credits: Jan Curtis, Fairbanks, Alaska

Artistic view of electrons, responsible for aurora, spiralling down magnetic field lines. The U-shaped potential structure illustrates the region where electrons get accelerated on their way down to the upper atmosphere. Here they are stopped by collisions with neutral atoms and molecules, primarily oxygen and nitrogen, at altitudes of a few hundreds kilometres down to 80 kilometres. Each collision transfers part of the electron energy to these atmospheric particles. In turn, they get rid of this energy excess by emitting visible emissions in specific wavelength (or colours) such as green (oxygen) or purple (nitrogen).

It has been observed that these electric potential structures are mainly of two types - symmetric (U-shaped) or asymmetric (S-shaped), and typically occur at the boundaries between magnetospheric regions with different properties.

The former type (U-shaped) was found at a plasma boundary between the so-called ‘central plasma sheet’, situated in the magnetotail at equatorial latitudes, and the ‘plasma sheet boundary layer’, an adjacent area located at higher latitudes. The latter type (S-shaped) was found at the boundary between the ‘plasma sheet boundary layer’ and the polar cap, further up in latitude.

Image Credits: ESA
Read more
Cluster – new insights into the electric circuits of polar lights
Auroras: Paintings in the sky from Exploratorium
Looking at exoplanet atmospheres from Centauri Dreams
Magnetic Explosions In The Distant Universe from Science Daily

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