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aurora, amazing northern lights
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Aurora, Amazing Northern Lights

Typically the aurora appears either as a diffuse glow or as "curtains" that approximately extend in the east-west direction. At some times, they form "quiet arcs"; at others ("active aurora"), they evolve and change constantly. Each curtain consists of many parallel rays, each lined up with the local direction of the magnetic field lines, suggesting that auroras are shaped by Earth's magnetic field. Indeed, satellites show electrons to be guided by magnetic field lines, spiraling around them while moving towards Earth.
The similarity to curtains is often enhanced by folds called "striations". When the field line guiding a bright auroral patch leads to a point directly above the observer, the aurora may appear as a "corona" of diverging rays, an effect of perspective.
Although it was first mentioned by Ancient Greek explorer/geographer Pytheas, Hiorter and Celsius first described in 1741 evidence for magnetic control, namely, large magnetic fluctuations occurred whenever the aurora was observed overhead. This indicates (it was later realized) that large electric currents were associated with the aurora, flowing in the region where auroral light originated. Kristian Birkeland (1908) deduced that the currents flowed in the east-west directions along the auroral arc, and such currents, flowing from the dayside towards (approximately) midnight were later named "auroral electrojets" (Birkeland currents).
On 26 February 2008, THEMIS probes were able to determine, for the first time, the triggering event for the onset of magnetospheric substorms. Two of the five probes, positioned approximately one third the distance to the moon, measured events suggesting a magnetic reconnection event 96 seconds prior to auroral intensification. Dr. Vassilis Angelopoulos of the University of California, Los Angeles, the principal investigator for the THEMIS mission, claimed, "Our data show clearly and for the first time that magnetic reconnection is the trigger."

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Date added:Feb 08, 2010
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