AURORA
An aurora (plural: aurorae or auroras; from the Latin word aurora, "sunrise" or the Roman goddess of dawn) is a natural light display in the sky particularly in the high latitude (Arctic and Antarctic) regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere (thermosphere). The charged particles originate in the magnetosphere and solar wind and, on Earth, are directed by the Earth's magnetic field into the atmosphere. Most aurorae occur in a band known as the auroral zone,[1][2]
which is typically 3° to 6° in latitudinal extent and at all local
times or longitudes. The auroral zone is typically 10° to 20° from the
magnetic pole defined by the axis of the Earth's magnetic dipole. During
a geomagnetic storm, the auroral zone expands to lower latitudes.
Aurorae are classified as diffuse and discrete. The diffuse aurora is
a featureless glow in the sky that may not be visible to the naked eye,
even on a dark night. It defines the extent of the auroral zone. The
discrete aurorae are sharply defined features within the diffuse aurora
that vary in brightness from just barely visible to the naked eye, to
bright enough to read a newspaper by at night. Discrete aurorae are
usually seen in only the night sky,
because they are not as bright as the sunlit sky. Aurorae occasionally
occur poleward of the auroral zone as diffuse patches or arcs,[3] which are generally subvisual.
In northern latitudes, the effect is known as the aurora borealis (or the northern lights), named after the Roman goddess of dawn, Aurora, and the Greek name for the north wind, Boreas, by Pierre Gassendi in 1621.[4]
Auroras seen near the magnetic pole may be high overhead, but from
farther away, they illuminate the northern horizon as a greenish glow or
sometimes a faint red, as if the Sun were rising from an unusual
direction. Discrete aurorae often display magnetic field
lines or curtain-like structures, and can change within seconds or glow
unchanging for hours, most often in fluorescent green. The aurora
borealis most often occurs near the equinoxes. The northern lights have had a number of names throughout history. The Cree call this phenomenon the "Dance of the Spirits". In Medieval Europe, the auroras were commonly believed to be a sign from God.[5]
Its southern counterpart, the aurora australis (or the southern lights),
has features that are almost identical to the aurora borealis and
changes simultaneously with changes in the northern auroral zone.[6] It is visible from high southern latitudes in Antarctica, South America, New Zealand, and Australia. Aurorae occur on other planets.
Similar to the Earth's aurora, they are visible close to the planet's
magnetic poles. Modern style guides recommend that the names of
meteorological phenomena, such as aurora borealis, be uncapitalized.[7]
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 that electrons are guided by magnetic field lines,
spiraling around them while moving toward 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)[13]
deduced that the currents flowed in the east-west directions along the
auroral arc, and such currents, flowing from the dayside toward
(approximately) midnight were later named "auroral electrojets" (see
also Birkeland currents).
Still more evidence for a magnetic connection are the statistics of auroral observations. Elias Loomis (1860) and later in more detail Hermann Fritz (1881)[14] and S. Tromholt (1882)[15]
established that the aurora appeared mainly in the "auroral zone", a
ring-shaped region with a radius of approximately 2500 km around
Earth's magnetic pole. It was hardly ever seen near the geographic pole,
which is about 2000 km away from the magnetic pole. The
instantaneous distribution of auroras ("auroral oval"[1][2])
is slightly different, centered about 3–5 degrees nightward of the
magnetic pole, so that auroral arcs reach furthest toward the equator
about an hour before midnight. The aurora can be seen best at this time, called magnetic midnight, which occurs when an observer, the magnetic pole in question and the Sun are in alignment.
In the 1970s, astrophysicist Joan Feynman
deduced that auroras are a product of the interaction between the
Earth's magnetosphere and the magnetic field of the solar wind.[16] Her work resulted from data collected by the Explorer 33 spacecraft.[17]
On 26 February 2008, THEMIS probes were able to determine, for the first time, the triggering event for the onset of magnetospheric substorms.[18] 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.[19] 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."[20]
source : file:///D:/Aurora%20%28astronomy%29%20-%20Wikipedia,%20the%20free%20encyclopedia.htm
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