Auroras, sometimes called
the northern and southern (polar)
lights or aurorae (singular: aurora),
are natural light displays in the
sky, usually observed at night, particularly
in the polar regions. They typically
occur in the ionosphere. They are
also referred to as polar auroras.
In northern latitudes, the effect
is known as the aurora borealis, named
after the Roman goddess of dawn, Aurora,
and the Greek name for north wind,
Boreas by Pierre Gassendi in 1621.
The aurora borealis is also called
the northern polar lights, as it is
only visible in the sky from the Northern
Hemisphere, the chance of visibility
increasing with proximity to the North
Magnetic Pole, which is currently
in the arctic islands of northern
Canada. Auroras seen near the magnetic
pole may be high overhead, but from
further away, they illuminate the
northern horizon as a greenish glow
or sometimes a faint red, as if the
sun was rising from an unusual direction.
The aurora borealis most often occurs
from September to October and from
March to April. The northern lights
have had a number of names throughout
history. The Cree people call this
phenomenon the "Dance of the
Spirits." Auroras can be spotted
throughout the world. It is most visible
closer to the poles due to the longer
periods of darkness and the magnetic
field.
Its southern counterpart,
the aurora australis or the southern
polar lights, has similar properties,
but is only visible from high southern
latitudes in Antarctica, South America,
or Australia. Australis is the Latin
word for "of the South."
Benjamin Franklin first
brought attention to the "mystery
of the Northern Lights." He theorized
the shifting lights to a concentration
of electrical charges in the polar
regions intensified by the snow and
other moisture
The Northern Lights
are one of nature's most spectacular
visual phenomena, and in this time
lapse video they provide a breathtaking
display of light, shape, and color
over the course of a single night
in Norway.
Auroral
mechanism
The phenomenon of aurora is an interaction
between the Earth's magnetic field
and solar wind.
Auroras are produced
by the collision of charged particles
from Earth's magnetosphere, mostly
electrons but also protons and heavier
particles, with atoms and molecules
of Earth's upper atmosphere (at altitudes
above 80 km (50 miles)). The particles
have energies of 1 to 100 keV. They
originate from the Sun and arrive
at the vicinity of Earth in the relatively
low-energy solar wind. When the trapped
magnetic field of the solar wind is
favorably oriented (principally southwards)
it connects with Earth's magnetic
field, and solar particles enter the
magnetosphere and are swept to the
magnetotail. Further magnetic reconnection
accelerates the particles towards
Earth.
The collisions in the
atmosphere electrically excite electrons
to take quantum leaps (a mechanism
in which the electron's kinetic energy
is converted to visible light); and
molecules in the upper atmosphere.
The excitation energy can be lost
by light emission or collisions. Most
auroras are green and red emissions
from atomic oxygen. Molecular nitrogen
and nitrogen ions produce some low
level red (pink) and very high blue/violet
auroras. The light blue and green
colors are produced by ionic nitrogen
and the neutral helium gives off the
purple colour whereas neon is responsible
for the rare orange flares with the
rippled edges. Different gasses interacting
with the upper atmosphere will produce
different colors, caused by the different
compounds of oxygen and nitrogen.
The level of solar wind activity from
the Sun can also influence the color
and intensity of the auroras
Northern lights over CalgaryTypically
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 aurora is 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" (see also 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, who is the principal investigator
for the THEMIS mission, claimed, "Our
data show clearly and for the first
time that magnetic reconnection is
the trigger." .
Still more evidence
for a magnetic connection are the
statistics of auroral observations.
Elias Loomis (1860) and later in more
detail Hermann Fritz (1881) 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", Yasha/Jakob Feldstein
1963[8]) is slightly different, centered
about 3-5 degrees nightward of the
magnetic pole, so that auroral arcs
reach furthest towards the equator
around midnight. The aurora can be
seen best at this time.
Solar wind
and the magnetosphere
Schematic of Earth's magnetosphereThe
Earth is constantly immersed in the
solar wind, a rarefied flow of hot
plasma (gas of free electrons and
positive ions) emitted by the Sun
in all directions, a result of the
million-degree heat of the Sun's outermost
layer, the corona. The solar wind
usually reaches Earth with a velocity
around 400 km/s, density around 5
ions/cm3 and magnetic field intensity
around 2–5 nT (nanoteslas; Earth's
surface field is typically 30,000–50,000
nT). These are typical values. During
magnetic storms, in particular, flows
can be several times faster; the interplanetary
magnetic field (IMF) may also be much
stronger.
The IMF originates on
the Sun, related to the field of sunspots,
and its field lines (lines of force)
are dragged out by the solar wind.
That alone would tend to line them
up in the Sun-Earth direction, but
the rotation of the Sun skews them
(at Earth) by about 45 degrees, so
that field lines passing Earth may
actually start near the western edge
("limb") of the visible
sun.
Earth's magnetosphere
is the space region dominated by its
magnetic field. It forms an obstacle
in the path of the solar wind, causing
it to be diverted around it, at a
distance of about 70,000 km (before
it reaches that boundary, typically
12,000–15,000 km upstream, a
bow shock forms). The width of the
magnetospheric obstacle, abreast of
Earth, is typically 190,000 km, and
on the night side a long "magnetotail"
of stretched field lines extends to
great distances.
When the solar wind
is perturbed, it easily transfers
energy and material into the magnetosphere.
The electrons and ions in the magnetosphere
that are thus energized move along
the magnetic field lines to the polar
regions of the atmosphere.
Frequency of
occurrence
Aurora australis 1994 from latitude
47 degrees southThe aurora is a common
occurrence in the Poles. It is occasionally
seen in temperate latitudes, when
a strong magnetic storm temporarily
expands the auroral oval. Large magnetic
storms are most common during the
peak of the eleven-year sunspot cycle
or during the three years after that
peak.[citation needed] However, within
the auroral zone the likelihood of
an aurora occurring depends mostly
on the slant of IMF lines (the slant
is known as Bz), being greater with
southward slants.
Geomagnetic storms that
ignite auroras actually happen more
often during the months around the
equinoxes. It is not well understood
why geomagnetic storms are tied to
Earth's seasons while polar activity
is not. But it is known that during
spring and autumn, the interplanetary
magnetic field and that of Earth link
up. At the magnetopause, Earth's magnetic
field points north. When Bz becomes
large and negative (i.e., the IMF
tilts south), it can partially cancel
Earth's magnetic field at the point
of contact. South-pointing Bz's open
a door through which energy from the
solar wind can reach Earth's inner
magnetosphere.
The peaking of Bz during
this time is a result of geometry.
The interplanetary magnetic field
(IMF) comes from the Sun and is carried
outward with the solar wind. Because
the Sun rotates the IMF has a spiral
shape. Earth's magnetic dipole axis
is most closely aligned with the Parker
spiral in April and October. As a
result, southward (and northward)
excursions of Bz are greatest then.
However, Bz is not the
only influence on geomagnetic activity.
The Sun's rotation axis is tilted
8 degrees with respect to the plane
of Earth's orbit. Because the solar
wind blows more rapidly from the Sun's
poles than from its equator, the average
speed of particles buffeting Earth's
magnetosphere waxes and wanes every
six months. The solar wind speed is
greatest — by about 50 km/s,
on average — around 5 September
and 5 March when Earth lies at its
highest heliographic latitude.
Still, neither Bz nor
the solar wind can fully explain the
seasonal behavior of geomagnetic storms.
Those factors together contribute
only about one-third of the observed
semiannual variation.
Auroral events
of historical significance
The auroras which occurred as a result
of the "great geomagnetic storm"
on both August 28 and September 2,
1859 are thought to be perhaps the
most spectacular ever witnessed throughout
recent recorded history. Balfour Stewart,
in a paper to the Royal Society on
November 21, 1861, described both
auroral events as documented by a
self-recording magnetograph at the
Kew Observatory and established the
connection between the September 2,
1859 auroral storm and the Carrington-Hodgson
flare event when he observed that
“it is not impossible to suppose
that in this case our luminary was
taken in the act.” The second
auroral event, which occurred on September
2, 1859 as a result of the exceptionally
intense Carrington-Hodgson white light
solar flare on September 1, 1859 produced
aurora so widespread and extraordinarily
brilliant that they were seen and
reported in published scientific measurements,
ship's logs and newspapers throughout
the United States, Europe, Japan and
Australia. It was reported by the
New York Times that in Boston on Friday
September 2, 1859 the Aurora was "so
brilliant that at about one o'clock
ordinary print could be read by the
light". One o’clock Boston
time on Friday September 2, would
have been 6:00 GMT and the self-recording
magnetograph at the Kew Observatory
was recording the geomagnetic storm,
which was then one hour old, at its
full intensity; this is amazingly
accurate news reporting. Between 1859
and 1862 Elias Loomis published a
series of nine papers on the Great
Auroral Exhibition of 1859 in the
American Journal of Science where
he collected world wide reports of
the auroral event. The aurora is thought
to have been produced by one of the
most intense coronal mass ejections
in history, very near the maximum
intensity that the Sun is thought
to be capable of producing. It is
also notable for the fact that it
is the first time where the phenomena
of auroral activity and electricity
were unambiguously linked. This insight
was made possible not only due to
scientific magnetometer measurements
of the era but also as a result of
a significant portion of the 125,000
miles (201,000 km) of telegraph lines
then in service being significantly
disrupted for many hours throughout
the storm. Some telegraph lines however,
seem to have been of the appropriate
length and orientation which allowed
a current (geomagnetically induced
current) to be induced in them (due
to Earth's severely fluctuating magnetosphere)
and actually used for communication.
The following conversation occurred
between two operators of the American
Telegraph Line between Boston and
Portland, Maine, on the night of September
2, 1859 and reported in the Boston
Traveler:[3]
