the aurora borealis main
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AURORA BOREALIS
COURSE: ENV 102
SUBMITTED TO: MR.
SHAHNEWAJ CHOWDHURY
SUBMITTED BY: AHMAD
MUFASSIR MASUM
I.D. 0821037
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Table of Contents
Acknowledgement 3
Aurora Borealis- Natures Paintings
4
Early Theories 5
Solar Wind & the Magnetosphere
6
Origin 7
Auroral Mechanism 10
Frequency of Occurrence 11
Pictures 12
Significance & Tourism
13
Conclusion 14
Reference 15
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First and foremost I would like to thank Almighty for
giving me the power to complete this assignment. I
consider it a privilege to thank my honorable course
instructor Mr. Shahnewaj Chowdhury for providing me this
wonderful opportunity of submitting an assignment of my
choice.
I would also like to extend my gratitude to Mr. Kazi Anwar
Hossain. I would never have known about the existence of
a natural phenomenon like Borealis Aurora if he had not
written a book with a reference to it.
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AURORA BOREALIS- NATURES PAINTING
he Aurora Borealis, sometimes called Northern Lights, are
eerie streaks of color that dance like flames across the
midnight arctic skies. This natures firework got its
scientific Latin name by Galileo Galilee; it translates into the red
dawn from the north. Some thoughts are that these are ancestral
spirits that circumference a fire; to others, it is a manifestation of
Gods blessing upon a lifetime marriage. The existence and its
occurrence went unexplained for years. Now, science has allowed
people to understand more how this exhilarating phenomenon is
developed into a nature light show. No pencil can draw it, no
colors can paint it and no words can describe it in all its
magnificence.
T
For millennia, people have watched them and worried about what
ill omens they represented: war, death or the wrath of God. It
wasn't until the mid-1800s that scientists finally began to discover
many of their mysteries. Like lightning and earthquakes, they
were natural events, not supernatural ones. Thanks to intensive
study by research satellites during the Space Age, aurora has
been substantially de-mystified, even as their ethereal beauty has
remained to dazzle us and inspire awe.
Scientists learned that aurora often accompanied magnetic
'storms' and an unsettled magnetosphere; they were produced by
flows of charged particles entering the atmosphere; they came
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and went with the sunspot cycle; and their colors were the
product of excited oxygen and nitrogen atoms hundreds of miles
above the surface of the Earth.
EARLY THEORIES
Many theories were proposed in order to answer questions about
Aurora Borelis. However, most of these theories were
insignificant. Some of these include:
Auroral electrons come from beams emitted by the Sun. This
claim was made around 1900 by Kristian Birkeland, whose
experiments in a vacuum chamber with electron beams and
magnetized spheres (miniature models of the earth or "terrellas")
showed that such electrons would be guided towards the Polar
Regions. Problems with this model included absence of aurora at
the poles themselves, self-dispersal of such beams by their
negative charge, and more recently, lack of any observational
evidence in space.
The aurora is the overflow of the Van Allen radiation belt ("leakybucket theory"). This was first disproved around 1962 by James
Van Allen (1914-2006) and coworkers, who showed that the high
rate at which energy was dissipated by the aurora would quickly
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drain all that was available in the radiation belt. Soon afterwards
it became clear that most of the energy in trapped particles
resided in positive ions, while auroral particles were almost
always electrons, of relatively low energy.
The aurora is produced by solar wind particles guided by the
Earth's field lines to the top of the atmosphere. This holds true for
the "cusp" aurora, but outside the cusp, the solar wind has no
direct access. In addition, the main energy in the solar wind
resides in positive ions; electrons have only about 0.5 eV
(electron volt), and while in the cusp this may be raised to 50100eV, that still falls short of auroral energies.
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SOLAR WIND & THE MAGNETOSPHERE
The 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 25 nT (nanoteslas; Earth's
surface field is typically 30,00050,000 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 ofsunspots,
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 about70,000 km (before it reaches that boundary, typically 12,000
15,000 km upstream, a bow shock forms). The width of the
magnetosphere obstacle, abreast of Earth, is typically
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http://en.wikipedia.org/wiki/Solar_windhttp://en.wikipedia.org/wiki/Coronahttp://en.wikipedia.org/wiki/Tesla_(unit)http://en.wikipedia.org/wiki/Geomagnetic_stormhttp://en.wikipedia.org/wiki/Interplanetary_magnetic_fieldhttp://en.wikipedia.org/wiki/Sunspothttp://en.wikipedia.org/wiki/Magnetismhttp://en.wikipedia.org/wiki/Magnetospherehttp://en.wikipedia.org/wiki/Bow_shockhttp://en.wikipedia.org/wiki/Solar_windhttp://en.wikipedia.org/wiki/Coronahttp://en.wikipedia.org/wiki/Tesla_(unit)http://en.wikipedia.org/wiki/Geomagnetic_stormhttp://en.wikipedia.org/wiki/Interplanetary_magnetic_fieldhttp://en.wikipedia.org/wiki/Sunspothttp://en.wikipedia.org/wiki/Magnetismhttp://en.wikipedia.org/wiki/Magnetospherehttp://en.wikipedia.org/wiki/Bow_shock -
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190,000 km, and on the night side a long "magneto tail" 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.
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ORIGIN
The ultimate energy source of the aurora is the solar wind flowingpast the Earth. The magnetosphere and solar wind consist of
plasma (ionized gas), which conducts electricity. When an
electrical conductor is placed within a magnetic field while
relative motion occurs in a direction that the conductor cuts
across, rather than along, the lines of the magnetic field, an
electrical current is said to be induced into that conductor and
electrons will flow within it. The amount of current flow is
dependent upon a) the rate of relative motion and b) the strength
of the magnetic field, c) the number of conductors ganged
together and d) the distance between the conductor and the
magnetic field, while the direction of flow is dependent upon the
direction of relative motion. Dynamos make use of this basic
process (the dynamo effect), any and all conductors, solid or
otherwise are so affected including plasmas or other fluids. In
particular the solar wind and the magnetosphere are two
electrically conducting fluids with such relative motion and are
able to generate electric currents by "dynamo action", in the
process also extracting energy from the flow of the solar wind.
The process is hampered by the fact that plasmas conduct easily
along magnetic field lines, but not so easily perpendicular to
them. So it is important that a temporary magnetic connection be
established between the field lines of the solar wind and those of
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the magnetosphere, by a process known as magnetic
reconnection. It happens most easily with a southward slant of
interplanetary field lines, because then field lines north of Earth
approximately match the direction of field lines near the north
magnetic pole and similarly near the south magnetic pole. Indeed,
active auroras are much more likely at such times. Electric
currents originating in such way apparently give auroral electrons
their energy. The magnetospheric plasma has an abundance of
electrons: some are magnetically trapped, some reside in the
magnetotail, and some exist in the upward extension of the
ionosphere, which may extend some 25,000 km around Earth.
Bright auroras are generally associated with Birkeland currents
which flow down into the ionosphere on one side of the pole and
out on the other. The ionosphere is an ohmic conductor, so such
currents require a driving voltage, which some dynamo
mechanism can supply. Electric field probes in orbit above the
polar cap suggest voltages of the order of 40,000 volts, rising upto more than 200,000 volts during intense magnetic storms.
Ionospheric resistance has a complex nature, and leads to a
secondary Hall current flow. By a strange twist of physics, the
magnetic disturbance on the ground due to the main current
almost cancels out, so most of the observed effect of auroras is
due to a secondary current, the auroral electrojet. An auroral
electrojet index is regularly derived from ground data and serves
as a general measure of auroral activity.
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http://en.wikipedia.org/wiki/Magnetic_reconnectionhttp://en.wikipedia.org/wiki/Magnetic_reconnectionhttp://en.wikipedia.org/wiki/North_magnetic_polehttp://en.wikipedia.org/wiki/North_magnetic_polehttp://en.wikipedia.org/wiki/South_magnetic_polehttp://en.wikipedia.org/wiki/Electronshttp://en.wikipedia.org/wiki/Magnetospherehttp://en.wikipedia.org/wiki/Ionospherehttp://en.wikipedia.org/wiki/Birkeland_currenthttp://en.wikipedia.org/wiki/Ohm's_lawhttp://en.wikipedia.org/wiki/Hall_currenthttp://en.wikipedia.org/wiki/Magnetic_reconnectionhttp://en.wikipedia.org/wiki/Magnetic_reconnectionhttp://en.wikipedia.org/wiki/North_magnetic_polehttp://en.wikipedia.org/wiki/North_magnetic_polehttp://en.wikipedia.org/wiki/South_magnetic_polehttp://en.wikipedia.org/wiki/Electronshttp://en.wikipedia.org/wiki/Magnetospherehttp://en.wikipedia.org/wiki/Ionospherehttp://en.wikipedia.org/wiki/Birkeland_currenthttp://en.wikipedia.org/wiki/Ohm's_lawhttp://en.wikipedia.org/wiki/Hall_current -
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However, ohmic resistance is not the only obstacle to current flow
in this circuit. The convergence of magnetic field lines near Earth
creates a "mirror effect" which turns back most of the down-
flowing electrons, inhibiting current-carrying capacity. To
overcome this, part of the available voltage appears along the
field line, helping electrons overcome that obstacle by widening
the bundle of trajectories reaching Earth; a similar "parallel
voltage" is used in "tandem mirror" plasma containment devices.
A feature of such voltage is that it is concentrated near Earth and
indeed, as deduced by Evans (1974) and confirmed by satellites,
most auroral acceleration occurs below 10,000 km. Another
indicator of parallel electric fields along field lines are beams of
upwards flowing O+ ions observed on auroral field lines.
While this mechanism is probably the main source of the familiar
auroral arcs, formations conspicuous from the ground, more
energy might go to other, less prominent types of aurora, e.g. the
diffuse aurora and the low-energy electrons precipitated in
magnetic storms.
Some O+ ions also seem accelerated in different ways by plasma
processes associated with the aurora. These ions are accelerated
by plasma waves, in directions mainly perpendicular to the field
lines. They therefore start at their own "mirror points" and cantravel only upwards. As they do so, the "mirror effect" transforms
their directions of motion, from perpendicular to the line to lying
on a cone around it, which gradually narrows down. In addition,
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the aurora and associated currents produce a strong radio
emission around 150 kHz known as auroral kilometric radiation.
Ionospheric absorption makes AKR observable from space only.
These "parallel voltages" accelerate electrons to auroral energies
and seem to be a major source of aurora. Other mechanisms have
also been proposed, in particular, Alfvn waves, wave modes
involving the magnetic field first noted by Hannes Alfvn (1942),
which have been observed in the lab and in space. The question is
however whether this might just be a different way of looking at
the above process, because this approach does not point out adifferent energy source, and many plasma bulk phenomena can
also be described in terms of Alfvn waves. Other processes are
also involved in the aurora, and much remains to be learned.
Auroral electrons created by large geomagnetic storms often
seem to have energies below 1 keV, and are stopped higher up,
near 200 km. Such low energies excite mainly the red line of
oxygen, so that often such auroras are red. On the other hand,
positive ions also reach the ionosphere at such time, with
energies of 20-30 keV, suggesting they might be an "overflow"
along magnetic field lines of the copious "ring current" ions
accelerated at such times, by processes different from the ones
described above.
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AURORAL MECHANISM
Aurora 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
magneto tail. 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 aurora. The light blue and green colors are produced
by ionic nitrogen and the neutral helium gives off the purple color
whereas neon is responsible for the rare orange flares with the
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FREQUENCY OF OCCURENCE
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 aurora.
The aurora is a common occurrence in the ring-shaped auroral
zone mentioned above. It is occasionally seen in temperate
latitudes, when a strong magnetic storm temporarily expands the
auroral distribution. Large magnetic storms are most common
during the peak of the eleven-year sunspot cycle or during the
three years after that peak. However, within the auroral zone, the
likelihood of an aurora occurring depends significantly on the
slant of the IMF lines, 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 the Earth's
seasons when polar activity is not. It is known, however, that
during spring and autumn, the Earth's magnetic field and the IMF
link up, as noted below. South-pointing IMF lines open a door
through which energy from the solar wind can reach Earth's inner
magnetosphere.
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Also, the Sun's rotation axis is tilted 7 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 September 5 and March 5, when Earth
lies at its highest heliographic latitude.
Still, neither the IMF lines nor the solar wind can fully explain the
seasonal behavior of geomagnetic storms. Those factors together
contribute only about one-third ofthe observed semiannual variation.
PICTURES
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SIGNIFICANCE & TOURISM
Historically, many Arctic peoples viewed the aurora borealis as a
positive economic indicator: following an auroral night, the next
day's hunt would provide plentiful game or fish. Modern times still
connect the aurora with good economic welfare: The solar winds
bring bounty to Alaska's winter economy by attracting visitors in
search of the waves of light. In the fall/winter season of 1997-
1998, Alaska received 209,600 visitors, according to a Division ofTourism report. While about half traveled strictly for business,
many of the remaining pleasure tourists came in search of the
aurora borealis. Over half visit from Washington and another 30
percent originate from the rest of the U.S. Canadian visitors and
other overseas visitors represented equal shares at 5 percent
each. Most of the overseas visitors originate from Japan.
Fairbanks is the supreme destination for those in search of the
northern lights. A study prepared by the School of Mangement at
UAF indicates the average Japanese visitor is a young, unmarried
women between 18 and 25 years of age. Many were students or
had four-year degrees and worked full-time. According to
Japanese culture, it is common for young, single women to remain
living at home. This gives them a high degree of disposable
income. The study indicated 34 percent of those visiting Alaska
earned income in the $20,000 to $39,000 range; 10 percent
earned $40,000 to $59,999; and 25 percent earned $60,000 to
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$79,999. Nearly 70 percent arrive on prepackaged tours, spend
three to four days in Fairbanks and one to two days in Anchorage,
reported the UAF study. Even though the tour is paid for in Japan,
each visitor spends an additional $523 per visit in Alaska, on the
average, noted the research.
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CONCLUSION
Amongst so many beautiful phenomenons of nature, Aurora
Borealis is one. They are the spectacle of a lifetime. They glow
and spin across the sky, mystically enchanting our imaginations.
Chroniclers have often described these as the light of heaven, the
line of souls etc. For ages they have been a source of story and
myth. Since the beginning of time, ancient civilizations have
taken notice of the northern lights, and each had a unique
explanation of them. Some believed they were caused as a result
of fungi on rotting wood, others believed they were magic, while
many believed they were in the presence of temperamental gods
and summoning spirits. In China, ancient people believed they
were seeing dragons, with brightly colored scales moving silently
in the night. Others in the Mediterranean region believed that the
red light in the night was blood flung onto the sky. The Vikings
believed the aurora was the beautiful maidens called Valkyries,
which escorted those killed in battle to the gods. The Sami people
of Lapland believed they had power over the lights, and whistling
under them would cause them to come closer. Many ancient
peoples would not stare at or speak of the aurora, due to a fear of
insulting their divine nature. The Finnish called them revontulet,which means fox fires, for the reason that an arctic fox whipping
its tail was responsible for throwing snow high into the air,
lighting up the sky. Others believed they were torches held by
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spirits to lead the way to the after life. To me these looks like
nature meaningless but beautiful paintings. Paintings that give a
divine feel mixed with an eerie experience. Paintings not even the
greatest of painters could draw.
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REFERENCE
Wikipedia.
Encyclopedia.
National Geography.
Alaska science websites.
Articles in various magazines.
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