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    COURSE: ENV 102





    I.D. 0821037

  • 8/6/2019 The Aurora Borealis Main


    Table of Contents

    Acknowledgement 3

    Aurora Borealis- Natures Paintings


    Early Theories 5

    Solar Wind & the Magnetosphere


    Origin 7

    Auroral Mechanism 10

    Frequency of Occurrence 11

    Pictures 12

    Significance & Tourism


    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


    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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    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



    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.


    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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    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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    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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    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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    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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    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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    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


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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.


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    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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    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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    National Geography.

    Alaska science websites.

    Articles in various magazines.

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