The Aurora Borealis Main

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    COURSE: ENV 102





    I.D. 0821037

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    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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