Auroras
Aurora Borealis. One of the wonders of the world, but did you know that this
name was given for the spectacular auroras found specifically in the North Pole? If you
find it a bit difficult to explore the North and see these dancing lights, have no fear! The
same phenomenon occurs in the South pole, the Aurora Australis, which is just as great at
inducing awe, fascination, and especially curiosity. How did these lights occur? Why are
they the colors that they are? In short, this spectacular phenomenon is due to the
collisions of charged particles from the Sun's atmosphere and the nitrogen and oxygen
molecules found in our very own.
The sun is so incredibly hot that its outer layers extend out and release electrons
and protons, along with it's magnetic field, into space. This is known as solar wind. The
production of solar winds is due to the magnetic fields created by the electrical currents
that are strong enough to pass the surface of the sun. Thus allowing it to move outward at
high speeds. It takes up to 1 to 2 days for the plasma to reach Earth's magnetic field,
traveling at speeds of up to 1000 kilometers per second. The boundary where they meet is
known as the magnetosphere, which is the magnetic field surrounding Earth. This wind
alters the magnetic field much like the way a soap bubble's geometry is altered after
being blown on it's surface. The side of Earth that faces the sun has a magnetic field that
is compressed whereas the magnetic field on the opposite side stretches out and is called
the magnetotail, creating a shape of a tear drop. (1) The purpose of the magnetic field is
to protect earth from solar particles, which guides these dangerous particles away. This
phenomenon, unfortunately, is invisible to the human eye. However, when some
particles become trapped at Earth's magnetic poles, what can be seen are the beautiful
aurora lights known today.
The pressure that builds up from the solar wind creates an electric voltage, and
causes the electrons to move towards Earth's two poles. As more electrons approach the
poles, they force through the upper layer of the atmosphere, the ionosphere. (1) This is
the region where solar particles and air molecules collide. These solar particles cannot
move around in any direction in the magnetic field. They can, however, move parallel to
the magnetic field. When the solar wind enters and collides with the Earth's atmosphere,
the charged particles are only confined to the magnetic field lines. So, as the solar wind
and magnetic field combine, it allows the solar wind to drag the magnetosphere field and
plasma along. This energizes the plasma in the magnetosphere and the magnetosphere
responds by transmitting all the electrons and protons from the solar wind into the upper
atmosphere of the two poles where energy can be released, thus creating an aurora that
looks like curtains or rays. This process is called reconnection. (1) The constant collisions
of the solar particles with the atmosphere can be seen through the quick fluidity of these
curtain-like lights.
As a solar molecule collides with a gas molecule an electron from that gas
molecule is sent to a higher energy. As the electron retreats back to its ground state, a
photon is released, which, in our eyes, is light that is emitted. There are many different
colors of the spectra that are seen in auroras. These colors depend on the atoms that
collided with the solar molecules and the altitude of these atoms. At higher altitudes,
oxygen is the most common gas molecule. As the altitude lowers, oxygen and nitrogen
molecules are being bombarded by solar molecules. (4) Although auroras may exhibit
colors such as white, pink, or purple, these are not the actual colors of the light emitted
from the nitrogen and oxygen molecules. Instead they are the combination of colors
emitted by oxygen, which are red and green, and nitrogen, which are blue and red.
The color of light emitted is also dependent on the altitude. The characteristic red
color in the upper region of the auroras is due to the oxygen being sparsely distributed.
When a solar molecule collides with an oxygen atom, the excited electron returns to its
ground state in 3/4 of a second to emit a green proton. (4) To emit a red photon, however,
it takes two minutes. During this time, a collision with another air particle might occur
and would convert the energy to the other particle. Because collisions between
atmospheric molecules and solar molecules are rare in higher altitudes, the emission of
red light can be visible at above 150 miles above Earth's surface. At up to 150 miles,
oxygen emits green light. At lower altitudes, nitrogen is prevalent, emitting purple light
above 60 miles from Earth's surface and blue light at an altitude of up to 60 miles. (3) At
these altitudes, the oxygen atom is prevented from emitting any photons, and nitrogen
emits red and blue light, leading to the characteristic purple color of the lower ends of the
auroras. There are even molecules that emit energy that cannot be perceived by the
human eye, which are more adapted to seeing green, orange, red, and yellow
colors. When solar molecules collide with lighter molecules such as hydrogen and
helium, they emit blue and purple colors and these can only be seen through high quality
photographs.
These auroras, as stated before, are not found only in the North pole. When in the
South, they are known as Aurora Australis. The energetic particles that collide with the
magnetic field that surround Earth have no preference towards which pole to congregate
in. Due to the rotation of the sun, the south pole can go through months with all-day sun
exposure. In these cases, auroras can only be seen during spring or fall, when the sun is
not more prominent. (1) Sky visibility and darkness are key in seeing auroras, so hours
around midnight are times when it they are mostly visible.
Aurora lights are can be described as light ebbing and flowing throughout the sky,
as if it were in water. The rapid movement of the aurora may look like a spiral of curtains
with curls or rays within the curtains. During periods of intense solar activity, the solar
wind may cause a sudden brightening of the aurora. In these cases, these aurora lights
become substorms, which usually last for about thirty to ninety minutes. (1) In begins by
a slow expansion of the aurora shape, usually an oval, and then a sudden brightening of a
small area, known as the “auroral breakup” (1) which continues to expand and stretch
outwards towards the ends of the auroral shape. This phenomenon is often found on the
opposite side of where the sun is currently hitting earth. As a spectator, this will look like
an aurora suddenly brightening and filling the entire sky in a matter of seconds, with
curtain-like lights rapidly moving. These substorms disturb the magnetic field and are so
strong that these solar winds can decrease the magnetic field strength.
The magnetic field is not only affected. The atmosphere at these high altitudes are
also being affected by the energy that is emitted through light. The solar wave particles
also heat and cause ionization of the atmosphere which increases electrical flow. The
currents of the charged particles flow within the magnetosphere and ionosphere and they
increase the heat of the gas in the atmosphere at altitudes were auroras are found. The
heat rises and causes convection in the atmosphere. This phenomenon also creates a
change in the wind speed. Temperature and wind speed can increase by tenfold and go as
fast as 1000 miles per hour. (1) Luckily, these effects do not occur near Earth's surface to
cause potential harm. There is still research considering the long term effects of the
climate change caused by auroras.
As stated earlier, the effects of electrons colliding with molecules in the Earth's
atmosphere results in visible light emitted. Though it may seem that electrons are the sole
molecules partaking in this phenomenon, photons also release a certain type of aurora
that varies from the typical curtains or rays caused by electrons. The shape of the aurora
is caused by the restrictions of the magnetic field that electrons must follow. Protons are
no different. However, in an atmosphere with many electrons and protons, these two
subatomic molecules are bound to collide and when they do, a hydrogen atom is created.
Because it is neutral, it is no longer bound by the magnetic field line. However, a proton
can undergo multiple collisions and thus become a proton once again and continue being
restricted by the magnetic field line. Throughout this process, the proton can spread
through the atmosphere and create a “diffuse glow” (1) which, due to the path being so
spread out, usually cannot be physically seen but is present in images taken.
These auroras are not just found in Earth. The sun extends it's solar wind
outwards towards other planets of our solar system and these planets have their own
atmosphere as well and thus produce their own auroras. Because some planets, like
Venus, do not have magnetic fields, it does have magnetotails which, on Earth or any
planet with a magnetic field, are the result of magnetic fields. A probe known as Venus
Express (5) was sent to this planet to investigate and discovered that the as the solar wind
from the sun reacted with the ions in the ionosphere, a “magnetic plasma bubble” (5)
stretched out for 2100 miles and lasted less than two minutes. This phenomenon
exhibited the process known as magnetic reconnection, though having a slightly different
process in its inception. As such, the auroras in Venus look irregular and are seen as dim
flashes. Though scientists are unsure of these dim flashes, the aurora explanation seems
to be favored, since the process of magnetic reconnection occurs even if there are no
magnetic fields. Otherwise, much like Earth, there are other planets (and even large
moons) that have magnetic fields that will give off auroras.
Of the planets in our solar system, Saturn and Jupiter are the two that have the
strongest magnetic fields and are surrounded by huge magnetospheres. To compare just
how large they are, if magnetic fields could be seen from Earth, Jupiter's magnetosphere
would be the same size as the Sun's and it is hundreds of millions of miles farther from
Earth! Ganymede, one of Jupiter's moons, is the only known moon that has it's own
magnetic field, thus creating a smaller magnetosphere with Jupiter's. These two large
planets rotate at rapid speeds and the plasma that is created in the inner magnetosphere,
due to the coupling between the ionosphere and magnetic field, is extended outward by
centrifugal force. However, it's rotational speed is slower than that of the planet's because
of conservation of angular momentum. (6) As the magnetic field lines attempt to increase
the plasma speed, they bend, which leads to electrical currents full of energy to flow
throughout and between the magnetosphere and ionosphere. These powerful currents
create auroras around each pole in Jupiter and are bright enough to be seen from Earth.
Saturn, however, have weaker currents are not as distinct, but can still be seen. Although
Saturn and Jupiter were the two main planets shown to have auroras, Mercury, Uranus,
and Neptune are also the remaining planets that have this phenomenon. Along with Mars,
who has a much weaker field and only partly interacts with the solar wind from the sun.
Auroras are a spectacle to be seen and is one of the few otherworldly phenomenon
that can be observed right here on earth. Many processes must occur for these rapidly
moving curtains of lights to appear in the sky, including the release of massive energy
from the sun which alters the atmosphere by modifying the magnetic fields of not only
Earth but other planets that contain one. The colors and shapes seen are the electrons and
protons from the sun bombarding molecules in the atmosphere and depending on the
molecules that collide with the solar wind, different wavelengths of light are emitted, thus
producing the array of colors found in the night sky. Although auroras are known to be
diffuse and discrete, it displays the sun's powerful effects onto earth's atmosphere and
ultimately earth itself.
Bibliography
1. http://odin.gi.alaska.edu/FAQ/
2. http://www.webexhibits.org/causesofcolor/4D.html
3. http://science.howstuffworks.com/nature/climateweather/atmospheric/question471.htm
4. http://www.sciences360.com/index.php/what-makes-the-color-of-the-auroraborealis-3577/
5. http://phys.org/news/2012-04-venus-aurora-magnetotails.html
6. http://www.sunearthplan.net/6/310/Extra-terrestrial-aurora