Hugh Robin Farquhar Campbell
What are the Northern Lights?
The three conditions necessary for the aurora borealis to occur, solar winds of charged
particles, the presence of a strong magnetic field, and a thick atmosphere, makes the northern lights
a unique display among the rocky planets in our solar system. As a visible manifestation of solar
activity, the Northern Lights signify great releases of energy from the sun, which cause great
geomagnetic storms.
As the root cause of the aurora borealis, the intensity of charged particles released by the
sun is the determining factor in the strength of the Northern Lights visible at high latitudes. Due to
the emission of extreme energy produced in the centre of the sun due to proton-proton fusion, the
outer layer of the sun, the solar corona, reaches temperatures of nearly 1.1x106K, (Feldman, Landi, &
Schwadron, 2005) at which temperatures, the highly ionised atoms and charged particles, such as
protons and electrons, are travelling at average speeds of 145 km.s-1. While the escape velocity of
the sun is 620km.s-1, due to the Maxwell-Boltzmann distribution of speeds, many particles have
speeds greater than the escape velocity (Encrenaz, Bibring, & Blanc, 2003).The emission of plasma
from the sun causes two types of solar wind, of which one is denominated the slow solar wind and
travels at speeds of 400 km.s-1, and the other is named the fast solar wind, and travels at speeds of
750 km.s-1 (Marshall, 2010). Above active areas of the sun, usually sun spots, electrons and charged
particles can be accelerated along the closed loops of magnetic fields called Helmet streamers, with
the result that some of the charged plasma can become detached where the peak of the streamer is
furthest from the sun (Kallenrode, 2004). If large amounts of plasma suddenly become detached,
this results in a large emission of particles in the sun known as solar wind. Since the emission of slow
solar wind depends on Helmet streamers, changing solar activity most affects emission of slow solar
wind. By contrast, the less dense fast solar wind occurs due to coronal holes, where open, nonlooped, magnetic field lines cause plasma to be accelerated away from the sun, leaving the corona
whence the plasma came cooler and less dense (NASA, 2013). The solar wind originating from
coronal holes are faster due to acceleration along magnetic field lines. The fast solar winds are also
affected by solar activity, and both the fast solar wind and slow solar wind peak every 11 year solar
cycle, and the changing amounts of plasma hitting the earth’s atmosphere
during the solar
cycle dictates the strength of the aurora borealis.
The earth’s magnetic field protects life on earth from bombardments of high energy solar
plasma, and the magnetosphere present is what causes the Northern lights to appear where they
do. The magnetic field caused by the electric currents produced due to movements of liquid iron in
the earth’s mantle causes a giant magnetic field pattern to extend from the planet (What causes the
Earth's magnetic field?). This magnetic field pattern is known as the magnetosphere, and the
strength of the solar wind is such that the magnetosphere is squashed at the side pointing to the
sun, while an elongated magnetotail is produced behind the earth. Incoming particles from the solar
wind are most often deflected away from the earth by the magnetosphere, which thus protects life
on earth from most charged solar wind particles, some of which have energies of 10 keV and could
cause mutations in genetic structures, leading to cancers and mutated offspring (Rodríguez-Pacheco,
2014). However, there are some particles that have enough energy or the correct orientation to
enter the magnetosphere. Charged particles entering from especially the magnetotail can be
directed by and accelerated along the earth’s magnetic lines towards the direction of the North Pole,
whence the magnetic field emanates, and the paths of most of these particles follow magnetic lines
Hugh Robin Farquhar Campbell
coming from latitudes of 80°, not 90°, as there are fewer magnetic field lines at 90° compared to 80.
However, in a geomagnetic storm, caused by, for example, a coronal mass ejection, the
magnetosphere is compressed and causes the magnetic field lines in the magnetosphere to become
more concentrated, with the result that in geomagnetic storms solar particles being deflected more,
so the Northern Lights can be seen in places of lower latitude (Balogh, 1999). Every eleven years, at
the peak of solar activity, the solar winds are much stronger, and more plasma is released as solar
wind, with the result that the Northern Lights are especially fantastic when solar activity is highest in
the solar cycle, as the colliding charged particles with the air molecules causes the colours best
associated with the Northern Lights.
The colours of the Northern Lights are due to the way in which incoming charged particles
collide with air molecules. Charged particles, comprising mainly electrons, due to lower mass hence
higher speed with similar kinetic energies, are accelerated in the final few thousand kilometres as
they align with the magnetic fields and are drawn in to the atmosphere (Reiff, et al., 1988). The
result of this is that incoming electrons have very high energies, so collisions with molecules present
in the air cause electronic transitions to occurs, which causes different frequencies of light to be
emitted. Through triangulation of various aurorae, scientists have found that most aurorae occur at
80 km above the surface of the earth (Hansen). In one case, when ionised nitrogen - ionised when an
incoming electron causes another electron to be removed - regains an electron, the energy released
in forming the more stable nitrogen atom takes the form of a photon of a certain frequency,
corresponding to a certain wavelength in according with the equation E=hf, where h is Plank’s
constant. Nitrogen is more stable as an atom than an ion as the total spin, hence multiplicity, of the
nitrogen atom is higher than in a nitrogen ion, resulting in greater stability, according to Hund’s Rule
of Maximum Multiplicity (ChemWiki). Other transitions are caused by excited nitrogen or oxygen
atoms returning to the ground state, with the energy drop released as a photon of frequency
corresponding to the energy of the electronic transition. As a result of Quantum Mechanics, some
electronic transitions are “forbidden” in Chemistry, and cannot occur. In particular, the Laporte
electronic selection rule states that in an electronic transition parity should be inverted, from gerade
to ungerade, and vice versa. Gerade in this case means that the orbital looks the same when
reflected in the line of inversion, with the opposite being true for ungerade. The result of the
Laporte selection rule is that a transition can occur between a p and an s orbital, which are ungerade
and gerade severally, but a transition cannot occur between an s and an s orbital. However, there is
a probability that if the atom is further raised to an excited state, the transition can occur
(Lancashire, 2006). Since this transition is a function of probability, there is an average time until the
electronic transition will happen from when the atom originally becomes excited. Very excited
oxygen has its electronic transitions noticeable affected by the Laporte selection rule, as it takes
approximate ¾ of a second to emit green light, and often 2 minutes to emit red light, since these
transitions are forbidden. When the oxygen atom emits green light, the atom remains excited,
which allows the further transition that causes red light. When excited atoms collide with unexcited
molecules, some energy is transferred. This means that for oxygen the red light is seldom emitted, as
many collisions with other molecules causes the oxygen atom to lose the excitation required to
release red light (Atmospheric Optics). However, in the very upper reaches of the atmosphere,
around 100 km above the ground, the air is nearly a vacuum and oxygen is atomised. Since oxygen
atoms here collide very seldom with other atoms, the oxygen atoms remain excited long enough to
emit the red light.
Hugh Robin Farquhar Campbell
During the peak of a solar cycle, when there are very many sunspots, Helmet streamers
allow large amounts of solar plasma to detach from the sun and travel as the solar wind. The
charged particles are mostly deflected by the magnetosphere surrounding the earth; however, some
particles manage to pass through and into the magnetosphere, and are accelerated along the
magnetic field lines towards the high latitudes. During a geomagnetic storm, continual
bombardment of the magnetosphere causes distortion of the magnetic field, and more particles are
able to enter the magnetosphere and hit the atmosphere at lower latitudes. The incoming charged
particles excite the oxygen and nitrogen molecules in the air, and cause the nitrogen, at lower
altitudes, to emit green and blue light when electronic transitions occur or an electron is gained,
respectively. However, Quantum Mechanical selection rules mean that some electronic transitions
take much longer to occur, and in oxygen atoms, these forbidden mechanisms can only occur at high
altitudes, where collisions are rare, leaving time to produce the red and green light.
Bibliography
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Balogh, A. G. (1999). Corotating Interaction Regions. Springer.
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Hugh Robin Farquhar Campbell
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