In order to understand the Aurora as well as to understand Emission Spectral
Lines, you have to know a little Chemisty and Physics of atoms. This page is
designed to teach you what happens to electrons that absorb energy. It is heavily
borrowed from Michael Richmond's Physics Class, and I have reproduced some of
the illustrations here with my explanations. I refer you to his
site for the original material and greater detail.
The four figures below will give you an idea of what happens to the electrons
of atoms when high energy photons strike them.
In this first image, the electron is orbiting at the lowest energy orbital
around the nucleus. It is worth noting here that electrons actually do not "orbit"
their nuclei, but instead are said to occupy an energy "orbital,"
which is a region around the nucleus where the electron can more frequently
be "found." It is a fundamental property of physics that states that
all electrons will occupy the lowest energy orbital possible, and in the case
of the first figure, the electron pictured is occupying that low state.
In this second image, an incoming photon from the Sun is heading for the electron.
Sunlight travels in waveform, but is also traveling indiscrete packets of energy
described as photons. Thus, the photon is traveling at light speed, and with
energy that can range from radio up to gamma.
In this third image, the photon's energy has been absorbed by the electron,
and the "energized" electron is now occupying a higher energy orbital.
The photon has ceased to exist, having been absorbed by the electron. However,
recalling what I was writing a few sentences ago, this energized electron is
in an unstable energy configuration and would be more stable in the lowest energy
Therefore, as seen in this fourth image, the electron releases the photon
and drops back to the lowest possible energy state. Of great interest here is
that the photon that is now re-emitted is in a slightly different energy form
than when it was first absorbed. This new energy form is dependent on the particular
properties of the atom whose electron absorbed it.
Electrons can be bounced up to a higher energy orbital only if particular
amounts of energy are absorbed, and this again is based on the properties of
the atom. The mathematic of this event is found Michael
Richmond's page. Therefore, if the right amounts of energy are absorbed,
the electron can be bounced up to higher and higher energy orbitals, and thus
the fall back to the original resting state will be more dramatic. Depending
on the atom and the energy levels, the photon that is re-emitted might be in
the form of ultraviolet, infrared, or other energy levels of the EM spectrum.
To the Aurora
Each atom will has unique properties, and therefore each atom which has an
electron that absorbs an photon will release the photon in an energy form unique
to it. In the case of some elements, the photon that is re-emitted from the
electron will be at a particular color.
Different gases give off different colors when they are excited.
Oxygen at about 60 miles up gives off the familiar yellow-green color, Oxygen
at higher altitudes (about 200 miles above us) gives the all red auroras. Ionic
Nitrogen produces the blue light and neutral Nitrogen gives off the red-puple
and the rippled edges. If the atomsphere were made of different gases, we would
see different colors of the aurora. For instance, if the atomsphere were neon
or sodium, we would see red-orange and yellow auroras!
In some instances, the energy that is absorbed is so great that the electron
is energized commpletely out of the atom. This atom is thus said to be "ionized."
The energized electron will release its photon when it attaches itself to another
atomic nucleus. This is what happens inside the core of stars. Gamma rays are
released during the process of thermonuclear fusion, but are promptly absorbed
by the dense concentration of the electrons that are packed in the plasma core.
The electrons may momentarily attach themselves to a nucleus and release the
photon, but the high internal temperatures of the core will almost just as quickly
ionize the electron out of all of that atom's orbitals.
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