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| The colours of the aurora – the auroral spectrum |
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In order to explain the colours it is necessary to use a model of the atom. This is explained in the listed quotation from Egeland et al. (1999) paper.
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Here it is also explained how one calculates the wavelength of the auroral emission from the following equation:
λ= h c / E1 – E2 (5.3)
where h = Planck’s constant (6,63 10-35 J s), c is the velocity of light, while E1 – E2 is the energy levels in eV, when an electron jumps from say orbital 3 to say 2.
The kinetic energy of the auroral particles can be deposited – through collisions – into translational, vibrational, or rotational energies of atoms and molecules. This happens when the impact-excited electron moves from the ground state to a higher level. |
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The distribution of energy among these initial options sets the stage for the energy subsequently liberated in the visible, UV or IR auroral emissions. Thus, the auroral emissions contain atomic-lines and molecular-band spectra of the primary constituents; mainly oxygen and nitrogen. The auroral emissions can therefore be considered as the "fingerprints" of the atmospheric constituents. When the auroral particles reach the upper atmosphere and smash into the gasses of the air, collisions occur. Energy is then transferred and the atmospheric particles are heated, or excited.
The excitation process with atomic nitrogen can be represented in the following way:
e + N → N* + e’ (5.4a)
and/or
i + N → N* + i’ (5.4b) |
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where excitation is marked by a * (star). In each collision, energy from the hot particle is given off to the cold atmospheric particles. An excited particle is very unstable, and will soon return to the ground state. The electron moves back to the original orbit, by radiating electromagnetic emissions, which we write as follows:
N* → N + auroral emission (5.5)
The collision often results in both excitation and ionization. Then the process related to molecular nitrogen can symbolically be written:
e + N2 → (N2+)* + en + e’ (5.6)
This process is followed by radiation of the first negative bands:
(N2+)* → N2+ + 391,4 and 427,8 nm auroral emissions (5.7)
The probability of Process (5.6) followed by (5.7) is nearly independent of energy for electrons between 0.5 and 20 keV. The intensity of these bands is therefore used to determine the net downward electron energy. |
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Quantitative considerations show that about 25 ion pairs are produced for each photon emitted in the λ 391,4 nm band. The corresponding figure for λ 427,8 nm band is 75 ion pairs per photons. For protons, the situation is more complicated. Using the energy concept, auroral emission is the result of converting kinetic energy, to excitation energy and then finally to radiation energy.
The brightest visible feature of the aurora is the "green line" – often observed as yellow-green, at 557,7 nm. Even if the green auroras dominate during night-time, many other colours are also present. Intense auroras contain a large number of different, distinct colours – lines and bands from ultraviolet (UV) to infrared (IR), but the auroral spectrum is not a continuum. |
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| Click to get a larger version. |
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The coloured bars above the spectrum of the Sun illustrate the auroral colour composition in the visible range of the spectrum. The gas which gives rise to that light is noted by each emission, - see the figure to the left.
The accurate wavelengths in nanometres (nm) are also plotted in the figure. The continuous spectrum of sunlight is shown below the auroral emissions.
The auroral colours are typically divided into layers by altitude. The blood-red aurora at 630 nm is found high up in the sky, normally above 200 km. This 630 nm (OI) emission is created by auroral "soft" (i. e. < 1 keV) primaries.
It often forms a diffuse background radiation in which the discrete arcs are embedded. Between about 110 and 180 km, the green or the yellowish-green auroras dominate, while blue and violet auroras – magenta colours, often occur toward the bottom of auroral forms. |
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| Click to get a larger version. |
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"Red lower borders" – usually fast moving, indicate the present of particles with energies greater than 10 keV. However, blue auroras can extend up to nearly 200 km. Unfortunately, blue and violet colours are hard to see against a dark sky. Because the northern lights often cover large parts of the sky, we observe a large range of colours.
When the intensity is low – below the colour threshold of the eye which corresponds to a low particelike flux, we only see a grey-white veil.
The auroral colours are typically divided into layers by altitude. While the 630 nm red emission is found at the top, a magenta colour is seen at the bottom. Between about 110 and 180 km, the green or the yellowish-green auroras dominate. |
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| The energy level of the oxygen atom. |
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The green and red lines are emitted by atoms of oxygen (OI) after they are hit by fast electrons. The energy level of the oxygen atom is shown in the figure to the left.
The terms as well as the ‘S and ‘D levels are indicated, and the wavelengths of the photons emitted in transitions between energy levels are shown. The 630 nm (OI) emission can also be excited by thermal excitation, when electron temperatures go much above 3000 K.
The energy terms as well as the 1S and 1D levels and their radiative half-life are indicated, and the wavelengths of the photons emitted in transitions between energy levels are shown. Note that only 4,17 eV is needed to move the oxygen atom from the ground level (3P) to the 1S-level, while only 1,96 eV is needed to reach the 1D level.
Each element emits its characteristic colours, and for rarefied oxygen, these appear to us green or red. Typically, a delay of 0.5-1 second exists in the green auroras between collision and the emission, and that is why the rays of the aurora brighten and fade so slowly. The beam of electrons which "excites" the oxygen atoms may only last a small fraction of a second, but the afterglow persists 0.5-1 second or more. |
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However, identification of the yellow-green line at 557,7 nm was elusive. Finally, H. Babcock’s precise measurements in 1923 allowed Jon McLennan to identify it as a megastable transition of atomic oxygen. Collision between particles can de-excite the molecules before they have a chance to radiate. Below 100 km, where collisions quench even the green line, the blue and the red nitrogen bands predominate.
The different colours come from different gases in the atmosphere. That is why the auroral spectrum could be called the finger print of the atmosphere. Auroral colours and their height distributions contain important information about the upper atmosphere.
Some weak, but important hydrogen lines, first discovered by Vegard in 1939, exist in the auroral spectrum. The protons (H+) arrive in helical paths (see the figure below) spiralling around the geomagnetic field lines with a given pitch angle.
The emission Hά at 656, 3 nm and Hβ at 486.1 nm result from excited hydrogen atoms that are produced when energetic protons bombard the atmosphere. The excitation mechanism can be written: |
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X + H+ → X+ + H* (5.8)
Followed by the auroral emission
H* → H + hf ( hydrogen/proton auroras) (5.9)
The hydrogen atom again collides:
H + X → H+ + X + en (5.10)
and process 5.8 can start again. H* has almost the same velocity and direction as the original H+.
Therefore H collides with an atmospheric particle and may be re-ionized – Eq. (5.10), or may be excited. Excitation is more likely to occur at low particle energy.
Thus, an energetic H+ goes through a great number charge exchange and excitation before it has lost its energy and is brought to rest in the upper atmosphere. The H+ particle spend a large time as a neutral atom.
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The protons (H+ ) follow – in spiral-shaped paths, along the geomagnetic field lines and collides with an atom or a molecule in the atmosphere (see the figure to the left).
A charge exchange occurs as the proton picks up an electron and becomes an excited H-atom (H*) – cf. Eq. 5.8. The excited H-atom will emit light with a wavelength characteristic of the hydrogen atom , where the wavelengths have been calculated. Since the H-atoms are moving when emitting proton auroras, we observe a Doppler shift in wavelength.
When a photon is both in motion and emitting, this yields another interesting physical effect - it shows a Doppler displacement that depends on the velocity of the emitting hydrogen atom and the angle between the velocity vector and the direction of the photon. Thus, the emitted wavelengths are not exactly those we have calculated; they are slightly shifted with respect to the "normal" hydrogen wavelengths.
The motion of H* and H+ are mainly downward – deeper into the ionosphere. The first realistic estimate of the auroral-particle energies were based on such Doppler profiles, before the space age. |
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When a distribution of an ensemble of protons of initial energies and pitch angles is known, we can estimate the total emission from hydrogen as a function of height, as well as the Doppler profile of the hydrogen light for any direction of observations. These computations are not difficult in principle, but cumbersome in practice. As a result of charge exchange, the proton auroras are more defocused (i. e. diffuse) than the incident precipitation. These auroras occur in an oval displaced dusk-ward of the electron oval and have a response time different from the electron auroras.
There are several auroral emissions in the UV- range, which cannot be recorded on the ground because they are absorbed by the dense atmosphere close to Earth. However, a satellite well above the auroras is an excellent platform for studies of auroras both in the UV and infrared band. The auroral lights also yield information about the temperature in the upper atmosphere since the intensity of light varies with the temperature of the gas. It was professor L. Vegard who saw this possibility to observe temperature from the auroras in the 1930s. The average temperature in the region 100 to 200 km varies from 200 to ~1 000 oC under normal conditions.
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