Proton aurora – the Doppler shift

I’ve mentioned in previous posts that there are two types of particle storms that lead to aurorae – electrons and protons. The two have very different effects on the atmosphere. Although electrons follow the direction of the magnetic field and cling to it hard, protons will occasionally leave the field lines as Hydrogen atoms and drift in the sky. Electron aurorae glow because electrons excite other electrons in the shells of gasses they hit on the way down, which then leads to emission of light when the atoms or molecules relax. Protons do the same, but because they also spend time as hydrogen atoms – with an electron in their own shell – they can emit light and we can record information about them from the light they emit.

Charge-exchange

Charge-exchange is the mechanism by which protons are able to leave the field lines. Depicted in the diagram below, protons clinging to the field line as all charged particles must, meet atmospheric species. They strike the species and ionise it, liberating an electron from the outer shell. That negatively charged electron is attracted to the positive proton and together they form a hydrogen atom. This atom then leaves the field line as it is now neutral and flies through the air emitting excess energy as light. At some point the atom will itself be ionised through collision with an atmospheric particle and will then be confined to the field line once more. This will continue until the proton has been slowed down to similar speeds as most atmospheric particles – a process known as thermalisation as at this point the speed of the proton is around the speed expected of protons in the atmosphere whose energy comes from heat – thermal energy.

Charge exchange

Charge exchange

Information from light

As mentioned in previous posts, unlike the Sun, which emits a full spectrum of all the colours of the rainbow, plus x-rays, radio waves and all the rest, single gases only emit light in a certain number of colours, known as their spectrum – a fingerprint in light. As a result, scientists looking at the light emitted by protons when they are in the form of hydrogen will normally start with a particular colour or wavelength that hydrogen is known to emit in.

There is plenty of hydrogen in the atmosphere, all of it excited through photochemistry, some of it reflecting direct sunlight from many kilometers away even in the middle of the night, and so choosing a good wavelength means taking all of this into account. When a good wavelength is chosen – Hydrogen-Beta is the name of one such wavelength at 486.13 nm – the shift in the wavelength due to velocity – the Doppler shift – becomes apparent and can be studied.

The Doppler shift is that familiar effect when a police car or ambulance comes haring towards you at some magnificent speed, and the siren sounds a little higher than it should. It passes by and the pitch seems to drop, so the receding siren sounds a little lower than it did before. The reason for this is the distance between each wave of sound hitting you. Imagine someone had mounted a tennis ball cannon on top of that car. The cannon fired a tennis ball once every few seconds. To the people on the vehicle, the distance between each ball is just the speed that the ball before travelled before the next ball was fired. For the person the car is driving towards, however, the distance between balls is the same distance minus however far the car had driven in the meantime – the wavelength appears lower, which in the case of sound means higher pitch and in the case of light means bluer light. If the car was travelling away from the observer, the wavelength would appear longer. If the car was travelling perpendicular to the observer and fired its cannon, the balls would appear to be the same distance apart as seen from the ground and the car. Einstein showed this isn’t true at very high velocities because time distortion can start to kick in – meaning the rate at which the cannon is firing would seem to slow down to the observer, which would lengthen the wavelength – however even at auroral velocities, this is usually not much of a problem. Even so, the Doppler shift is a measurement of the line of sight velocity only.

Below are a few graphs of Doppler profiles. The signals from many different protons travelling at a range of velocities combine together to form a single profile around the central wavelength. In this profile, there is information about the total range of speeds and the range of angles to the observer that the speeds were directed to. If the speeds to the observer were all zero, the shift would be zero and the profile would look like one of the lines along the bottom line of graphs – the peak shape of the line is due to how instruments record single wavelengths of light. The middle line is the same as the bottom line, but with Doppler profiles added and the top line is the sort of thing you get in the atmosphere, when lots of different things in the air emit at the same time and mix in with the Doppler profile you want. All taken together, you can find out how the protons normally far in the magnetosphere have been scattered towards you and to what velocities they have all been accelerated, which might help compare to theories saying why and how they have been scattered and accelerated.

Doppler profile of proton aurorae

Doppler profile of proton aurorae

Although it was well known that aurorae take on different forms and that the equatorward moving aurora during strong geomagnetic disturbances tended to be of the red cloud version, it wasn’t until the 1940s that Vergard first made spectrographic measurements of the hydrogen Doppler shift profile available in proton aurora. Although the instruments have improved since that time, the basic measurements mentioned above would’ve been extremely familiar to him.

Since that time, several things have been determined including that the auroral oval for protons is slightly offset and equatorward of the auroral oval for electrons (see the images below from the IMAGE satellite, showing the proton oval on the left, electrons in the middle and the combined oval on the right). That the protons come in at lower energies and in lower fluxes 90% of the time. Within the last ten or fifteen years it has become apparent that the ionisation and heating effect of the two aurorae are so different that they cannot be modelled in identical ways.

Proton, electron and combined auroral ovals

Proton, electron and combined auroral ovals

Do proton aurorae happen elsewhere?

This is an interesting question as proton aurorae have not definitively been observed on planets other than the Earth, though every now and again a detection is claimed. The reason for this is that the largest aurorae in the solar system happen on the gas giants and that is where people look for strong proton aurorae. When observing the proton aurorae on the Earth, people tend to wait for long Moonless winter nights in order to cut out the interference from other sources, even then the small amount of hydrogen present in the atmosphere is enough to emit plenty of contaminating light in order to ruin certain lines – this is known as geocoronal interference. In the case of the Gas Giants, the amount of hydrogen in the atmosphere is greatly increased, the disc we see of the gas giants is always on the dayside and the reflected light comes from a massive ball of hydrogen called the Sun – as a result the search for Doppler shifts in a massively contaminated system is difficult to put it mildly.

Protons are however known to be in the Io torus, which is mostly formed of ionised hydrogen and sulphur dioxide group ions. As electrons from the Torus are spun in, it is a pretty safe bet that protons must be too. At very high energies, ions are known to go in as they charge exchange at such extreme energies that x-rays are released and can be seen from the Earth, but aurorae of normal energies remain an observational target – perhaps for a future space mission that will at least visit the night side? To end on here’s an image of Saturn’s (mostly electron) aurorae, including the nightside as seen by one such space mission, Cassini.

Saturn's infrared aurora

Saturn's infrared aurora

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