Shining Curtains

When the Vikings swept across Europe in the 7th and 8th centuries, they brought with them many stories; stories of their Gods, of the birth of the world, of Asgard, of Valhalla, of Jormundur the world-serpent, of Loki the trickster, Odin the father and of Ragnarok- the end of this world and the beginning of the next. However, the reason I mention the Vikings today is in reference to one particular set of stories they brought with them; of shining curtains of brilliant, heavenly fire, dancing across the northern sky as the Gods fought with one another. Such lights were not common in Europe, but they were certainly known, and throughout history have provoked terror at the anger of the various Gods that was clearly being displayed across the heavens. Now, we know these shining curtains as the aurora borealis (Aurora was the Roman goddess of the dawn, whilst boreas was the Greek name for the north wind (because the aurora was only observed in the far north- a similar feature known as the aurora australis is seen near the south pole). The name was acquired in 1621).

Nowadays, we know that the auroras are an electromagnetic effect, which was demonstrated quite spectacularly in 1859. On the 28th of August and 2nd of September that year, spectacular auroras erupted across much of the northern hemisphere, reaching their peak at one o’clock in the morning EST, and as far south as Boston the light was enough to read by. However, the feature I am interested here concerns the American Telegraph Line, stretching almost due north between Boston, Massachusetts, and Portland, Maine. Because of the great length and orientation of this line, the electromagnetic field generated by the aurora was sufficient to induce a current in the telegraph, to the extent that operators at both ends of the line communicated to decide to switch off their batteries (which were only interfering) and operate solely on aurora-power for around two hours. Aside from a gentle fluctuation of current, no problems were reported with this system.

We now know that the ultimate cause of the aurorae is our sun, and that two loads of exceptional solar activity were responsible for the 1859 aurora. We all know the sun emits a great deal of energy from the nuclear fusion going on in its core, but it also emits a whole lot of other stuff; including a lot of ionised (charged) gas, or plasma. This outflow of charged particles forms what is known as the solar wind, flowing out into space in all directions; it is this solar wind that generates the tail on comets, and is why such a tail always points directly away from the sun. However, things get interesting when the solar wind hits a planet such as Earth, which has a magnetic field surrounding it. Earth’s magnetic field looks remarkably similar to that of a large, three-dimensional bar magnet (this picture demonstrates it’s shape well), and when a large amount of charged particles passes through this magnetic field it is subject to something known as the motor effect. As every GCSE physics student knows, it is this effect that allows us to generate motion from electricity, and the same thing happens here; the large mass of moving charge acts as a current, and this cuts across the earth’s magnetic field. This generates a force (this is basically what the motor effect does), and this force points sideways, pushing the solar wind sideways. However, as it moves, so does the direction of the ‘current’, and thus the direction of the force changes too; this process ends up causing the charge to spin around the magnetic field lines of the earth, causing it to spiral as this mass of charged particles moves along them. Following these field lines, the charge will end up spiralling towards the poles of the earth, at which point the field lines bend and start going into the earth itself. As the plasma follows these lines therefore, it will come into contact with the Earth’s atmosphere, and one section of it in particular; the magnetosphere.

The magnetosphere is a region of our atmosphere that covers the upper level of our ionosphere which has a strong magnetic field. Here, the magnetic fields of both the charged plasma and the magnetosphere itself combine in a rather complicated process known as magnetic reconnection, the importance of which will be discussed later. Now, let us consider the contents of the plasma, all these charged particles and in particular high energy electrons that are now bumping into atoms of air in the ionosphere. This bumping into atoms gives them energy, which an atom deals with by having electrons within the atoms jump up energy levels and enter an excited state. After a short while, the atoms ‘cool down’ by having electrons drop down energy levels again, releasing packets of electromagnetic energy as they do so. We observe this release of EM radiation as visible light, and hey presto! we can see the aurorae. What colour the aurora ends up being depends on what atoms we are interacting with; oxygen is more common higher up and generates green and red aurorae depending on height, so these are the most common colours. If the solar wind is able to get further down in the atmosphere, it can interact with nitrogen and produce blue and purple aurorae.

The shape of the aurorae can be put down to the whole business of spiralling around field lines; this causes, as the field lines bend in towards the earth’s poles, them to describe roughly circular paths around the north and south poles. However, plasma does not conduct electricity very well between magnetic field lines, as this pattern is, so we would not expect the aurora to be very bright under normal circumstances. The reason this is not the case, and that aurorae are as visible and beautiful as they are, can be put down to the process of magnetic reconnection, which makes the plasma more conductive and allows these charged particles to flow more easily around in a circular path. This circular path around the poles causes the aurora to follow approximately east-west lines into the far distance, and thus we get the effect of ‘curtains’ of light following (roughly) this east-west pattern. The flickery, wavy nature of these aurora is, I presume, due to fluctuations in the solar wind and/or actual winds in the upper atmosphere. The end result? Possibly the most beautiful show Earth has to offer us. I love science.

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