Dr Sami Mikhail, a lecturer in Earth Sciences at the University of St Andrews, explains the Aurora Borealis. On Sunday the 6th March 2016 the people of Scotland and northern England witnessed a beautiful natural phenomenon in the night sky, the Aurora Borealis. Living in Fife (colloquially known as the Kingdom of Fife) in Scotland, I was one of the lucky ones. For me, they brought to mind a particular quote from the Old Testament: “And the bow shall be in the cloud; and I will look upon it, that I may remember the everlasting covenant between God and every living creature of all flesh that is upon the Earth” – Genesis 9:16. This quote is about the colourful heavenly phenomenon known as a rainbow, which is simply (or not) the result of reflection, refraction and dispersion of light through water. Think of it like shining a white light through a crystal prism – i.e. the cover of Pink Floyd’s classic album, the Dark Side of the Moon. However, if the author(s) of Genesis understood science (geophysics to be precise), then he/she/they would have decided to cite the Aurora as proof of the “everlasting covenant between God and every living creature.” Because the Aurora is evidence that we are protected from harmful solar rays, and without this protection, we’d all die. The Aurora is a magnificent and mystical-looking light show seen at night in the northern or southern hemisphere known as Aurora Borealis or Aurora Australis (or the Northern and Southern Lights.) I should state that contrary to the depth of colour commonly seen on Twitter, Facebook, and Google images, the Aurora Borealis isn’t a vivid spectacle, in fact they’re relatively faint to what is commonly assumed (note, this misconception can lead to disappointment, so rid yourself of this before viewing them.) The photos flying around the web showing the Aurora Borealis as an intense band of colour in the night sky are taken using long-exposure photography (or photo layering on some smartphones.) However, despite being relatively faint to the naked eye, they are undeniably beautiful, and invoke a sense of awe. So what are they? Not surprisingly, every person who has seen them has pondered this question. Several ancient civilizations interpreted them as anything from spirits of the afterlife dancing in the sky, to harbingers of impending death, to literal gaps in the sky providing a glimpse into the heavens. Even famous (genius) revolutionaries like Benjamin Franklin had a go at explaining them – he thought they were something to do with the large volumes of snow and ice on the poles – note, this is wrong. The truth is actually provided by the etymology of their name. Aurora is named after the ancient Greek word for sunrise and Borealis is derived the ancient Greek word for wind (Boreas). Scientifically, one would say they are the result of solar wind interacting with Earth’s magnetosphere. Bear with me…
The word ‘wind’ usually means the flow of gases on a large scale. On Earth’s surface this means the bulk movement of air. However, solar wind is moving through space, and it is not technically a gas. Solar Wind is a stream of charged particles flowing outward from the Sun (through the vacuum of space) and crashing into anything in their path, including planets (and any astronauts performing a space walk – note, this is a genuine threat to astronaut Tim Peake when he is outside of the protective shielding of the ISS). The word ‘magnetosphere’ denotes the region of space surrounding anything in space (planet, star, moon, or Tim Peake) where charged particles are controlled by that object's magnetic field. Which sounds pretty abstract. In short, the magnetic field of a planet is like the bar-magnet we all played with in primary school, where one end is positive and the other end is negative. We usually refer to the ends of Earth’s bar-magnet as north and south, and compass needles point to the one closest to you at the time (depending on if you’re in the northern or southern hemisphere.) So how does the solar wind and Earth’s magnetosphere combine to produce the wonder of the northern lights? And why does this mostly occur in high latitudes (the Arctic and Antarctic)? Firstly, Earth’s magnetic field is shaped like the letter ‘O’ between two brackets, like this: (O) So when the solar wind encounters Earth's magnetic field it is deflected around the planet like water around the bow of a ship. Now, at the top and bottom of our planet (polar areas), the magnetic field has less strength, and is therefore less effective at deflecting the solar wind. Some of the solar wind can penetrate into the atmosphere and literally crash into atoms and compounds that make up air. When they hit the atmosphere, some of the oxygen (green or orange-red) and nitrogen (blue or red) get really energized to the point where they cannot retain their energy, so they must release some of their energy in the form of visible light and heat (like a candle flame). This causes the lovely phenomenon where we see the beautiful moving lights in the night sky.
So what links the aurora with the Sun, Earth’s core, earthquakes, and life? The link with the sun is obvious, because the solar wind comes from the sun. Earth’s core is the reason we have a magnetic field – this is called the geodynamo (by geophysicists). The first thing to point out is that Earth’s core is made of two parts, a solid inner core and a liquid outer core. The composition of both is effectively iron plus some nickel (alongside some other stuff, like carbon, silicon, and sulfur.) The liquid outer core isn’t static; instead it is vigorously convecting (moving) which causes motion of an electrically conductive material (in this case, iron.) The electric current produced results in the generation of a magnetic field that is aligned along the axis of Earth’s rotation (i.e. north and south.) So what do earthquakes have to do with any of this? Well, one of the key driving forces of the geodynamo is the large temperature difference between the Earth’s core and mantle (the latter being the part of the Earth sandwiched between our feet and the core, which is about 2,900 kilometers thick.) In short, the core is hotter than the mantle, and it therefore loses heat to the mantle because the laws of thermodynamics dictate that whenever there is a temperature difference, heat will move from high to low temperatures (like opening a window.) Now, most earthquakes on Earth are the consequence of plate tectonics (an established theory of global tectonics in which the lithosphere is divided into a number of crustal plates, each of which moves over the surface more or less independently to collide with, move past, or slide under adjacent plates.) Plate tectonics has created every mountain range, all the lovely chains of fertile islands like Japan and Hawaii, over 85% of Earth’s volcanoes, and is responsible for most of Earth’s earthquakes. The important part here is that plate tectonics involve colder plates from the surface being injected into the relatively warmer mantle, which causes heat loss from the mantle (because of the aforementioned laws of thermodynamics.) This has a net cooling effect on the mantle and constitutes to the large temperature difference between the Earth’s core and mantle. The greater the temperature difference between the core and mantle, the more vigorous the convection will be in the outer core, and the faster the outer core moves around and around is directly proportional to the strength of Earth’s magnetic field.
Venus is a planet roughly the same size and composition as Earth, but lacks both plate tectonics and a magnetic field. So it is assumed by some that without plate tectonics, Earth’s magnetic field would have ceased several billion years ago (this is what happened to Venus, and also Mars.) Importantly, without the protective magnetic field we would be unprotected from the solar wind, and totally screwed. In a scenario without a protective magnetic field, the same solar wind that provides the beautiful natural phenomenon in the night sky – which provides Iceland with a lot of its winter tourism – would wreak havoc with our DNA by irradiating us to the point that we’d all rapidly develop terminal cancer. So if you’re ever lucky enough to see the Aurora Borealis (or the Aurora Australis), try to remember to reflect upon what they mean. While staring up in awe at the light show dancing across the sky, consider that every living creature of all flesh that is upon the Earth should be grateful for the geodynamo, and all that constitutes towards its existence. #allhailgeology Follow Dr Sami Mikhail on Twitter