字幕表 動画を再生する 英語字幕をプリント The Aurora Borealis, otherwise known as The Northern Lights are one of nature’s most beautiful displays of omnipotence. Despite what its name suggests, The Northern Lights can be seen almost anywhere in the world, even as far south as the equator. The phenomenon has been witnessed in Honolulu and Singapore. However, sightings that far south are extremely rare, it’s most commonly witnessed within the Arctic Circle. The South Pole also has its own version of the Aurora Borealis, called the Aurora Australis. And Earth isn’t the only planet with aurora, you would witness a similar phenomenon if you were to stand on Jupiter or Saturn. Uranus has aurora too, but unlike the other planets with their wave-like aurora, the aurora on Uranus would look like glowing dots… and yes, I am talking about the planet. But what exactly causes these mysterious light shows? Let’s find out. Some Inuit believe that the spirits of their ancestors can be seen dancing within the lights of the Aurora. The Vikings believed that the Aurora was a bridge of fire to the sky, forged by the gods. Well, spoiler alert, according to modern science, the northern and southern lights are not dancing spirits or a bridge of fire. They are actually caused by something much more tangible, the Sun. Our Sun is a 4.57 billion-year-old ball of hydrogen and other gasses fusing away in space, 150 million kilometres away from planet Earth. And every once in a while it kicks up a bit of a storm, a Solar Storm to be precise. The Sun produces energy by smashing hydrogen atoms together in its core, under such immense temperatures and pressures, to cause the hydrogen atoms to fuse, to form helium atoms. This process is known as nuclear fusion. When atoms are under these kinds of pressures they turn into a mushy soup of freely moving particles, which is a fourth state of matter known as plasma. The Sun is essentially a gigantic ball of swirling plasma. Using the process of nuclear fusion, the Sun is able to turn small amounts of matter into enormous quantities of energy. It’s how the Sun has managed to keep producing energy for billions of years without running out of fuel. If human-kind were able to consistently replicate nuclear-fusion here on Earth, we would be able to make almost infinite amounts of energy, using relatively little resources. In fact, if you could convert the mass of one bag of sugar into energy it would be enough to drive a car, non-stop, for 100,000 years. All this plasma swirling around within the Sun generates huge magnetic fields. These magnetic fields are under such immense pressures within the Sun, that sometimes the lines of force of theses magnetic fields meet. When this happens, the magnetic field is forced outwards, towards the surface of the Sun. Once it reaches the Sun’s surface it ejects outwards into space at great speeds, taking extremely hot gasses and charged particles, also known as plasma, with it. This is known as a solar flare and they can be witnessed on the Sun’s surface as a very bright spot followed by a cloud of gas. When a very large solar flare occurs containing a significantly huge amount of energy, it is referred to as a Coronal Mass Ejection or CME for short. The gas clouds produced by a CME can sometimes be larger than the Sun itself. CME’s and solar flares don’t stop just outside the Sun’s surface. Charged particles ejected from the Sun continue to travel outwards through space over enormous distances. After travelling through space for around two days they will reach planet Earth. This is known as the solar wind and it can be extremely dangerous. If the magnetic field released from a solar flare or CME were to hit planet Earth it could cause the extinction of the human race. So why hasn’t it? After all solar flares are taking place regularly on the Sun’s surface. Well the solar wind does actually hit Earth, often, but our trusty planet is well-prepared. We have a defence system to protect us from the huge amounts of energy the Sun bombards us with on a daily basis. Just like the Sun, the Earth produces its own magnetic field. At the Earth’s core is a ball of solid iron, the heat of this iron turns the surrounding outer core into flowing liquid iron. The movement of this outer liquid core produces a magnetic field, that, luckily for us, encompasses the Earth. The Earth’s magnetic field protects us from all sorts of harsh destructive forces that come from far away in space, including solar flares. This protective barrier is called the Earth’s magnetosphere and it extends thousands of kilometres into space. That sounds large, but it’s actually rather modest, Jupiter’s magnetosphere extends over seven million kilometres into space on each side of the planet. In fact, on Jupiter’s nightside, the side facing away from the Sun, the magnetic field which is emitted is so large, that it reaches Saturn. Anyway, back to Earth. Most of the Sun’s solar winds simply bounce off the Earth’s magnetosphere when it reaches us. However, the magnetosphere has two weak spots, at the north and south poles. At these two locations the magnetic fields which protect planet Earth are much less prevalent than nearer the equator. This means that a very small percentage of the charged particles that come from the Sun do make their way into the Earth’s atmosphere via the two weak spots at the poles. When this happens the electrons in the solar wind collide with oxygen and nitrogen atoms in the Earth’s atmosphere. During this interaction energy is transferred from the atoms in the solar wind to the Earth’s oxygen and nitrogen atoms. Raising these atom’s energy states and exciting them. When an atom gets excited its electrons move into an orbit further away from the nucleus. These newly excited atoms need to release this new found energy to “calm themselves down” and return to their baseline energy state. The oxygen and nitrogen atoms release this energy in the form of particles of light or photons. The light they give out is what we call the aurora. This process of exciting atoms to cause them to release energy in the form of light is the exact same way that neon lights work. The aurora are simply mother nature’s neon lights, executed on a far grander scale. The only difference is that nature uses this phenomenon in a far subtler way than to advertise strip bars. The aurora typically appears in vast curtains or waves of light following a distinct line across the sky. They appear this way because they are following lines of force in the Earth’s magnetic field. Yep, that’s right, the aurora uses the force. The different colours of the aurora are caused by different gases in the atmosphere. Each gas emits a unique colour when it is excited. For example, oxygen gives off a green light when it’s excited, which is the most common colour seen in the aurora. Nitrogen on the other hand, gives of blue and red colours when excited. So to summarise, when charged particles released from the solar flares on the Sun’s surface hit our planet at either the north or south pole, they interact with atoms in the Earth’s atmostphere. Causing their electrons to move to a higher energy state. When the electrons drop back down to a lower energy state they release photons, tiny little particles of light, which light up the night’s sky in a spectacular display of painted waves, following the natural magnetic field lines of planet Earth. Knowing how the aurora are formed may spoil the beauty for you, but I believe there’s an even greater beauty in understanding the science behind one of nature’s most breathtaking performances. So if you’re ever lucky enough to experience the aurora for yourself, just take a moment to contemplate on how truly marvellous and magnificent the universe really is …and then take selfies.