Northern lights, or Aurora Borealis, is a natural phenomenon caused by high-speed electrically charged particles bombarding Earth’s atmosphere. As a result of this bombardment the oxygen and nitrogen atoms in the atmosphere emit light which we then see as the aurora (a similar process happens in neon light tubes).
The bombarding particles actually mainly originate all the way from the Sun. As a result of the Sun’s energy production there is a continuous flow of electric particles, or plasma, which we call the solar wind, flowing from the Sun into the interplanetary space and beyond. The solar wind carries, along with its plasma, the magnetic field of the Sun. As the solar wind reaches Earth, it encounters the magnetic shield that is formed by Earth’s magnetic field, deforming it by pushing it from the dayside and stretching it into a long tail on the night side. Under certain circumstances the solar wind then interacts with Earth’s magnetic field in a way that allows some particles to reach Earth’s atmosphere on the dayside polar regions, causing the so-called daytime aurora (these can be seen, for example, in Svalbard during winter when it is dark even during the daytime, even though they are not as strong as the nigh-time aurora). As the solar wind blows towards the tail of Earth’s magnetosphere, the resulting pressure in the tail can energize some particles and following Earth’s magnetic field back to the polar regions, they reach the atmosphere causing the usual night time aurora. The regions where the particles finally hit Earth’s atmosphere causing aurora are called the auroral ovals, located around the northern and southern geomagnetic poles. Although we currently have a good overall understanding of how the aurora are formed, some details still remain a mystery. For example, we actually do not quite know the details of what happens in the interaction between the solar wind and Earth’s magnetic field, or what exactly takes place in the magnetic field’s tail region.
While the solar wind indeed flows continually, it also changes constantly (these ever-changing conditions of the space environment are collectively referred to by the term space weather). Very slow and steady solar wind does not usually cause aurora to form, but either high speed streams or types of shock fronts caused by solar eruptions are needed. When there is an eruption in the Sun, it will take roughly 1-3 days for it to reach Earth. The larger the disturbance in the solar wind (and the eruption in the Sun) is, the more it warps Earth’s magnetic field, causing the auroral ovals to expand, and the further south we can see the aurora. In this way the occurrence of aurora is closely tied to the level of solar activity. While there are constant changes in the level of solar activity, on a longer timescale the activity follows an 11-year cycle. Luckily this does not mean that the Aurora would completely disappear even during low solar activity, and even now, in 2019, we are still enjoying spectacular auroral displays even though we are close to the solar activity minimum. Solar eruptions large enough to cause aurora can happen any time!
Depending on the disturbance in Earth’s magnetosphere, the auroras can be very faint or very bright, ranging from invisible to the human eye to even illuminating the ground below in shades of green. Most often auroras are seen as green bands in the sky familiar from most photographs of northern lights, but other colors appear also. Sometimes there is red above the green, and sometimes there is purplish red below the green. The different colors are a result of the energy absorption by different chemical elements of the atmosphere at different altitudes. The most common color green and the occasionally visible red above the green are both coming from oxygen atoms. The purplish color below the green, however, is coming from nitrogen molecules. Because the auroras occur quite high in the atmosphere, at roughly 100km, the same auroral display can be seen over a large area on ground. On the other hand, unfortunately, because they are above the clouds, one needs a cloudless sky to enjoy the show.
For more information on how to see the aurora please see the page “How can I see the Northern Lights?“.