Neons up in the atmosphere

Auroras, or the polar lights, are magnificent light shows held in the Earth’s atmosphere. The curtains of shimmering light stir the imagination and feature prominently on people’s bucket lists. What causes this remarkable phenomenon and why is it so much more common in the polar regions than in the tropics? Can we ever hope to see it in Poland? What are the colours of the polar lights and why do they die down only to light up again a moment later?

By definition, an aurora is light emitted by plasma excited by electric current. We have long been aware of the effect and have used it in neon lamps for over a century! But what exactly does it mean and how can we visualise it, especially if we parted ways with physics in high school (and not on friendly terms)?

Solar gusts

The auroras start on the Sun. The Sun is a ball of plasma, or highly ionised gas, which is incredibly dense and scorching hot. Just like the Earth, it has a magnetic field, but it’s a lot more complicated and dynamic than ours. Solar activity changes periodically, reaching its maximum every 11 years. Every time it happens, the Sun’s magnetic poles switch places (in a process referred to as magnetic reversal). The Sun’s outer layer, known as the photosphere, radiates light, without which there would be no life on Earth. But the Sun doesn’t stop at that. Out of its corona and into space shoot out bits of plasma, creating the solar wind. Large expulsions of plasma are known as Coronal Mass Ejections (CME). The solar wind consists mostly of electrons, protons and the accompanying, frozen-in magnetic field, whose lines retain the direction they followed on the Sun’s surface. When travelling through space, the solar wind may encounter various objects, like the Earth. It takes the wind up to four days to reach it, while the light covers the distance in a little over 8 minutes.

Swirling particles

The Earth has its own magnetic field, which originates in the planet’s outer core, made up of liquid iron and nickel. The magnetic field extends outwards and envelopes the Earth and its atmosphere in a giant protective shield, the magnetosphere. As a result, much of the solar wind passes our planet and continues its journey through space. If, however, the solar wind carries with it a frozen-in magnetic field whose lines run opposite to the Earth’s magnetic field (a situation known as an antiparallel arrangement), the two fields combine through the process of magnetic reconnection [1]. When this happens, the particles carried by the solar wind enter the Earth’s magnetosphere. They comprise electrons and protons, which are electrically charged and cannot therefore move about as they please. Instead, their movement within the magnetic field is governed by stringent rules. First of all, they can only travel in the directions indicated by magnetic field lines, without ever crossing any of them. Secondly, they do not move in straight lines but in spirals. This is why, once the magnetic fields have reconnected, the solar wind particles do not fall straight towards the Earth’s surface, but flow around the planet, swirling along the magnetic lines, and gather on the opposite, “night” side of the magnetosphere.

The pressure of the solar wind deforms the magnetosphere, flattening it on the Sun-facing side and stretching it out on the other. The magnetic tail which forms as a result is where the electrons from the Sun (which enter the magnetosphere during magnetic reconnection) flow to. Such a stretched-out shape, however, is very unstable. The Earth’s magnetic field counteracts the deformation by shrinking and partially regaining a more symmetrical form. Consequently, the electrons from the magnetic tail are once again accelerated and sent back towards the Earth along the magnetic lines, which is to say, towards the poles.

The Earth’s atmosphere is made up of layers, the outermost of which – the ionosphere – is characterised by a high content of O⁺ and N²⁺ ions. When entering the atmosphere, the electrons, which have gained considerable speed in the magnetosphere, collide with the ions giving them energy. This extra energy excites the ions, which – after a while – return to their original state by emitting any excess energy as light. It is this light that forms the aurora. Its colour depends on the type of ion that emits the light, with O⁺ ions responsible for the red and the green, and N²⁺ ions for the purplish-red (magenta).

Beyond the beauty

Auroras and their intensity are directly related to the strength with which the solar wind acts upon the Earth’s magnetic field. The level of the disturbance is described by means of the K_P index, which ranges from 0, for low activity, to 9, which indicates a magnetic storm. Three-day K_P index forecasts help aurora hunters decide when to stay at home and when to grab their cameras and rush to the north [2, 3]. When the K_P index goes beyond 6, auroras may be visible in the northern sky over the Baltic Sea. With the index over 7, you may get a chance to observe the spectacle from Mazury!

The K_P index forecasts are based on solar wind data provided by the DSCOVR satellite, which sits at the point of gravitational equilibrium between the Earth and the Sun. The polar lights, however, are not the only or even the most important reason behind the interest in the K_P index. Disturbances in the Earth’s magnetic field have profound consequences on electrical grids, the Global Positioning System (GPS), aviation, telecommunication and radio communication [4].

On 1 September 1859, a giant coronal mass ejection on the Sun wrought havoc with the telegraphic system, with two telegraphers injured by electrocution. At the same time, extraordinarily bright auroras were visible around the globe, including the Caribbean Islands, Mexico, southern China and Japan [4]. In March 1989, at the end of the Cold War, a fierce magnetic storm caused a nine-hour blackout in the Canadian province of Quebec, over 200 less severe electricity failures across the United States, and a widespread anxiety, as many people thought the disruption signalled the outbreak of a nuclear war with the Soviet Union. The resulting auroras could be observed over much of the USA, including Texas and Florida [5]. Nowadays, magnetic storms can be detected up to 60 minutes before they hit the Earth and electrical grids are equipped with various safety features to minimise the risk of potential system failure.

The impact of the solar wind, however, is not limited to the Earth. The polar lights occur on other planets of the solar system, with ultraviolet auroras observed, among others, around the magnetic poles of Jupiter and Saturn [6]. The situation looks slightly different on Mars, whose cooling core does not produce such a powerful magnetic field. As a result, observed on Mars are ultraviolet proton auroras, widespread in the planet’s Sun-facing hemisphere [7].

The topic of auroras is broad, fascinating and full of exciting surprises, even for the scientists. If you too find it intriguing, go ahead and explore it! The following bibliography (available in English) is a good place to start.

Author: Anna Myśliwiec

Scientific consultation: Prof. Wojciech Miloch

Translation: Barbara Jóźwiak