In order to understand the mechanism behind the aurora, we must first start with our Sun. Our Sun and all stars are mainly made of gas and plasma, which is a special type of matter made of charged particles. Unlike normal gas, plasma can have magnetic fields and can conduct electricity. Radiation from the Sun is a combination of many kinds, including light, infrared, x-rays and ultraviolet radiation. Stars also come in different colours based on their different temperatures. Hotter stars appear blue or white and cooler ones are red and orange.
Much of what we know about stars comes from studying our own star at the centre of the solar system, the Sun. It provides the main source of energy to power the Earth’s weather, climate and life. Without it, we simply would not exist. The radiation from the Sun takes only 8 minutes to reach us, which is much closer and more accessible than studying the stars from next nearest star system to us, Alpha Centauri. Even though light travels at roughly 300,000 kilometers per second, it takes nearly 4.5 years for light from this star system to reach us from 40 trillion kilometres away (this is roughly 300,000 times the distance from the Earth to the Sun).
Our Sun is also useful because the knowledge we gain about it can be applied to other stars in the universe. It seems that our Sun is a rather typical star. In terms of its mass, size, brightness and composition, it is generally in the middle of most of the stars that we have discovered and studied so far. Despite being average, a whopping 1.3 million Earths can fit inside our Sun and this glowing ball of gas takes up 99.8% of the mass of our entire solar system. What is unusual about it however is that the Sun stands alone. Most stars are part of what’s called a binary system that has two stars that orbit around the same point in space.
Like the Earth, the Sun is divided into layers. The inner core creates the energy required to keep the star burning, which is followed by layers that move the core’s energy through radiation and convection. The part of the Sun that we can see is called the photosphere and it is essentially the surface of the star, or the lowest level of its atmosphere. Temperatures here are around 5,500 degrees Celsius. The higher you go into the Sun’s atmosphere, the higher the temperatures rise, all the way up to the corona: the hot, low-density layer of the upper atmosphere. The corona is significantly hotter than the layers below it, which can experience temperature from 1 million to 10 million degrees Celsius. This hotter outer later is still quite a mystery to scientists. It is much like walking away from a campfire but feeling warmer as you walk further away. Because of these extreme temperatures on and above the Sun's surface, collisions between gas molecules occur not only often, but also violently. This means that protons and free electrons can be thrown out in to space from the star's atmosphere. When these energized and charged particles are blown throughout the solar system, we call it “solar wind”.
The Sun also has what is called “sunspots”, which can be seen through telescopes as dark areas against the star’s bright surface and are each usually comparable to the size of the Earth. These develop when magnetic fields crawl through the solar surface and last for a couple of days to a few weeks. Sometimes, this magnetic energy is released, causing an intense burst of radiation known as a “solar flare”. These explosive events appear very bright and last up to a couple of hours. Solar flares can sometimes be followed by a “coronal mass ejection”, which violently releases radiation from the sun’s outer plasma layer, or corona.
The Earth’s magnetic field protects the planet from these streams of charged particles like water around the bow of a ship. This veil extends tens of thousands of kilometres into space to protect us from harmful radiation. However, at the Earth’s north and south poles, this field is weaker so it sometimes allows some charged particles to pass into our atmosphere. Here, the charged particles collide with the atmosphere’s gas molecules to emit light in a variety of colours, commonly known as the auroras. The auroras are referred to as northern lights (or aurora borealis) when seen in the northern hemisphere, and southern lights (or aurora australis) when seen from the southern hemisphere. They are most commonly observed against the dark night sky in rural, open regions that are not obstructed by light pollution from cities and urban areas. In the north, this phenomenon is often seen from northern Canada, southern Greenland, northern Siberia, Alaska, and northern Scandinavia. In the south, the aurora can be observed in the high latitudes of places like Australia, Argentina, Chile, New Zealand and Antarctica.
Myself pictured with the northern lights in Tromso, Norway as part of the European Space Agency's #AuroraHunters event in March 2019. Credit: M.C. Sarac
The colour of the aurora depends on the type of particles that are colliding together. For example, the common yellow/green aurora is caused by the interaction with oxygen molecules around 100 kilometres above the Earth’s surface. Nitrogen molecules that interact with these charged particles from the Sun create auroras that are blue and purple. The rarer red auroras are caused by the charged solar particles interacting with oxygen molecules at high altitudes of around 300 kilometres.
While widely appreciated from Earth (and by the lucky astronauts who observe these lights from the International Space Station), the auroras have also been discovered and pictured on other planets. The auroras seen on Jupiter are significantly larger and stronger, and they are also fuelled by the charged particles from nearby objects (such as Jupiter’s orbiting moons). Saturn’s auroras can last for days and have been photographed by the Hubble Space Telescope and the Cassini spacecraft. They typically appear blue because an ultraviolet camera is used to capture them, but these auroras would appear red to the human eye. Auroras have also been on observed on Mars, Venus, Uranus and Neptune.
Aurora pictured on Jupiter's northern pole. Credit: NASA, ESA, and J. Nichols (University of Leicester)
When a high-speed stream of radiation from the Sun arrives to Earth, it knocks our magnetic field and can decrease the field’s strength. This pushing and pulling of the field can cause undesirable electrical fluctuations in conductors like electrical cables and power grids. Often called “geomagnetic storms”, these events can cause interference in satellite operations, such as high-frequency radio communications and GPS navigation, degradation to satellite hardware including solar arrays and microchips, pipeline corrosion, and obstruction in communications with airplanes and ships. One of the most recognized geomagnetic storm events took place in March of 1989 when Hydro-Québec's electricity transmission system was interrupted, causing a blackout of nine hours for over 6 million residents. Intense auroras were viewed at both of the Earth’s poles during this storm and some satellites over these polar regions lost control for several hours.
Today, spacecraft are used to watch the Sun’s activity to predict the possible effects on Earth when violent flares occur. Since its launch in 1995, the Solar and Heliospheric Observatory has been watching the Sun closely, which is a joint effort between the American and European space agencies. Similarly, NASA’s Solar Dynamics Observatory and Solar Terrestrial Relations Observatory spacecraft have been studying the relationship between the Earth and the Sun since 2010 and 2006 respectively.