Solar particles dancing in the sky

The skies were lit up once again in many parts of Europe, even the UK, with spectacular aurorae! A beautiful sight, and one with much fascinating physics behind it.

At time of writing, I have about an hour before my friends pick me up to go out for dinner, so I've paused the sombre political podcast I was listening to in order to jot down a quick article about a spectacular phenomenon of the natural world instead. In dark times like these, we all need a little light and wonder. So, what are the aurorae, or as we call it in this part of the world, 'northern lights' all about?

Aurorae above Göttingen, Germany, taken on the night of 19th January 2026. Image credits to one of my wise astrophysicist friends and fellow nerds, Jude Simmons.

The Sun, on which almost all life as we know on Earth depends, is responsible for these beautiful skies. Charged solar particles are emitted in huge numbers from the Sun constantly as solar wind in all directions, travelling at vast speeds. Thankfully, only a tiny fraction of these reach Earth, and we are mostly protected by our magnetosphere, a spherical magnetic field that extends around our planet. The magnetic field captures these charged particles, where they are accelerated towards the northern and southern poles, as this is where the field is strongest. The illumination of the sky is caused by interactions between solar particles and our atmosphere, where these high speed particles slam into atmospheric molecules, exciting their atomic shell electrons and subsequently releasing this excess energy in the form of light as photons, thus allowing the shell electrons to return back to their lower, favoured states. These amounts of energy are discrete, or fixed, taking values depending on the atomic structure of the atmospheric molecule, which in turn dictates the wavelength, and hence colour, of the light emitted. From this, we can deduce that green light is caused by the excitation of (monatomic) oxygen atoms, red by (diatomic) nitrogen as well as (monatomic) oxygen, and purple/blue by hydrogen and helium. This does also depend slightly on atmospheric height. For example, blue/purple aurorae are relatively rare as hydrogen and helium form small parts of the atmospheric composition and are near the top of it, so these are not usually visible except with very dark skies or particularly strong solar storms. Other colour combinations can be created by secondary reactions, where electrons from molecules excited by solar particles subsequently excite other molecules around them.

On the phone to a wise friend the other day, I wondered out loud why aurorae seemed so much more frequent nowadays than when I was younger, or even a few years ago. My wise friend reminded me that in approximately 11 year cycles, the Sun's activity reaches a maximum, and we are currently near this point in the cycle. At maximum activity, solar flares, sunspots and events called coronal mass ejections, where a large amount of plasma is ejected from the Sun's surface into space, reach a peak, and Earth is bombarded with charged particles. These can cause geomagnetic storms, potentially disrupting satellites and communications systems (although who wouldn't mind a bit less social media these days!) as well as spectacular aurorae. In October 2024, the National Oceanic and Atmospheric Administration (NOAA), NASA and the Solar Cycle Prediction Panel announced that the Sun had reached its solar maximum period, which could continue for another year. Like Earth, the Sun also has a magnetic field. Bizarrely, roughly every 11 years, the Sun's magnetic poles flip, and the Sun transitions from being calm to an active and stormy state, an appropriate metaphor for the current political climate. This can be nicely seen in the image below, which shows the Sun at solar minimum, where it appears spotless, compared to solar maximum, where sunspots show areas of intense magnetic activity, associated with solar phenomena such as flares, and coronal mass ejections as mentioned earlier.

Visible light images from NASA’s Solar Dynamics Observatory highlight the appearance of the Sun at solar minimum (left, Dec. 2019) versus solar maximum (right, August 2024). During solar minimum, the Sun is often spotless. Sunspots are associated with solar activity, and are used to track solar cycle progress. Credit: NASA/SDO

As one might imagine, given the potential for damage to our satellites and global communication systems upon which much of modern life depends, there is a great deal of interest in researching solar activity, so that we can better predict and prepare for geomagnetic storms. Observations of the Sun are done in multiple wavelengths, and the below image shows the difference between the Sun at different points in the solar cycle in the ultraviolet wavelength range (which we cannot see with our human eyes), which reveals further intricate details of the dynamics on the Sun's surface.

More images from the Solar Dynamics Observatory showing the appearance of the Sun in ultraviolet, 171 Ångstrom wavelength, where the left side is at solar minimum in December 2019, and the right side from May 2024. Credit: NASA/SDO

My friends will be ringing the doorbell to scoot off to a Moroccan restaurant any time soon, so I'd better leave it there with the whistle stop tour of the Sun and the activities of its particles in our atmosphere. So there we have it, the first post of 2026 bringing light and wonder. Here's to more of both in the year to come :)

A more classic image of aurorae, also taken by Jude in Germany.