The science and the northern lights

The northern lights, also known as the Aurora Borealis, is a breathtaking natural phenomenon that has captivated humanity for centuries. These mesmerizing displays of colorful lights illuminate the night sky, predominantly in polar regions, particularly those north of the Arctic Circle. The science behind the northern lights is a fascinating tale of interactions between the Earth’s atmosphere, charged particles from the Sun, and our planet’s magnetic field—a perfect blend of natural wonder and cosmic science.

This was a big one

At the core of this spectacular light show is the Sun, a massive star that continuously emits charged particles, mainly superheated electrons and protons, in what is known as the solar wind. This solar wind is a constant stream of energy-rich particles flowing out into space. Occasionally, the Sun experiences solar eruptions known as coronal mass ejections (CMEs) — often accompanied by solar flares — which release enormous bursts of charged particles toward Earth. These energetic outbursts from the Sun significantly increase the likelihood of witnessing the northern lights, as they propel a dense wave of particles towards Earth. If you're interested in tracking these events, I recommend subscribing to updates on the Space Weather Live webpage, where you can receive timely alerts about solar activity - or make it even easier by downloading our app for curated information on the Northern Lights and life in Tromso. This is a great companion for your stay.

As these charged particles approach Earth, they encounter our planet's magnetic field, known as the magnetosphere. The magnetosphere is generated by the motion of molten iron in Earth’s outer core, which creates a protective magnetic shield around the planet. This shield plays a crucial role in deflecting most of the solar wind particles away from Earth, safeguarding us from the harmful effects of these high-energy particles.

However, some of these particles manage to penetrate the magnetosphere, particularly near the polar regions where the magnetic field lines converge. Guided by these magnetic field lines, the particles are funneled toward the poles, where they ultimately collide with gas molecules in Earth’s atmosphere. These collisions primarily occur with oxygen atoms and nitrogen molecules, and it's these interactions that give rise to the stunning light displays known as the northern (and southern) lights.

When these energetic particles collide with gas molecules, they excite the molecules, causing their electrons to move to higher energy levels. As the electrons return to their normal energy levels, they release the excess energy in the form of light, or photons. The color of the emitted light depends on the type of gas involved (atomic oxygen or molecular nitrogen) and the altitude at which the collision occurs.

The structure is never the same

For example, oxygen atoms typically emit green and red light, which accounts for the predominantly green and red hues often seen in the Aurora. On the other hand, nitrogen molecules can produce a range of colors, including pink, purple, and blue. The altitude of these interactions also influences the appearance of the auroras, with most occurring between 80 and 300 kilometers (50 to 186 miles) above the Earth’s surface.

The intensity, color, and shape of the northern lights vary based on several factors, including the energy of the incoming solar particles, the altitude of the interactions, and the composition of the atmosphere in the region. Additionally, the occurrence of the northern lights follows an approximately 11-year cycle known as the solar cycle, which corresponds to the Sun’s varying activity. During periods of solar maximum, when solar activity is at its peak, the Northern Lights become more frequent and vibrant, offering more opportunities to witness this awe-inspiring spectacle - at least in theory.

What the numbers mean

If you're using an aurora tracking app, like Guide to Tromsø or SpaceweatherLive during your stay, you'll come across a few key numbers worth understanding — they're what separate a lucky glance at the sky from knowing "exactly" when to step outside.

The Kp-index (rated 0 to 9) is the most commonly cited number, and it measures how disturbed Earth's overall magnetic field is at a given moment. Higher Kp means a stronger, more widespread aurora that can be seen further south than usual. But here in Tromsø, sitting directly beneath the auroral oval, you don't need a high Kp for a great show — Kp 1 or 2 is often enough on a clear night. Don't wait for a big number before heading outside.

Bz is a more specific and, for chasers, arguably more useful measurement: it tracks the north-south direction of the magnetic field carried by the solar wind, measured in nanotesla (nT). When Bz points south (shown as a negative number), it links up with Earth's own magnetic field and allows solar wind energy to flow into the atmosphere — this is often what triggers a sudden brightening or an active substorm. As a rule of thumb, Bz at -3 nT or lower is a good sign, and -5 nT or lower is very favorable. When Bz is positive, the "gate" is essentially closed, no matter how promising other numbers look.

Hemispheric Power (HP), measured in gigawatts (GW), is a real-time estimate of how much energy is currently being deposited into the upper atmosphere by aurora-producing particles. Think of it as a live snapshot of how "switched on" the aurora is right now, as opposed to Kp's broader 3-hour average. Quiet nights typically sit around 10–20 GW; values climbing above 50 GW usually mean a noticeably brighter and more active display.

None of these numbers guarantee anything on their own — but a negative Bz, a modest Kp, and a rising Hemispheric Power reading, combined with clear skies, is about as good a signal as you'll get that it's worth bundling up and heading outside.

The Northern Lights are more than just a beautiful light show; they are a vivid reminder of the complex and dynamic interactions between the sun, Earth’s magnetic field, and our atmosphere. This phenomenon serves as a stunning example of the interconnectedness of our universe, captivating both scientists and spectators alike with its ethereal dance across the polar skies.

For those eager to witness the northern lights in all their glory, Tromsø, Norway, is one of the best places on Earth to do so. Situated directly under the Northern Lights oval at 69 degrees north, Tromsø offers numerous opportunities to catch a glimpse of this natural wonder. The city, which is one of the largest north of the Arctic Circle with a population of around 80,000, is well-equipped to cater to aurora chasers. Visitors can spend several days hunting for the lights, and when the skies are clear, Tromsø provides an ideal vantage point. Moreover, the city offers a variety of activities to complement the aurora experience, such as dog sledding, snowshoeing, and exploring the unique Arctic culture.

Get expert advice on when to travel, and see what other guests have experienced on tour in the north. Visit and become a member of our Facebook group "Tromsø Northern Lights".