Aurora Tonight

Space weather explained

What causes the northern lights?

Aurora borealis is produced when charged particles from the Sun collide with gases in Earth's upper atmosphere. The physics behind it connects events 150 million kilometres away to the green glow on your northern horizon.

It starts at the Sun

The Sun constantly sheds a stream of charged particles - protons and electrons - called the solar wind. This stream flows outward at roughly 400-800 km/s, reaching Earth in two to four days. Even during quiet periods, the solar wind is present. Aurora at high Arctic latitudes - Tromsø, Svalbard, northern Canada - occurs most nights as a result of this continuous flow.

What causes the more dramatic aurora visible at lower latitudes - Scotland, northern England, even southern Europe during the strongest events - is a larger, faster eruption: a coronal mass ejection (CME). A CME is a cloud of magnetised plasma ejected from the Sun's corona. When an Earth-directed CME arrives, it can increase geomagnetic activity by an order of magnitude above normal solar wind conditions.

The strongest aurora displays in the UK in recent years - including the widespread sightings in May 2024 - resulted from a series of powerful CMEs arriving in quick succession during a period of high solar activity near solar maximum.

Earth's magnetic field and the auroral oval

Earth's magnetic field acts as a shield, deflecting most of the solar wind around the planet. But it is not a perfect barrier. Where the field lines converge at the magnetic poles, charged particles can enter the atmosphere. The region where this happens most intensely forms a ring around each magnetic pole - the auroral oval.

During quiet conditions, the oval sits at around 65-72°N magnetic latitude. From northern Scandinavia or Alaska, aurora is visible on most clear nights as a result. During a geomagnetic storm - triggered by a CME impact - the oval expands equatorward. At Kp 7, it reaches roughly 50°N. At Kp 9, it can extend to 40°N or below.

The expansion is not uniform. The oval is brighter and more active on the nightside of Earth, between roughly 21:00 and 03:00 local time. This is when the geometry of the magnetosphere creates the strongest particle acceleration toward the atmosphere.

Why auroras have different colours

Green

The most common colour. Produced by oxygen atoms at 100-150 km altitude. The 557.7 nm emission is exactly what the human eye detects most sensitively in the dark, which is why green dominates most aurora displays.

Red

Produced by oxygen atoms at higher altitudes - above 200 km. Rarer than green because the atmosphere is thinner up there. Red aurora often appears at the top of green curtains, or as a faint crimson sky during very strong storms.

Blue and purple

Come from nitrogen molecules lower in the atmosphere, below about 100 km. Often appear at the lower edges of curtains. Cameras tend to capture blue and purple more readily than the naked eye.

Pink

A mix of red oxygen and blue-purple nitrogen emissions appearing at the lower edges of strong displays. Often the last colour visible before an active arc fades.

Arcs, rays, and coronae

Aurora takes several distinct forms depending on the level of activity and the particle energy involved. A quiet arc - a green band low on the northern horizon - is the most common starting point. As activity increases, the arc may develop rays extending upward from it, then begin to fold and ripple as the magnetic field fluctuates.

During active periods, multiple arcs can appear simultaneously, moving rapidly across the sky. At the peak of a strong geomagnetic storm (Kp 7+), a corona can form directly overhead - rays converging to a point above you as the auroral oval passes across your latitude.

The most active phase of a display - called a substorm - can develop within minutes. This rapid brightening is caused by a sudden release of energy stored in the stretched nightside of the magnetosphere. Substorms can occur multiple times during a single storm event.

Related pages

What Is the Kp Index?

The scale used to measure geomagnetic activity and predict aurora visibility by latitude.

Tips for Viewing the Northern Lights

Practical advice on dark skies, timing, and what to look for.

Northern Lights Photography

Camera settings and technique for capturing aurora on film.

Northern Lights UK - Live Forecast

Current aurora visibility status for UK locations based on live Kp data.

Aurora Forecast Tonight

Live Kp index and geomagnetic storm status from NASA space weather data.

Common questions

More on the physics and geography of aurora borealis.

What causes the northern lights?
The northern lights - aurora borealis - are caused by charged particles from the Sun colliding with gases in Earth's upper atmosphere. When these particles, carried by the solar wind or ejected in a coronal mass ejection (CME), reach Earth, they are funnelled toward the poles by the planet's magnetic field. Collisions with oxygen and nitrogen at altitudes of 100-300 km release energy as light.
Why are auroras green?
Green is the most common aurora colour and comes from oxygen atoms at altitudes of around 100-150 km. When an energetic particle excites an oxygen atom, it releases energy at a wavelength of 557.7 nm - green. At higher altitudes (above 200 km), excited oxygen emits red light instead. The less common blue and purple shades come from nitrogen molecules lower in the atmosphere.
What is a coronal mass ejection and how does it cause aurora?
A coronal mass ejection (CME) is a large eruption of plasma and magnetic field from the Sun's corona. When an Earth-directed CME arrives - typically 1-3 days after leaving the Sun - it compresses Earth's magnetic field on the dayside and stretches it on the nightside. This disturbs the magnetosphere and accelerates particles toward the poles, producing aurora that is often far more intense than the usual solar wind aurora.
Why do auroras form a ring around the poles?
Earth's magnetic field converges at the magnetic poles. Charged particles from the Sun follow magnetic field lines and are funnelled down into the atmosphere in an oval-shaped band around each magnetic pole - called the auroral oval. This ring sits at around 65-72°N during quiet conditions. During geomagnetic storms, the oval expands equatorward, bringing aurora to lower latitudes.
Do the southern lights exist?
Yes. Aurora australis - the southern lights - occur simultaneously with aurora borealis. They are a mirror image, appearing in a ring around the south magnetic pole. Because the south magnetic pole is located in Antarctica - a largely uninhabited and inaccessible continent - southern lights are seen far less often than their northern counterpart, but they are equally real.
Can auroras happen during the day?
The aurora itself can and does occur during daylight hours - geomagnetic activity does not switch off at sunset. However, the sky needs to be dark for aurora to be visible to the naked eye. During summer at high latitudes, continuous twilight can make it difficult to see even active aurora. The optimal viewing window runs from late September through to late March in the UK.
What is the solar cycle and how does it affect aurora?
The Sun follows an approximately 11-year cycle of activity, moving between solar minimum (fewer sunspots, quieter) and solar maximum (more sunspots, more flares and CMEs). Aurora is more frequent and intense near solar maximum. Solar cycle 25 peaked around 2024-2025, making this a particularly active period for aurora globally.