Why the Aurora Is Green (and Sometimes Red, Blue or Pink): Colours, Shapes and Visibility

What determines the colour of the aurora, why it forms an oval around the magnetic poles, and how the Kp index maps that oval down to latitudes like the UK.

▶ Open the simulation

The basic recipe

The aurora is fundamentally an atmospheric light show: charged particles - mostly electrons, accelerated along Earth's magnetic field lines - collide with oxygen and nitrogen atoms and molecules high in the atmosphere, transferring energy that those atoms briefly hold before releasing it again as a photon of light. The process is broadly similar to how a neon sign works, with Earth's upper atmosphere standing in for the glass tube and the solar wind standing in for the electrical current, though the altitudes, energies and physics of exactly how particles get accelerated into the atmosphere are considerably more involved.

Why colour depends on altitude

Colour is set mainly by which gas is struck and at what altitude, because different gases at different atmospheric pressures release light at different wavelengths. Green, by far the most commonly seen colour, comes from oxygen atoms at altitudes of roughly 100-200 kilometres, emitting at a specific wavelength (557.7 nanometres) that the human eye is particularly sensitive to - part of why green dominates naked-eye impressions even when other colours are also present. Above about 200 kilometres, oxygen instead tends to emit a deep red at 630.0 nanometres, a slower and fainter process that requires the very low particle density found at those higher altitudes to occur before the excited oxygen atom loses its energy some other way; red aurora is generally associated with the most energetic storms and often appears as a faint cap above the main green display, easy to miss with the naked eye but often obvious in a long-exposure photograph. Below roughly 100 kilometres, energetic particles instead excite nitrogen molecules, producing blue and, in mixtures with the red and green above it, magenta and pink fringes along the lower edge of a bright display - typically only visible during strong storms, when particles penetrate deep enough into the atmosphere to reach that altitude.

Why the aurora forms an oval around the poles

Earth's magnetic field lines converge at the magnetic poles, funnelling incoming charged particles toward roughly circular zones centred there - the auroral ovals, one around each geomagnetic pole. Under quiet conditions, the oval sits at a radius of only about 15-20 degrees from the pole, keeping the aurora confined to high-latitude regions like northern Scandinavia, Alaska and northern Canada. As geomagnetic activity increases, energy pumped into the magnetosphere pushes the oval outward and it can expand to encompass 40-50 degrees of latitude during the most severe storms - which is the physical reason aurora becomes visible from much lower latitudes, including on rare occasions from southern England or the Mediterranean, only during strong storms.

Reading the Kp index as an aurora forecast

Because the size of the auroral oval tracks geomagnetic activity fairly reliably, the Kp index doubles as a rough aurora visibility forecast. As a working rule of thumb, each step up in Kp pushes the edge of the visible oval a few degrees of latitude further from the pole. In practice this means Kp 3-4 keeps aurora largely confined to northern Scandinavia, Iceland and northern Canada; Kp 5-6 brings a realistic chance to Scotland and northern England; and Kp 7 or higher has, on documented occasions, brought a naked-eye display within reach of southern England and central Europe. It is only a rule of thumb, though - actual visibility on any given night also depends on cloud cover, light pollution, and exactly where the oval sits relative to your location at the time.

Shapes, movement, and a look-alike worth knowing

A display typically starts as a quiet, stable arc low on the horizon and, as activity increases, develops into folded bands, vertical curtain-like rays known as draperies, and - when the display sits directly overhead - a dramatic radiating pattern called a corona. The apparent motion comes not from any single patch of gas moving, but from different regions of the upper atmosphere being excited moment to moment as the underlying particle precipitation shifts, which is also why displays can change dramatically within seconds. One frequently photographed but scientifically distinct phenomenon, a narrow mauve-and-white ribbon called STEVE (an intentionally informal acronym, discovered largely through citizen-scientist photographs and formally studied only since around 2016), is now understood to arise from a different mechanism - fast-flowing charged particles in the upper atmosphere rather than the particle precipitation that produces ordinary aurora - even though it often appears alongside a genuine auroral display.

Frequently Asked Questions

Why is the aurora usually green?

Green comes from oxygen atoms at 100-200 kilometres altitude emitting at a wavelength the human eye is especially sensitive to, and this is the altitude range where most auroral particle collisions occur, which is why green dominates most displays.

What causes red aurora?

Red comes from oxygen at higher altitudes, above about 200 kilometres, where the low particle density allows a slower emission process to complete before the excited atom loses energy another way. It is generally associated with stronger storms and often forms a faint cap above the main green band.

What Kp index do you need to see the aurora from the UK?

As a rough guide, Kp 5-6 gives Scotland and northern England a realistic chance of a naked-eye display, while Kp 7 and above has occasionally brought visible aurora as far south as southern England, weather and light pollution permitting.

Is STEVE the same thing as the aurora?

No. STEVE looks like a narrow mauve ribbon and often accompanies a genuine aurora, but it is produced by a different physical process involving fast-moving charged particles in the upper atmosphere, rather than the particle precipitation that creates ordinary auroral light.

Can you see aurora from the equator?

Only in the most extreme storms on record, such as the 1859 Carrington Event, when the auroral oval expanded so far that a red glow was reported near the Caribbean and other near-equatorial locations. Under normal conditions the oval stays confined to high latitudes.