The sky — astronomy

Auroras: The Sun’s Light Show in Our Atmosphere

An aurora is light produced when charged particles from the Sun, guided by Earth’s magnetic field, collide with gases in the upper atmosphere. It usually appears near the polar regions, but strong geomagnetic storms can push it to much lower latitudes.

Auroras are among the most striking sights in nature, and a clear reminder that our planet is connected to the Sun. The shifting green, red and violet curtains do not come from deep space. They are glowing gases in our own atmosphere.

The solar wind and Earth’s magnetic field

The Sun constantly releases a stream of charged particles, mostly electrons and protons, known as the solar wind. Its speed is typically hundreds of kilometres per second and varies with solar activity.

Earth is shielded by a magnetic field that creates a protective region around the planet called the magnetosphere. It deflects most of the solar wind. But when the magnetic field carried by the wind connects with Earth’s own field, energy and particles can enter the magnetosphere. The particles then travel along magnetic field lines toward the regions around the magnetic poles.

There they plunge into the upper atmosphere and collide with atoms and molecules of oxygen and nitrogen. Each collision excites the atom, which then releases that energy as light. That light is the aurora. It is most often seen in ring-shaped zones around the magnetic poles called auroral ovals.

In the north the display is called the aurora borealis, or northern lights; in the south it is the aurora australis. They are the same process at opposite ends of the planet. Similar auroras have been observed on other planets that have magnetic fields, such as Jupiter and Saturn.

The colours of an aurora

Colour depends on which gas is struck and at what altitude.

• Green – the most common colour. It comes from atomic oxygen at roughly 100–150 kilometres up, and our eyes are especially sensitive to it.
• Red – also from atomic oxygen, but higher, above about 200 kilometres. This light is fainter and harder to see with the naked eye, yet during strong storms it is what people at lower latitudes most often notice.
• Blue and violet – mainly from molecular nitrogen, often seen along the lower edges of the curtains.

Shapes vary as well: quiet arcs, ribbons, rays, and sometimes a flickering corona when the display spreads overhead. Cameras usually show more colour than the eye, because they gather light over longer exposures. That is why photos often look brighter and more vivid than the real sight.

Geomagnetic storms

The brightest and farthest-reaching auroras occur during geomagnetic storms, temporary disturbances of Earth’s magnetosphere. There are two main drivers:

1. Coronal mass ejections (CMEs) – huge clouds of plasma and magnetic field thrown out by the Sun.
2. High-speed solar wind streams flowing from regions of the solar corona called coronal holes.

Size is not the only thing that matters. The orientation of the cloud’s magnetic field is important too: when it points southward, opposite to Earth’s field at the front of the magnetosphere, the connection is more efficient and the storm can be stronger.

Solar activity follows a cycle of about 11 years. Strong storms are more frequent around solar maximum, although good auroras can occur at other times as well.

Storm strength is described by the Kp index (a scale from 0 to 9) and by the five-level G1–G5 scale of the U.S. National Oceanic and Atmospheric Administration (NOAA). During weak storms, auroras stay near the polar regions. During the strongest, the oval expands and the lights can be seen far toward the equator. The powerful storm of May 2024 is a good example: auroras were photographed at unusually low latitudes, including in southern Europe. At such latitudes you normally see only a faint reddish glow low on the poleward horizon.

How to follow aurora forecasts

Auroras cannot be predicted to the hour, but several tools help:

• NOAA’s Space Weather Prediction Center (SWPC) publishes alerts, observations and forecasts of geomagnetic activity, including the Kp index.
• Solar missions such as SOHO (a joint ESA and NASA mission) and NASA’s Solar Dynamics Observatory (SDO) watch the Sun and reveal mass ejections. Spacecraft stationed between Earth and the Sun, about 1.5 million kilometres away, measure the solar wind shortly before it arrives, giving a warning of only tens of minutes.
• A CME typically takes from about one to several days to reach Earth, so a storm is often anticipated in advance, but its strength remains uncertain until the last moment.

Practical tips for watching, wherever you live:

1. Find a dark place away from city lights. See also how to watch the sky.
2. Face the poleward horizon and give your eyes 15–20 minutes to adapt to darkness.
3. Check the cloud forecast. Even a strong storm is invisible behind thick cloud.
4. Use a phone in night mode, or a camera on a tripod with a long exposure. Faint red auroras often show up better in photos than to the eye.
5. Be patient. Displays come in waves and can last minutes or hours.

An aurora is a reminder that Earth is part of a larger system. Its magnetic field, its atmosphere and the Sun’s energy together create a light show people have watched for thousands of years.

Positions in Astronativa are astronomically exact. The interpretation is for inspiration. · Glossary · Terms · Privacy