Aurora Chaser

The science

What causes the aurora?

The southern lights are the last step in a chain that starts 150 million kilometres away, on the Sun. Here’s how it works, and why southern Australia is one of the better places on Earth to see it.

The short version

The aurora is light given off by oxygen and nitrogen high in the atmosphere when they are struck by fast-moving electrons. Those electrons get their energy from the solar wind, the stream of charged particles flowing out from the Sun. Earth’s magnetic field steers them into rings around the magnetic poles. When the Sun sends a burst of material our way, the rings grow brighter and expand towards the equator, far enough to be seen from Australia.

From the Sun to the aurora The Sun sends out the solar wind and coronal mass ejections. They meet Earth’s magnetosphere, which is squashed on the side facing the Sun and stretched into a long tail on the night side. Energy stored in the tail drives particles down magnetic field lines into rings around both magnetic poles, where the aurora forms. Sun Solar wind (300–800 km/s) Coronal mass ejection Magnetosphere Magnetotail → Auroral ovals Not to scale
The solar wind and coronal mass ejections meet Earth’s magnetosphere. Energy stored in the magnetotail drives particles down field lines into the auroral ovals around each magnetic pole.

It starts at the Sun

The Sun constantly blows a thin, hot wind of charged particles (a plasma) into space at around 300–800 km/s, carrying the Sun’s magnetic field with it. Two kinds of solar event drive most aurora:

  • Coronal mass ejections (CMEs): billions of tonnes of magnetised plasma blasted off the Sun, often alongside a solar flare. A CME aimed at Earth usually arrives one to four days later. CMEs cause the biggest storms, like the one in May 2024 when the aurora was seen across much of Australia.
  • Coronal holes: regions where the Sun’s magnetic field opens out into space and lets fast solar wind escape. The Sun turns about once every 27 days as seen from Earth, so a long-lived coronal hole can bring aurora back on a roughly monthly cycle.

Solar activity rises and falls over a cycle of about 11 years. Solar Cycle 25 reached its maximum around late 2024. Large storms remain common for several years after the peak, so there’s still plenty to see.

Earth’s magnetic shield

Earth’s magnetic field forms a protective bubble, the magnetosphere, that deflects most of the solar wind. The solar wind squashes it on the side facing the Sun and stretches it into a long magnetotail on the night side.

What matters most is the direction of the magnetic field carried by the solar wind. When it points southward (space weather forecasters call this a negative Bz), it’s opposite to Earth’s field at the front of the magnetosphere and the two connect through a process called magnetic reconnection. That lets solar wind energy pour in and build up in the magnetotail. When the tail can’t hold any more it snaps back in a substorm, firing particles towards Earth along the magnetic field lines.

The recipe for a big display: fast solar wind and a strongly southward Bz that lasts for hours. Spacecraft sitting 1.5 million km towards the Sun measure both, giving roughly 30–60 minutes’ warning before the solar wind reaches Earth.

Particles light up the sky

Guided by the field lines, electrons (and some protons) dive into the upper atmosphere in two rings called the auroral ovals, one around each geomagnetic pole. Between about 100 and 400 km up they collide with oxygen atoms and nitrogen molecules and give them extra energy. As the atoms and molecules settle back down they release that energy as light, much like the gas in a neon sign.

On a quiet night the southern oval sits over the Southern Ocean and Antarctica. Macquarie Island, halfway to Antarctica, is right underneath it. During a geomagnetic storm the oval brightens, thickens and expands towards the equator, bringing it into view from Australia.

The colours

Each colour comes from a particular gas at a particular height:

  • Green: oxygen, about 100–250 km The most common colour and the brightest to the eye (557.7 nm).
  • Red: oxygen, above about 200 km The oxygen state that gives off red light (630.0 nm) takes almost two minutes to do so. Lower down, collisions with other molecules steal the energy first, so red only survives in the thin air at the top of the display.
  • Blue and purple: nitrogen Seen in strong displays, especially in the tops of rays still lit by the Sun after dusk.
  • Pink fringe: nitrogen, about 90–100 km Very energetic electrons reaching low down can give a bright curtain a pink lower edge.

Why Australia?

Aurora australis is Latin for “southern dawn”, the southern twin of the northern hemisphere’s aurora borealis. Your chance of seeing it depends less on your geographic latitude than on your magnetic latitude: how close you are, magnetically, to the auroral oval.

Earth’s magnetic field is tilted and lopsided. In the southern hemisphere its poles are shifted towards the Australian side of Antarctica, which puts southern Australia magnetically closer to the oval than its geographic latitude suggests:

  • Hobart (42.9° S) has a magnetic latitude of about 54°, the same as Edinburgh or Seattle.
  • Melbourne (37.8° S) sits at about 49° magnetic latitude, level with Berlin.
  • Darwin, at about 22°, is too close to the magnetic equator to ever see the aurora.

The aurora sits in the oval around the south magnetic pole, so from Australia you look south. In the biggest storms it can climb overhead over Tasmania.

Why it often looks red from the mainland

From southern Tasmania a strong display can fill the sky with green curtains. From further north the same display is hundreds or even thousands of kilometres away, and the curve of the Earth hides its lower part. The green light (around 100–250 km up) drops below the horizon, and only the high red light shows. Geometrically, light from 100 km up is hidden beyond about 1,100 km, but red light from 300 km up can still peek over the horizon from about 1,900 km away.

Why distant aurora looks red An aurora curtain stands above the curved Earth, green at the bottom and red at the top. An observer close to it sees the whole curtain high in the sky. A distant observer’s horizon line passes through the middle of the curtain, so only the red top is visible. Red 200–400 km Green 100–250 km Distant observer’s horizon Mainland: only the red top shows above the horizon Tasmania: the whole display, high in the sky ← North South → Heights exaggerated
The further you are from the aurora, the more of it is hidden below your horizon. Only the high red light is left, which is why mainland photos are often red and pink rather than green.

Cameras see more than eyes

In dim light our eyes switch to rod cells, which are sensitive but colour-blind and poor at deep red. So faint aurora often looks like a grey-white glow or a pale cloud to the naked eye, while a camera’s long exposure collects light for several seconds and records the colour. Phone night modes do this well.

A quick test: if a “cloud” low in the south looks green, red or pink in a photo, it’s the aurora.

Kp, K and the G-scale

  • K-index: how disturbed Earth’s magnetic field is at one magnetometer over a three-hour period, on a scale from 0 (quiet) to 9 (extreme).
  • Kp: the “planetary” K-index, combined from stations around the world. It’s reported in thirds, for example 5−, 5 and 5+.
  • Kaus: the Bureau of Meteorology’s Australian-region K-index, built from Australian magnetometers. It’s the most relevant measure for local conditions and is usually close to Kp.
  • G-scale: NOAA’s geomagnetic storm scale, from G1 (minor, Kp 5) to G5 (extreme, Kp 9).

How the map works

  1. Each point’s magnetic latitude comes from the AACGM-v2 model (altitude-adjusted corrected geomagnetic coordinates) at 110 km altitude for 2026. This model accounts for the real, lopsided shape of Earth’s field, which a simple tilted-dipole model gets wrong by several degrees over Australia.
  2. For the chosen Kp, the map estimates how far towards the equator the auroral oval reaches. This is about 66.5° magnetic latitude at Kp 0 and moves about 2° closer to the equator with each step of Kp, to around 48° at Kp 9.
  3. It then compares the two. Inside the oval, the aurora can be overhead. Up to 4° outside it, it’s visible to the naked eye; 4–8° outside, low on the horizon; 8–12° outside, usually only by camera. These distances were tuned to match the Bureau of Meteorology’s guide (K3 southern Tasmania, K4 all of Tasmania, K5 the Victorian coast, K6 the south-west coast of WA, K7 and above southern NSW).

It’s a guide, not a forecast. The oval is rarely a neat ring. It reaches furthest towards the equator around local midnight, and substorms make it surge and fade within minutes. Kp is a three-hour global average. And none of it matters if it’s cloudy. For what’s actually happening, use the forecasts and alerts.