Aurora Borealis and Australis: Causes and Viewing Conditions

On the night of November 5, 2023, skywatchers as far south as Texas and Florida witnessed curtains of green and red light rippling across the sky — a geomagnetic storm rated G4 on NOAA's five-point scale, the strongest in nearly two decades. That event brought the aurora borealis to latitudes where most people had never expected to see it. This page covers the physical mechanism that produces both the northern lights (aurora borealis) and their southern counterpart (aurora australis), the conditions that control where and when they appear, and what distinguishes a memorable display from a disappointing grey smudge on the horizon.

Definition and scope

The aurora borealis and aurora australis are optical phenomena produced when energetic charged particles from the Sun collide with gases in Earth's upper atmosphere, releasing energy as visible light. They are mirror phenomena — one centered on the north magnetic pole, one on the south — and together they define what scientists call the auroral ovals: roughly circular bands of auroral activity that encircle each geomagnetic pole at latitudes typically between 65° and 72°.

These ovals are not fixed. During periods of heightened solar activity, the ovals expand equatorward, which is why a powerful storm can push auroras deep into the continental United States or southern Australia. The key dimensions and scopes of astronomy relevant here span plasma physics, atmospheric chemistry, and geomagnetism simultaneously — aurora science sits at an unusually productive intersection of disciplines.

How it works

The chain of causation starts at the Sun. Solar wind — a continuous stream of electrons and protons flowing outward from the solar corona at roughly 400 kilometers per second under quiet conditions — interacts with Earth's magnetosphere. The magnetosphere deflects most of this flow, but when the solar wind carries a southward-directed magnetic field (called a negative Bz component), it can partially merge with Earth's field in a process known as magnetic reconnection.

Reconnection channels particles down along magnetic field lines toward the polar regions. Those particles then collide with nitrogen and oxygen atoms and molecules at altitudes between approximately 100 and 300 kilometers. The collisions excite the atoms to higher energy states; when the atoms return to their ground states, they emit photons at specific wavelengths:

1. Green (557.7 nm) — oxygen atoms at altitudes of roughly 100–150 km; the most commonly observed aurora color.
2. Red (630 nm) — oxygen atoms at higher altitudes, above approximately 200 km, where collisions are less frequent and the excited state persists longer.
3. Blue and purple — nitrogen molecules, most visible at the lower edge of the aurora, below about 100 km.
4. Pink lower borders — a mix of nitrogen and oxygen emissions compressed near the base of a display.

The how it works framework for space weather follows this same particle-field-atmosphere chain, which is why aurora forecasting relies on real-time solar wind data rather than atmospheric weather models. NOAA's Space Weather Prediction Center publishes 3-day geomagnetic storm forecasts and Kp-index readings that aurora chasers treat the way sailors treat barometric pressure.

Common scenarios

Most aurora activity clusters around two predictable drivers.

Coronal mass ejections (CMEs) are large expulsions of plasma and magnetic field from the Sun's corona. A fast CME can reach Earth in as little as 15 hours, though 1–3 days is more typical. When a CME arrives with a strong southward Bz, it drives the most dramatic displays — the kind that reach mid-latitudes and produce reds and purples alongside the usual green.

Coronal holes produce a steadier, lower-intensity enhancement. When Earth's field line connects to an open coronal hole on the Sun, a high-speed solar wind stream — often reaching 600–800 km/s — produces recurring geomagnetic activity that can repeat on roughly 27-day solar rotation cycles.

The aurora australis behaves identically in mechanism but differs sharply in human accessibility. The southern auroral oval sits over Antarctica and the Southern Ocean, leaving only the southern tips of South America (particularly Tierra del Fuego), New Zealand's South Island, and Tasmania within reach during moderate activity. For a side-by-side comparison with other atmospheric light phenomena, the astronomy frequently asked questions page addresses common points of confusion between aurora, airglow, and noctilucent clouds.

Decision boundaries

Whether a given night produces a viewable aurora depends on four interacting factors, and understanding which ones are negotiable separates patient observers from perpetually disappointed ones.

Geomagnetic activity (Kp index): The Kp index runs from 0 to 9. At Kp 5 (the threshold for a geomagnetic storm), auroras become reliably visible from roughly 55° geomagnetic latitude — southern Alaska, northern Canada, northern Scandinavia. Kp 7 pushes visibility to about 50°, covering Seattle and London. Kp 9 can bring aurora to 40° or below.

Light pollution and horizon clarity: Even a strong aurora can be drowned out by urban skyglow. A site with a Bortle scale rating of 4 or better — meaning a dark rural sky — makes the difference between seeing structure and seeing a vague green tint.

Moon phase: A full moon near the horizon suppresses faint auroral curtains. New moon windows within a predicted storm period are the optimal overlap.

Local time: Auroral activity peaks around magnetic midnight, when the observer is on the nightside of Earth directly beneath the auroral oval. This is typically 1–3 hours after geographic midnight depending on longitude.

The interaction between these factors is why two observers 100 kilometers apart can have completely different experiences on the same night. Those interested in the broader context of how Earth fits into the solar system's dynamic environment will find the key dimensions and scopes of astronomy reference useful for situating auroral science within heliospheric physics. For practical observing strategy and equipment considerations, the how to get help for astronomy page covers resources available to both beginners and experienced skywatchers.

References

• Chandra X-ray Center, Harvard-Smithsonian
• Harvard-Smithsonian Center for Astrophysics, Multiple Star Catalog context
• LASP / University of Colorado, SORCE mission data
• LIGO Scientific Collaboration
• LIGO Scientific Collaboration, 2017 announcement
• LIGO Scientific Collaboration, Technical Overview
• MAST