Substorm Detection & Tracking

Substorms are the main event for aurora watchers. They're sudden releases of energy stored in Earth's magnetotail, and when one fires off, it dumps energetic particles into the upper atmosphere — producing bright, fast-moving, dynamic aurora that can last 30–60 minutes. Lumina tracks the whole substorm lifecycle in near-real-time.

What actually is a substorm?

Picture the magnetotail — the part of Earth's magnetic bubble that's stretched out behind the planet like a windsock — as a giant rubber band. When the IMF points southward, the solar wind's magnetic field connects to Earth's field on the dayside, then drags it around to the nightside. The rubber band stretches. Energy builds up.

Eventually the stretched field lines in the tail snap back together in a process called magnetic reconnection — the rubber band lets go. That's the substorm onset. The energy that was stored over tens of minutes gets released in a few minutes, accelerating particles along field lines down into the atmosphere. Result: aurora.

On the Space Weather Visualiser you can watch this happen in the animation — the plasma sheet in the tail thickens and brightens during the loading phase, then a flash at the tail X-point marks the substorm release.

How Lumina detects substorms

Lumina uses two independent data sources to build confidence that a substorm is actually happening:

1. GOES geosynchronous satellites

GOES satellites sit in geosynchronous orbit (~36,000 km up) and carry magnetometers. During a substorm, the magnetic field at GOES suddenly "dipolarises" — it snaps back from a stretched-out tail-like configuration to a more normal dipole shape. Lumina watches the Hp component (the one parallel to Earth's dipole axis) for these signatures.

2. Geoscience Australia ground stations

Three Antarctic and sub-Antarctic observatories feed real-time magnetometer data into Lumina:

• Macquarie Island (~54°S magnetic) — sub-auroral. When Macquarie Island shows a negative bay in the H-component, the substorm electrojet is active and aurora is likely overhead at sub-auroral latitudes. This is the most relevant station for Australian and NZ viewers.
• Casey (~66°S magnetic) — deep in the auroral oval. Direct, high-confidence substorm confirmation.
• Mawson (~68°S magnetic) — also deep in the oval. Provides multi-station agreement.

When two or more of these stations simultaneously show a negative ΔH deflection (the horizontal field component dipping below its quiet-day baseline), Lumina raises the substorm confidence level.

Why southern hemisphere stations matter

Most aurora apps rely on northern hemisphere magnetometers — stations in Canada, Scandinavia, and Alaska. Those tell you what's happening over the northern auroral oval. But the two hemispheres aren't mirror images.

Aurora in the north and south are broadly conjugate (they happen at the same time, driven by the same magnetospheric processes), but there are significant inter-hemispheric asymmetries :

• IMF B_y tilts the reconnection pattern. The east-west component of the IMF (B_y) introduces a dawn-dusk asymmetry in how the solar wind's field connects to Earth's. Under a given B_y polarity, magnetic reconnection is enhanced on one side of the magnetosphere in the north and the opposite side in the south. The result: a substorm can fire harder in one hemisphere than the other, or with a timing offset. Northern magnetometers might show a strong onset while the southern signature is delayed or weaker — or vice versa.
• Ionospheric and ground conductivity differs. When it's winter in the south, the southern ionosphere sits in darkness for longer — less sunlight means lower conductivity, which changes how the substorm electrojet closes its current loop. On top of that, the southern oceans provide very different ground conductivity than the northern continents. Both effects alter the magnetometer signal you'd measure at the surface.
• Earth's internal field is asymmetric. The geomagnetic field itself isn't a perfect dipole — it's stronger near the south magnetic pole than the north, and has features like the South Atlantic Anomaly that have no northern counterpart. This skews global substorm occurrence rates, the altitude of particle precipitation, and even the latitude where the oval sits for a given level of activity. A hemispheric-average forecast based on northern data can be off for southern watchers.

This is why Lumina goes straight to the source: Casey, Mawson, and Macquarie Island are measuring the actual southern hemisphere electrojet that drives the aurora you're trying to see. When Macquarie Island shows a negative bay, that's the substorm current system directly overhead at sub-auroral latitudes — it's not a proxy inferred from Canadian data. For Australian and New Zealand aurora watchers, that's a much more reliable signal than anything measured in the northern hemisphere.

The substorm lifecycle

Lumina classifies the current state into one of five levels:

Substorm vs geomagnetic storm

These are different things, and it's worth knowing the difference:

• A substorm is a localised, short-lived (30–60 min) release of energy in the magnetotail. It can happen multiple times in one night and produces bright, dynamic aurora.
• A geomagnetic storm is a global disturbance of Earth's magnetic field, usually triggered by a CME hitting the magnetosphere. Storms last hours to days and can drive repeated substorms.

You can have substorms without a storm (common during moderate activity) and storms that drive many substorms in succession (the really good nights). Lumina tracks both, and the substorm state feeds directly into the Field mode probability.

Reading the signal on the dashboard

In the Aurora Probability card's activity drivers panel, you'll find a substorm status row. It'll show the current alert level, which stations are active, and the detection confidence. When you see that row turn green with "Active" and high confidence from multiple stations — that's when you really want to be somewhere dark with a clear southern view.