Aurora watch

Bz is the gatekeeper

The solar wind's magnetic field has to point south to connect with Earth's field and let energy in. Northward Bz, and almost nothing happens no matter how fast the wind is blowing. Rough guide to the southward numbers:

• 0 to −5 nT — quiet. Not worth a cold drive.
• −5 to −10 nT — worth watching, especially if it holds.
• −10 to −20 nT — promising. Southern horizon glow likely from the deep south.
• below −20 nT — rare and serious. Drop what you're doing.

Read Bz against the envelope, not on its own

Bt sets the ceiling. Bz of −5 inside an envelope of ±5 means the field is fully south but feeble. Bz of −15 inside an envelope of ±25 is a strong field that has turned hard south — far more productive. The gap between the Bz line and the bottom of the envelope tells you how much more there is to give if the field rotates further.

Duration beats peak

A single southward spike does little. The magnetotail needs loading, so roughly thirty to sixty minutes of sustained southward Bz is what builds toward a display. Watch for the line sitting below zero, not dipping below it.

Speed and density set the intensity

• Speed near 300–400 km/s is ordinary; 500–600 is elevated; 700+ is a strong stream.
• A density spike and a speed jump arriving together, with Bt jumping too, is a shock front — the leading edge of a coronal mass ejection. That's the classic setup.
• High density alone, with Bz north, mostly just compresses the magnetosphere without producing much.

GOES tells you when it actually lands

The wind traces are a forecast; GOES is the confirmation. Watch Hp on GOES-18 sag over an hour or two as the tail stretches and stores energy, then snap sharply upward. That snap is substorm onset, and the visible display usually brightens within minutes of it. If the snap comes close to where your shifted Bz predicted the front, the propagation was good.

Then there's the sky itself

• Local magnetic midnight is the most active window, and for New Zealand that falls a little before clock midnight.
• You need a genuinely dark southern horizon — no town glow, no moon if you can help it.
• Southern aurora is often faint to the naked eye while showing clearly on a long exposure. A test frame of a few seconds at high ISO will reveal a glow you can't yet see.
• Give it time. Displays pulse and fade over hours, and the substorm that matters may be the third one, not the first.

None of this is a guarantee — it's a probability read off upstream measurements, and the magnetosphere gets a vote. The panel tells you when the odds are worth the drive.