| Phase | Start | Peak |
|---|
A magnetospheric substorm is a burst of energy released from Earth's magnetotail. When the solar wind's magnetic field turns southward (negative Bz), energy builds up in the magnetosphere. When it releases, it drives currents in the ionosphere that produce the aurora.
The substorm cycle has three phases:
The AL index (Auroral Lower) is the most important single number for aurora watchers. It measures the strength of electrical currents flowing in the ionosphere during a substorm.
It's calculated by taking the minimum horizontal magnetic field perturbation across a ring of magnetometer stations at auroral latitudes. When a substorm fires, the westward electrojet current intensifies, pulling the magnetic field negative at stations below it.
This standalone page shows Bz (the driver) because it can fetch NOAA data directly from your browser. The full Node-RED version at /aurora.html shows the real AL index calculated from 5 USGS magnetometer stations (College AK, Barrow AK, Sitka AK, Newport WA, Fredericksburg VA).
| Source | Data | Update | Lag |
|---|---|---|---|
| NOAA SWPC | Bz, Bt (solar wind from DSCOVR at L1) | 1 min | ~5 min |
| NOAA SWPC | Kp index (estimated, 1-min) | 1 min | ~2 min |
| NOAA SWPC | Hemispheric Power (POES satellites) | 5 min | ~10 min |
| USGS Geomag | AL index (ground magnetometers) | 1 min | ~2 min |
New Zealand sits at ~45-47° south geomagnetic latitude, which is well equatorward of the typical auroral oval. For aurora to be visible from NZ, you generally need:
| Feature | This Page | Glendale |
|---|---|---|
| Headline number | Bz (solar wind driver) | Substorm strength (AL-like index) |
| Substorm index | Available on /aurora.html (USGS magnetometers) | Proprietary calculation from global magnetometers |
| Phase detection | Based on Bz + Kp thresholds | Based on magnetometer AL index |
| Update rate | Every 60 seconds | Every ~1 minute |
| Data source | NOAA SWPC (free, public APIs) | Proprietary (likely SuperMAG or similar) |
The Bz and Bt values between this page and Glendale match closely (within ~0.2 nT), confirming the same upstream NOAA data. The substorm index differs because Glendale uses a proprietary magnetometer-based calculation, while we derive ours from USGS ground stations.