Data Sources & Credits ↓
The AL Index: The Headline Number
The AL index (Auroral Lower) is the single most important number for aurora watchers. It directly measures the westward electrojet, the electrical current flowing in the ionosphere during a substorm. The more negative the value, the stronger the substorm and aurora.
How to read it: AL near zero = quiet. The more negative, the stronger the aurora.
Quiet: > -50 nT | Minor substorm: -50 to -200 nT | Moderate: -200 to -500 nT | Strong: < -500 nT | Major: < -1000 nT
Unlike Bz (which shows what's driving activity from the solar wind), AL shows what's actually happening at Earth. A substorm can fire even after Bz recovers northward.
This page calculates an AL proxy in real-time from 7 USGS magnetometer stations at auroral latitudes. The AL value is the minimum delta-H (horizontal field perturbation) across all stations at each timestamp. Updates every 60 seconds with ~2 minute lag.
USGS Auroral Zone Stations (AL Calculation)
The AL index is derived from stations positioned under the northern auroral oval. When a substorm fires, the westward electrojet intensifies, pulling H negative at whichever station sits beneath it.
Barrow (BRW)
Northernmost US station, deep in the auroral zone.
71.3°N · Alaska
Deadhorse (DED)
North Slope, under the auroral oval.
70.3°N · Alaska
College (CMO)
Near Fairbanks, key auroral latitude station.
64.9°N · Alaska
Sitka (SIT)
Southeast Alaska, catches equatorward expansion.
57.1°N · Alaska
Shumagin (SHU)
Aleutian Islands, sub-auroral coverage.
54.9°N · Alaska
Newport (NEW)
Pacific Northwest, mid-latitude reference.
48.3°N · Washington
Fredericksburg (FRD)
East coast reference, responds only during major storms.
38.2°N · Virginia
Limitations: The official AL index uses 12 stations distributed around the auroral oval. Our proxy uses 7 USGS stations concentrated in North America. This means we may miss substorms centered over Scandinavia or Siberia. If SuperMAG (300+ stations) comes back online, it would replace this calculation with the far more accurate SML index.
What is a Substorm?
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 four phases:
Quiet
No significant geomagnetic activity. AL near zero, Bz neutral or northward.
Growth
Bz has turned southward. Energy is loading into the magnetotail. AL still near zero, the substorm hasn't fired yet.
Expansion
Substorm onset: AL drops sharply negative (< -200 nT). Aurora brightens and expands equatorward. This is when you look up!
Recovery
AL recovering toward zero. Aurora fading and retreating poleward. Can still be visible but weakening.
Key Metrics
AL Index (nT)
The headline number. Minimum H-component perturbation across 7 auroral stations. Measures actual substorm strength in real-time.
Quiet: > -50 | Minor: -50 to -200 | Moderate: -200 to -500 | Strong: < -500
IMF Bz (nT)
North-south component of the solar wind magnetic field. Negative Bz = substorm fuel. This is the cause; AL is the effect.
Quiet: > -2 | Active: -5 to -15 | Strong: < -15
Kp Index
Global 3-hour geomagnetic activity index (0-9). NZ needs Kp 5+ for visible aurora. Slower to respond than AL.
Quiet: 0-2 | Unsettled: 3-4 | Storm: 5+ | Major: 7+
South HP (GW)
Hemispheric Power: total energy deposited by particles into the southern atmosphere, measured by POES satellites.
Quiet: < 20 GW | Active: 20-50 GW | Storm: > 50 GW
Solar Wind & Propagation
θ (Clock Angle)
Direction of the solar wind magnetic field. >120° = geoeffective southward coupling.
0-60° safe | 60-120° moderate | 120-180° geoeffective
Speed (km/s)
Solar wind bulk flow speed. Determines L1-to-Earth propagation delay and energy transfer.
Slow: < 400 | Normal: 400-500 | Fast: > 500 | Very fast: > 700
Density (p/cm³)
Proton number density. With speed, determines dynamic pressure on the magnetosphere.
Low: < 5 | Normal: 5-10 | High: > 15
Bt (Total Field)
Total solar wind magnetic field strength. Bz is a component of Bt.
Typical: 2-8 nT | Strong: > 15 nT
Propagation delay: DSCOVR sits at L1, ~1.5M km upstream. Delay = 1,500,000 / speed / 60.
At 400 km/s: ~63 min. At 600 km/s: ~42 min. At 800 km/s: ~31 min.
The propagation table shows three windows: Inbound (hasn't reached Earth yet), Arriving (hitting the magnetosphere now), and Prior (already passed).
NZ Ground Magnetometers (MANA)
The MANA array is a network of 5 magnetometer stations operated by the University of Otago, spread across New Zealand from Southland to Northland. This is unique ground-truth data showing what the geomagnetic field is actually doing directly beneath our feet.
The chart shows delta-H (deviation from a quiet baseline) for each station. The baseline is calculated as the average of the first 30 minutes of the displayed window. Negative delta-H values indicate geomagnetic disturbance caused by ionospheric currents driven by substorm activity.
How to read the chart: When lines drop below zero, the ground magnetic field is being disturbed. The further negative, the stronger the substorm currents overhead. Southern stations (Awarua, Swampy) typically respond first and strongest during aurora visible from NZ.
dH/min (rate of change) shows how rapidly the field is changing. Values above 5-10 nT/min indicate active geomagnetic conditions.
The NZ Disturbance index (max |delta-H| across all stations) feeds into the Aurora Chance calculation. Note: NZ stations typically show only minor perturbations even when aurora is clearly visible on the southern horizon. Strong negative readings here indicate an extreme event with the auroral oval directly overhead, which is rare at NZ latitudes.
Awarua (Southland)
Southernmost station. First to detect aurora-related disturbances in NZ.
46.5°S
Swampy (Dunedin)
Near Dunedin, second from south.
45.8°S
Eyrewell (Canterbury)
Canterbury plains, mid-South Island.
43.4°S
Oakview (Waikato)
North Island central station.
37.8°S
Donnelly (Northland)
Northernmost station. Activity here means a major event.
35.3°S
Southern Hemisphere Magnetometers (INTERMAGNET)
The INTERMAGNET network is the global standard for ground-based magnetometer data, operated by geological surveys worldwide. We monitor four southern hemisphere stations to track geomagnetic disturbances closer to the auroral oval.
Like the MANA chart, this shows delta-H (deviation from quiet baseline). Stations at higher geomagnetic latitudes (MCQ, MAW, CSY) respond first and strongest during substorms. Data is served via the British Geological Survey (BGS) GIN.
Macquarie Island (MCQ)
Sub-Antarctic island, key auroral zone station.
54.5°S · ~1 min lag
Casey (CSY)
Australian Antarctic station, deep south.
66.3°S · ~3 min lag
Canberra (CNB)
Mid-latitude reference, similar to NZ.
35.3°S · ~0 min lag
Mawson (MAW)
Deep Antarctic, directly under auroral oval.
67.6°S · ~3 min lag
Data Sources
| Source | Data | Update | Lag |
|---|
| NOAA SWPC | Bz, By, Bt (solar wind mag from DSCOVR at L1) | 1 min | ~5 min |
| NOAA SWPC | Solar wind plasma (density, speed, temperature) | 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 |
| U. of Otago | MANA array (5 NZ ground magnetometers, H + dH) | 1 min | ~1 min |
| INTERMAGNET | Southern hemisphere magnetometers (MCQ, CSY, CNB, MAW) | 1 min | 1-3 min |
Aurora Visibility from NZ
New Zealand sits at ~45-47° south geomagnetic latitude, well equatorward of the typical auroral oval. You see aurora by looking toward the expanded oval to the south, not by being under it. This means:
What drives NZ visibility:
- Global AL is the primary indicator (substorm intensity at the source)
- Kp 5+ expands the auroral oval far enough south to see from NZ
- Bz < -5 nT sustained keeps the magnetosphere open
- South HP > 50 GW means significant energy input
NZ AL (MANA magnetometers) typically stays near zero even during visible aurora events. It only goes strongly negative during extreme storms (Kp 8-9) when the oval reaches NZ latitude. A positive NZ AL does NOT mean no aurora is visible.
Camera detection: A live night-sky camera in Nelson monitors for aurora in real-time. When active, the Aurora Score feeds directly into the likelihood calculation, providing ground-truth confirmation of NZ-visible aurora.
Typical minimum conditions: Kp ≥ 5, Global AL ≤ -300 nT. Good display: Kp ≥ 6, AL ≤ -500. Clear skies, dark site, low southern horizon required.
Credits & Data Sources
This monitor uses 16 ground magnetometer stations across 3 networks, plus satellite-based solar wind data, all from free public APIs. Updates every 60 seconds.
| Source | Data | Lag | Link |
|---|
| NOAA SWPC | IMF Bz/By/Bt, Kp, Hemispheric Power, solar wind plasma (DSCOVR at L1) | ~5 min | swpc.noaa.gov |
| USGS Geomag | 7 auroral zone magnetometers (CMO, BRW, DED, SIT, SHU, NEW, FRD) for Global AL | ~2 min | geomag.usgs.gov |
MANA Array
(U. of Otago) | 5 NZ ground magnetometers (Awarua, Swampy, Eyrewell, Oakview, Donnelly) for NZ AL | ~1 min | solartsunamis.otago.ac.nz |
INTERMAGNET
(BGS GIN) | 4 southern hemisphere magnetometers (MCQ, CSY, CNB, MAW) for Southern AL | 1-3 min | imag-data.bgs.ac.uk |
How our AL compares to other apps: Leading aurora apps typically use proprietary indices derived from 300+ stations (e.g. SuperMAG SML). Our Global AL uses 7 USGS stations focused on North America, which may miss substorms centred over Scandinavia or Siberia. The NZ AL and Southern AL are unique to this page, calculated from stations in and around NZ and the southern auroral zone.