How to read NOAA's OVATION aurora map

What OVATION is

OVATION stands for Oval Variation, Assessment, Tracking, Intensity, and Online Nowcasting. Patrick Newell and co-workers at the Johns Hopkins University Applied Physics Laboratory developed it as an empirical model of the intensity of the aurora. NOAA made a real-time version in 2011, and the current version of the OVATION Prime model went into NOAA operations in 2020.

Aurorix reads the product NOAA calls "Aurora - 30 Minute Forecast": a global grid of 1 degree by 1 degree cells, each with a value for aurora intensity. The map on the Aurorix home page draws the northern hemisphere part of that grid on a polar projection.

Why the forecast is only 30 to 90 minutes

The model takes the solar wind speed and the magnetic field carried by the solar wind from a satellite at the L1 point, about 1.6 million km upstream from Earth. The wind needs time to cover that distance. At 800 km/s the trip takes about 33 minutes; at 400 km/s about 67 minutes; at 300 km/s about 89 minutes. That is NOAA's lead time, and it explains the range.

Sometimes the solar wind data are contaminated or missing. NOAA then drives the model from the current Kp instead, and the forecast has no lead time at all.

What the value means

NOAA says that an aurora viewing probability can be derived by assuming a linear relationship with the modelled intensity. Aurorix therefore shows the grid value as a percentage. A value of 10 means roughly a 10 percent modelled chance of seeing aurora in that cell's sky at that moment; 70 means very likely.

The map colours are our own steps, not NOAA's: cells from 5 up are drawn, with brighter greens at 10, 20, 40 and 70. Cells below 5 are left empty so that noise does not look like a forecast.

What the grid cell is

A one-degree cell is about 111 km tall. Its width shrinks toward the pole: near 69 degrees north one degree of longitude is about 40 km, and near 62 degrees north about 52 km. Aurorix reads the cell nearest to the place centre, so the value represents the neighbourhood and not an exact street.

What the map does not know

• Daylight. The model has no sun. The oval can be bright over a sunlit sky.
• Cloud. Overcast hides any value. Use the cloud forecast next to it.
• Distance. NOAA notes aurora does not need to be directly overhead and can be seen from as much as 1,000 km away when it is bright and conditions are right. A low value at your place with a bright oval to the north can still mean a glow on the horizon.
• Local light and terrain. Street lights, the moon and mountains decide what you actually see.

Quellen

• NOAA SWPC: Aurora - 30 Minute Forecast
• NOAA SWPC: Tips on viewing the aurora