How to read an aurora forecast: Kp, Bz, solar wind and the OVATION map

What does Kp mean in an aurora forecast?

Kp is a 0 to 9 index of how disturbed Earth’s magnetic field is over three hours. The higher it goes, the further the aurora reaches toward the equator: about 227 km per step of Kp.

Kp 5 and up is a geomagnetic storm (G1, rising to G5 at Kp 9). Kp is global and three-hourly, so it lags fast changes; GFZ Potsdam’s Hp30 is the same idea every half hour and shows a substorm sooner. Whether a Kp is enough depends on your place: what Kp you need from your spot.

Why does Bz matter for the aurora?

Bz is the north-south part of the solar wind’s magnetic field. When it points south (negative) for 20 minutes or more, energy pours into Earth’s field and the aurora brightens; when it points north, even a fast wind does little.

Bz around −10 nT or lower, held for a while, is the classic sign of a good night. Values flip within minutes, so treat one reading as a hint and a steady southward stretch as the signal. Coupling functions such as Newell’s combine speed, Bz and By into one number for how much energy gets in; Sunreach shows it as a multiple of an ordinary hour.

What do solar wind speed and density tell you?

Speed sets how hard the wind pushes and how soon it arrives: at 400 km/s the air measured at the L1 spacecraft reaches Earth in about an hour, at 800 km/s in about half that. Density adds pressure; a sudden jump in both is a CME or a fast stream arriving.

• Slow (under 450 km/s): quiet unless Bz is strongly south.
• Fast (500–700 km/s): coronal-hole streams, often Kp 4–5 for a day or two.
• Very fast with a density spike: a CME shock; the strongest storms come when its field also turns south.

What is the OVATION aurora map?

NOAA’s OVATION model turns the last hour of solar wind into the chance of aurora overhead on a 1° grid, 30 to 90 minutes ahead. Green is a small chance, yellow and red a strong one.

The map shows the aurora overhead, but you can see it from several hundred kilometres further toward the equator, low on the horizon toward the pole, and a camera sees more than the eye (about 1.5 Kp’s worth). Sunreach draws the eye and camera lines on its globe and checks your skyline toward the pole, so you know whether the glow on the map is above or behind your ridge.

Which aurora forecast should you trust, and when?

Each one for its own time scale: the 27-day outlook and the 3-day Kp forecast for a trip; the Kp forecast over your dark hours and any CME on its way for tonight; the solar wind (Bz and speed) for the next hour; the OVATION map and Hp30 for right now.

Forecasts a day or more ahead often miss by a Kp or two, and a CME’s strength only shows when it passes the spacecraft. Sunreach keeps the forecasts it saw and compares them with what came, puts the oval’s turn through the night into the need (why the aurora is strongest near magnetic midnight), and adds cloud from Apple Weather and the Moon. Read more about the tool on the aurora forecast in Sunreach.