Northern lights

The Northern lights tool tells you whether a polar aurora can be seen or photographed from your place, at what time of the night and in which direction, and which settings to use. It also works in the southern hemisphere, for the southern lights. It has three tabs: Night, Session and Forecast.

What is a polar aurora?

The Sun constantly sends out a stream of particles, the solar wind. When these particles reach the Earth, its magnetic field guides them toward the polar regions, where they excite the gases of the upper atmosphere, which start to glow: green and red for oxygen, blue and purple for nitrogen.

Geomagnetic activity is measured with the Kp index, from 0 (quiet) to 9 (extreme storm). The higher it is, the further the auroral oval spreads from the poles. What matters is not your geographic latitude but your geomagnetic latitude, measured from the magnetic pole.

Place and date

Place: by default, the phone’s GPS position. Tap it to search an address or enter coordinates.
Date: the arrows move to the previous or next night, Today goes back to the current night.
Explore the night: a strip coloured by the light of the sky, with bands showing when the Moon is up and when the aurora is within the camera’s reach.

Night tab

Aurora tonight: a verdict sums up your chances (overhead, visible to the eye, camera, low on horizon, unlikely, night too bright or no forecast), with:

your geomagnetic latitude, the Kp needed here for a photo and for the naked eye, and the direction to look in (toward the geomagnetic pole);
the forecast activity, the darkness of the night and the Moon, which brightens the sky.

The aurora window is the time when it is dark enough (Sun more than 12° below the horizon) and the forecast Kp reaches your place’s threshold. Show moves the strip to that time, Plan opens the Planner and the bell sets a reminder.

Geomagnetic activity (Kp): a chart of the planetary Kp published by NOAA (SWPC), measured then forecast in 3-hour periods. The dashed lines show the Kp needed here for a photo and to the naked eye; the dark background marks the night. Tap a bar to move the strip to that time.

Clear sky in the coming nights? Once the option is on, a forecast gives the cloud cover, fog and rain during the aurora window. It is indicative and needs a connection.

Moments of the night: sunset, start of sufficient darkness, moonrise and moonset, magnetic midnight (usual activity peak, give or take two hours), end of darkness, sunrise.

Session tab

Choose the expected aurora intensity, from very faint to intense, and how fast the curtains move: slow, active or fast.
Set the focal length, aperture and ISO.
The app gives the shutter speed needed and checks two limits: pinpoint stars (NPF rule) and sharp curtains, which call for a shorter exposure when the aurora moves fast. If the exposure is too long, it raises the ISO and warns you when they get very high: open the aperture or choose a shorter focal length.

When the exposure is longer than one second, a floating countdown button times it.

Forecast tab

Your location: geomagnetic latitude, viewing direction and hemisphere.
What each Kp gives here: for each Kp value, what you can hope for (unlikely, camera, naked eye, overhead).
Coming nights: the strongest Kp during each night’s dark hours. The first three days come from the 3-hour forecast, the rest from the 27-day outlook (one maximum per day, indicative, marked ≈). Tap a night to open it in the Night tab.
Alert me on aurora evenings: a notification in the evening of each night when the forecast Kp is enough here, for the next 27 nights (renewed each time you open the tool). Weather is not included: check the sky in the Night tab.

Geomagnetic activity data needs a connection.

Shooting tips

Where and when: away from lights, with a clear horizon in the direction shown. Long winter nights give more chances; activity often peaks around magnetic midnight.
Equipment: a sturdy tripod, a remote release (remote control or self-timer) and a fast wide-angle lens. Bring spare batteries kept warm: the cold drains them quickly.
Settings: manual focus on a bright star, white balance around 3,500 to 4,000 K. Shorten the exposure when the curtains move fast so they stay sharp.
Composition: a foreground (mountain, lake, house) gives the scale; calm water reflects the aurora.