How long do northern lights last?

Substorms vs sustained displays

A substorm is a short burst of auroral activity driven by a sudden release of stored magnetic energy in Earth's magnetotail. Substorms last 15-45 minutes and produce bright, fast-moving aurora. They can occur without a large Kp reading - a Kp 3 night might produce one or two substorms. The display intensifies, fades, and leaves behind a dimmer diffuse glow.

A sustained display occurs during elevated, continuous Kp - typically Kp 5 or above driven by a coronal mass ejection (CME) or prolonged high-speed solar wind. Aurora remains active for hours, with periods of higher intensity interspersed with quieter intervals. During a G3 storm, activity can persist across the whole night.

Typical duration at different Kp levels

These estimates apply from Scotland, Iceland, or Norway at Bortle 3 or lower. Mid-latitude observers need higher Kp to see anything and will see shorter windows.

The best time window

10 pm to 2 am local solar time is the strongest period for auroral activity at any longitude. This corresponds approximately to magnetic midnight - the point when your location is on the nightside of Earth most directly facing the auroral oval.

During a substorm, the strongest phase often lasts only minutes. The aurora brightens, forms distinct structures, and then fades. If you arrive at 11 pm and the substorm peaked at 10:40 pm, you will see the quiet aftermath. Do not leave after 20 minutes on site - you are more likely in between substorms than past the only one of the night.

What dimmer phases look like

Between active bursts, the aurora often settles to a diffuse low-arc glow on the northern horizon. This is still aurora, just in a quiet phase. The sky directly overhead may show faint structure if you look carefully.

At Kp 3 from Scotland, this quiet phase can be almost invisible to the naked eye but shows clearly in a 10-second camera exposure. Pointing your camera north and taking a test shot is the most reliable way to confirm whether activity is present. Give it at least 90 minutes before concluding nothing will happen.

Does Kp level predict duration?

Partially. A high Kp reading (5+) driven by a CME usually means elevated activity for hours, because the solar wind disturbance driving it takes time to pass Earth. A brief spike driven by a co-rotating interaction region (CIR) may produce a shorter, sharper burst.

The 7-day forecast on this site shows the predicted Kp trajectory, which gives an indication of whether activity will be sustained or brief. A Kp 6 reading that is expected to hold for 12 hours is a stronger signal than a short spike to Kp 6 that drops back to Kp 3 within 3 hours.