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.
Kp 3-4 (quiet to moderate)
Expect 1-3 substorms of 15-40 minutes each, with dim diffuse aurora between. Total active time: 30-90 minutes across a 4-hour watch window. Easy to miss if you go out late or leave early.
Kp 5-6 (G1-G2 storm)
Elevated activity for 2-6 hours, with stronger bursts within. Most displays stay active for the evening and may continue through the night. Clear skies for several hours give a good chance of catching a bright episode.
Kp 7+ (G3+ storm)
Aurora can persist for 8-12 hours or more. Multiple intense episodes separated by periods of activity at the horizon. May be visible from sunset to sunrise at high latitudes. These are the nights worth staying out for in full.
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.
