Explaining how the Kp index relates to geomagnetic activity and the Northern Lights
The Kp-index is used to characterise and describe the magnitude of geomagnetic disturbance caused by solar wind interacting with Earth’s magnetic field on a scale from 0-9. The Kp index does not describe how strong or bright the Northern Lights will appear and is therefore not very useful for aurora chasers in the field.
The Kp value is determined by averaging global magnetic measurements over three hour intervals, from 09-12, 12-15, 15-18 etc. Data collected by Earth-based magnetic observatories are then used to make predictions based on the days and weeks past.
Each value in the index is as following:
0 Quiet
1 Quiet
2 Unsettled
3 Unsettled
4 Active
5 Minor storm (G1)
6 Moderate storm (G2)
7 Strong storm (G3)
8 Severe storm (G4)
9 Extreme storm (G5)
The letter K is from the German word Kennziffer which means "characteristic digit" while p stands for planetary. The scale was developed in 1949 by Julius Bartelsand.
Honestly, the Kp-index is not the most useful indicator for aurora chasers.
At best, the predicted Kp-value can give you a rough idea of what to expect, and so it can be handy for planning.
If the predicted Kp-value for a given night is 1, it is unlikely you’ll see a colourful and dynamic aurora. However, if the predicted Kp-value is 5, odds are you'll see a beautiful display. This is a bit more complicated though.
Auroras occur around the world the aurora oval. Regional substorms, which are flare-ups within this oval, cannot be captured by a global Kp value.
When you are outside chasing auroras, it is way more useful to look at the real time space weather data, the speed and density of the solar wind (high is good), the direction and value of it’s magnetic field (Bz south is good) and the local magnetic disturbance at Leirvogur Magnetic Observatory (more shaking is better).
An a quiet night, a sudden gust of solar wind can lead to a lovely display seemingly out of nowhere. For 10-30 minutes on average, the activity might increase such that beautiful Northern Lights can be seen. At that particular moment, the measured local K-value is a lot higher than the resulting global Kp-average of the same three hour interval. It’s like measuring the average wind speed between three hours.
A sudden gust of wind doesn’t necessarily have a large effect on the average. This is especially true in spring and autumn when even slow moving solar wind is enough to spark beautiful aurora, even though the Kp value says the night is quiet. The image on the left shows a lovely burst of aurora activity when the measured Kp value only reached 1.
So please, don't focus too much on the Kp-index. In fact, ditch it.
What to use then instead? Check the real time space weather data instead. It's a much better indicator of what to expect for the next hour or so.
