The Kp Index, and Why It Misleads You

Kp is the number everyone quotes and almost nobody explains. It gets treated as an aurora meter, as though 5 means a good night and 2 means stay in. It is not that, and using it that way will cost you nights.

Here is what it is, where it comes from, and the three properties that make it much less useful for planning an evening than its popularity suggests.

Julius Bartels introduced the local K index in 1938 at the Niemegk observatory near Potsdam, and the planetary version, Kp, in 1949. It has been calculated the same way ever since, which is a large part of its value: it is one of the longest consistent records in geophysics.

The method is to measure how much the horizontal component of the magnetic field wobbles at a set of observatories, compared with what a quiet day looks like at each of them, over a three-hour window. Those local values get standardised and combined into one planetary figure from thirteen observatories.

Kp runs from 0 to 9, in thirds. So the real scale is 0, 0+, 1-, 1, 1+, and so on, which is why you sometimes see values like 4- quoted. It is a measure of how disturbed earth's magnetic field has been, used as a proxy for how much energy the solar wind has been putting in.

A clear sky at Kp 2 beats a cloudy sky at Kp 6. Every time, without exception, because you cannot see through cloud.

This sounds obvious written down and yet the whole retail apparatus around the aurora is built the other way round. People refresh Kp apps, book on the strength of a forecast number, and stand under solid overcast. Our cloud statistics exist because that is the variable you can actually plan for, months ahead, and Kp is not.

There is a second reason not to fixate on Kp if you are going somewhere like Tromso. Sitting inside the auroral oval means the display can be overhead at Kp 1 or 2. High Kp values matter enormously if you are trying to see the aurora from Scotland or Germany, because they mean the oval has expanded far enough south to reach you. From inside the oval, a big number mostly means the show has moved to somewhere further south than you are.

If you want to actually use forecasts on the night, in rough order of usefulness:

1. The cloud forecast. Genuinely the most important input, and the one your guide will be using to decide where to drive.
2. Bz, the north-south orientation of the interplanetary magnetic field measured upstream. When it is pointing south, the field can connect with earth's and energy starts loading. This is a live measurement with roughly 15 to 60 minutes of lead time, and it is a far better short-term signal than Kp.
3. Solar wind speed and density, from the same satellites. Faster and denser is better.
4. The oval maps, which show where the ring is sitting right now rather than reducing everything to one integer.
5. Kp, as a general indication of whether the last few hours have been active. Useful context, poor trigger.

Because it has been computed identically since 1949, Kp is excellent for comparing across years and across solar cycles. Statements about this decade being more active than the last are usually built on Kp or its close relatives, and they are on firm ground.

It is a climate instrument that people have pressed into service as a weather instrument. That is the whole misunderstanding in one line.

Alongside Kp you will see NOAA's G scale, running from G1 to G5. It is not a competing measurement, it is a severity rating for geomagnetic storms, and it maps onto the top half of the Kp range.

The scale exists for infrastructure operators rather than for tourists: it describes the risk to power grids, satellites and radio, and the aurora is a side effect that happens to be visible.

Which is worth remembering when an app pushes a G1 alert. From inside the auroral oval, G1 is a good evening rather than an event, and the same conditions that make a G1 exciting in Germany make it ordinary in Tromso. It is another version of the same mistake this page is about: a planetary number describing a local experience.

If you want to follow the same signals a good guide is following on the night, the useful sources are public and free.

The 30-minute aurora forecast. NOAA publishes a map of the oval driven by the OVATION model, fed by solar wind measured at the L1 point about 1.5 million kilometres upstream. The lead time is simply how long the wind takes to cover that distance, which is why it is 15 to 60 minutes rather than hours. This is a nowcast of where the aurora is, not a number describing where it has been.

Bz. The north-south orientation of the interplanetary magnetic field, from the same spacecraft. Sustained negative values mean the field can connect with earth's and energy is loading into the system. If you learn one number to replace Kp, learn this one.

The three-day forecast. NOAA's outlook for the coming days, useful for deciding which night of your trip to prioritise.

Your national met service. Cloud remains the deciding factor, and the Icelandic Met Office in particular publishes a combined aurora and cloud forecast, which is the correct pairing of the two variables.

Notice what these have in common: they are either live measurements with a short horizon, or they are cloud. Kp sits awkwardly between the two, describing the recent past of the whole planet, and that is the reason it disappoints people who use it as a booking signal.

• GFZ German Research Centre for Geosciences, which produces the official Kp index: kp.gfz.de and the index description. The index was introduced by Julius Bartels in 1949, following the local K index in 1938.
• Matzka, J. et al. (2021), The Geomagnetic Kp Index and Derived Indices of Geomagnetic Activity, Space Weather, doi 10.1029/2020SW002641 — the reference description of what the index is and how it is derived.
• NOAA Space Weather Prediction Center: planetary K-index, 30-minute aurora forecast, 3-day forecast and the NOAA space weather scales for the G ratings.
• Icelandic Met Office, for the combined aurora and cloud forecast: vedur.is.