When you’re trying to catch the Northern Lights for the first time, it’s natural to wonder when you can see them and where. The answer to these questions, it turns out, is not so easy. Even our best indicators in Aurora forecasting come with strengths and weaknesses.

Enter the K-index.

In simple terms, the K-index is a measure of Aurora strength on a scale of 0 to 9, with K9 being the highest possible strength. When given as Kp, this denotes a planetary average of the K value over a span of three hours.

The K/Kp values also correspond to geomagnetic storm conditions. When the K-index reaches a value of 5, for example, it is synonymous with a G1 Storm, or a minor geomagnetic storm.

Be careful how you use the K-index. Seasoned Aurora Chasers know that this indicator works best when compared with other types of information to gauge the possibility of Northern Lights. In fact, many Aurora groups and amateur forecasters avoid using the K-index altogether, because they don’t view the predicted Kp as a solid indication that Northern Lights will appear.

So, what is a solid indicator? That’s a topic for another day, because many conditions have to align in order for us to see the Northern Lights. A good discussion might involve the Bz, Bt, Speed, Density, and Hemispheric Power, or a real-time forecasting model, at the very least.

Let’s stick to Kp for now.

An Aurora Chaser’s guide to understanding Kp

Keep in mind, there are many ways Kp can help you on the chase. The trick is to learn the meanings, know what to look for in your region, and take K-index measures with a grain of salt.

It’s the gold standard. The K-index is the standard used in forecasts by NOAA Space Weather Prediction Center and other government agencies. It’s what they use to alert subscribers in both their 3-day forecasts and the 27-day outlook, which makes it a valuable prediction tool.

It always refers to the same places on the map. The way K/Kp forecasts are depicted, the values correspond to a location on the map, designated by latitude. This relationship stays the same. So, if you learn that you typically need a Kp4 to view Aurora at your latitude, you will always look for Kp4 or stronger.

It’s a forecasting tool. Also known as space weather, Aurora is a difficult phenomenon to predict. Forecasting space weather is not unlike forecasting the weather here on Earth. Potential impacts can move in a different direction and conditions can change quickly, rendering some of the advice from forecasters irrelevant.

I would recommend reading such forecasts the same way you might read an iffy Earth weather forecast — hope for the best, but be prepared for the worst. We never know which way the space weather will turn!

It’s data gathered from magnetometers across the globe. To calculate the K-index, scientists use measurements from magnetometers around the world, like those in Boulder, Colorado, or Kiruna, Sweden. They generate the K value on an index that’s based on fluctuations in the geomagnetic field detected in near real-time. Geomagnetic activity creates Aurora activity, so this is key!

It’s an average. When referred to as the Kp, the K-index gives a planetary “p” average of geomagnetic activity, and it indicates the average of the activity that occurred over a span of three hours.

It sometimes refers to the past. Aurora forecasts and Kp alerts are distributed based on activity that happens in three-hour increments of Universal Coordinated Time. That means when you check the Kp, you could very well be seeing the average Aurora strength from three hours in the past. While Kp is useful for understanding the trends of the Aurora on any given night, it does not necessarily indicate future activity.

It may change. A space weather forecast is just that: a forecast. Conditions may change unexpectedly, not unlike the way weather in our atmosphere often takes unexpected turns for the better or worse. The NOAA Space Weather Prediction Center issues many Watches for potential activity, but they also issue stronger Warnings for conditions they are able to detect. That said, a forecast of K/Kp can change at any time.

It’s often an over-prediction. The reason Aurora forecasts exist is to protect systems and infrastructure from powerful pulses and impacts coming from outside the Earth’s atmosphere. Strong impacts can cause drag on satellites, power grid fluctuations, false alarm triggering, and transformer damage, to name a few. The agencies that predict this type of activity try to warn against the strongest possible conditions at any given time. That means a 3-day forecast could be exaggerated, and we could see much lower activity than what was forecast.

It’s available on many apps, which might delay or lag. One of the reasons Kp is so popular is because it is widely distributed on many apps, with the option to have your phone or mobile device send alerts when Kp reaches a certain level. A potential pitfall is that apps need to be maintained and updated too, and they are drawing information from other sources before relaying that information to you. So, there may be delays or a lag in receiving this information. That’s why I often recommend getting your information from a direct source, like the NOAA Space Weather Prediction Center.

It’s not an exact science. Just like a weather forecaster might not be able to tell you that it is going to start raining exactly at 11:03 p.m. Eastern, it’s difficult for forecasters to pinpoint the exact time that Aurora may appear. The NOAA Space Weather Prediction Center issues their best Aurora forecasts 2-3 days in advance of a potential display.

The timing of these forecasts is given in three-hour increments of Universal Coordinated Time, but that doesn’t mean the Northern Lights will begin appearing at exactly 03:00 UTC. On a good day, it could! More than likely, though, the timestamps will mark a window of time when active conditions begin to build, and windows of time when the Aurora become extremely active.

That’s why I recommend using forecast times more like a guide for when to be alert, not a schedule.

It’s arguably the wrong data to consider in real-time. While the K/Kp can be useful in Aurora forecasting, the best data comes in real-time. As darkness approaches, it’s often best to turn your attention to other indicators and look at other types of data. How much speed are we getting? How much density is it packing? Is the Bz (a component of the Interplanetary Magnetic Field) cooperating so that we can witness an Aurora display?

It’s not the only indicator of active conditions. There are many different methods and sources that can be used to gauge Aurora activity, and Kp is just one of them. In fact, when other conditions align, such as a high speed and density of the solar wind combined with a negative/southward Bz, you might catch Aurora right in your backyard — even with a low Kp.

In a decade of Aurora Chasing, I have witnessed times just like this. The forecasts and reported Kp came in very low, say Kp2, yet devoted photographers captured the Northern Lights in the mid-latitudes. You just never know!

Author’s Note: The goal of this blog is to give practical advice for viewing the Northern Lights to beginners and amateur space weather enthusiasts, using the simplest terms, common topics, and popular sources. I draw upon my experience as a journalist and an Aurora Chaser, though I do not have formal training in the field. Photo by Nelly Volkovich/Unsplash.