How to read Aurora forecast
Many people have trouble reading the various factors that determine the outcome of the Northern lights. In this post, I will go over the main objects that I use to forecast the visibility of the Northern lights. I will explain each factor and try to do it with examples we can understand. I will start with what I believe to be the most significant factor.
I recommend a website auroraforecast.is or similar websites to examine solar wind speed-Interplanetary magnetic field-leirvogur magnetic observatory center and the sun observation. Ignore the KP forecast.
Solar wind strength. The strength of the solar wind tells me a lot of things about what I can expect on any given night. Let's say average Solar wind speed is 350 kms. That could give no lights to rather faint lights with a good short peak, given other factors that I will go into later will compensate the average solar wind speed. Substantial increase in the solar wind, for example. If the solar wind increases from 350 kms to, for example, 500 or more, I know a solar flair from a coronal hole will hit earth atmosphere or has already, and with solar wind above 500, I can almost always expect to see good lights at some point during dark hours. KP forecast, for example, could be forecasting a KP 1 for the night, but if I see the solar wind increase during the day, I know for sure the KP forecast will be wrong.
On multiple occasions, when I pick up people for northern lights tour, and they look in the beginning a bit hopeless as they have seen that the KP forecast is 1 and they don't expect anything, and I tell them the chances are actually good. Although I usually try to keep the expectation intact, as many things can go wrong. But in my mind, I am almost sure we will experience great northern lights. In the end, they will go on and tell their friends they saw excellent lights on a KP 1 night and sometimes write something nice about me in reviews. When, in fact, the real KP became higher due to the increase in the solar wind and KP possible became 4 or more. And then there is the other side of the dice, when the KP forecast is forecasting, for example, KP5 and newspapers are advertising a solar storm. Hotel employees have told my customers that a great night lays ahead with the northern lights illuminating the whole sky. But then if I see no increase in the solar wind, I know for sure that nothing like that will happen unless miraculously the solar wind stream hits precisely at the time of the tour. In fact, I hardly ever look even at the KP index forecast. It is much better to examine the solar wind.
2. Interplanetary magnetic field. The Interplanetary magnetic field or the IMF is a bit more complicated thing to explain but has a massive impact on what we will see during the night. Basically, what you want is the IMF to be negative south and not positive north. This will often change during the day and night and flip from minus to plus. If negative south the solar wind stream is being drawn into the magnetic field and a faint northern light bow will start to form, with time and if more solar wind stream is continuously adding to the magnetic line or aurora oval the northern lights will explode at some point and then fade out and recharge depending on how long the IMF is south. The stronger the solar wind, the better ofc, but even on average solar wind speed, if the IMF has been more on the negative side, we can expect to see good lights at some point during the dark. If the solar wind is above average and the IMF is mostly north, it will take a long time for the aurora oval or a bow looking aurora in the sky to form, so you could have to wait 3-4 hours and then boom; finally, you see them. So when I see the solar wind speed is above average, I know already we will see something, not necessarily a great show. Still, at least we have seen them and usually some movement too, then I look at the IMF. If I see it is negative, I know the lights are forming. If I see it positive, I know I have to wait. So seeing a faint bow with the camera even not visible with the naked eye, I need the IMF to be going negative, and in some-time 1-3 hours, I can expect to see decent lights. So IMF and Solar wind and how they react to each other are the 2 most significant factors, in my opinion, on what to expect on any given night.
3 Geomagnetic observation. It is a result of a Solar wind stream and IMF flipping hard south and hard north. if I see the Geomagnetic observation line is rattling during the day, I can be hopeful for the night as it is a result of the above factors being good. Geomagnetic observation is as well the main thing KP and K indexes are calculated from. The geomagnetic line still can look flatlined during the day and then drop during dark hours, which is the most common thing. That will result in the K index, which is a local geomagnetic index, meaning KP is a global scale and K a local scale. K INDEX 0 during the day that could become 2 or 3 during the night when the line drops. Resulting in a decent outburst of the lights over a short time. This is the most common situation and an average night, nothing like vivid photographs but still beautiful, especially on peaks and if they start to move. So why do people not just monitor the GML (Geomagnetic line)? The problem is, what you will see in the sky will not show on the GML until about 15 minutes later, and if you are inside waiting for it to drop and then it drops, and you go out to see, it is a high chance the outburst is already over, and you have missed it. After a significant drop in the GML it usually takes a long time for the lights to recharge, and possibly you need to wait another night. That's why I sometimes get nervous if I see the GML drop early for example around 20:00 and my pickup starts at 21:00 it might result in the sky being green on camera but the lights aren't forming, more like idling like a fog in the sky and there could take some time for another peak to happen.
4. KP and K Index. The Kp-index is the global geomagnetic activity index that is based on 3-hour measurements from ground-based magnetometers around the world. Each station is calibrated according to its latitude and reports a certain K-indice depending on the geomagnetic activity measured at the location of the magnetometer. The K-index itself is a three-hour-long quasi-logarithmic local index of the geomagnetic activity at the given location and time compared to a calm day curve. A magnetometer measures the maximum deviation of the horizontal component of the magnetic field at its location and reports this. The global Kp-index is then determined with an algorithm that puts the reported K-values of every station together. The Kp-index ranges from 0 to 9, where a value of 0 means that there is very little geomagnetic activity, and a value of 9 means extreme geomagnetic storming. This will as well tell us at which latitude Northern lights will be visible; for example, KP 2 is in general good for Iceland, and the lights will appear about 25 degrees up from the northern horizon. If you traveled further north, the KP 2 lights would appear higher on the horizon the further north you go, and if you would travel further south, they will be seen lower on the horizon and not visible if too far south. In Iceland, KP 1 is visible very low on the horizon, and 5 about straight overhead, 6,7,8,9, and you will see the lights as well looking south. Unfortunately, this is the scale most people look at as a probability scale, and the forecast of the Kp is, in my opinion, usually not accurate. Long term KP forecast is based on the 27-day solar circle and therefore is forecasting last month's activity. It is useful for planing big coronal events that tend to last for few months, so if you are looking for dates to come to see the northern lights in their best manners, you could plan around that. But for typical nights, I find it inaccurate. Same as with various apps and information people have, it's frankly annoying how KP forecast is presented as something accurate and as a probability of what we will see. There is a huge difference in actual KP and forecasted KP, and then there is the K-Index, the local one. Witch, for example, on latitudes as Iceland will be possible 4 when the KP is 2. So overall, ignore the KP forecast and all apps that use KP as their primary source as a probability scale. Learn to read the solar wind and the IMF, and you will feel like an all-knowing wizard. That being said, looking at the K index showing good numbers during the day is a really good sign of activity during the night; still, a K index 0 could give decent lights during the night, given the solar wind and IMF are good. Low K index during the day, followed by low solar wind and IMF being mostly on+north, is an indicator of a very weak aurora, and most likely, nothing will be seen unless with a camera.
5. Solar wind density. The higher, the better, although I have seen great lights that look very dens with low density on the density meter when the solar wind is strong. The solar wind is plasma with solar particles, and if the density is high, it takes the lights less time to recharge when the IMF flips to -south and more plasma to enter the magnetic oval and thus give better lights. High-density lights 8nt or above will compensate slower moving solar wind, and great northern lights can happen with high density and rather slow solar wind, given the IMF is frequently on - south. An excellent example of this is November 28 and 29, 2019. Both days had good density and similar solar wind speed just above average, although 29 had a slightly faster wind. November 28 resulted in very faint lights during the evening and dark hours, except for a small burst that happened around 18:00-19:00, which we did not see here in Iceland, but I saw on live camera from Norway, and I could read on magnetic meters a small drop. Now one day later 29 November, during the day, the almost identical data is showing average solar wind but still decent density, frankly not much excitement, given what we saw the night before. Later in the day, we started to see something different when examining the various data we have at our disposal. We notice the IMF is flipping south and is staying south for a reasonable amount of time, resulting in the dense solar wind to enter the aurora oval. You could see that on the aurora oval model from NOOA. This resulted in a great northern lights show from around 21-23:30, and the main difference was the IMF going -south opposite what was happening on the 28 where the IMF was mostly on+north.
6. Sun observation. By looking at ultraviolet pictures from the sun, we can see coronal holes and sunspot that will spew out the mighty solar wind that will most likely hit earth in about 2-3 days from facing the earth and expect great lights. If there are no holes visible on the earth facing sun, we will most likely not experience much in the coming days. However, this is why we monitor the solar wind, and why we will know when the coronal hole solar wind reaches earth is when there is an increase in the solar wind. However, this is an exciting factor for us because we can see the holes, and it gives us great hope and excitement when we see it coming; however, we don't know how it will turn out until it lands. For example, this season, we have been experiencing a great coronal hole now for consecutive 3 months in a row. It is linked to the 27-day cycle of the sun. Let's say tonight a big hole landed that was visible pointing towards earth 2 days ago and is giving good lights tonight and for the next 2 nights. We could forecast, 27 days from tonight, this hole will be facing us again and giving good lights. We will see the coronal hole way before it will be giving us the lights on the left side of the ultraviolet picture of the sun, so if I see that I can call my friend in Australia and say hey, the hole is still here take the flight and come over.
Conclusion: most important, learn to read the solar wind data and the IMF, Happy hunting.
About the author. My name is Þorkell H Haraldsson know as Thor or Icethor in the business. I am not a space physicist. Still, an enthusiastic Northern lights hunter, my opinions solely are based on my real-time observation on the northern lights that I have compared to the data showing on those meters almost every night. I have been doing my own forecasts sometimes here on my website but every day on a note paper, this season. I find my forecast to be relatively accurate and never far off. Technical information is taken here and there from various sources and sometimes changed to my wording and what makes sense to me.