The Ovation forecast layer appears on many aurora-related websites, including our own Aurora Map page. It’s a convenient way of graphically showing where an aurora might be seen, but we know it can cause some confusion about what is being shown and intrigue in how it is calculated. So let’s have a look at what it is (and what it isn’t!), how it works, and why it’s useful.
What the Ovation Aurora Forecast isn’t
Let’s get this one out and dealt with straight away: the Ovation aurora forecast product is not the exact extent/location of the visible aurora. It is not a satellite image of where an aurora is, and the colours often used to present the forecast do not represent the colours of the aurora.
As we’ll come on to, it is a very useful tool but it is not a perfect guarantee that you’ll see the aurora.
So what is it?
The Ovation aurora forecast product is an estimate of the “probability of visible aurora” for different locations as shown on the maps. NOAA’s Space Weather Prediction Center (SWPC) produce the forecast and make it freely available on their site, both as images they create and as a text file for others to use (like we do).
At its heart, the forecast uses the OVATION (Oval Variation, Assessment, Tracking, Intensity, and Online Nowcasting) Prime (2010) auroral oval model. This model estimates the amount of particle precipitation (i.e. the amount of electrically-charged particles entering the earth’s atmosphere) which drives the aurora. It does this by looking at the current solar wind conditions (e.g. the direction of the Bz-component of the interplanetary magnetic field) and computing a value known as the Newell coupling function. Using previous data, the model then knows approximately how much particle precipitation there should be based on the value of this coupling function.
SWPC run the OVATION Prime (2010) model using near-real time data from the DSCOVR satellite. Since it takes the solar wind about 30 minutes to travel from the DSCOVR satellite to Earth, the model produces a 30 min forecast. In reality, the exact timing depends on the solar wind velocity and so the travel time can be shorter or longer than this.
Additionally, SWPC convert particle precipitation values into “probability of visible aurora”. As described here, the values are simply scaled into probability percentage using an empirical formula, i.e. it’s not built on physics, but rather by previous observations. SWPC also smooth the model output (to reduce noise in the data) and clip the aurora at the day/night terminator (as, although aurora do occur during daylight hours, they can only been seen in darkness).
Why is this useful?
The Ovation aurora forecast provides a good estimate of where the aurora might be seen directly overhead, and a “best guess” at how likely that is. This allows the aurora-hunting community to produce interactive maps and alerts and, ultimately, it is another tool to be used in determining whether to head out or not.
But it has its weaknesses
The forecast is based on a model. While this model is pretty good (though there is an improved and updated version available which SWPC does not yet use), it is still fundamentally an estimate of what might happen based on what has happened in the past.
As mentioned before, the model is based on statistical averages of what the aurora was like at different values of the Newell coupling function. Recall that the Newell coupling function is driven solely by the solar wind conditions. It does not take into account any time-history of the magnetospheric drivers behind the aurora (e.g. magnetic substorms). Think of it as trying to predict the water level in a river based solely on how heavy the rain is, without knowing exactly how much water was in the river already.
The “probability” part of the forecast really is just a best guess. It has no physics behind it and cannot take into account local conditions (e.g. cloud cover, light pollution, or obstructions) because of its fixed, large-scale nature. Also, the aurora happens at hundreds of kilometers in altitude, so can often be seen much further away than the forecast will show. This is why SWPC draw the red “view-line” on their images but it is not something that you will see on many other sites. It is also not particularly accurate.
So what does all that mean?
The Ovation aurora forecast is absolutely a very handy tool. It provides a (roughly) 30 min forecast of how likely it is to see an aurora overhead from across the globe. But it should be “taken with a pinch of salt”, and used in conjuction with other data, such as our magnetometer-based alert status and other people’s observations. The best way to know for sure, though, is to go out and look for yourself.