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How to read an Aurora forecast

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Usually the first sign of any potential aurora activity chatter, comes from the Kp indices forecast, such as the one below.

These values indicate the expected geomagnetic activity for any given 3-hour period for the next three days. This is the fastest way to quickly find out what kind of geomagnetic conditions are to be expected over the next 3 days. The times are in UTC, which is a time standard used across the globe. For NZ daylight savings we're 12 hours ahead, so 21:00 on the 28th UTC, would be 09:00 on the 29th NZST.

The Kp number system is a scale of planetary geomagnetic activity. It is not a plot map where you can definitely say auroras will be visible.

The Kp number is a system of measuring aurora. It goes from 0 to 9 (0 being very weak, 9 being a major geomagnetic storm with strong auroras visible). It is not an accurate gauge of how strong the aurora will be, more a case of how much potential there is to see one.

I also relate it to the old earthquake Richter scale. For those of us who've experienced many earthquakes recently, you can have a Mag 4 and you can have a totally different Mag 4 that appears a lot stronger, yet both are measured the same on the Richter scale. So you as you get into it, you look at other scales, in the earthquake instance, the Mercalli scale. Aurora are no different.

So when you're looking for Aurora, you want to see high Kp numbers. The higher the better. Anything above (and including) Kp5 is classed as a geomagnetic storm.

I also view it as the chance of seeing an event rather than the strength of a given event. And yes, you can photograph aurora in New Zealand at Kp1 (with beams) and in the same instance we've had Kp9 and not seen a thing.

If there is one piece of advice I can give you: DO NOT rely solely on Kp index when considering if there will be an aurora or not.

I use the Kp forecast from



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What are these G numbers you speak of?

Geomagnetic storms are labelled G1 to G5. It is a period when there is strong to very strong geomagnetic activity due to a lot of build up in Earth’s magnetotail. Here is a table which converts the G into Kp.

G1 = Kp5
G2 = Kp6
G3 = Kp7
G4 = Kp8
G5 = Kp9

It's for this reason you hear a lot of people say they wouldn't get out of bed for anything less than KP5. They like their aurora potential to be strong, just like their coffee. If you waited for a KP5 event every time before going aurora hunting you'd rule out 60% of your opportunities of seeing one. Let's face it a single shot Americano is still a coffee and Kp3 and 4 are readily visible from the Christchurch area.

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Aurora On : Aurora Off

In my experience, the Interplanetary Magnetic Field, or Bz is fundamentally the aurora on and off switch. Although there are exceptions, they are few and far between and this is applicable for all but the most hardened scientific aurora hunter.

So if Bz is reading +'ve or northerly, you are unlikely to see an aurora at all. The stronger the northerly reading the less likely you are to see anything. I've been in Kp 9 conditions with a northerly Bz and not seen anything at all.

However, if Bz is reading -'ve or south then you are likely to see one - if other conditions allow. Any -'ve reading is good, but the stronger the better.

Bz changes in an instant, so DO NOT write of a night hunting because Bz is not currently favourable.

The instrumentation for this reading sits on a satellite called DSCOVR, some 1.5 million km closer to the Sun from Earth. So depending on the speed of the solar winds, its arrival time on Earth can differ from the time it reaches the satellite. However that gives us advance warning of the conditions, so everything to the right of the Earth line is what's coming.


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The north-south direction of the interplanetary magnetic field (Bz) is the most important ingredient for auroral activity. When the north-south direction (Bz) of the the interplanetary magnetic field is orientated southward, it will connect with Earth's magnetosphere which points northward. Think of the ordinary bar magnets that you have at home. Two opposite poles attract each other! With a southward Bz, solar wind particles have a much easier time entering our magnetosphere. From there they are guided into our atmosphere by Earth's magnetic field lines where they collide with the oxygen and nitrogen atoms that make up our atmosphere, which in turn causes them to glow and emit light.

For a geomagnetic storm to develop it is vital that the direction of the interplanetary magnetic field (Bz) turns southward. Continuous values of -10nT and lower are good indicators that a geomagnetic storm could develop but the lower this value goes the better it is for auroral activity. Only during extreme events at higher latitudes and with high solar wind speeds it is possible for a geomagnetic storm (Kp5 or higher) to develop with a northward Bz.



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Why do we see those stunning lights in the northern- and southernmost portions of the night sky? The Aurora Borealis and Aurora Australis occur when high-energy particles are flung from the Sun's corona toward the Earth and mingle with the neutral atoms in our atmosphere -- ultimately emitting extraordinary light and colour. Michael Molina explains every step of this dazzling phenomenon.



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GOES magnetometer, is the next important thing and we're doing some studies as to how the effect on GOES becomes indicative of BEAMS or the potential to see beams.

There used to be a choice of magnetometers, but  NOAA’s Geostationary Operational Environmental Satellites (GOES) are currently limited to 1, and that is number 16, which is positioned somewhat inconveniently on the Eastern seaboard of the US at 75.2° West.

On a GOES plot, GOES has a natural curve in its graph, based on the rotation of the Earth and the position of the satellite in relation to the sun ( i.e. day or night). What we're looking for are spikes. On this chart, I've marked all of them, but we're actually more interested in the ones below 50nT as these tend to give stronger results.


Our localised findings are the correlation between the upward springs and the appearance of beams in an aurora (or at the very least heightened activity). Of course, the rest of the conditions also have to be met, particularly a south Bz. The stronger the bounce back (upward) direction the stronger the beam activity gets, as the graph settles, the beams disappear. There is also a short delay between the satellite receiving the data and the arrival of the beams, this again depends on the speed of the wind, as the satellite is 6 Earth radii away.

GOES is a great indication tool for NZ conditions, especially when conditions are less than KP5. However remember we need a southerly Bz to see aurora.

In basic terms we need the GOES16 magnetometer to head towards 50 nT. Think of this as an elastic band being stretched back. If you recall the images from the previous units, we're looking for a snap back of the magnetic flux. So the lower the number the further back the band is being stretched.

Now lets release the band. If it jumps back up quickly, like on this graph, it may produce beams, they could be bright, but it would be very short lived, because the snap back was quick. A slower snap back, would produce an effect for a longer period of time, but the effect may not be as bright, or may not "beam".

The correlation was first noticed in NZ Aurora circles by Geoff Cloake, so we affectionately call this the "Cloake Effect". I've not had it fail me, which is why I use it as the primary indicator for Aurora potential, even when KP is as low as one. In those conditions we may only get a low grade photo quality green arc, but it would nevertheless be an aurora.



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