Let's simplify for a minute. Let's say that we're going to look at the component of the geomagnetic field that is running horizontally to the Earth's surface at any given point. Now because the Earth is curved, this is a tangent line that is continually changing as you move around the Earth. Now let's look at the disturbances from normal, caused by solar weather – coronal holes, CMEs, what have you.
So we have these variations, that are going to be different for different parts of the Earth for the same event. How do we measure it? It's a little like a Richter scale for geosolar storms. It runs from zero to nine, and there's a special formula that enables it to be calculated regardless of the location of the observatory, just like the Richter magnitude of a quake can be determined from seismographs on the opposite side of the globe. This scale for solar-induced geomagnetic activity is called the K-index. Zero is essentially no activity; anything above 5 is considered a storm level of activity. The bigger the number, the greater the effects seen on the ground, and the farther south the auroral oval can be seen. At a K=9, the aurora can be seen...in the TROPICS.
(Just for the sake of more information, the letter K was derived from the German word “kennziffer,” which apparently means “characteristic number.” Us scientists, we love our imaginative names, you know?)
Now if we reference the Kp index, we're talking about the interplanetary K index, not the geomagnetic K index. This is an average of all the K indices from all of the observatories, weighted as appropriate (remember, you won't get the same measurements from the various observation sites, so you have to factor that in, as well as the fact that the geomagnetic field is constantly changing). This gives us an indication of what the interplanetary magnetic field (IMF) is doing. BUT – not all of the stations report in at the same time. So then scientists have to calculate something called the “estimated Kp” which is just what it sounds like – an estimate for those stations that haven't reported in yet. This can sometimes be a very good predictor of what the magnetic field is going to do, and sometimes not so much. We're still very much learning this particular science.
But we're not done with indexes. There's also something called the a index. This is based on the AMPLITUDES (yep, there's the reason for using an a) of the deviations from geomagnetic normal, taken over a three-hour period. Then there's the A index, which is an AVERAGE (yep, that's where the A came from) of all the a-indices for a 24-hour period.
One more index we need to look at is the G scale, which is the National Oceanic and Atmospheric Administration's (NOAA) way of quantifying the strength of the geomagnetic disturbance. For any K index of 4 or less, the scale shows G0. At K=5, we jump to G1 – minor storming. For K=6, we have G2. For K=7, G3. At K=8, we have a storm level of G4, and at the maximum K=9, we have maximum storming of G5. Think of it like the Earth's solar DefCon level.
Next week we'll go into those DefCon levels in detail.