Solar, Space, and Geomagnetic Weather, Part V: Solar Activity and the Activity Indices
By Stephanie Osborn
“Interstellar Woman of Mystery”
Rocket Scientist and Novelist
Okay, back to bar magnets again. Because the Earth has one. But of course it’s three-dimensional, not like our iron filings on paper example. Imagine picking up the bar magnet with the iron filings and paper attached, and rotating it 360º, letting the iron filings remain in the areas they move through. Now you have an image of what a three-dimensional dipolar (2-pole) magnetic field looks like — sort of like a giant pumpkin.
With the solar wind (which is probably the largest influence on the interplanetary magnetic field) pushing on it from the Sun direction, the side of the pumpkin facing the Sun tends to smush in, but the side away from the Sun tends to stretch out and form a long tail. (You can see a really good animation of how this works here: http://en.wikipedia.org/wiki/File:Animati3.gif) This is all to say that you HAVE to think of the geomagnetic field three-dimensionally. And if it is three-dimensional, then each part of the field has an x-, a y-, and a z-coordinate component.
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.
Yes, this is the Aurora Australis observed from space.
(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.
Most of the time, it looks kinda like this:
Each vertical bar represents a 3-hour period. The bars are color-coded to show you how much effect it will have on Earth. Green means little to no effect. Yellow means the geomagnetic field will be unsettled, and you might see a few minor effects. Red means the geomagnetic field will be “storming,” and you WILL see effects. The taller the red bar, the stronger the effects.
So, occasionally, it looks more like this:
If it ever looks like this, we’re probably in trouble:
It actually DID look like this, though. This is from the big geomagnetic storm that occurred October 28-29, 2003. According to NASA, “During the height of the solar activity, more than half of the deep space and near-Earth space science missions experienced the effects of the Halloween storms of 2003. The Solar and Heliospheric Observatory (SOHO) satellite, a collaboration between NASA and the European Space Agency (ESA), failed temporarily. NASA’s Advanced Composition Explorer (ACE) satellite experienced damage, and instruments aboard many spacecraft had to be shut down temporarily.”
“The effects of these storms were ghoulish enough that [aircraft controllers] had to re-route aircraft, it affected satellite systems and communications, and it also caused a power outage in Sweden for about an hour,” said Dr. Holly Gilbert, a solar scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md. “The aurorae are normally limited to the higher latitudes, and these storms were so powerful they created aurorae that could be seen as far south as Florida.”
And here is an aurora photograph, taken during that huge geomagnetic storm…from just outside HOUSTON, TX, on October 29, 2003.
But we’re not done with indices. 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 time we’ll go into those DefCon levels in detail.
Comet Tales blog/Osborn Cosmic Weather Report: http://stephanie-osborn.blogspot.com/