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.
-Stephanie
Osborn
