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Magnetometer question


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Hello, I want to make sure I am looking at the X or H data right right way on a magnetometer. Off of the attached picture where I added some extra lines, would you go off of the black or red lines to see how far south the aurora can go, or is it a different way?




Edit: Never mind, after going though a bit of old data, I think I've figured it out.

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  • 3 weeks later...
On 12/1/2015 at 1:52 AM, Marcel de Bont said:

Marcel, if i'm right, the K-index is based on the horizontal component (H) of the magnetic field. See this source from the gfz-potsdam. Magnetic fields are measured by XYZ components or HDF components. This picture explains the components of the magnetic field.


The following formulas exist :

H = sqrt(X^2 + Y^2)

D = arctan (Y / X)

F = sqrt(X^2 + Y^2 + Z^2) = sqrt(H^2 + Z^2)

or if you have HDZ components:

X = H * cos(D)

Y = H * sin(D)

Z = H * tan(I)

So, if you want to calculate the K-index, you take the maximum fluctuations in the H-component within a 3-hour period and convert it to K-index via a conversion table that varies per observatory.

The table for the Boulder magnetometer is:


So, in the picture in the opening post, the maximum fluctuation is around 200nT, making the K-index 6.

Was this helpfull ?

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Yep, its helpful. I'm still partly confused from on the 10th, there was a drop of 35nT in H on the Boulder magnetometer and was K6 while the Fredericksburg magnetometer had a drop of 60 nT and was K4 (K9 is >500nT for both stations). The plot here, http://www.swpc.noaa.gov/products/boulder-magnetometer, has the D component too, so I'm guessing it may have something to do with Boulder having a drop of 21deg and Fredericksburg having a drop of 10deg at the time.

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I cannot explain the K6 for boulder. If I look at your graphic and also the magnetometer data for bou and frd, the K should be both 4. The maximum H variation for Boulder is 47.69, the maximum for Frediricksburg is 68.14, making K-index = 4.
When I look at the data from cmo (College), that had a maximum H variation of 283.57 nT, making K=6 (cmo k9 value is 1000). Maybe you are mistaken with this?

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Some station K indices are in the files here, ftp://ftp.swpc.noaa.gov/pub/lists/geomag/201512AK.txt.


Edit: After going through some of the data again, the pattern isn't the drop in the D component, it looks like it has something to do with the rise or how high the D component angle is. Then that would go along with the variation in the H component.

Another edit: So, while searching for something else I managed to come across this, http://k9la.us/Where_Do_the_K_and_A_Indices_Come_From.PDF. I'm guessing it has changed slightly over 12 years, but that might solve a big chunk of what the D component has to do with the K index. And by using Boulders current chart on the SWPC's page, http://www.swpc.noaa.gov/products/boulder-magnetometer, and the real data, the variation in the D component needs to be multiplied by ~6, not sure about other stations.

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