Question:
I have downloaded some magnetometer data from the website and was wondering if you could answer some questions regarding the co-ordinate system of the data. I notice from the files that the co-ordinates are (H, D, Z) all in nT. Could you tell me exactly how the H and D co-ordinates are defined?

Answer:
We try to set up the magnetometers during a magnetically quiet period. We position the sensor head such that the D-component is close to zero (usually +/-300 but as much as +/-500). This is call geomagnetic alignment. The deviations to the magnetic filed are dominantly in the N/S (H-component) direction and the Z-component, (vertical just like X,Y,Z).

If you are using the NetCDF files you will see a large possibly overwhelming amount of header information. Decreases in the H-component is Southward usually seen as a negative values on the plot. We recently started computing quiet time values for the data but for now, you will still need to go to our on-line plots to find those values was so you can subtract it yourself.

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Question:
The unit of the D parameter should be degrees. There is however some discrepancy in the GIMA data files; the D parameter column header is listed as "D-Comp(nt)" implying the unit is nanoTesla's. I suspect this is a typographical error.

Answer:
No, the D-component IS in nT. The magnetometer head is oriented magnetic north, hence there is no mean value for that component. This was done before the magnetic pole started on its long hike. Also, due to shifting of the heads it is difficult to maintain it. We consider +/- 500 to be acceptable before bother to adjust it assuming we can even get to it. For degrees from Magnetic North at any

I hope that answers at least some of your questions. We welcome any that you still have.

Last Modified: 10 February 2004
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