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I'm trying to convert geomagnetic coordinate to geographic, but there are some steps in the paper which I'm reading, that I don't understand.

I have the geomagnetic latitude centered around the magnetic pole, but now I want to express that in geographic lat/lon.

It says that the In local magnetic time the magnetic longitude $ML$ is expressed as

$$ML = 2\pi t/24 +\delta ML(t),$$

where $\delta ML(t)$ is the 

longitudinal difference between the sub­solar point and the magnetic poles at time $t$ (hours).

What does this mean and is there a formula to calculate this?

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The subsolar point is the point on the earth's surface where the sun is directly overhead. The formula you give uses the longitude of this. Take the difference of this from the longitude for the poles of whichever geomagnetic reference model you are using.

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  • $\begingroup$ THanks! I think i understand, as an example, for IGRF model the North geomagnetic pole longitude is approximately -72.62. Would I then take the longitude position of Sun at Zenith at time t, say from this site (timeanddate.com/worldclock/sunearth.html) and find the difference between those two? Is it always a positive value or can it also be negative? $\endgroup$
    – erotavlas
    Commented Mar 24, 2014 at 18:18
  • $\begingroup$ You should be able to obtain the longitude of the solar point at t from that calculator. Check the reference where you obtained that formula to determine the sign convention - otherwise you might try a simple case you know the answer to determine the sign convention. Also be sure to take care with your units. $\endgroup$
    – Mark Rovetta
    Commented Mar 24, 2014 at 19:08

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