Two types of magnetic measurements are made at IRF - absolute and short-term variations. Absolute measurements are used to monitor the strength and direction of the magnetic field which is generated inside the Earth. This changes slowly, on a time scale of 10's of years, due to poorly-understood internal processes.
Absolute measurements are made with proton-precession magnetometer (magnitude) and with the standard DI flux (theodolite) method (see IAGA document page 197-198 and NOAA site). Readings are made every week. Short-term variations with time scales from a few seconds to a year are due to electric currents flowing in the ionosphere (primarily 100-130 km altitude) and induced in the surface layers of the solid Earth. At IRF, DMI (Danish Meterological Institute) flux-gate magnetometers, are used to measure these variations. Data is recorded with 1 s time resolution, directly onto a PC at the measurement site.
Two magnetometers are operated at IRF, one primary and one backup to be able to backup up data gaps of one of them. A copy of the data is sent directly to the IRF main building over a radio link.
Each month, the flux-gate data are calibrated against the absolute measurements, quiet days are identified and local K-indices calculated. Kiruna digital magnetic data is also sent to the WDC-C2 in Japan, as one of few stations that provides 1 sec value (WDC = World Data Center).
The direction and strength of the magnetic field are usually given in one of three ways: three orthogonal components (X,Y and Z), one in cylindrical coordinate system with horizontal component, vertical component, and declination angle from north (H, D and Z), and one in polar coordinate system (D, I and F) (http://www.ngdc.noaa.gov/geomag/geomaginfo.shtml). These are illustrated below.
The magnetic field at a point on the Earth's surface: The magnetic elements