Magnetometers
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 Earths. This changes slowly, on a time scale of 10's of years due to poorly-understood internal processes. Absolute measurements are made by
proton-precession magnetometer (magnitude) and by small magnets suspended on torsion fibres (direction). Readings are made about twice a month. 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,
flux-gate magnetometers are used to measure these variations. Data is normally recorded with 20 s time resolution, directly onto a PC at the measurement site. 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 and is used in producing global AE indices. Microfilm copies of Kiruna magnetograms are archived at WDC-C1 in Copenhagen. (WDC = World Data Center)
The direction and strength of the magnetic field are usually given in one of two ways : three orthogonal components, X,Y and Z or one horizontal component plus two direction angles (H, D, I). These are illustrated below.
The magnetic field at a point on the Earth's surface:
The magnetic elements
