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Measuring the magnetic field

Each magnetic observatory consists of a vector magnetometer, which measures the three components of the magnetic field, and an absolute scalar magnetometer, which measures the total field intensity. These data are recorded once per second, before being automatically processed (filtering, etc.). The vector magnetometer is very sensitive to temperature. This is why its thermal environment must be as stable as possible. For instance, at the Chambon-la-Forêt National Observatory, magnetometers are located in a temperature-controlled cellar, varying by less than a few tenths of a degree during the day. The data first undergo a quality control test by comparing the total intensity deduced from the three vector magnetometer recordings with that provided by the scalar magnetometer. Below is an image of the magnetic data (a magnetogram), available in real time from all our observatories.

Magnetogram of Chambon-la-Forêt from 1 June 2014. H, D and Z are the three components of the magnetic field provided by the vector magnetometer. F is the total field strength measured with the scalar magnetometer. Fs-Fv represents the total field strength measured with the scalar magnetometer (Fs) minus the total field strength measured with the vector magnetometer (Fv). (Values in nT are displayed every minute).

The vector magnetometer does not provide absolute measurements, and so must be calibrated on a regular basis. To do so, absolute measurements are performed by hand with a fluxgate magnetometer fixed on a theodolite.

Principle of absolute measurements.
In Chambon-la-Forêt's pavilion of absolute measurements (building on the left), the observer aims at a target, whose direction from true north is known. The target is attached to the building above the cellar, where the vector and scalar magnetometers are located. Using a theodolite magnetometer, we can measure the declination (as shown at the bottom right of the page for 2004) and inclination of the magnetic field. These data are used for the calibration of vector magnetometer data. The correction curve is called the baseline
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The INTERMAGNET standard requires absolute measurements to be made at least twice a week. The calibration curve in time is called the baseline and all the data coming from absolute measurements are used to establish this curve. Data management for each observatory thus requires much time and precision, because the values obtained by the magnetometers must be examined (and de-spiked if necessary, etc), in order to correct them with the baseline at the end of the year, and to ensure daily access to them in real time.

2019-07-02