4.1. Definition and Frequency

Absolute control at a magnetic observatory requires periodic measurements of the absolute value of the geomagnetic field. At present, this requires manually operated instruments; a single observation takes several minutes.

Absolute control is required in order to correct for the long-term instability in the variometer installation, and thereby enabling the determination of very long-term changes in the magnetic field. The frequency with which these measurements should be made depends on several factors: the intrinsic and thermal stability of the variometer electronics, sensor, and piers; the method of temperature stabilization and monitoring; the accessibility of the observatory; availability of staff; and other considerations.

The choice of period between measurements should be influenced by:

• the change in baseline values from measurement to measurement,

• the consistency between primary and alternative variometers (including intermediate Reference Measurements),

• the consistency between total field derived from the vector variometer Fv, and those measured using a recording absolute scalar magnetometer Fs, and

• known disruptive events.

INTERMAGNET recommends a maximum period of one week between absolute measurements.

Observations should be obtained for different levels of magnetic activity and for different magnetic field values.