## 3 System Description
### Geomagnetic Recording System  
Due to the wide amplitude and frequency ranges of the field sources monitored by observatories, highly sensitive and stable instruments (generally termed magnetometers) are operated in an environment free from artificial magnetic disturbance. Two sets of magnetometers are usually required: on the one hand, variometers are automatic instruments recording high-resolution, high-frequency variations of the Earth's magnetic field; however, this feature is counter-balanced by the lack of long- or medium-term stability, e.g., against temperature or pillar movements; on the other hand, absolute instruments (whose measurements in theory do not depend on physical parameters other than the magnetic field itself) refer the recorded variations to the geographic reference frame. Automatic recording is accomplished with diverse instrumentation based on different physical properties. The ones in use at LIV are based on fluxgates and proton precession magnetometers, whose technologies are summarized here:* The fluxgate technology is based on the electric current induced in a coil submitted to both, the natural and a known artificial magnetic field in the presence of a magnetically susceptible core close to saturation. Three mutually orthogonal fluxgate bars can be used to refer vector variations, each determining the magnetic field projection along its axis. The fluxgate magnetometer at LIV (Figure 3) is a triaxial one; it is sampled at a rate of once per second and provides about 0.1 nT effective resolution. * Proton precession magnetometers (PPM) are based on the intrinsic dipole magnetic moment of protons precessing around the ambient magnetic field, thus measuring the total magnitude F of the geomagnetic field with typical accuracies of 0.2 nT. When coils (usually two mutually perpendicular pairs of Helmholtz coils) are placed around them, a known current produces a known magnetic field that is superimposed to the natural one. Measurement of the resulting total field with the PPM allows determining variations of the declination and inclination angles. Two PPMs are used at LIV, which are sampled every ten seconds: one of them is a single scalar PPM to just measure F; the other is placed within two sets of Helmholtz coils, which produce a complete cycle of measurements including F, I and D every minute, constituting a variometer known as Proton Vector Magnetometer in \(\delta\)D/\(\delta\)I (or deltaD/deltaI) configuration. Currently, there are two widely-used types of absolute instruments found in most geomagnetic observatories:  
* The scalar PPM again, for measurements of the \"total force\", F.
* The D/I fluxgate magnetometer, or D/I-flux. The fluxgate technology explained above is also used in this case, where the sensor is tightly coupled to the telescope of a theodolite. The angles thus measured are referred to previously determined geographic references, allowing the angles intrinsic to the magnetic field, D and I, be established. This instrument requires skilled manual operation, though some automated copies, known as AUTODIF, are already available for use [24]. Our short-term plans at LIV include deploying an automated D/I-flux, which would allow absolute measurements theoretically available during the whole year (_i.e._, including the unattended periods). Data processing from both variometers and absolute instruments roughly entails (1) thorough detection of spurious records (e.g., spikes and artifacts) in the series; and (2) translation of automatically recorded vector component variations to their absolute level (see [25] and references therein). Robust hardware has been designed to manage the acquisition of the variometers, both by the companies/institutes providing the instruments and by the technical staff at Ebre Observatory. We have almost doubled our control and logging systems to avoid data loss in case a hypothetical failure is produced during the unattended period.