# On a Novel Approach to Correcting Temperature Dependencies in Magnetic Observatory Data
## 2 Existing Approaches to Assessing the Quality of Observatory Data  
In the manual for long-term magnetic observations [9], temperature correction of data is one of the stages of processing component variations recorded at observatories. In [16], it was experimentally shown that the temperature coefficient of the sensor depends not only on the physical principle of the magnetic field measurement, but also on the variometer design. In addition, the temperature dependence of the electronic components of the signal amplifiers and analog-to-digital converters can have a significant effect on the measurements. All these factors lead to temperature drifts of the vector variometers comparable to the required EMF measurement accuracy for building main field models. In order to control the long-term stability of variometer readings, magnetic observatories continuously record the modulus of the total EMF vector using proton- or Overhauser-type scalar magnetometers [26], which are least affected by temperature. A key indicator of the quality of magnetic observatory measurements is the difference between the full vector modulus calculated from variometer data \(F(v)\) and the full vector modulus measured by a scalar magnetometer \(F(s)\):  
\[G=F(v)-F(s). \tag{1}\]This value is called deltaF (\(G\) record), and it has been provided by INTERMAGNET observatories along with definitive data sets since 2009 [5]. Ideally, \(G\) values are in the range of \(-0.5\) to \(0.5\) nT during periods of low geomagnetic activity, but can reach \(\pm 1\)-\(2\) nT during periods of strong geomagnetic disturbances.  
Since modern variometers are not capable of measuring the full values of the EMF components with a sufficient absolute accuracy [27], regular absolute measurements, supplementing the variations with baseline values, are an integral part of the observatory operation. Absolute measurements are made manually with a declinometer/incinometer with a single-component fluxgate sensor (Dflux) fixed on the optical axis. The results are the values of the total components of the EMF vector at the time of the measurement \(t_{0}\). The observed baseline values at time \(t_{0}\) are:  
\[X_{b}(t_{0})=X(t_{0})-X_{v}(t_{0}), \tag{2}\]  
\[Y_{b}(t_{0})=Y(t_{0})-Y_{v}(t_{0}), \tag{3}\]  
\[Z_{b}(t_{0})=Z(t_{0})-Z_{v}(t_{0}), \tag{4}\]  
where \(X_{v}\), \(Y_{v}\), and \(Z_{v}\) are variations of the EMF vector recorded by the variometer, with the addition of some offsets providing \(\mid\)G\(\mid\) values within \(5\) nT.  
Thus, the baseline values are nothing more than variometer calibration corrections that eliminate long-term drifts. The stability of the baseline values and baseline-dependent G record throughout the year is a basic indicator of the quality of the data provided by the observatory [28; 29].  
Temperature correction in observatory data processing can be accomplished in two ways. The most obvious way to reduce the influence of temperature is to adjust the temperature of the sensor using active (heating) and passive (thermal insulation) temperature control. In one way or another, all INTERMAGNET magnetic observatories maintain a constant temperature inside the variation pavilion. However, this method does not allow for the complete elimination of temperature variations in sharply continental and arctic climates due to considerable temperature variations both on seasonal and diurnal scales. When processing variometer data, the temperature dependence can be eliminated automatically by adding daily baseline values to the variations. However, in some cases, this procedure fails due to an insufficient frequency of the baseline values compared to the temperature variation frequencies. In these cases, a local temperature correction becomes necessary.