Principal component analysis as a tool to extract Sq variation from the geomagnetic field observations: Conditions of applicability.

We analyzed the applicability of the principal component analysis (PCA) as a tool to extract the Sq variation of the geomagnetic field (GMF) taking into account different geomagnetic field components, data measured at different levels of the solar and geomagnetic activity, data from different months. The validation of the method was performed with geomagnetic data obtained at the Coimbra Magnetic Observatory in Portugal (40° 13' N, 8° 25.3' W, 99 m a.s.l., IAGA code COI). GMF variations obtained with PCA were "classified" as SqPCA using reference series: (1) obtained from the observational data (SqIQD), (2) simulated by ionospheric field models. While our results show that both the data-based and model-based reference series can be used, the DIFI3 model performs better as a reference series for GMF at middle latitudes. We also recommend to estimate the similarity of the series with a metric that account for possible local stretching/compressing of the compared series, for example, the dynamic time warping (DTW) distance. Since the validation of the method was performed on the geomagnetic series obtained at a mid-latitudinal European observatory, we recommend performing additional tests when applying this method to data obtained in other regions/latitudes.•For the Y and Z components of the geomagnetic field PCA can be used to extract Sq variations from the observations without any additional procedures and SqPCA is equals to PC1.•For the X component PCA can be used to extract Sq variation from the observations of the X component, but further analysis, for example, a comparison to a set of reference curves either obtained from the data analysis or generated using models, is always needed to classify PCs of the X component.•We recommend to use data generated by DIFI-class models as reference series and the dtw metric (dynamic time warping distance) to classify SqPCA.

Datasets of the solar quiet (Sq) and solar disturbed (SD) variations of the geomagnetic field from the mid latitudinal Magnetic Observatory of Coimbra (Portugal) obtained by different methods.

The datasets of daily variations is obtained from the geomagnetic field raw observations at the Coimbra Magnetic Observatory (COI, Portugal). The data set was obtained for the 01.01.2007-31.12.2017 time interval and covers almost the entire solar cycle 24. The raw data were processed using two methods to extract daily variability. The first method uses the so-called "geomagnetically quiet days" to calculate S-type variations as daily means resulting in the data sub-set named "IQD Sq and SD". The second method uses the principal component analysis (PCA) to extract main variability modes of the original data. The first three modes produced by PCA and explaining up to 98% of the variability of the raw data are in the data sub-set named "PCA modes". Both methods allow to extract regular geomagnetic field variations related to daily variations (S-type variations) in the ionospheric dynamo region and some magnetospheric currents (e.g., field-aligned currents). The COI location in middle latitudes near the mean latitude of the ionospheric Sq current vortex's focus allows studying its seasonal and decadal variability using the S-type regular variations of the geomagnetic field measured near the ground. The S-type variations for the X and Y components of the geomagnetic field obtained at the COI observatory can also be re-scaled and used to analyze geomagnetic field variations obtained at other European geomagnetic observatories at close latitudes. The S-type variations for the Z component of the geomagnetic field obtained at the COI observatory can be compared to similar variations observed at more continental regions to study the so-called "coastal effect" in the geomagnetic field variations.