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Here we present the results of the comparative analysis of the regular dailyvariations of the geomagnetic field, Sq and SD, obtained from the observationswith two methods: (1) using the so-called geomagnetically quiet day (QD) tocalculate mean daily variations and (2) using the principal component analysis(PCA) to decompose original series into main variability modes. The originalgeomagnetic field series were obtained at the Coimbra Magnetic Observatory(COI, Portugal) in 01.01.2007-31.12.2017. The data were analyzed separately foreach of 12 months. The geomagnetic field components X, Y and Z were analyzedindependently. The Sq and SD variations obtained with QD were compared with theprincipal components from PCA (PCs, daily variation of the geomagnetic field ofdifferent types) using the correlation analysis. In this study only the PCAability to extract daily variations similar to ones acquired using the standardapproach with QD was analyzed. The results for X, and Y and Z are essentiallydifferent. The Sq variation is always filtered to the 1st PCA mode for Y and Z,whereas the SD variation is filtered either to the 2nd or 3rd mode. For X theSq variation tends to be filtered to the 2nd mode, while the SD variation ismore often filtered to the 1st mode. The number of the PCs classified either asSq or SD follows seasonal and decadal variations of the geomagnetic activity.The analysis showed that PCA can automatically extract the Sq variation fromthe observations of Y and Z; however complementary analysis (for example, acomparison to a reference curve) is needed to identify a PC corresponding tothe SD variation. For X component, the automatic extraction of the Sq or SDvariations is not possible, and the complementary analysis is always needed.The dataset used in this paper is described in detail in a companion paper byMorozova et al. (2021a).

Comparison of the solar variations of the geomagnetic field at the  Coimbra Magnetic Observatory (COI) obtained by different methods: effect of  the solar and geomagnetic activity