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 Sq_PCA using reference series: (1) obtained from the observational data (Sq_IQD), (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 Sq_PCA.

Equinoctial Asymmetry in Solar Quiet Fields along the 120° E Meridian Chain

Equinoctial asymmetry of the range of the solar quiet day variation (Sq) of the horizontal geomagnetic field (H) has been found in some low latitude geomagnetic observatories. This study conducted an investigation of its latitude distribution and the relationship with the solar cycle by using the H field measurements from six observatories along the 120° E meridian chain in the years 1957–2013. Results illustrate a significant equinoctial asymmetry of the SqH range at all observatories. Three main features were identified. First, the signature of the equinoctial asymmetry of the SqH range is opposite for observatories located at the northern and southern sides of the Northern Hemisphere Sq current focus. It shows larger values around spring than autumn equinox at southern observatories, and the converse is seen at northern observatories. Second, the asymmetry increases with the distance from the Sq current focus, suggesting the stronger sensitivity of the distant observatories than observatories around the focus. The result of linear fitting presents a positive dependence of the asymmetry coefficient on geographic latitude, with a reversal of the asymmetry occurring at 28.1° N near the focus of the average Sq current. Third, there is no obvious dependence of the equinoctial asymmetry of the SqH range on solar activity, suggesting a possible cause from some regional factors related to the ionospheric dynamo process.