The flow of liquid metal inside the Earth's core produces the geomagnetic field and its time variations. Understanding the variability of those deep currents is crucial to improve the forecast of geomagnetic field variations and may provide relevant information on the core dynamics. The main goal of this study is to extract and characterize the leading variability modes of core flows over centennial periods, and to assess their statistical robustness. To this end, we use flows that we invert from two geomagnetic field models (‘gufm1’ and ‘COV-OBS’), and apply principal component analysis and singular value decomposition of coupled fields. The quasi-geostrophic (QG) flows inverted from both geomagnetic field models show similar features. However, ‘COV-OBS’ flows have a less energetic mean and larger time variability. The statistical significance of flow components is tested from analyses performed on subareas of the whole domain. Bootstrapping methods are also used to extract significant flow features required by both ‘gufm1’ and ‘COV-OBS’. Three main empirical circulation modes emerge, simultaneously constrained by both geomagnetic field models and expected to be robust against the particular a priori used to build them (large-scale QG dynamics). Mode 1 exhibits three large vortices at medium/high latitudes, with opposite circulation under the Atlantic and the Pacific hemispheres. Mode 2 interestingly accounts for most of the variations of the Earth's core angular momentum. In this mode, the regions close to the tangent cylinder and to the equator are correlated, and oscillate with a period between 80 and 90 yr. Each of these two modes is energetic enough to alter the mean flow, sometimes reinforcing the eccentric gyre, and other times breaking it up into smaller circulations. The three main circulation modes added to the mean flow account for about 70 per cent of the flows variability, 90 per cent of the rms total velocities, and 95 per cent of the secular variation induced by the total flows. Direct physical interpretation of the computed modes is not straightforward. Nonetheless, similarities found between the two first modes and time/spatial features identified in different studies of core dynamics, suggest that our approach can help to pinpoint the relevant physical processes inside the core on centennial timescales.

Three long-term temperature data series measured in Portugal were studied to detect and correct non-climatic homogeneity breaks and are now available for future studies of climate variability. Series of monthly minimum (T_min) and maximum (T_max) temperatures measured in the three Portuguese meteorological stations of Lisbon (from 1856 to 2008), Coimbra (from 1865 to 2005) and Porto (from 1888 to 2001) were studied to detect and correct non-climatic breaks. These series, together with monthly series of average temperature (T_aver) and temperature range (DTR) derived from them, were tested in order to detect breaks, using firstly metadata, secondly a visual analysis, and thirdly four widely used homogeneity tests: von Neumann ratio test, Buishand test, standard normal homogeneity test, and Pettitt test. The homogeneity tests were used in absolute (using temperature series themselves) and relative (using sea-surface temperature anomalies series obtained from HadISST2.0.0.0 close to the Portuguese coast or already corrected temperature series as reference series) modes. We considered the T_min, T_max and DTR series as most informative for the detection of breaks due to the fact that T_min and T_max could respond differently to changes in position of a thermometer or other changes in the instrument's environment; T_aver series have been used mainly as control. The homogeneity tests showed strong inhomogeneity of the original data series, which could have both internal climatic and non-climatic origins. Breaks that were identified by the last three mentioned homogeneity tests were compared with available metadata containing data such as instrument changes, changes in station location and environment, observation procedures, etc. Significant breaks (significance 95% or more) that coincided with known dates of instrumental changes were corrected using standard procedures. It was also noted that some significant breaks, which could not be connected to known dates of any changes in the park of instruments or stations location and environment, were probably caused by large volcanic eruptions. The corrected series were again tested for homogeneity; the corrected series were considered free of non-climatic breaks when the tests of most of monthly series showed no significant (significance 95% or more) breaks that coincide with dates of known instrument changes. Corrected series are now available within the framework of ERA-CLIM FP7 project for future studies of climate variability (doi:10.1594/PANGAEA.785377).

The Coimbra Magnetic Observatory (International Association of Geomagnetism and Aeronomy code COI) in Portugal has a long history of observation of the geomagnetic field, spanning almost 150 yr since the first geomagnetic measurements in 1866. These long instrumental geomagnetic records provide very important information about variability of geomagnetic elements and indices, their trends and cycles, and can be used to improve our knowledge on the sources that drive variations of the geomagnetic field: liquid core dynamics (internal) and solar forcing (external). However, during the long life of the Coimbra Observatory, some inevitable changes in station location, instrument's park and electromagnetic environment have taken place. These changes affected the quality of the data collected at COI causing breaks and jumps in the series of geomagnetic field components and local K index. Clearly, these inhomogeneities, typically shift-like (step-like) or trend-like, have to be corrected or, at least, minimized in order for the data to be used in scientific studies or to be submitted to international databases. In this study, the series of local K index and declination of the geomagnetic field are analysed: the former because it allows direct application of standard homogenization methods and the latter because it is the longest continuous series produced at COI. For the homogenization, visual and statistical tests (e.g. standard normal homogeneity test) have been applied directly to the local geomagnetic K index series (from 1951 to 2012). The homogenization of the monthly averages of declination (from 1867 to 2012) has been done using visual analysis and statistical tests applied to the time series of the first differences of declination values, as an approximation to the first time derivative. This allowed not only estimating the level of inhomogeneity of the studied series but also detecting the highly probable homogeneity break points. These points have been cross-checked with the metadata, and the COI series have been compared with reference series from the nearest geomagnetic stations and, in the case of declination series, from the recent geomagnetic field model COV-OBS to set up the required correction factors. As a result, the homogenized series measured in COI are considered to be essentially free of artificial shifts starting from the second half of the 20th century, and ready to be used by the scientific community.

The Coimbra Magnetic Observatory (COI), Portugal, established in 1866, has provided nearly continuous records of the geomagnetic field elements for more than 150 years. However, during its long lifetime inevitable changes to the instruments and measurement procedures and even the relocation of the observatory have taken place. In our previous work (Morozova et al., 2014) we performed homogenization – elimination of the artificial changes – of the measured declination series (D) for the period from 1866 to 2006. In this paper we continue work on applying homogenization procedures to the measured series of the absolute monthly values of the horizontal (H, 1866–2006), vertical (Z, 1951–2006) and inclination components (I, 1866–1941). After homogenization of all measured series for the 1866–2006 time interval, we performed the homogenization of the series of all geomagnetic field elements (X, Y, Z, H, D, I and F) to the level of the 2015 epoch. Since all series except D have a gap of about 10 years in the middle of the 20th century, splitting each of them into two, the homogenization to the level of 2015 was done only for the series available after 1951 (with the D series homogenized for the whole time interval of 1866–2015). The COI geomagnetic field elements are available via the following addresses: https://doi.org/10.5281/zenodo.4308022 (Ribeiro et al., 2020) for the original COI data (ASCII and XLSX formats) and https://doi.org/10.5281/zenodo.4308036 (Morozova et al., 2020) for the homogenized COI data (ASCII and XLSX formats).

The total electron content (TEC) over the Iberian Peninsula was studied using data from two locations obtained both by Global Navigation Satellite System receivers and an ionosonde. The principal component analysis applied to the TEC data allowed us to extract two main modes. Each mode is characterized by a daily TEC variation of a certain type (principal component [PC]) and its amplitude for each of the studied days (given by the empirical orthogonal functions [EOFs]). The variations of these modes as well as the original TEC data were studied in relation to four strongest geomagnetic storms of 2015 and three geomagnetic disturbances of lower amplitude observed during the same months. EOFs were found to correlate well with space weather parameters characterizing solar UV and XR fluxes, number of the solar flares, parameters of the solar wind, and geomagnetic indices. Multiple regression models were constructed to fit EOFs using combinations of the space weather parameters with a lag from 0 to 2 days. Combining the regression models of EOFs with the corresponding PCs, we reconstructed TEC variations as a function of space weather parameters observed in previous days. The possibility to use such reconstructions for the TEC forecasting was also studied.