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Abstract. The Coimbra Magnetic Observatory (International Association of Geomagnetism and Aeronomy code COI) inPortugal 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.

While the Sun is generally more eruptive during its maximum and declining phases, observational evidence shows certain cases of powerful solar eruptions during the quiet phase of the solar activity. Occurring in the weak Solar Cycle 14 just after its minimum, the extreme space weather event in 1903 October -November was one of these cases.Here, we reconstruct the time series of geomagnetic activity based on contemporary observational records. With the mid-latitude magnetograms, the 1903 magnetic storm is thought to be caused by a fast coronal mass ejection (~1500 km/s) and is regarded as an intense event with an estimated minimum Dst' of ~−513 nT The reconstructed time series has been compared with the equatorward extension of auroral oval (~44.1° in invariant latitude) and the time series of telegraphic disturbances. This case study shows that potential threats posed by extreme space weather events exist even during weak solar cycles or near their minima.

The Azambuja fault is a NNE trending structure located 50 km NE of Lisbon, in an area of important historical seismicity. It is sited in the Lower Tagus Basin, a compressive foredeep basin related to tectonic inversion of the Mesozoic Lusitanian Basin in the Miocene. The fault is evident in commercial seismic reflection data, where it shows steep thrust geometry downthrowing the Cenozoic sediments to the east. It has also a clear morphological signature, presenting a NNE-SSW trending, east facing, 15 km long scarp, reaching a maximum height of 80 m. The fault scarp is the geomorphic appearance of a flexure expressed as a zone of distributed deformation, where Miocene and Pliocene sediments are tilted eastwards and are cut by steeply dipping meso-scale faults presenting reverse and normal offsets, with a net downthrow to the east. This pattern at the surface is compatible with a steep fault in the basement that tilts and branches through the overlying Cenozoic sedimentary cover. In order to constrain the neotectonic activity of this structure, detailed geological studies were conducted. Morphotectonics was studied through aerial photo interpretation, analysis of topographic maps and digital mapping. Those studies indicate Quaternary slip on the fault in the ranges of 0.05-0.06 mm per year. Seismogenic behaviour was assumed for the Azambuja fault based on the evidence of Quaternary tectonic activity and its location in an area of significant historical seismicity. M w 6.4-6.7 maximum earthquakes, with recurrence intervals of 10000-25000 years, were estimated based upon the displaced morphological references, cumulative offsets and fault length.

[1] The great solar storm that took place on 24-25 October 1870 is not well known and has been almost absent from previous studies. In this work, a large amount of information that was registered at the time is compiled and analyzed, including early geomagnetic data and several comprehensive descriptions of the auroras observed during these two nights. These descriptions reveal unusual characteristics for a typical lowlatitude aurora. For example, unlike most low-latitude auroras (generally red and diffuse), this event was mostly characterized by a variable palette of colors, including greenish and white. The geomagnetic records analyzed from Lisbon and Coimbra (Portugal), Greenwich (United Kingdom), Munich (Germany), and Helsinki (Finland) clearly show an intense geomagnetic disturbance on 24-25 October. The Coimbra magnetograms reveal that this disturbance consisted of two distinct geomagnetic storms, the first on 24 October (with amplitudes of 37 0 in D and 182 nT and 48 nT in H and Z, respectively), and the second on 25 October (with amplitudes of 33 0 in D and 281 nT and 192 nT in H and Z, respectively). Finally, from early photographic solar observations made during 1870, we have identified a long-lived group of sunspots that are most likely related to the solar source of this great event of space weather.