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.

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A detailed palaeomagnetic, rock-magnetic and palaeointensity study has been carried out on a Miocene volcanic sequence which consists of 39 consecutive lava flows recording a polarity transition in La Gomera (Canary Islands, Spain). In addition, new 40 Ar/ 39 Ar ages were obtained in two flows, yielding 9.63 ± 0.06 Ma in the lower and 9.72 ± 0.08 Ma in the upper part of the sequence. Palaeomagnetic results allowed determining a ChRM direction in all studied lavas: The 25 lowermost flows of the sequence display normal polarity directions and above, a sequence of 14 flows correspond to a transitional geomagnetic regime. If considered together with palaeomagnetic results from a previous study, which were obtained on the flows immediately overlying the upper part of the sequence analysed in this work, these results indicate that the reversal recorded in the Hermigua sequence corresponds to the normal to reverse C4Ar2n to C4Ar3r polarity transition. The lower-lying 25 normal-polarity flows yield a mean direction D = 359.6 • , I = 42.4 • (α 95 = 5.1 • ; k = 33) which agrees well with the expected values. Above, a sequence of 14 flows displays a more irregular directional behaviour, including several transitional directions, suggesting the occurrence of a precursor to the transition. The presence of a virtual geomagnetic pole (VGP) cluster in the western Atlantic Ocean observed in this study coincides with previous records of Miocene transitions. Interestingly, this region corresponds to a near-radial flux centre of the present-day non axial dipole field. Angular dispersion of VGPs calculated for the 25 lowermost normal polarity flows of the sequence shows a lower than expected result. Palaeointensity determinations were carried out using a Thellier type double heating method. 27 of the 48 analysed samples measured yielded successful results. Mean VDMs mean values range from 1.1 ± 0.5 to 8.8 ± 0.9 × 10 22 Am 2 . The intensity values decrease significantly on approaching the directional transitional zone, suggesting an earlier start of the polarity transition in the intensity record, typical of a decreasing axial dipole.

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