The Extreme Space Weather Event in 1903 October/November: An Outburst from the Quiet Sun

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 solar activity. Occurring in the weak Solar Cycle 14 just after its minimum, the extreme space weather event in 1903 October–November is 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−1) and is regarded as a superstorm with an estimated minimum of the equivalent disturbance storm time index (Dst') of ≈−531 nT. The reconstructed time series has been compared with the equatorward extension of auroral oval (≈44fdg1 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.