On the intencity of the Magnetic Superstorm of September 1909

Analysis is made of solar observations and ground-based magnetometer data recording space weather before and during the magnetic superstorm of 25 September 1909. From these data, it is inferred that the storm was initiated by an interplanetary coronal-mass ejection having a mean Sun-to-Earth velocity of similar to 1,679 km/s. The commencement pressure on the magnetopause was similar to 32.4 nPa, sufficient to compress the subsolar magnetopause radius to similar to 5.9 Earth radii. Early on in the evolution of the storm, low-latitude geomagnetic disturbance exhibited extreme longitudinal asymmetry, something that can be attributed to substorm activity extending to low latitudes. For this storm, Dst attained a minimum of -595 nT, comparable to that of the great magnetic storm of March 1989 (-589 nT; the most intense storm in terms of Dst of the space age). These results inform projects focused on understanding and mitigating the deleterious effects of extreme space-weather events. Plain Language Summary Historical records of solar observations and ground-level geomagnetic disturbance are analyzed for the magnetic superstorm of 25 September 1909. This storm was initiated by the arrival, at Earth, of a shock wave in the solar wind that can be linked to the ejection of plasma from a sunspot active region. The 1909 storm was one of the most intense of the twentieth century. It exhibited violent levels of geomagnetic disturbance (with a minimum Dst value of -595 nT), caused widespread interference to telegraph systems, and brought spectacular aurorae to the nighttime sky. Results reported here inform projects focused on understanding and mitigating the deleterious effects of extreme space-weather events to technological systems of importance to modern society.