On the intensity 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 ~1,679 km/s. The commencement pressure on the magnetopause was ~32.4 nPa, sufficient to compress the subsolar magnetopause radius 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.