The extreme space weather event in September 1909

Monthly Notices of the Royal Astronomical Society
By: , and 



We evaluate worldwide low-latitude auroral activity associated with the great magnetic storm of September 1909 for which a minimum Dst value of −595 nT has recently been determined. From auroral observations, we calculate that the equatorward boundary of the auroral oval in the 1909 event was in the range from 31°–35° invariant latitude (assuming auroral height of 400 km) to 37°–38° (800 km). These locations compare with satellite-based observations of precipitating auroral electrons down to 40° magnetic latitude for the March 1989 storm with its comparable minimum Dst value of −589 nT. According to Japanese auroral records, bluish colour started to appear first, followed by reddish colour. The colour change can be attributed to the transition from sunlit aurora to the usual low-latitude reddish aurora. Telegraph communications were disrupted at mid/low latitudes, coincidently with the storm main phase and the early recovery phase. The telegraphic disturbances were caused by geomagnetically induced currents associated with the storm-time ring current and substorm current wedge. From the calculated CME energy ─ based on the 24.75 hr separation between the flare-associated magnetic crochet and the geomagnetic storm sudden commencement and interplanetary conditions inferred from geomagnetic data ─ and consideration of the ∼−40 nT crochet amplitude, we estimated that the soft X-ray class of the 24 September 1909 flare was ≥X10. As is the case for other extreme storms, strong/sharp excursions in the horizontal component of the magnetic field observed at low-latitude magnetic stations were coincident with the observation of low-latitude aurora.

Publication type Article
Publication Subtype Journal Article
Title The extreme space weather event in September 1909
Series title Monthly Notices of the Royal Astronomical Society
DOI 10.1093/mnras/sty3196
Volume 484
Issue 3
Year Published 2019
Language English
Publisher Oxford Academic
Contributing office(s) Geologic Hazards Science Center, Geomagnetism Program
Description 17 p.
First page 4083
Last page 4099
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