The Extreme Space Weather Event of 1872 February: Sunspots, Magnetic Disturbance, and Auroral Displays

We review observations of solar activity, geomagnetic variation, and auroral visibility for the extreme geomagnetic storm on 1872 February 4. The extreme storm (referred to here as the Chapman-Silverman storm) apparently originated from a complex active region of moderate area (approximate to 500 mu sh) that was favorably situated near disk center (S19 degrees E05 degrees). There is circumstantial evidence for an eruption from this region at 9-10 UT on 1872 February 3, based on the location, complexity, and evolution of the region, and on reports of prominence activations, which yields a plausible transit time of approximate to 29 hr to Earth. Magnetograms show that the storm began with a sudden commencement at approximate to 14:27 UT and allow a minimum Dst estimate of <= -834 nT. Overhead aurorae were credibly reported at Jacobabad (British India) and Shanghai (China), both at 19.degrees 9 in magnetic latitude (MLAT) and 24.degrees 2 in invariant latitude (ILAT). Auroral visibility was reported from 13 locations with MLAT below divide 20 divide degrees for the 1872 storm (ranging from divide 10.degrees 0 divide - divide 19.degrees 9 divide MLAT) versus one each for the 1859 storm ( divide 17.degrees 3 divide MLAT) and the 1921 storm ( divide 16.degrees 2 divide MLAT). The auroral extension and conservative storm intensity indicate a magnetic storm of comparable strength to the extreme storms of 1859 September (25.degrees 1 +/- 0.degrees 5 ILAT and -949 +/- 31 nT) and 1921 May (27.degrees 1 ILAT and -907 +/- 132 nT), which places the 1872 storm among the three largest magnetic storms yet observed.