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

TL;DR

This paper reviews the 1872 February extreme geomagnetic storm, analyzing historical observations of sunspots, magnetic disturbances, and auroras to understand its origin, strength, and significance among the largest storms recorded.

Contribution

It provides a detailed reconstruction of the 1872 storm's characteristics, origins, and compares its intensity to other historic extreme geomagnetic storms.

Findings

The 1872 storm had a minimum Dst of approximately -834 nT.

Auroras were observed at low magnetic latitudes, indicating a very strong storm.

The storm was among the three largest geomagnetic storms recorded to date.

Abstract

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 (\approx 500 {\mu}sh) that was favorably situated near disk center (S19{\deg} E05{\deg}). 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 \approx29 hr to Earth. Magnetograms show that the storm began with a sudden commencement at \approx14:27 UT and allow a minimum Dst estimate of {\pounds} -834 nT. Overhead aurorae were credibly reported at Jacobabad (British India) and Shanghai (China), both at 19{\deg}.9 in magnetic latitude (MLAT) and 24{\deg}. 2 in invariant latitude (ILAT). Auroral visibility was reported from 13 locations with MLAT below |20|{\deg} for the 1872 storm (ranging from |10{\deg}. 0|--|19{\deg}. 9| MLAT) versus one each for the 1859 storm (|17{\deg}. 3| MLAT) and the 1921 storm (|16.{\deg}2| MLAT). The auroral extension and conservative storm intensity indicate a magnetic storm of comparable strength to the extreme storms of 1859 September (25{\deg}.1 \pm 0{\deg}.5 ILAT and -949 \pm 31 nT) and 1921 May (27{\deg}.1 ILAT and -907 \pm 132 nT), which places the 1872 storm among the three largest magnetic storms yet observed.