The Extreme Space Weather Event in February/March 1941

TL;DR

This paper analyzes the extreme geomagnetic storm of February/March 1941, reconstructing its intensity and effects, and compares it with other major storms to understand its significance and characteristics.

Contribution

It reconstructs the 1941 storm's intensity using an alternative Dst measure and analyzes its solar origin and global geomagnetic impact, providing new insights into historic extreme space weather events.

Findings

The 1941 storm had a minimum Dst* < -464 nT, comparable to the most extreme storms since 1957.

The storm caused significant low-latitude aurorae and an equatorward boundary at 38.5° invariant latitude.

Equatorial stations recorded enhanced storm amplitudes, affecting Dst-based measurements.

Abstract

Given the infrequency of extreme geomagnetic storms, it is significant to note the concentration of three extreme geomagnetic storms in 1941, whose intensities ranked fourth, twelfth, and fifth within the aa index between 1868-2010. Among them, the geomagnetic storm on 1 March 1941 was so intense that three of the four Dst station magnetograms went off scale. Herein, we reconstruct its time series and measure the storm intensity with an alternative Dst estimate (Dst*). The source solar eruption at 09:29 - 09:38 GMT on 28 February was located at RGO AR 13814 and its significant intensity is confirmed by large magnetic crochets of 35 nT measured at Abinger. This solar eruption most likely released a fast interplanetary coronal mass ejection with estimated speed 2260 km/s. After its impact at 03:57 - 03:59 GMT on 1 March, an extreme magnetic storm was recorded worldwide. Comparative analyses on the contemporary magnetograms show the storm peak intensity of minimum Dst* < -464 nT at 16 GMT, comparable to the most and the second most extreme magnetic storms within the standard Dst index since 1957. This storm triggered significant low-latitude aurorae in the East Asian sector and their equatorward boundary has been reconstructed as 38.5{\deg} in invariant latitude. This result agrees with British magnetograms which indicate auroral oval moving above Abinger at 53.0{\deg} in magnetic latitude. The storm amplitude was even more enhanced in equatorial stations and consequently casts caveats on their usage for measurements of the storm intensity in Dst estimates.