Temporal Variations of the Three Geomagnetic Field Components at Colaba Observatory around the Carrington Storm in 1859

TL;DR

This study reconstructs and analyzes the geomagnetic field variations at Colaba during the 1859 Carrington storm, providing revised timings, magnitudes, and insights into the storm's impact and preconditioning effects.

Contribution

It offers the first detailed chronological reconstruction of the 1859 geomagnetic disturbances at Colaba, including new measurements and analysis of the storm's magnitude and timing.

Findings

Revised the peak disturbance times for geomagnetic components.

Identified magnitude discrepancies between hourly and spot measurements.

Suggested preconditioning of the geomagnetic field prior to the storm.

Abstract

The Carrington storm in 1859 September has been arguably identified as the greatest geomagnetic storm ever recorded. However, its exact magnitude and chronology remain controversial, while their source data have been derived from the Colaba H magnetometer. Here, we have located the Colaba 1859 yearbook, containing hourly measurements and spot measurements. We have reconstructed the Colaba geomagnetic disturbances in the horizontal component ({\Delta}H), the eastward component ({\Delta}Y), and the vertical component ({\Delta}Z) around the time of the Carrington storm. On their basis, we have chronologically revised the ICME transit time as =< 17.1 hrs and located the {\Delta}H peak at 06:20 -- 06:25 UT, revealing a magnitude discrepancy between the hourly and spot measurements (-1691 nT vs. -1263 nT). Furthermore, we have newly derived the time series of {\Delta}Y and {\Delta}Z, which peaked at {\Delta}Y ~ 378 nT (05:50 UT) and 377 nT (06:25 UT), and {\Delta}Z ~ -173 nT (06:40 UT). We have also computed the hourly averages and removed the solar quiet (Sq) field variations from each geomagnetic component to derive their hourly variations with latitudinal weighting. Our calculations have resulted in the disturbance variations (Dist) with latitudinal weighting of Dist Y ~ 328 nT and Dist Z ~ -36 nT, and three scenarios of Dist H ~ -918, -979, and -949 nT, which also approximate the minimum Dst. These data may suggest preconditioning of the geomagnetic field after the August storm ({\Delta}H =< -570 nT), which made the September storm even more geoeffective.