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

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 in India. 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 interplanetary coronal mass ejection transit time as <= 17.1 hr and located the Delta H peak at 06:20-06:25 UT, revealing a magnitude discrepancy between the hourly and spot measurements (-1691 nT versus -1263 nT). Furthermore, we have newly derived the time series of Delta Y and Delta Z, which peaked at Delta Y approximate to 378 nT (05:50 UT) and 377 nT (06:25 UT), and Delta Z approximate to -173 nT (06:40 UT). We have also computed their hourly averages and removed their solar quiet field variations in each geomagnetic component to derive their hourly disturbance variations (Dist) with latitudinal weighting. Our calculations have resulted in disturbance variations with latitudinal weighting of Dist Y approximate to 328 nT and Dist Z approximate to -36 nT, and three scenarios of Dist H approximate to -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.