Dst of the Carrington storm of 1859

Abstract

The super-storm of 1859 gives an opportunity to apply models to predict Dst that have been exercised mostly on non-extreme cases. The exercise gains significance through a Bombay magnetogram that Tsurutani et al. (2003) recently published showing a negative H excursion of ∼1600 nT, which is unprecedented for the latitude of the station, and which presents difficulties of interpretation if the negative excursion is taken to be equivalent to Dst. Following a suggestion by Li et al. (2006), we have replaced the original Bombay magnetogram, which has many points per hour during the interesting phase of the storm, by hourly averages, thereby constructing a time profile that is closer to a Dst profile as it is usually calculated. Then, the maximum H-depression is ∼−850 nT, which lies not so astonishingly outside the officially observed range. The Bombay magnetogram exhibits two major H-depressions, which we interpret to mean that the event was caused by a geoeffective ICME-sheath followed by a magnetic cloud across which the magnetic field rotated from north to south. On this interpretation, the ICME-sheath caused the extraordinary −1600 nT excursion in H. Then, the issue is whether this large excursion was produced by ionospheric currents (in which case it is not so exceptional) or by magnetospheric currents (in which case it is unprecedented)? To explore the second possibility, we use empirical models that relate the measured shock transit time to the ICME speed and peak magnetic field strength at 1 AU together with average pre-shock interplanetary conditions to construct geoeffective parameters throughout the ICME-sheath. We use the Burton et al. equation as modified by O’Brien and McPherron to calculate Dst. The resulting Dst profile lies reasonably close to hourly averaged Bombay magnetogram, especially if one uses the upper limit allowed for field strength in the ICME-sheath or if one discards the most extreme outlier of the Bombay measurements. We conclude that it is possible to interpret the Bombay magnetogram as having been produced by the magnetospheric currents.