Analyses of Equatorward Auroral Extensions during the Extreme Geomagnetic Storm on 15 July 1959

Intense solar eruptions occasionally trigger extreme geomagnetic storms, expand the boundaries of the auroral oval, and facilitate equatorward extensions of the auroral visibility. It is important to analyse such events, to better understand the extremity of space weather and its impact on the technological infrastructure of the modern civilization. However, unlike other extreme geomagnetic storms, little is known about the auroral activity associated with the extreme geomagnetic storm on 15/16 July 1959, the second largest geomagnetic storm in the space age. This study acquired and analysed two Chinese accounts and one Russian account of auroral visibility at low (≤40°) magnetic latitudes (MLATs). These records allowed us to conservatively reconstruct the equatorward boundaries of the auroral visibility and the auroral oval at 27.4° MLAT and 35.4° invariant latitude, respectively. Our analysis chronologically contextualized these auroral records slightly before the peak of the extreme geomagnetic storm. Moreover, their coloration indicates the excitations of, at least, nitrogen at 427.8 nm and oxygen at 557.7 nm at these low MLATs. Our results allow us to contextualize this extreme geomagnetic storm within other extreme events, based on the equatorward boundary of the auroral oval, thereby facilitating the improvement in existing empirical models for correlations of the auroral extension and the storm intensity.