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

Abstract

Abstract Intense solar eruptions occasionally trigger extreme geomagnetic storms, expand the boundaries of the auroral oval, and facilitate equatorward extension of auroral visibility. Analyses of such events are important to better understand the extremity of space weather and its impact on the technological infrastructure of modern civilisation. However, unlike other extreme geomagnetic storms, little is known about the auroral activity associated with the extreme geomagnetic storm that occurred on 15/16 July 1959, which was the second largest geomagnetic storm in the space age. In this study, two Chinese accounts and one Russian account of auroral visibility at low (≤ 40°) magnetic latitudes (MLATs) were acquired and analysed. These records allowed us to conservatively reconstruct the equatorward boundaries of the auroral visibility and auroral oval at 27.4° MLAT and 35.4° invariant latitude, respectively. Our analysis chronologically contextualised them, revealing that the main phase occurred slightly before the peak of the extreme geomagnetic storm. Moreover, their coloration indicates the excitation of, at least, nitrogen at 427.8 nm and oxygen at 557.7 nm at these low MLATs. Our results allow us to contextualise this extreme geomagnetic storm based on the equatorward boundary of the auroral oval, thereby facilitating the improvement in existing empirical correlation models.