Recreating the Horizontal Magnetic Field at Colaba during the Carrington Event with Geospace Simulations

Abstract:

An intriguing aspect of the famous 2 September 1859 geomagnetic disturbance (or `Carrington' event) is the horizontal magnetic ($B_H$) dataset measured in Colaba, India (magnetic latitude approximately 20 degrees N). This dataset exhibits a sharp decrease of over 1600 nT and a quick recovery of about 1300 nT, all within a few hours during the daytime. The mechanism behind this has previously been attributed to magnetospheric processes, ionospheric processes or a combination of both. In this paper, we outline our efforts to replicate this low-latitude magnetic dataset using the Space Weather Modelling Framework (SWMF). By simulating an extremely high pressure solar wind scenario, we can emulate the low-latitude surface magnetic signal at Colaba. We find that magnetospheric currents adjacent to the near-Earth magnetopause are the primary contributors to the large Colaba $B_H$, as are strong Region~1 field-aligned currents (FACs). The rapid recovery of $B_H$ in our scenario is due to the retreat of these magnetospheric currents as the magnetosphere expands, as opposed to ring current dynamics. In addition, we find that the scenario that accurately recreated surface magnetic field observations during the Carrington event had a minimum calculated Dst value between -431 and -1191~nT, indicating that Dst may not be a suitable estimate of storm intensity for this kind of event.