September 2017 Space-Weather Events: A Study on Magnetic Reconnection and Geoeffectiveness

Abstract

September 2017 was an extremely active space-weather period with multiple events leading to varying impacts on the Earth’s magnetosphere. The geoeffectiveness of a space-weather event largely depends on the magnetic reconnection between the southward interplanetary magnetic field and the day-side northward geomagnetic field. In this work, we estimate the reconnection rates during two intense (SYM-H peak ≤ − 100 nT) and two moderate (−50 nT ≥ SYM-H > − 100 nT) geomagnetic storms, and a high-intensity long-duration continuous auroral electrojet (AE) activity (HILDCAA) event in order to assess the contribution of the reconnection to resultant geomagnetic effects. Strong reconnection rates led to intense geomagnetic storms, while moderate-intensity geomagnetic storms were associated with discrete and weaker reconnection events. Comparatively weak magnetic reconnection continuing for a long interval of time led to the HILDCAA event. On average, a significant correlation was observed between the reconnection rates and geomagnetic-activity indices. However, the relationships are found to be more complex on shorter time-scales, varying from event to event. The importance of a quantitative study of the reconnection process for the prediction of geomagnetic activity is demonstrated.