Research Article

Geomagnetically induced currents during the 07–08 September 2017 disturbed period: a global perspective

¹ British Antarctic Survey (UKRI-NERC), High Cross, Madingley Road, Cambridge, CB3 0ET England, United Kingdom
² Department of Physics, University of Otago, PO Box 56 Dunedin, New Zealand
³ Transpower New Zealand Limited, 96 The Terrace, PO Box 1021 Wellington, New Zealand
⁴ British Geological Survey (UKRI-NERC), The Lyell Centre, Currie, Edinburgh, EH14 4BA Scotland United Kingdom
⁵ Scottish Power Energy Networks Holdings Ltd, 10 Technology Ave, Blantyre, Glasgow, G72 Scotland, United Kingdom

^* Corresponding author: macl@bas.ac.uk

Received: 21 October 2020
Accepted: 31 March 2021

Abstract

Measurements from six longitudinally separated magnetic observatories, all located close to the 53° mid-latitude contour, are analysed. We focus on the large geomagnetic disturbance that occurred during 7 and 8 September 2017. Combined with available geomagnetically induced current (GIC) data from two substations, each located near to a magnetic observatory, we investigate the magnetospheric drivers of the largest events. We analyse solar wind parameters combined with auroral electrojet indices to investigate the driving mechanisms. Six magnetic field disturbance events were observed at mid-latitudes with dH/dt > 60 nT/min. Co-located GIC measurements identified transformer currents >15 A during three of the events. The initial event was caused by a solar wind pressure pulse causing largest effects on the dayside, consistent with the rapid compression of the dayside geomagnetic field. Four of the events were caused by substorms. Variations in the Magnetic Local Time of the maximum effect of each substorm-driven event were apparent, with magnetic midnight, morning-side, and dusk-side events all occurring. The six events occurred over a period of almost 24 h, during which the solar wind remained elevated at >700 km s⁻¹, indicating an extended time scale for potential GIC problems in electrical power networks following a sudden storm commencement. This work demonstrates the challenge of understanding the causes of ground-level magnetic field changes (and hence GIC magnitudes) for the global power industry. It also demonstrates the importance of magnetic local time and differing inner magnetospheric processes when considering the global hazard posed by GIC to power grids.