GIC produced by Solar Wind transients

Abstract

Geomagnetic storms are potentially hazardous to the activities and technological infrastructure of modern civilisation. The storms cause large and comparatively rapid changes in the geomagnetic field at the Earth’s surface, leading to Geomagnetically Induced Currents (GICs) in electrical power transmission networks. GICs can disrupt and potentially damage equipment in power transmission networks; examples of such damage have occurred in Canada, South Africa, Sweden, USA, and also New Zealand. Large geomagnetic storms are triggered by reconfiguration of the geomagnetic field due to forcing from space, and in particular the plasma which flows from the Sun termed the solar wind.

In this thesis we examine the link between processes in space, geomagnetic field variations, and GIC observations. We utilise the large dataset of GIC measurements which we have created from the long-lived and spatially extensive set of Direct Current (DC) observations undertaken by Transpower New Zealand. The goal of our research is to better understand the link between processes in space and GIC in New Zealand. This is a step towards providing warning of events likely to cause large and potentially damaging GICs at mid-latitude locations.

This research particularly focuses on solar wind magnetosphere-ionosphere coupling. In this topic it is critical to understand how the solar wind and the frozen-in Interplanetary Magnetic Field (IMF) couple to activity at the Earth’s surface. We compare the timing of spikes in ground-based magnetic field observations with OMNIweb predicted arrival time at Earth of solar wind shock events, but find these times rarely agree. Instead, we use a case-by-case test to link the SOHO reported shock events to Earth arrival times. This produces an empirical relationship between solar wind pressure measurements and the associated horizontal magnetic field rate of change (H') measurements, which is found to strongly depend on IMF orientation and magnetic latitude.

The arrival of the solar wind shocks drive H' spikes in New Zealand, as reported by the magnetic field observatory at Eyrewell. The shock-produced H' spikes lead to GIC enhancements detected at transformers throughout the South Island of New Zealand. The magnitude of these GIC events is found to strongly depend on local time; larger GIC magnitudes are found during local midday compared to local midnight events, for the same level of H'. Sunward facing locations typically experience 50% larger currents than nightside locations. Various possible causes of this are investigated, with mixed results. It is determined that public/non-public holidays, IMF orientation, and the directional change in the declination angle strongly affect the local time dependence.