Geomagnetic Pulsations Driving Geomagnetically Induced Currents

Abstract

Geomagnetically induced currents (GICs) are driven by the geoelectric field induced by fluctuations of Earth's magnetic field. Drivers of intense GICs are often associated with large impulsive events such as coronal mass ejections. To a lesser extent fluctuations from regular oscillations of the geomagnetic field, or geomagnetic pulsations, have also been identified as possible drivers of GICs. In this work we show that these low-frequency pulsations are directly observed in measured GIC data from power networks. Due to the low-pass nature of GICs, Pc5 and lower-frequency pulsations drive significant GICs for an extended duration even at midlatitudes. Longer-period Ps6-type disturbances apparently not typical of midlatitudes are seen with GIC amplitudes comparable to the peak GIC at storm sudden commencement. The quasi-ac (alternating current) nature of the sustained pulsation driving affects the power system response and cannot be properly modeled using only direct current (dc) models. A further consideration is that the often used dB/dt GIC proxy is biased to the sampling rate of the geomagnetic field measurements used. The dB/dt metric does not adequately characterize GIC activity at frequencies in the low ultralow-frequency (ULF) range, and a frequency-weighted proxy akin to geoelectric field should be used instead.Plain Language Summary Geomagnetically induced currents (GICs) are naturally occurring currents induced in conductive media, such as the Earth, by fluctuations of the geomagnetic field. When large grounded conductors such as power networks are present, these currents also enter the network and pose serious risk to the stability of the network. In extreme cases, the GICs can result in total network collapse. Particular fluctuations of the local geomagnetic field are geomagnetic pulsations, which occur when the magnetic field lines are perturbed and ring, causing oscillations. These oscillations have not previously been thought to be effective in driving large GICs, but now measured GIC data have shown this is not always the case, and the power grid couples particularly well to low-frequency pulsations. Essentially, the power grid acts as an antenna, and pulsations have been picked up where not previously expected. Understanding the effectiveness of these pulsations and including them in GIC modeling is vital for protection of the grounded power networks we rely on.

Frequency Considerations in GIC Applications

“…variations with high amplitude are the ones creating the most severe power system impacts. The specific importance of GIC estimations during Storm Sudden Commencement (SSC) has been emphasized by Kappenman, (2003Kappenman, ( , 2006, Marshall et al, (2012), and the research of GIC during pulsations has been recently presented in Yagova et al, (2020) and Heyns et al, (2020). Thus, it would be valuable to have further clarification of questions (1) and (2) above.…”

Forecasting the Probability of Large Rates of Change of the Geomagnetic Field in the UK: Timescales, Horizons, and Thresholds

“…Though geomagnetic storms are often broadly cited as a cause of severe space weather, there are numerous physical processes that occur during the extended intervals of strong coupling between the solar wind and magnetosphere. Examples of these physical process include the SSC at the start of the storm (e.g., Clilverd et al., 2018; Rodger et al., 2017), substorms that occur during the main and recovery phases (e.g., Dimmock et al., 2019; Ngwira et al., 2015, 2018; Pulkkinen et al., 2003; Pulkkinen et al., 2015; Viljanen et al., 2006), and ULF pulsations (e.g., Heyns et al., 2020). Such magnetic ULF waves with periods of between 2.5 and 10 min, termed Pc5 waves (Jacobs et al., 1964), may be observed in the post‐noon sector as a result of the Kelvin‐Helmholtz instability operating on the dusk flank (Mann et al., 1999; Rae et al., 2005), or related to the impact of an interplanetary shock (Zhang et al., 2010).…”

Statistics on Omega Band Properties and Related Geomagnetic Variations

“…Inductive coupling between dB/dt and electric field that drives GIC is not linear due to the finite Earth's conductivity, so a large but short dB/dt impulse does not necessarily produce large GIC (Cagniard, 1953; Oyedokun et al., 2020). This nonlinearity in the dB/dt ‐GIC relationship is less pronounced for the lower frequency magnetic field variations with periods more than 1 min, including Pi3/Ps6 range of pulsations (Heyns et al., 2021) inherent to omega bands (Gustafsson et al., 1981; Jorgensen et al., 1999; Opgenoorth et al., 1983; Saito, 1978; Viljanen et al., 2001). This gives the confidence to use dB/dt as a proxy for GIC in the case of omega bands.…”