Geoelectric Fields and Geomagnetically Induced Currents Related to Magnetospheric Ultra Low Frequency Waves

Geomagnetic field variations related to magnetospheric Ultra Low Frequency (ULF) waves are frequently observed during geomagnetically active conditions, and they induce geoelectric fields that ultimately drive geomagnetically induced currents (GIC) in power systems. The properties of these waves – including frequency, amplitude, and polarization – vary widely due to many factors including local time, latitude, phase of geomagnetic storm, state of magnetosphere-ionosphere system, and type of solar wind driving condition. Additionally, measurements of geomagnetic fields, geoelectric fields, and GIC with sampling intervals needed to detect many ULF waves (~1s) are sparse during major historical storms. For these reasons, it is challenging to quantitatively assess extreme ULF wave amplitudes and determine which conditions lead to the largest wave fields and GIC. In this research, we use recently improved ground conductivity constraints and an expanded catalog of 1s measurements during past geomagnetic storms to estimate moderate, large, and extreme ULF wave geoelectric field amplitudes, primarily focusing on mid- and low-latitude regions and comparing with direct GIC measurements in several cases. We further describe the conditions that lead to the largest amplitude ULF wave geoelectric fields and GIC.