Abstract
Past studies found that large‐amplitude geomagnetically induced current (GIC) related to magnetospheric Ultra Low Frequency (ULF) waves tend to be associated with periods >120 s at magnetic latitudes >60°, with comparatively (a) smaller GIC amplitudes at lower latitudes and shorter wave periods and (b) fewer reports of waves associated with GIC at lower latitudes. ULF wave periods generally decrease with decreasing latitude; thus, we examine whether these trends might be due, in part, to the undersampling of ULF wave fields in commonly available measurements with 60 s sampling intervals. We use geomagnetic field (B), geoelectric field (E), and GIC measurements with 0.5–10 s sampling intervals during the 29–31 October 2003 geomagnetic storm to show that waves with periods <∼120 s were present during times with the largest amplitude E and GIC variations. These waves contributed to roughly half the maximum E and GIC values, including during times with the maximum GIC values reported over a 14‐year monitoring interval in New Zealand. The undersampling of wave periods <120 s in 60 s measurements can preclude identification of the cause of the GIC during some time intervals. These results indicate (a) ULF waves with periods ≤120 s are an important contributor to large amplitude GIC variations, (b) the use of 0.1–1.0 Hz sampling rates reveals their contributions to B, E, and GIC, and (c) these waves' contributions are likely strongest at magnetic latitudes <60° where ULF waves often have periods <120 s.
Plain Language Summary
Geomagnetic field variation can induce geoelectric fields in the Earth that interfere with the operation of grounded electric power transmission systems. There are many sources of geomagnetic field variation, including plasma waves in the near‐Earth space environment. Past studies have found that wave periods between 120 and 600 s in high latitude regions led to the largest electrical currents in power systems. The waves responsible for driving these electrical currents have periods that decrease with decreasing latitude. We examine whether these trends may be partly due to the wide use of measurements with sampling rates that cannot resolve wave periods below 120 s. We use measurements sampled every 0.5–10 s during a major geomagnetic storm to show that wave periods shorter than 120 s can drive large amplitude geoelectric field and electrical current variations. These results suggest (a) waves with periods <120 s are an important contributor to electrical currents in power systems, (b) the use of sampling intervals of 1–10 s reveals their contributions, and (c) these waves' contributions are likely strongest at middle and low latitude regions where wave periods are often <120 s.
Key Points
Waves with periods <∼2 min occur at magnetic latitudes <60° throughout the 29–31 October 2003 storm
The waves drive or contribute significantly to geomagnetically induced currents that are comparable to 14‐year peak values
0.1–1.0 Hz sampling rates are required to determine wave contributions to currents at magnetic latitudes <60°