Great geomagnetic storms and extreme impulsive geomagnetic field disturbance events An analysis of observational evidence including the great storm of May 1921
Impulsive geomagnetic field disturbances are an important aspect of the geomagnetic storm environment for electric power grids and other ground-based infrastructures that can be impacted by geomagnetically induced currents (GIC). Significant power grid impacts in present day networks have been observed at relatively low levels of intensity, for example the Quebec grid blackout during the March 1989 storm occurred at a peak intensity of ∼480 nT/min and permanent damage to large power transformers have occurred at even lower intensity levels. An analysis of both contemporary and historic storm data and records indicates d Bh/d t impulsive disturbances larger than 2000 nT/min have been observed at latitudes of concern for power grid infrastructures. In extreme scenarios available data suggests that disturbance levels as high as ∼5000 nT/min may have occurred during the great geomagnetic storm of May 1921. A comparative evaluation of this storm using morphology-based techniques indicates that large electrojet intensifications exceeding those observed during the March 1989 superstorm were present, further that the Dst range for the May 1921 storm could be as much as -900 nT or ∼50% larger than the March 1989 Dst ranking which is the largest on record.