GIC at mid-latitudes under extreme Dst scenarios

The Dst index is primarily a measure of the magnitude of the equatorial magnetospheric ring current, based on an hourly average of the variation of the Horizontal (H) component from a number of low and mid-latitude ground-based observatories. Extreme values of this index are rare; for example the largest in recent decades occurred during the March 1989 storm, with Dst peaking at -589 nT. Larger values of Dst have been established for the Carrington event – in September 1859 the Bombay observatory recorded a peak of about -850 nT. However, some researchers have suggested that Dst may have been even larger still (-1760 nT), based on one value from Bombay during the 1859 storm. More recent theoretical work has suggested that -2500 nT is the largest possible value physically achievable, given our knowledge of the magnetosphere. Although the ring current variation affects low to mid-latitudes directly, at higher latitudes most variation of H is due to auroral electrojet activity. Motivated by the largest theoretical possible Dst value, we investigate the size of the H variation at high latitudes that might occur during large Dst excursions, by extrapolating relationships observed between H and Dst from previous large storms. To test the consequence of these large H variations we compute the likely GIC in mid-latitude European high-voltage power distribution networks, based on a set of conductivity and grid models of similar complexity, during geomagnetic storms with Dst values of -800, -1700 and -2500 nT.