The Interplanetary and Magnetospheric Causes of Geomagnetically Induced Currents (GICs) > 10 A in the Mäntsälä Finland Pipeline: 1999 through 2019

The interplanetary and magnetospheric causes of intense geomagnetically induced current (GIC) > 10 A (and > 30 A) events during 21 years (1999 through 2019) at the Mäntsälä, Finland (57.9° magnetic latitude) gas pipeline have been studied. Although shocks and substorms are predominant causes of intense GICs, some newly discovered geoeffective interplanetary features are: solar wind plasma parcel impingements, interplanetary magnetic field (IMF) northward (Bn) and southward (Bs) turnings, interplanetary coronal mass ejection (ICME) filaments and reverse shocks. From a study of > 30 A GIC events, it is found that supersubstorm (SML < -2500 nT) and intense substorm (-2500 nT < SML < -2000 nT) auroral electrojet intensifications are the most frequent (76%) cause of all of these GIC events. These events occur most often (76%) in superstorm (SYM-H ≤ -250 nT) main phases, but they can occur in other storm phases and lesser intensity storms as well. All of the > 30 A GICs were complex events involving a combination of two or more of shock, substorm, pressure pulse, and IMF Bs and Bn turnings. Shock-related GICs are observed to occur at all local times. The first and second “Halloween” superstorms (29-30 and 30-31 October 2003) combined, produced by far the greatest number of GICs in the interval of study (9 > 30 A GICs and 168 > 10 A GICs). In the first Halloween superstorm, a shock- triggered supersubstorm (SML < -3548 nT) caused 33, 57, 51 and 52 A GICs. The 57 A GIC was the most intense event of the superstorm and of this study. Equally intense magnetic storms were also studied but their related GICs were far less numerous.