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

The interplanetary and magnetospheric phenomena time-coincident with 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 forward shocks and substorms are predominant causes of intense GICs, some newly discovered geoeffective interplanetary features are: solar wind plasma parcel (PP) impingements, possible interplanetary magnetic field (IMF) northward (Bn) and southward (Bs) turnings, and reverse shocks. The PPs are possibly the loop and filament portions of coronal mass ejections (CMEs). From a study of > 30 A GIC events, it is found that supersubstorm (SSS: 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. After substorms, PPs were the most frequent causes of Mäntsälä GIC > 30 A events. Forward shocks were the third most frequent cause of the > 30 A events. Shock-related GICs were observed to occur at all local times. The two “Halloween” superstorms of 29–30 and 30–31 October 2003 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 SSS (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. It is possible that this SSS is a new form of substorm. Equally intense magnetic storms were also studied but their related GICs were far less numerous and less intense.