Extremely intense (SML ≤–2500 nT) substorms: isolated events that are externally triggered?

We examine particularly intense substorms (SML less than or equal to -2500 nT), hereafter called "supersubstorms" or SSS events, to identify their nature and their magnetic storm dependences. It is found that these intense substorms are typically isolated events and are only loosely related to magnetic storms. SSS events can occur during super (Dst less than or equal to -250 nT) and intense (-100 nT greater than or equal to Dst >-250) magnetic storms. SSS events can also occur during nonstorm (Dst greater than or equal to -50 nT) intervals. SSSs are important because the strongest ionospheric currents will flow during these events, potentially causing power outages on Earth. Several SSS examples are shown. SSS events appear to be externally triggered by small regions of very high density ( similar to 30 to 50 cm-3) solar wind plasma parcels (PPs) impinging upon the magnetosphere. Precursor southward interplanetary magnetic fields are detected prior to the PPs hitting the magnetosphere. Our hypothesis is that these southward fields input energy into the magnetosphere/magnetotail and the PPs trigger the release of the stored energy.