Extremely intense substorms/supersubstorms: interplanetary origin and characteristics

Abstract:

Extremely intense substorms or supersubstorms (SSSs) may be associated with the strongest geomagnetically induced current (GIC) flows in the ionosphere, potentially causing power outages on Earth. A statistical study is performed on 74 SSS events with SuperMAG AL (SML) peak intensities < -2500 nT occurring during 1981-2012. These are often found to be isolated SML peaks, only loosely related to geomagnetic storms. SSS events can occur during super- (Dst ≤ -250 nT) and intense- (-100 nT ≥ Dst > -250) magnetic storms. SSSs can even occur during non-storm (Dst ≥ −50 nT) intervals. The long-term study reveals that SSSs occur during all phases of the solar cycle with the highest occurrence (3.8 year^-1) in the descending phase. SSSs exhibited an annual variation with equinoctial maximum altering between spring in solar cycle 22 and fall in solar cycle 23. All SSS events were associated with strong southward interplanetary magnetic field Bs component as a part of interplanetary magnetic clouds in 46% and of interplanetary sheath fields in 54% of the cases. SSS events appear to be externally triggered by small regions of very high density solar wind plasma parcels impinging upon the magnetosphere. We will show case studies of two SSS events induced by interplanetary shocks. These SSSs have a different auroral evolution than a nominal Akasofu-type substorm. The auroras associated with the SSSs did not have the standard midnight onset and following expansion. Instead, at the time of the SML index peak, the midnight sector was generally devoid of intense auroras, while the most intense auroras were located in the premidnight and postmidnight magnetic local times. Precursor energy input through magnetic reconnection was insufficient to balance the large ionospheric energy dissipation during the SSSs. It is argued that besides the release of stored magnetotail energy during the SSSs, these were powered by additional direct driving through both dayside magnetic reconnection and solar wind ram energy.