SuperMAG SML-LT Parameter as an Indicator of Substorm Triggering Mechanism

Magnetospheric substorms tend to occur during intervals of southward Interplanetary Magnetic Field (IMF Bz) orientation. Magnetic reconnection of the interplanetary magnetic field (IMF) and the Earth's magnetic field on the dayside magnetopause removes magnetic flux from the dayside magnetosphere and stores it in the magnetotail. At some point, the energy stored within the magnetotail is abruptly released in the form of a geomagnetic substorm, perhaps triggered by an instability internal to the magnetotail or a further northward or southward IMF turning. However, interplanetary conditions other than northward and southward IMF turnings can also trigger substorms. Alternative triggering agents include abrupt changes in the dawn/dusk IMF By component and sudden pressure pulses. There may be different ground signatures for substorms triggered by different mechanisms. Whereas IMF Bz turnings lead to substorms with particle precipitation centered on midnight, IMF By turnings and pressure pulses can lead to particle precipitation away from midnight. The SuperMAG SML-LT parameter can be helpful in identifying substorm triggering agents. For IMF Bz triggers, SML-LT maximizes around midnight as expected for standard Akasofu-type substorms. Although onsets for changes in IMF By can occur at locations away from midnight, they exhibit maximum SML-LT variations near midnight. On the other hand, SML-LT exhibits maxima at local time sectors far away from midnight for pressure induced substorms. This is because during impulse-induced substorms particle injection takes places via a viscous interactions, namely the KH instability, whose occurrence is favoured at dawn and dusk.