Burst Geomagnetic Pulsations as Indicators of Substorm Expansion Onsets During Storms

We report on the dynamics of field-aligned currents (FACs), broadband geomagnetic pulsations, and airglow obtained from the Irkutsk (IRK), Mondy (MND), and Borok (BOX) midlatitude geomagnetic observatories and the Tory (TOR) optical observatory during storm substorms. For the first time, using the short duration, Delta t < 0.5 min, high-frequency component of the burst pulsations (Pi1B), we determined the substorm double expansion phase (EP) onsets <5 min apart, which is hardly possible by means of the low-frequency (periods of 2-5 min) Psc/PiB pulsations. We argue that the observed burst pulsations are the result of prompt changes in the solar wind dynamic pressure and/or the current circuit related to the westward electrojet. Each pulsed source can excite short bursts of broadband electromagnetic modes of the ionospheric Alfven resonator in the range of short-period pulsations with a periodic resonance structure of the spectrum characteristic of the observed Pi1B/Psc pulsations. Plain Language Summary We explored geomagnetic and optical midlatitude observations during storms. In addition to the common low-frequency (periods of 2-5 min) Psc/PiB pulsations, we used the short-period part (Pi1B) of burst pulsations with T < 10 s that allowed us to determine not only isolated substorm expansion onsets but also double substorm onsets and series of onsets of short substorm activations or pseudobreakups during storms. This is hardly possible with the commonly used long-period pulsations (Pi2, Pi3) that are problematic due to their long duration.