Global impulse burst of geomagnetic pulsations in the frequency range of 0.2–5 Hz as a precursor of the sudden commencement of St. Patrick’s Day 2015 geomagnetic storm

We have analyzed a short-term (3–4 s) burst of geomagnetic pulsations in the frequency range of 0.2–5 Hz observed during the commencement of a magnetic storm on March 17, 2015. The burst was observed by a network of observatories in different sectors of local time and at different latitudes. The spectra of pulsations involves a resonant structure with a global maximum at a frequency of 2.78 ± 0.38 Hz, despite some differences at different observatories. There is a delay by almost 4 s in the maximum of the train amplitude at nightside observatories with respect to a dayside observatory. The burst of pulsations has been shown to be on the front of the magnetic disturbance associated with sudden storm commencement (SSC) and, therefore, can be considered as a precursor. The observations of particle fluxes by low-orbit satellites have shown that the SSC is accompanied by a dramatic increase in the fluxes of precipitating protons and electrons. We have suggested that the mechanism of oscillation generation may be the ion–cyclotron instability of ring current protons and the resonant structure of pulsations may be associated with the ionospheric Alfvén resonator.     

Possible Origin of Ultralow-Frequency Whistlers

The sonographic analysis of records of ultralow-frequency emissions recorded by the induction magnetometer at the Mirnyi observatory (Antarctica) in 1981–1985 has revealed the presence of a special class of signals in the frequency band 0.25–5 Hz having a characteristic dispersion reminiscent of the dynamic spectra of LHR-whistlers observed in the VLF band. The ULF whistlers are observed, as a rule, at morning and evening hours of local time at moderate values of the K _p-index (0 < K _p < 4). The analysis of the frequency–time behavior of observed signals shows that the canalized propagation of short broadband pulses as magnetosonic waves in the layered medium, for example, in the magnetospheric current sheet can serve as a possible cause of the dispersion. Such sporadic phenomena on the magnetopause as microreconnections or FTE-events may be sources of these pulses.