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.