Satellite observations of banded VLF emissions in conjunction with energy‐banded ions during very large geomagnetic storms

Frequency‐banded electromagnetic VLF waves up to 2000 Hz are observed concurrently with warm (10 s to 10,000 s of eV) energy‐banded ions in the low latitude auroral and sub‐auroral zones during every large geomagnetic storm encountered by the FAST and DEMETER satellites. The banded ions and waves persist for several FAST or DEMETER orbits, lasting up to 12 h, in both dawn and dusk sectors, in both the northern and southern hemispheres. If the waves are generated at harmonics of the proton gyrofrequency, the inferred source region would be ∼4000 km altitude. Previous investigations have shown that such waves can propagate from this source region to the locations of both spacecraft. An investigation into the growth of waves at harmonics of f ci in the inferred source region suggests that these emissions could be generated by ion bands similar to those observed at the same time as the waves. Magnetospheric waves such as these play a role in energy transfer between distinct particle populations and may contribute to ion heating and ion outflow as well as electron energization. All of these phenomena occur during the strongest magnetic storms. The appearance of the banded ions and associated wave activity suggests that there may be distinct changes in the geospace system that characterize large magnetic storms.