Lightning-induced lower-hybrid turbulence and trapped Extremely Low Frequency (ELF) electromagnetic waves observed in deep equatorial plasma density depletions during intense magnetic storms

摘要

During the early phase of the intense magnetic storm of 7–11 November 2004, theDEMETER satellite encountered large-scale equatorial plasma density depletions withdensity decreases of two or three orders of magnitude. Wave measurements carried outinside these depletions show the occurrence of broadband and localized lower-hybridturbulence triggered by whistlers propagating from thunderstorm lightning occurringbelow the orbit path. High-sample-rate waveforms reveal that this lower-hybrid turbulencecan evolve into localized large-amplitude quasi-monochromatic wave packets similar tolower-hybrid structures that were, up to now, only observed in the auroral regions, usuallyon high-latitude magnetic field lines associated with discrete aurora. These equatorialstructures have typical amplitudes of up to 20 mV/m and durations of 20–30 ms.Simultaneous thermal ion measurements show that these bursts are often correlated withsmall-scale density depletions of 5–10%. Although the lower-hybrid structures are lessintense than those observed in the auroral zone and although their energy source isdifferent, our observations lend support to t he idea that the formation of lower-hybridstructures is an universal mechanism operating in inhomogeneous magnetized spaceplasmas in the presence of VLF whistler mode turbulence. Besides the lower-hybridturbulence, another interesting feature is the occurrence of strong narrowbandelectromagnetic ELF emissions with amplitudes of a few millivolts per meter atfrequencies below the proton gyrofrequency. They are continuously observed throughoutthe entire depletion. These emissions occur not only within the depletions but also,although less intense, outside of them over a large latitudinal range. They are tentativelyidentified as magnetospheric line radiations (MLRs) commonly observed duringmagnetically disturbed periods. Similar events were observed on 15 May 2005 andon 24 August 2005 during two other intense magnetic storms.