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

Abstract

[1] During the early phase of the intense magnetic storm of 7-11 November 2004, the DEMETER satellite encountered large-scale equatorial plasma density depletions with density decreases of two or three orders of magnitude. Wave measurements carried out inside these depletions show the occurrence of broadband and localized lower-hybrid turbulence triggered by whistlers propagating from thunderstorm lightning occurring below the orbit path. High-sample-rate waveforms reveal that this lower-hybrid turbulence can evolve into localized large-amplitude quasi-monochromatic wave packets similar to lower-hybrid structures that were, up to now, only observed in the auroral regions, usually on high-latitude magnetic field lines associated with discrete aurora. These equatorial structures 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 with small-scale density depletions of 5-10%. Although the lower-hybrid structures are less intense than those observed in the auroral zone and although their energy source is different, our observations lend support to the idea that the formation of lower-hybrid structures is an universal mechanism operating in inhomogeneous magnetized space plasmas in the presence of VLF whistler mode turbulence. Besides the lower-hybrid turbulence, another interesting feature is the occurrence of strong narrowband electromagnetic ELF emissions with amplitudes of a few millivolts per meter at frequencies below the proton gyrofrequency. They are continuously observed throughout the 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 tentatively identified as magnetospheric line radiations (MLRs) commonly observed during magnetically disturbed periods. Similar events were observed on 15 May 2005 and on 24 August 2005 during two other intense magnetic storms. Copyright 2008 by the American Geophysical Union.