Selective Attenuation of Lightning‐Generated Whistlers at Extralow Frequencies: DEMETER Spacecraft Observations

Abstract

Frequency-time spectrograms measured by the Detection of Electro-Magnetic EmissionsTransmitted from Earthquake Regions (DEMETER) spacecraft (2004)(2005)(2006)(2007)(2008)(2009)(2010), altitude about 700 km) at frequencies below 20 kHz sometimes contain lightning-generated whistlers whose intensity is significantly reduced at specific frequencies. The frequencies of the reduced intensity vary smoothly over the event duration, forming apparent curves in frequency-time spectrograms. Events at frequencies higher than the Earth-ionosphere waveguide cutoff frequency were explained by the interference of the first few waveguide modes of lightning-generated spherics propagating therein (Záhlava et al., 2015, https://doi.org/10.1002/2015JA021607. Here we present an analysis of events observed at frequencies lower than about 1 kHz. Altogether, we analyze 263 events identified at the times when DEMETER operated in the burst mode. The vast majority of the events (95%) took place during the nighttime, and they occurred more frequently during spring/autumn than during winter/summer. We present an overview of event properties. Moreover, measurements of all six electromagnetic field components performed by DEMETER allow us to perform a detailed wave analysis. It is shown that the emissions propagate with high wave normal angles inclined toward the Earth. We suggest that the events might be due to the wave propagation in the ionospheric waveguide formed around the refractive index maximum at the altitude of about 105 km.

VLF Emissions With Banded Structure in the 16‐ to 39‐kHz Frequency Range Measured by a High‐Latitude Ground‐Based Receiver

“…First, they might be due to plasma instabilities in the magnetosphere, as in the case of structured auroral hiss (Titova et al, 2007). Second, they could be formed in the Earth-ionosphere waveguide, as in the case of long-distance propagation of lightning generated VLF emissions (Záhlava et al, 2015(Záhlava et al, , 2018.…”

“…An example of this is shown in Figures 1e and 1f. We point out that although the time duration of a single sferic or a preliminary breakdown pulse is much lower than the typical length of observed events, for long time series a sequence of many consecutive sferics or preliminary breakdown pulses may form nearly continuous emission bands (Záhlava et al, 2015(Záhlava et al, , 2018. Thus, the event formation may be related to the propagation of VLF lighting emissions in the Earth-ionosphere wave-guide and to changes in the emission's frequency spectrum.…”

“…Here, we report for the first time ground-based observations of VLF emissions with banded structure (BS) occurring in the 16-to 39-kHz frequency range. We discuss their possible formation mechanisms, suggesting two different hypotheses: (i) they might be due to plasma instabilities in the magnetosphere, as is the case of structured auroral hiss (Titova et al, 2007), or (ii) they could be formed in the Earth-ionosphere waveguide as a result of a long-distance propagation of lightning generated VLF emissions (Záhlava et al, 2015(Záhlava et al, , 2018.…”