Very low frequency radio events with a reduced intensity observed by the low‐altitude DEMETER spacecraft

Zãhlava, J; Němec, F.; Santolı́k, O.; Kolmašová, Ivana; Parrot, M.; Rodger, Craig J.

doi:10.1002/2015ja021607

Cited by 2 publications

(13 citation statements)

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“…These are used as a starting point of the presented analysis. We note that only four of the ELF events took place at the same time as the VLF events analyzed by Záhlava et al ().…”

Section: Data Setsmentioning

confidence: 50%

“…Figure compares seasonal dependences of the analyzed ELF events (red) with VLF events studied by Záhlava et al (; green). The fractions of the total number of events which took place in individual months are displayed; that is, the sum of values in individual columns is by definition equal to 1.…”

Section: Resultsmentioning

confidence: 89%

“…Those events consist of many nearly horizontal curves of a reduced power spectral density of electric field fluctuations. Their detailed properties and possible formation mechanism were described by Záhlava et al (). An example of such event measured on 22 September 2007 between 22:56:07 and 23:29:28 UT is shown in Figure .…”

Section: Introductionmentioning

confidence: 99%

“…Parrot et al () used a full‐wave calculation of this propagation in order to model the V‐shaped events in detail. Basic properties of the events can be, however, reproduced using a simplified waveguide mode theory (e.g., Budden, ) as shown by Záhlava et al (). They evaluate the power spectral density of the sum of first two modes of the Earth‐ionosphere waveguide as a function of distance between the spacecraft and the source storm obtaining a spectrogram qualitatively similar to the events analyzed.…”

Section: Introductionmentioning

confidence: 99%

“…Frequency‐time spectrogram of power spectral density of electric field fluctuations measured by the Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions spacecraft on 22 September 2007 between 22:56:07 and 23:29:28 UT. The red rectangle shows a frequency‐time interval containing an example of the very low frequency events studied by Záhlava et al ().…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2018

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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.

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“…These are used as a starting point of the presented analysis. We note that only four of the ELF events took place at the same time as the VLF events analyzed by Záhlava et al ().…”

Section: Data Setsmentioning

confidence: 50%

Section: Resultsmentioning

confidence: 89%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

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

et al. 2018

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

Macotela

Němec

Manninen

et al. 2019

Geophysical Research Letters

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Very low frequency (VLF) emissions of natural origin were identified for the first time by analyzing 1-hr ground-based magnetic field spectrograms in the 0.2-to 39-kHz frequency range. Data were used from the Kannuslehto radio receiver (L-shell~5.5), recorded during different campaigns between 2006 and 2019. The spectrograms exhibit banded structures, which consist of several strip elements that vary in time and frequency over the event duration. Statistical analysis of 95 events shows that they are observed in the frequency range that extends from 2 to~37 kHz, and mainly appearing above 16 kHz. The events span from 4 to 110 min and occur in the evening sector (~17-01 magnetic local time), mostly during quiet geomagnetic conditions. Furthermore, they are primarily left-handed polarized and are associated with bursts of lightning-related radio emissions such as sferics and tweeks.Plain Language Summary Advances in modern technology have created tangible scientific and societal benefits, allowing us to make new discoveries, perform better observations, and monitor important parameters of our Earth's condition. In this study, new digital processing tools are used to uncover a natural, namely, non-man-made, very low frequency (VLF) radio wave emission never observed before. These emissions consist of several strips as appear on our detector screens, with structures having frequency drifts that vary during the event duration. Moreover, the range of frequencies in which these emissions are observed, 16-39 kHz, is higher than the typical one used for VLF observations. The measurements were made in Northern Finland during different campaigns, between 2006 and 2019, using~10-m size square-loop radio antennas installed on the ground. The duration of the structured emissions ranges from about four to a hundred minutes. These emissions occur during evening and early night hours, and when the Earth's magnetic conditions are calm. Additionally, we learned that the field perpendicular to the wave propagation rotates clockwise when looking at the incoming wave. A remarkable characteristic of these emissions is that they are coincidental in time with the occurrence of bursts of lightning VLF emissions.

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Very low frequency radio events with a reduced intensity observed by the low‐altitude DEMETER spacecraft

Cited by 2 publications

References 28 publications

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

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

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

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