Electromagnetic Radiation Spectrum of a Composite System

Dwyer

2020

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Brief bursts of high-frequency (HF) and very high frequency (VHF) radio emissions unaccompanied by strong low-frequency radiation have been observed during initiation and propagation of lightning or thunderstorm electrical breakdown without leading to fully fledged lightning. This paper investigates a physical mechanism to generate such radio bursts by electrical discharge activity inside a thundercloud. When a discharge consists of many high-frequency emission sources, such as streamers, that generate currents in random directions, its radiation spectrum peaks in the HF and VHF bands, and the spectral magnitudes in low frequencies are much smaller or even negligible. Combined with recent observational findings, the present study suggests that lightning initiation may begin with a short burst of many randomly occurring small-scale discharges in a localized thundercloud region. Plain Language Summary How exactly lightning forms from electrical breakdown activity inside thunderclouds remains unclear. To answer this question, radio frequency (RF) radiation originating from thunderstorms is routinely measured and analyzed to investigate lightning dynamics. Recent multichannel radio observations clearly indicate that brief bursts of high-frequency RF radiation unaccompanied by lower-frequency waves can be generated during lightning formation. Such radio bursts have different spectral properties than well-documented RF radiation from lightning. This paper investigates a way for electrical breakdown activity to generate such radio bursts. We find those radio bursts can be generated by an electrical discharge consisting of a large number of elements that propagate in random directions. Our finding suggests that lightning may begin with many randomly occurring small-scale electrical discharges in a localized thundercloud volume. Recent multiband radio observations have also shown that bursts of HF and VHF radiation with only weak or even no accompanying sferic pulses (i.e., weak lower-frequency radiation) can be generated during thunderstorms. Examples of such RF bursts include the microsecond-long radiation from the initial breakdown of precursor events reported by Rison et al. (2016), which are thundercloud discharges not leading to fully fledged lightning, and narrow (e.g., ≤0.5 μs) VHF pulses that are the initiating events of a large fraction of lightning (Lyu et al., 2019; Marshall et al., 2019) and that also occur during lightning formation RESEARCH LETTER

Section: Geophysical Research Lettersmentioning

confidence: 87%

“…Taking Fourier transform of E z , we obtain

{\overset{E}{true}}_{z} false(\overset{r}{true}, t false) = \frac{ω {\overset{M}{true}}_{s} \sin θ}{4 π ε_{0} c^{2} R} ((), \overset{θ}{true} \cdot \sum_{i} e^{- j ω T_{i}} {\overset{n}{true}}_{i}),

where

{\overset{M}{true}}_{s}

false| \overset{E}{true} {false|}^{2}

(Liu et al, 2019; Shi et al, 2019), we need to compute

⟨ false| {\overset{E}{true}}_{z} {false|}^{2} ⟩

.…”

Section: Theorymentioning

confidence: 99%

Section: Theorymentioning

confidence: 99%

“…For an ensemble of discrete streamers, each streamer generates a current moment pulse and so an electric field pulse (Liu et al, 2019, 2020). The above equation can then be written as

E_{z} false(\overset{r}{true}, t false) = \frac{\sin θ}{4 π ε_{0} c^{2} R} ((), \overset{θ}{true} \cdot \sum_{i} ([], {\overset{\overset{M}{true}}{true}}_{i})),

where

([], {\overset{\overset{M}{true}}{true}}_{i}) = {\overset{\overset{M}{true}}{true}}_{i} false(t - R_{i} false/ c false)

is the time derivative of the current moment of the i th streamer evaluated at the retarded time.…”

Section: Theorymentioning

confidence: 99%

See 2 more Smart Citations

Thunderstorm High‐Frequency Radio Bursts With Weak Low‐Frequency Radiation

Dwyer

2020

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“…Broadband very high frequency (VHF) radio interferometer observations indicate that NBEs are produced by a process termed fast breakdown that propagates at a speed of ∼5 × 10 7 m/s (Huang et al., 2021; Lyu et al., 2019; Rison et al., 2016; Shao et al., 2018; Stock et al., 2017; Tilles et al., 2019), which is thought to consist of a large system of streamers. Theoretical and modeling studies have found that a large system of streamers can indeed explain various electrical and spectral properties of NBEs (Liu & Dwyer, 2020; Liu et al., 2019, 2020). The other lightning initiation pathway begins with an “initiating event” that generates a brief (≤1 μs) burst of VHF radiation (Lyu et al., 2019; Marshall et al., 2019) without detectable lower frequency pulses, which amounts to 88% of the initiation events analyzed by Lyu et al.…”

Section: Introductionmentioning

confidence: 99%

LOFAR Observations of Lightning Initial Breakdown Pulses

Scholten

Hare

et al. 2022

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This paper reports an observational study of lightning initial breakdown pulses (IBPs) using the low‐frequency array radio telescope and a broadband magnetic field sensor. The data show that the overall spatiotemporal evolution of the electrical breakdown causing an IBP is rather complex. During an IBP, spatially and temporally separated bursts of very high frequency (VHF) electromagnetic radiation occur in a volume on the order of 1003 m3, and they are coincident with brief magnetic field pulses, indicating that the location of the active breakdown region can change suddenly. Furthermore, recurrent breakdown activity is observed, especially at the location of the VHF burst. Interpreting each VHF burst as being generated by a corona burst, an IBP pulse appears to start off from an initial corona burst and subsequent corona bursts enhance it. We further suggest that the generation of IBPs likely involves multiple space stems/leaders and connections between them.

Radio Frequency Emissions From Streamer Collisions in Subbreakdown Fields

Koile

Dwyer

2021

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Lightning emits short wavelength radio waves from the very high frequency (VHF, 30–300 MHz) to ultrahigh‐frequency (UHF, 0.3–3 GHz) range. A lightning subprocess that has recently been shown to emit in this frequency range is streamer collisions. In this work, we report a modeling study of streamers colliding in ambient electric fields ranging from subbreakdown to overbreakdown values. The streamers are initiated from isolated hydrometeors similar in size to those typically seen in thunderstorms. In every case presented, the collision produces emissions extending into the UHF range. The emission spectrum from the subbreakdown ambient field cases falls off faster as frequency increases compared with the overbreakdown cases. It appears that the length of the streamers upon colliding, under the same ambient field, has a negligible effect on the shape of the spectrum. The results are important for interpreting observations of lightning processes that involve streamer collisions.