On the numerical modelling of the VLF Trimpi effect

Nunn, D.

doi:10.1016/s1364-6826(96)00048-x

Cited by 31 publications

(38 citation statements)

References 49 publications

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“…Theory and numerical calculations predict strong inter-modal scattering between dominant TM modes upon incidence on lower ionospheric ionisation enhancements (e.g. see Nunn, 1997). These modes can be attenuated over the long propagation distances to the receiver and thus the VLF perturbations can be masked by noise.…”

Section: Summary and Concluding Commentsmentioning

confidence: 99%

Early VLF perturbations observed in association with elves

Mika

Haldoupis

Neubert³

et al. 2006

Ann. Geophys.

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Abstract. VLF remote sensing is used to detect lowerionospheric electron density changes associated with a certain type of transient luminous events known as elves. Both ground-and satellite-based observations of elves are analysed in relation to VLF data acquired at various receiver sites in Europe, the United States and Antarctica. Ground-based observations were performed during the EuroSprite2003 campaign, when five elves were captured by low-light cameras located in the Pyrenees. Analysis of VLF recordings from Crete shows early VLF perturbations accompanying all of the elves. A large dataset consisting of elves captured by the ISUAL (Imager of Sprites and Upper Atmospheric Lightning) payload on Taiwan's FORMOSAT-2 satellite over Europe and North America has also been analysed. Early/fast VLF perturbations were found to accompany some of the elves observed over Europe. However, no VLF perturbations were detected in relation to the elves observed by ISUAL over North America. The present analysis -based on the largest database of optical elve observations used for VLF studies so far -constitutes evidence of processes initiated by the lightning EMP (electromagnetic pulse) causing electron density changes in the lower ionosphere in line with theoretical predictions. It also proves that sub-ionospheric electron density changes associated with elves can intrude to lower heights and thus perturb VLF transmissions. The possibility of VLF detection, however, depends on several factors, e.g., the distance of the elve from the receiver and the transmitterreceiver great circle path (GCP), the altitude of the ionised region and the characteristics of the VLF transmitter, as well as the EMP energy, which occasionally may be sufficient to cause optical emissions but not ionisation.

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Section: Summary and Concluding Commentsmentioning

confidence: 99%

Early VLF perturbations observed in association with elves

Mika

Haldoupis

Neubert³

et al. 2006

Ann. Geophys.

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“…The linear Born theory of 3-D VLF scattering from an ionospheric plasma perturbation is fully described in Nunn (1997). The Born approximation involves assuming that the total incident field at a given point in the LIE is the incident zero order field (E 0 ).…”

Section: Scattering Theory and Implementationmentioning

confidence: 99%

“…The Born approximation involves assuming that the total incident field at a given point in the LIE is the incident zero order field (E 0 ). As shown by Nunn (1997), a better linearization may be achieved by assuming that the perturbation of the electric displacement vector is zero within the LIE rather than that the perturbation of the electric field is zero. The modified region is covered by a 3-D spatial grid, and the zero-order incident field E 0 calculated for each grid point.…”

Section: Scattering Theory and Implementationmentioning

confidence: 99%

Investigating radiation belt losses though numerical modelling of precipitating fluxes

2004

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Abstract. It has been suggested that whistler-induced electron precipitation (WEP) may be the most significant inner radiation belt loss process for some electron energy ranges. One area of uncertainty lies in identifying a typical estimate of the precipitating fluxes from the examples given in the literature to date. Here we aim to solve this difficulty through modelling satellite and ground-based observations of onset and decay of the precipitation and its effects in the ionosphere by examining WEP-produced Trimpi perturbations in subionospheric VLF transmissions. In this study we find that typical Trimpi are well described by the effects of WEP spectra derived from the AE-5 inner radiation belt model for typical precipitating energy fluxes. This confirms the validity of the radiation belt lifetimes determined in previous studies using these flux parameters. We find that the large variation in observed Trimpi perturbation size occurring over time scales of minutes to hours is primarily due to differing precipitation flux levels rather than changing WEP spectra. Finally, we show that high-time resolution measurements during the onset of Trimpi perturbations should provide a useful signature for discriminating WEP Trimpi from non-WEP Trimpi, due to the pulsed nature of the WEP arrival.

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“…This is particularly the case for conversion from the dominant mode to the Earth-detached mode [Wait, 1996]. The importance of mode conversion in scattering problems in the Earth-ionosphere waveguide has recently been discussed by Nunn [1997] and Baba et al [1998]. However, in general, intermodal scattering is less important than in the case of a "classic" electron concentration perturbation in the Earth-ionosphere waveguide caused by whistlerinduced electron precipitation [Rycroft, 1973].…”

Section: Scattering Situationmentioning

confidence: 99%