[1] VLF signal perturbations recorded on the Holographic Array for Ionospheric/ Lightning Research (HAIL) are quantitatively related to a comprehensive model of lightning-induced electron precipitation (LEP) events. The model consists of three major components: a test-particle model of gyroresonant whistler-induced electron precipitation, a Monte Carlo simulation of energy deposition into the ionosphere, and a model of VLF subionospheric signal propagation. For the two representative LEP events studied, the model calculates peak VLF amplitude perturbations within a factor of three of those observed, well within the expected variability of radiation belt flux levels. The phase response of the observed VLF signal to precipitation varied dramatically over the course of the two nights and this variability in phase response is not properly reproduced by the model. The model calculates a peak in the precipitation that is poleward displaced $6°f rom the causative lightning flash, consistent with observations. The modeled precipitated energy flux (E > 45 keV) peaks at $1 Â 10 À2 (ergs s À1 cm À2 ), resulting in a peak loss of $0.001% from a single flux tube at L $ 2.2, consistent with previous satellite measurements of LEP events. The precipitation calculated by the model is highly dependent on the near-loss-cone trapped radiation belt flux levels assumed, and hence our main objective is not to compare the model calculations and the VLF signal observations on an absolute basis but is rather to develop metrics with which we can characterize the VLF signal perturbations recorded on HAIL in terms of the associated precipitation flux. Metrics quantifying the ionospheric density enhancement (N ILDE ) and the electron precipitation (G) along a VLF signal path are strongly correlated with the VLF signal perturbations calculated by the model. A conversion ratio Y, relating VLF signal amplitude perturbations (DA) to the time-integrated precipitation (100-300 keV) along the VLF path (Y = G/DA), of 1.2 ± 0.3 Â 10 10 (el m À1 /dB) is suggested for precipitation events of similar location and characteristics to those examined. The total precipitation (100-300 keV) induced by one of the representative LEP events is estimated at $1.8 ± 0.4 Â 10 16 electrons, calculated directly from the conversion ratio Y and observations of VLF signal perturbations.
[1] Ionospheric effects of energetic electron precipitation induced by controlled injection of VLF signals from a ground based transmitter are observed via subionospheric VLF remote sensing. The 21.4 kHz NPM transmitter in Lualualei, Hawaii is keyed ON-OFF in 30 minute periodic sequences. The same periodicity is observed in the amplitude and phase of the sub ionospherically propagating signals of the 24.8 kHz NLK (Jim Creek, Washington) and 25.2 kHz NLM (LaMoure, North Dakota) transmitters measured at Midway Island. Periodic perturbations of the NLK signal observed at Palmer, Antarctica suggest that energetic electrons scattered at longitudes of NPM continue to be precipitated into the atmosphere as they drift toward the South Atlantic Anomaly. Utilizing a model of the magnetospheric waveparticle interaction, ionospheric energy deposition, and subionospheric VLF propagation, the precipitated energy flux induced by the NPM transmitter is estimated to peak at L $ 2 and $ 1.6 Â 10 À4 ergs s À1 cm À2 . Citation: Inan,
[1] DEMETER spacecraft detects short bursts of lightninginduced electron precipitation (LEP) simultaneously with newly-injected upgoing whistlers, and sometimes also with once-reflected (from conjugate hemisphere) whistlers. For the first time causative lightning discharges are definitively geo-located for some LEP bursts aboard a satellite. The LEP bursts occur within <1 s of the causative lightning and consist of 100-300 keV electrons. First in-situ observations of large regions of enhanced background precipitation are presented. The regions are apparently produced and maintained by high rate of lightning within a localized thunderstorm.
[1] Two different 4-hour sequences of subionospheric Very Low Frequency (VLF) signal perturbations are examined to characterize electron precipitation induced by nonducted obliquely propagating whistlers. These lightning-induced electron precipitation (LEP) events are typically associated with cloud-to-ground lightning discharges. The temporal and spatial signatures of LEP events associated with two disparate storms occurring over two 4-hour periods are examined. A distributed set of VLF observing sites, known as the Holographic Array for Ionospheric and Lightning Research (HAIL), captures the full latitudinal extent of the events, providing evidence that 90% of the precipitation occurs over a region 8°± 1°and 9°± 1°in latitudinal extent for the two time periods. The measured peak of the precipitation is poleward displaced (6°45 0 ± 30 0 and 7°45 0 ± 30 0 for the two case studies) from the causative discharge. Analysis indicates that the onset delay and the duration of precipitation steadily increase with increasing L-value, while the signal recovery time is independent of L-value for the LEP events associated with both storms. The causative lightning discharges associated with the two storms were located at different latitudes. For lightning occurring in the storm at higher latitudes, the associated LEP events are of longer duration and exhibit precipitation in a smaller area displaced less from the causative discharge. The onset delays and event durations increase more rapidly with increasing L-value for events associated with lightning occurring in the storm at higher latitudes. The general spatial and temporal signatures are consistent with those expected for LEP events induced by nonducted whistlers.
We examine the effects on the midlatitude ionospheric D region of the 7 April 2000 storm and the “Halloween storm” of late October 2003 by means of the associated perturbations of several subionospheric VLF signals propagating in both the northern and southern hemispheres. We use VLF nighttime data from the Holographic Array for Ionospheric/Lightning Research (HAIL), located in the United States (L = 2–3), as well as data from Palmer Station, Antarctica (L = 2.4). On 7 April 2000, a ∼5 dB depression in VLF amplitudes is observed at multiple HAIL stations, with a depression onset that occurs later for VLF signal paths at lower latitudes. On both 7 April 2000 and 31 October 2003, fluctuations in the amplitude of the VLF signals are first observed in the premidnight sector and persist through the end of the data‐recording period (dawn). The frequency content of the fluctuations is predominantly in the 0.01 to 0.02 Hz range but extends up to ∼0.03 Hz. Increases in the energetic electron flux in the loss cone as measured by the NOAA‐POES satellites are observed on both 7 April 2000 and 31 October 2003. We suggest that both the signal depressions and subsequent fluctuations are associated with variations in the precipitation flux of energetic electrons onto the upper atmosphere. Auroral activity patterns based on data from the NOAA‐POES satellites show that the equatorward edge of the auroral oval expanded equatorward to lower L shells (L < 3) during both geomagnetic storms. Using the auoral activity patterns and multiple VLF/LF signal paths, we provide evidence that the fluctuations and the signal depression coincide with the equatorward edge of the auroral oval extending over the perturbed VLF/LF Great Circle Paths. Quantitative modeling of subionospheric VLF wave propagation incorporating energetic electron flux measurements (and the associated altitude profiles of secondary ionization) yields results consistent with the variations in the VLF signal amplitude observed.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.