A method for determining intracloud lightning and ionospheric heights from VLF/LF electric field records

Abstract

[1] The Los Alamos Sferic Array (LASA) recorded VLF/LF electric-field-change signals from over ten million lightning discharges during the period from 1998 to 2001. Using the differential-times-of-arrival of lightning sferics recorded by three or more stations, the latitudes and longitudes of the source discharges were determined. Under conditions of favorable geometry and ionospheric propagation, sensors obtained ionospherically reflected skywave signals from the lightning discharges in addition to the standard groundwave sferics. In approximately 1% of all waveforms, automated processing identified two 1-hop skywave reflection paths with delays indicative of an intracloud (height greater than 5 km) lightning source origin. For these events it was possible to determine both the height of the source above ground and the virtual reflection height of the ionosphere. Ionosphere heights agreed well with published values of 60 to 95 km with an expected diurnal variation. Source height determinations for 100,000+ intracloud lightning events ranged from 7 to 20 km AGL with negative-polarity events occurring above $15 km and positive-polarity events occurring below $15 km. The negativepolarity events are at a suprisingly high altitude and may be associated with discharges between the upper charge layer of a storm and a screening layer of charge above the storm. Approximately 100 of the intracloud events with LASA height determinations were also recorded by VHF receivers on the FORTE satellite. Independent FORTE source height estimates based on delays between direct and ground-reflected radio emissions showed excellent correlation with the VLF/LF estimates, but with a +1 km bias for the VLF/LF height determinations.

Low-frequency ionospheric sounding with Narrow Bipolar Event lightning radio emissions: regular variabilities and solar-X-ray responses

“…Modification 2: Our model imposes a fixed low-pass filter on the input (groundwave) waveform. Rather than generate an echo model using a partially-conducting reflector (Smith et al, 2004), our present approach is to impose a frequency roll-off that is found to work better than the approach of Smith et al at mimicking the data, over the majority of events analyzed. By this, we mean that the modified approach results in a modeled ionospheric echo that more closely resembles the observed ionospheric echo, relative to that of an echo model using a partially-conducting reflector (Smith et al, 2004).…”

“…The two vertical dashed lines indicate the expected echo-arrival times for, respectively, the ionospheric and the ground-then-ionospheric echoes, based on a model which assumes a discrete mirror at an ionospheric height H i =61.2 km and a source height H s =13.1 km. The standard LASA height-retrieval (Smith et al, 2004) uses this approach and has obtained serviceable estimates of H s for lightning research. The standard retrieval idealizes the ionosphere as a sharp boundary between vacuum (underneath) and a finiteconductivity (e.g., 2.2 micro-mhos/m, as recommended by VLF experience (Wait and Spies, 1964)), isotropic conductor.…”

“…The strong rolloff at range >300 km is due in part to the two selection criteria we enforce: First, we select only for data lying within 400 km from the array center (28.5 deg N, −81.5 deg E). Second, we perform height retrievals only in the distance interval 200 km<range<800 km from the recording station (Smith et al, 2004). (The rationale for this distance interval is as follows: For shorter ranges than ∼200 km, the ionospheric echo signal is suppressed due to antenna nulls at zenith of both the vertical-monopole-on-groundplane receiver antenna, and the vertical-dipole NBE transmitter (Smith et al, 2002(Smith et al, , 2004 1999).…”

“…(c) Recording of the NBE ground wave, followed by both the ionospheric and the ground-then-ionospheric echoes, allows retrieval of the source and "ionospheric reflector" heights (Smith et al, 2004), for lightning-to-receiver ranges 200-800 km.…”

“…Modification 3: The standard LASA retrieval of heights (Smith et al, 2004) performs a correlation between the modeled and observed echo waveforms, to determine the "best" echo delays. In the present work, which seeks to minimize systematic errors in H i , we avoid correlation between two waveforms that are intrinsically somewhat dissimilar, and instead compare derived peak times from the observations and from the model.…”