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Compact Intracloud Discharges

Narrow bipolar electric field change pulses (NBEs) associated with powerful RF radiation have previously been described by several researchers [ LeVine, 1980; Willet et al., 1989; Smith, 1998; Smith et al., 1999; Rison et al., 1999]. Smith et al. [1999] showed that the discharges occur in clouds and stated that the sources, referred to as compact intracloud discharges (CIDs), emit distinct fast and isolated bipolar electric field change signatures. CIDs are excellent targets for FORTE, which regularly records RF radiation from CIDs in the form of transionospheric pulse pairs [Holden et al., 1995; Massey and Holden, 1995].


   NBE Sferic Example
Figure 3: Field change waveforms from a narrow negative bipolar pulse recorded by the New Mexico TU, RO, LA, and SO stations on July 8, 1998 at 02:24:00.414327. The ionospheric reflections of the signal can be seen in the waveforms of all four stations.

Fig. 3 shows a LASA example of multiple-station narrow negative bipolar pulse (NNBP) recorded by the New Mexico TU, RO, LA, and SO stations on July 8, 1998 from distances of 388, 544, 607, and 702 km respectively. The event occurred in Oklahoma east of the Texas Panhandle. The pulse is so temporally narrow that it is not possible to determine the polarity of the pulse from the plots of the entire 8 ms record. Depending on the source/receiver distance, ionospheric reflection may provide multiple pulses in the NBE waveforms due to the different travel times of the multiple paths for the VLF signal from lightning to a single LASA station. This is illustrated in Fig 4. The multiple paths allow the determination of both the source height and the reflecting ionospheric height. Ionospheric reflections are visible in all three waveforms immediately following the groundwave signal. The three dimensional source location of the CIDs is routinely determined for NBEs observed by LASA.

Among the distinguishing characteristics of NBEs are their fast rise and fall times and their isolation within our 8 ms duration electric field change records. Indications of intracloud activity are occasionally observed in the 8 ms LASA records.


   Earth Ionosphere with multiple VLF
     paths
Figure 4: Depiction of the source-receiver geometry leading to multiple paths for the VLF radiation from lightning.

This paper describes a comparison between LASA and the National Lightning Detection Network (NLDN) in order to characterize the accuracy of LASA geolocations. Also, initial results of a comparative study between LASA and the Kennedy Space Center LDAR are presented.


next up previous
Next: Results Up: Introduction Previous: LDAR and LMA description
Matthew J. Heavner
2000-10-04