Lightning-associated VLF perturbations observed at low latitude: Occurrence and scattering characteristics
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2013
Received: 16 November 2011
Accepted: 30 May 2012
Published: 19 February 2013
The occurrence of short-timescale (∼1–100 s) perturbations (early VLF events) on four Very Low Frequency (VLF) transmitter signals (call signs: NWC, NPM, VTX, NLK), recorded at Suva (18.1°S, 178.5°E, L = 1.16), shows the most frequent occurrence on the NWC signal and least on the VTX. Daytime early/fast events on the NWC transmission are (0.2–0.5 dB) with only negative amplitude perturbations with comparatively lower recovery times (10–30 s) as compared with most nighttime events with amplitude perturbations of 0.2–1.5 dB and recovery times of 20–80 s. The World-Wide Lightning Location Network detected causative lightnings for 74 of 453 early VLF events out of which 54 (73%) were produced due to narrow-angle scattering, and by 20 (27%) due to wide-angle scattering. The recovery (decay) of the scattered amplitude of early/fast events on the NWC signal shows both exponential and logarithmic forms, but the linear correlation coefficient is better with a logarithm fit. The first observations of early/slow events in daylight propagation are presented. Initial results on early/fast events with unusually long recoveries (≥5 min) and strong perturbations (≥1 dB) indicate that they are mainly observed on the transmissions from NPM and NLK in the nighttime only, with rare occurrence on other transmissions. Such unusually long recovery of early/fast events may be associated with large ionic conductivity perturbations associated with gigantic jets.
Very Low Frequency (VLF; 3–30 kHz) radio waves propagate through the atmospheric waveguide formed between the Earth’s surface (ocean or ground) and the lowest region of the ionosphere (D-region), which is called the Earth-Ionosphere Waveguide (EIWG). The VLF propagation from navigational transmitters undergoes short-timescale (∼1–100 s) perturbations called “early Trimpis” or “early VLF events”, which are believed to be caused by direct lightning-induced effects in the D-region ionosphere. Armstrong (1983) first reported the VLF perturbations that occurred within 100 ms (i.e., within the time resolution of the instrument) of the causative lightning discharges, which were thus termed “early” or “early/fast” events by Inan et al. (1988). Early VLF events are caused by scattering from the localized regions of the conductivity enhancements in the lower ionosphere associated with lightnings, producing Transient Luminous Events (TLEs) such as sprites, halos and elves (e.g., Dowden et al., 1994; Inan et al., 1996a; Sampath et al., 2000; Rodger, 2003; Haldoupis et al., 2004; Mika et al., 2005, 2006; Marshall et al., 2006; Kumar et al., 2008; Inan et al., 2010). However, from a comparison of early/fast VLF events with sprite observations in the USA on three dates (during 1995–2000) with high sprite activity, it was concluded that early/fast VLF events and sprites were not well correlated as many sprites occurred even without any early/fast event and vice versa (Marshall et al., 2006). It has been shown that the electromagnetic pulses (EMPs) from successive in-cloud lightning discharges associated with cloud-to-ground discharges can produce appreciable D-region electron density perturbations which, in some cases, can produce weak early/fast VLF events (Marshall et al., 2008) even if there might not be any TLE associated with electron density perturbations. From the modeling results on the lower ionospheric modification by lightning EMPs, Rodger et al. (2001) have shown both electron density enhancement and reductions, depending upon the relative occurrence of weak or strong lightnings. From a study on the modeling of relaxation (recovery) times of early/fast VLF events, Haldoupis et al. (2009) found that most of the early VLF events are associated with electron density (cm−3) enhancements of the order of 102–104 at altitudes of 75–85 km. Recently, strong early/fast events due to large electron density enhancements in the D-region of the ionosphere associated with gigantic jets have been reported (van der Velde et al., 2010).
Inan et al. (1995) first reported the connection between early/fast events and a small subset of sprites that occurred near the Transmitter-Receiver Great Circle Path (TRGCP), but at large distances from the receiver (>2000 km). They attributed these VLF perturbation events to narrow-angle forward scattering from enhanced ionization associated with lightning discharges located ±50 km off the TRGCP and having a lateral extent of ∼100–150 km. Dowden et al. (1996) observed early/fast events in a one-to-one relationship with sprites located within ~500 km around the receiver and attributed early/fast events to wide-angle scattering from sprite-generated columns of ionization with a shorter scale than the VLF wavelength. Early/fast events on the HWU and HWV transmitters, recorded at Crete station (35.31°N, 25.08°E), were observed in a one-to-one association with sprites in conjunction with those positive cloud-to-ground (+CG) flashes that lead to the production of sprites (Haldoupis et al., 2004). Recently, from the analysis of optical and narrowband VLF data recorded simultaneously during Euro-Sprite campaigns 2003–2009, Haldoupis et al. (2010) have further concluded a one-to-one association between early VLF events and sprites. The concept of TLE-associated ionization changes producing narrow-or wide-angle scattering of VLF transmissions has been under debate for many years. Some researchers have reported that a large fraction of early/fast events are associated with sprites due to wide-angle scattering (Dowden et al., 1996; Hardman et al., 1998) while others found that only a very small subset of sprites (∼5%) showed characteristics of VLF backscatter (wide-angle scattering) from the sprite body (Mika et al., 2005; Marshall et al., 2006).
Early VLF events can be further classified into early/fast, early/slow, rapid onset rapid decay (RORDs), early step-like and unusually long recovery events. However, all types of early VLF events provide evidence of direct coupling between lightning and the lower ionosphere, observed to occur within 20 ms of a causative lightning discharge (Inan et al., 1988). Early/fast events are currently the most widely-studied subionospheric VLF perturbations, mainly at the mid-latitudes. A subset of early/fast events with larger onset durations termed “early/slow” events, were first reported by Haldoupis et al. (2004). In contrast to early/fast events, early/slow events initially have a gradual growth in the observed VLF perturbations, and thus “slow” onset durations which range from about 0.5 to 2.5 s (Haldoupis et al., 2004, 2006).
Another subset of early/fast perturbations are step-like early/fast events mentioned by Sampath et al. (2000) and first studied by Kumar et al. (2008) who reported that they mainly occurred for TRGCPs in the daytime. Unusually long recovery early/fast events were first studied by Cotts and Inan (2007), but were mentioned in earlier publications (Inan et al., 1988, 1996a; Dowden et al., 1997) without any attention given to the unusual nature of the recovery times. Unusually long recovery events may, in practice, appear to be very similar to step-like events, due to the very slow recovery (Cotts and Inan, 2007).
In the current paper, we study the occurrence and scattering features of subionospheric early VLF events on transmissions from four VLF broadcast sites (call signs; NWC, NPM, VTX, and NLK) received at the low-latitude station Suva (18.2°S, 178.3°E). We follow the convention of referring to the VLF transmitters by their call-signs. Observational results have been derived from VLF data recorded during the period November 2006–January 2007, a period of high lightning activity in the South Pacific Region. Earlier studies on this subject were limited to short campaigns and for nighttime periods only. This is the first report of when both day and nighttime narrowband data have been continuously recorded and analysed for three months from four transmitters, and supplemented with lightning and broadband data. We report the first evidence of early/slow events occurring when the TRGCPs were in daylight. We also present the initial observations from the Suva receiver station of early/fast events with an unusually long recovery of 5–20 min or more, and we discuss the possible mechanisms.
2. Experimental Data
To identify the causative lightnings of early VLF events, we analysed World-Wide Lightning Location Network (WWLLN) data (Dowden et al., 2008) for lightnings located within 500 km off the TRGCPs, and which occurred within 100 ms of the occurrence of early VLF events. In this study, we make use of WWLLN data processed by the Reloc-A algorithm (Rodger et al., 2006). In 2006, WWLLN Reloc-A data included 36.5 million lightning locations, which equates to a global lightning detection efficiency ~2.6% (personal communication, Craig J. Rodger, 2011). With increasing WWLLN station numbers and improved processing algorithms, WWLLN global lightning rates have now increased by ∼4 times, relative to the 2006 Reloc-A algorithm (personal communication, Craig J. Rodger, 2011). In addition, wideband VLF data recorded using the Suva WWLLN station for 5 minutes at every hour in the nighttime (18:00–06:00 LT, LT = UT + 12 hrs) during the months of November and December 2006 have also been analysed to examine the sferics from possible causative lightnings at the time of occurrence of early/slow events (see Subsection 3.2).
3. Results and Discussion
3.1 Occurrence and scattering patterns of early VLF events
Early VLF events classified as early/fast, early/slow, RORDs, and early step-like events were all observed on the four transmitters monitored. The most common types were early/fast events. Early/fast events with an unusually long recovery were observed mainly on NPM and NLK sea-based paths, and rarely on the signal from NWC and VTX transmitters. We discuss these unusual events in Subsection 3.3. Analysis of early/slow events is presented in Subsection 3.2, while RORDs have not been considered in the current study, and typical examples of step-like events have been reported by Kumar et al. (2008). For each day, all early VLF events (early/fast, early/slow, step-like, unusually long recovery events) were visually inspected for the period November 2006 to January 2007, and a count of perturbation events (≥0.2 dB) along with their time of occurrence and amplitude changes (Δ A) in dB was made. Δ A is the difference between the pre-event signal level and the signal level at the peak of perturbation. Overall, we found that 353 early VLF events occurred on NWC, 245 on NPM, 40 on NLK, and 17 on VTX signals associated with 453 lightning events. In the case of simultaneous early VLF events on two or more transmitter signals, the causative lightning has been counted as one lightning event. Clearly, early VLF events occur most frequently on the NWC signal and least on the VTX signal. The difference in the occurrence of early VLF events can be mainly attributed to the stability of signals, the occurrence of lighting discharges near the TRGCPs, the TRGCP distances, and the transmitter power and frequency. SoftPAL recording indicates that, at any time, NWC and NPM signals are highly stable, whereas VTX and NLK signals have a considerably larger signal variability. Due to the smaller signal-to-noise ratio, some of the weak early VLF events on VTX and NLK signals cannot be identified. The coupling among VLF modes over the long propagation paths may wash out the weaker early VLF events (Marshall et al., 2006) which are most likely to happen on the VTX and NLK signals due to longer TRGCPs. In addition, Christian et al. (2003) found that approximately 78% of lightnings on the Earth occur between 30° around the geographic equator. Continental, island and coastal regions contribute 88% of the global total production. It is likely that the higher occurrence of early VLF events on the NWC signal is due to the higher lighting rate in the Australia region near the NWC-Suva TRGCP, and the high signal-to-noise ratio of transmissions from NWC.
Parameters of early/fast VLF events on the NWC signal: change in amplitude (Δ A), change in phase (Δθ), nose duration of the events (τ n ), correlation coefficient of scattered amplitude for logarithmic fit(rMlog), correlation coefficient of scattered amplitude for an exponential fit(rMexp).
Time (UT) (hh:mm:ss)
Δ A (dB)
τ n (Amp) (sec)
Early VLF events due to lightning discharges located ±50–100 km off the TRGCPs have been attributed to narrow-angle forward scattering (Inan et al., 1995), whereas those due to lightning discharges located ±100–500 km off the TRGCP (or even behind the receiver) have been attributed to wide-angle scattering (Dowden et al., 1996). The lightning discharges located ±50–100 km around the receiver and/or transmitter have also been attributed to narrow-angle forward scattering, as the area with perturbed electron density due to lightning discharges would overlap the transmitter or receiver. To identify the causative lightnings for all early VLF events (early/fast, early/slow, steplike early and unusually long recovery events) and hence the nature of the associated scattering, we analysed WWLLN lightning data for locations within 500 km off the TRGCPs, coincident with the perturbations. WWLLN data provides the time and location of the global lightnings with return stroke currents of more than ~50 kA, with a spatial and temporal accuracy of roughly 10–20 km and 10 μs, respectively (Rodger et al., 2006). WWLLN detected causative lightnings for 74 of our early VLF events. Under the above criteria, we found that about 73% (54 out of 74) events were associated with narrow-angle scattering and about 27% (20 out of 74) with wide-angle scattering. The locations of lightnings producing early VLF events by narrow-angle, and by wide-angle, scatterings, with respect to TRGCPs are indicated by solid red and blue circles in Fig. 1, respectively. In addition, there were 5 WWLLN detected lightnings which were located more than 600–800 km from the respective TRGCPs. These were coincident with Suva-observed early VLF events. However, these events have not been included in our study as they fall outside the literature-based criteria we have used to define narrow-angle, and wide-angle, scatterings. It is possible that these events may be extreme examples of wide-angle scattering.
The physical nature and conductivity profile of ionospheric disturbances involved in early VLF events and the occurrence of these events with narrow-angle, and wide-angle, scattering is still under dispute (Dowden, 1996; Dowden et al., 1996; Inan et al., 1996b; Dowden and Rodger, 1997; Hardman et al., 1998; Marshall et al., 2006) and requires more experimental data from different regions. From the simultaneous observations of early/fast events at nine closely-spaced (∼65 km) stations in the USA, and using a numerical model of the wave propagation and scattering of VLF signals in the Earth-ionosphere waveguide, Johnson et al. (1999) concluded that the ionospheric disturbance exhibits narrow forward VLF scattering patterns with 15 dB beam widths of less than 15°, consistent with their horizontal extent of 90±30 km. Mika et al. (2005) reported about 5% of backscatter (wide-angle scattering) from the sprite body, in part supporting the earlier observations reported by Dowden et al. (1996), who concluded backscattering was possible. However, Dowden et al. (1996) and the subsequent papers from that group concluded that wide-angle scattering of VLF waves from TLE-produced plasma was very common. From early/fast events and sprite observations in the USA, Marshall et al. (2006) reported that only a very small subset (9 out 250) of sprites shows characteristics of the VLF wide-angle scattering from the sprite body, and that all such cases correspond to multiple sprites or horizontally expansive sprites. Early/fast events associated with wide-angle scatterings have been reported under the EuroSprite-2007 (NaitAmor et al., 2007). The narrowangle scattering of VLF signals has also been reported to be associated from smoother and larger regions of the enhanced electron density (Dowden et al., 1996) and wide-angle scattering associated with columns of enhanced ionization due to strong sprites (Dowden and Rodger, 1997). More experimental work is required to better understand the link between the location of the TLE and the way narrow-angle, and wide-angle, VLF scattering can lead to associated early VLF events. In the current study, we have analysed early VLF events associated with narrow-angle, or wide-angle scatterings on a statistical basis. WWLLN-detected lightning locations within 100 km off the TRGCP show that about 73% (54 out of 74) were associated with narrow-angle scattering. In contrast, about 27% (20 out of 74) were produced by wide-angle scattering from conductivity perturbations associated with WWLLN-detected causative lightning discharges located within 100–500 km off the TRGCPs. About 20% of early VLF events, both on NWC and NPM transmitters, occurred simultaneously, and in the period 00–24 UT on 30 January 2007, 38 early/fast events (not shown here) were observed at Suva on two, or more than two, VLF signals simultaneously. While the electron density disturbances largely lead to scattering in the forward direction, the simultaneous occurrence of early/fast VLF events supports the possibility of early/fast events at Suva due to wide-angle scattering. This indicates that in some instances, wide-angle scattering of VLF signals on one or more transmitters can cause the occurrence of many events over a long period. Our observations of early/fast events on 30 January 2007 are similar to those presented by Dowden et al. (1996) from their observations of early/fast events on NAA, NSS, and NLK signals recorded at Boulder, USA. Also during EuroSprite-2008, on the HWU-Algiers TRGCP that passed within ~350 km from the storm center, 28 of the 35 sprites (80%) were associated with early VLF perturbations that displayed wide-angle scattering to an angle of about 70° (Haldoupis et al., 2010), whereas the other paths displayed narrow-angle scattering.
3.2 Early/slow events and sferic activity
3.3 Unusually long recovery early/fast events
Unusually long recovery early/fast events seem to be associated with conductivity changes at the altitudes where relaxation times are longer than 5 min. This may be possible at higher altitudes of about 80–90 km due to electronic conductivity perturbations, or at a lower altitude of about 50–60 km due to ionic conductivity perturbations (Cotts and Inan, 2007). The theoretical model reported by Marshall and Inan (2010) has shown that strong lightning EMPs can lead to ionization at elve-altitudes (~80–90 km) which can yield measurable VLF perturbations. The large smoothly-varying ionization can lead to early/fast events with long recovery times due to the long relaxation times of electrons at such altitudes. Based on a modeling study of EMP effects on the lower ionosphere Rodger et al. (2001) have shown that a two-fold increase in the ionization at 90-km altitude would take about 30 min to return to 10% of the ambient value, indicating that such events could be associated with elves. However, elve-related perturbations observed during the EuroSprite 2003 had weaker amplitude perturbations of 0.15–0.4 dB with recoveries of 2–3 minutes (Mika et al., 2006). Cotts and Inan (2007) have suggested the possibility that the slow recovery of ionic conductivity perturbations at 65 km could occur in association with gigantic jets (GJs). All the unusually long duration early/fast events observed at Suva were found to have strong amplitude perturbations of ≥ 1 dB (those 4 early/fast events presented here in Fig. 7 have strong perturbations >2 dB) indicating that these events are most likely associated with strong ionic conductivity perturbations, possibly due to gigantic jets as suggested by Cotts and Inan (2007). From an analysis of TLEs observed by ISUAL, Chen et al. (2008) have shown a global rate of 3.23, 0.50, 0.39, and 0.01 events per minute for elves, sprites, halos, and GJs, respectively. Thus, the fewer occurrences of unusually long recovery events are consistent with the lowest global occurrence (0.01 per minute) of GJs. No GJs were recorded by ISUAL along the NPM-Suva path within 200 km off the TRGCP during the period of VLF observations presented in this study. Lehtinen and Inan (2007) proposed a new chemistry model for the stratosphere/lower ionosphere, and attributed long recoveries (~103 to 104 s) of early/fast events to the persistent ionization generated by gigantic jet discharges at low altitude. The recent observations of a GJ-followed sprite by van der Velde et al. (2010), and associated strong early/fast VLF amplitude perturbations (> 1 dB) observed concurrently with a GJ, confirm the production of large electron density enhancements in the D-region of the ionosphere due to GJs. However, a part of the electron density enhancement was most likely due to the associated sprite. Large electron density perturbations at a higher altitude may produce strong perturbations but not the unusual long recovery as the relaxation time is less for a higher-order of electron density enhancements (Haldoupis et al., 2009). The initial faster recovery in some cases, as evident in Fig. 7(a) and (b), could be due to larger electron density enhancements with faster relaxation times in the D-region of the ionosphere, but the unusually long recovery times of events presented here is most likely associated with the persistent ionic conductivity changes in the lower D-region ionosphere associated with GJs, as suggested by Cotts and Inan (2007), or due to the complex events having both sprites and GJs. This is clearly an area for further experimental and theoretical research.
4. Summary and Conclusions
Early VLF events coincident with WWLLN-detected lightnings, located within 500 km off the TRGCPs, show the possibility of VLF perturbations produced by wide-angle scattering. The analysis shows that about 73% (54 out of 74), and about 27% (20 out of 74), of early VLF events coincident with WWLLN lightnings were associated with narrow-angle and wide-angle scatterings, respectively. This is also supported by the simultaneous occurrence of early VLF events on the NWC and NPM signals. However, this is a topic for continued research in view of the lower detection efficiency of WWLLN.
Daytime early/fast events are weak (0.2–0.5 dB) with only negative amplitude perturbations with shorter recovery times (10–40 s), indicating the occurrence of D-region electron density perturbations possibly associated with daytime TLEs (mainly sprites) producing ionization at a lower altitude where the electron density relaxation time is fast due to a comparatively higher electron collision frequency.
Early/slow events occur on the signals mainly when their TRGCPs are in the dark. An example of an early/slow event associated with a very strong sferic with ELF components (even after a propagation distance of about 1100 km), and a few others without an ELF component, is shown. The onset of this event indicates that TLEs (sprites) due to strong CG lightning triggered the onset of these events, and that EMPs associated with sferic clusters (IC discharges and/or due to in-cloud components of CG) contributed to the slow onset, which is consistent with earlier results on early/slow events.
We have presented the first evidence of daytime early/slow events, which occurred both on amplitude and phase. The recovery time (∼45 s) of these events is comparable with nighttime early/fast and early/slow events. Such a recovery indicates the occurrence of sprites at a higher altitude, or daytime elves where relaxation times are larger, and the possibility of secondary ionization due to strong EMPs in the daytime. Such early/slow events are very rare.
An initial study on unusually long recovery (5–20 min or more) early/fast events at our station, Suva, indicates that they mainly occur on NPM and NLK to Suva (sea-based) paths, when their TRGCPs are in darkness. Such events make up about 7% of total events on the NPM signal and have strong amplitude and phase perturbations. There is the possibility that these events are associated with strong ionic conductivity perturbations in the lower part of the D-region possibly associated with gigantic jets, as suggested by Cotts and Inan (2007), or due to complex events having both sprites and GJs. However, this is clearly an area for further experimental and theoretical research.
The authors are thankful to the Faculty Research Committee, The University of the South Pacific (USP), for providing support through a research grant and wish to thank WWLLN for providing the lightning location data. One author (Sushil Kumar) thanks the Department of Physics, Otago University, New Zealand, for providing the facilities under Ratu Sir Kamisese Mara Visiting Fellowship in which he could complete this work. We are grateful to Craig J. Rodger of Otago University for helpful discussions.
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