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Article

Lower Ionospheric Perturbations Associated with Lightning Activity over Low and Equatorial Regions

by
Dayanand Bhaskar
1,
Rajat Tripathi
1,
Mahesh N. Shrivastava
2,
Rajesh Singh
3,
Sudipta Sasmal
4,
Abhirup Datta
5 and
Ajeet Kumar Maurya
1,*
1
Department of Physics, Babasaheb Bhimrao Ambedkar University, Vidya Vihar, Raebareli Road, Lucknow 226025, India
2
Department of Geosciences, Universidad Católica del Norte (UCN), Av. Angamos 0610, Antofagasta 1270709, Chile
3
KSK Geomagnetic Research Laboratory, Indian Institute of Geomagnetism, Prayagraj 221505, India
4
Institute of Astronomy Space and Earth Science, P177, CIT Road, Kolkata 700054, India
5
Department of Astronomy, Astrophysics and Space Engineering, Indian Institute of Technology Indore, Indore 453552, India
*
Author to whom correspondence should be addressed.
Atmosphere 2025, 16(7), 832; https://doi.org/10.3390/atmos16070832
Submission received: 27 April 2025 / Revised: 7 June 2025 / Accepted: 6 July 2025 / Published: 9 July 2025
(This article belongs to the Section Upper Atmosphere)
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Abstract

We present lightning-induced ionospheric perturbations in narrowband very-low-frequency (VLF) signals from the transmitters NWC (21.82° S, 114.17° E, 19.8 kHz) and VTX (8.4° N, 77.8° E, 18.6 kHz) recorded at the low-latitude station Dehradun (DDN; 30.3° N, 78.0° E) over a 12-month period from September 2020 to October 2021. Early/slow VLF events, VLF LOREs, and step-like VLF LOREs associated with lightning were analyzed for their onset and recovery times. This study utilized data from the World Wide Lightning Location Network (WWLLN), which provides lightning locations and energy estimates. The results show that early/slow VLF events occur most frequently, accounting for approximately 68% of cases, followed by VLF LOREs at 12%, and step-like VLF LOREs at 10%. Furthermore, we observed that 100% of the VLF perturbing events occurred during the nighttime, which is not entirely consistent with previous studies. Moreover, more than 60% of VLF LOREs were associated with lightning energies of approximately 1 kJ, and about 40% were associated with lightning energies of ~10 kJ. Step-like VLF LOREs were linked to WWLLN energies between 1 and 5 kJ. The observed WWLLN energy range is somewhat lower than the energies reported in previous studies. Scattering characteristics revealed that 87.3% of events were associated with wide-angle scattering, while approximately 12.6% were linked to narrow-angle scattering. LWPC version 2.1 was used to simulate these perturbing events and to estimate the reflection height (H′, in km) and the exponential sharpness factor (β, in km−1) corresponding to changes in D-region electron density. The reflection height (H′, in km) and the exponential sharpness factor (β, in km−1) of the D-region varied from 83 to 87 km and from 0.42 to 0.79 km−1 for early/slow VLF events, from 83 to 85 km and from 0.5 to 0.75 km−1 for step-like VLF LOREs, and from 81 to 83 km and from 0.75 to 0.81 km−1 for VLF LOREs, respectively.

1. Introduction

The D-region of the ionosphere, which constitutes its lowest portion, exhibits diurnal variations, ranging from 60 to 75 km during daytime hours and from 75 to 95 km during the nighttime. This region is primarily responsible for reflecting the majority of very-low-frequency (VLF) signals. When additional ionization is induced in this region due to a lightning strike, the VLF signal becomes scattered, resulting in a perturbation observed at the receiver [1]. The rapid fluctuations in VLF signals caused by lightning strokes that directly interact with the lower ionosphere are referred to as early VLF events [2,3]. These abrupt alterations occur almost instantaneously (<20 ms) following a lightning strike and persist for between 20 ms and 2 s; they are categorized as early events [4,5]. The recovery time of these events has been reported to range from tens of seconds to several minutes in previous studies [6,7,8,9,10,11,12]. Early VLF events are further classified based on their onset and recovery time into the following types: early/fast events, early/slow events, long-recovery early events (LOREs), and step-like long-recovery early events (step-like LOREs). The term onset duration refers to the time interval between the initial occurrence of the VLF perturbation and the subsequent maximum amplitude or phase change [5,6,13].
This study focuses primarily on three types of VLF event: early/slow events, LOREs, and step-like LOREs. The onset time for early/slow events ranges from 0.5 to 2.5 s, with recovery times ranging from 10 to 180 s. In contrast, LOREs have recovery times lasting up to 20 min [6,7,8,9]. Preliminary studies have indicated that these VLF events are triggered by sprites—transient luminous events caused by lightning [3,14,15]. A detailed analysis of the correlation between early VLF events and the first documented sprites indicated that VLF dispersion occurred within ±50 degrees relative to the transmitter–receiver great circle path (TRGCP) [2], a scattering pattern termed narrow-angle scattering. Conversely, other studies have reported VLF scattering at distances greater than 100 km away from the TRGCP, referred to as wide-angle scattering [16,17].
A substantial body of research provides strong evidence for a robust correlation between early VLF perturbations and sprite-producing lightning. This relationship shows a high degree of linearity, suggesting a near one-to-one mapping between the two phenomena [14,18,19]. However, further analysis of datasets containing simultaneous VLF perturbations and sprite-producing lightning revealed that only a small proportion of sprites exhibited backscattering due to these perturbations [10,20,21]. Several factors influence the observation of early VLF perturbations and signal behavior. These include the frequency of the transmitter, the distance between the transmitter and the perturbation, the modal structure of various TRGCPs at the site of the ionospheric disturbance, and the scattering process itself. Additionally, the distance between the sprite and the receiver significantly affects VLF signal perturbation parameters, with greater amplitude and longer recovery time observed when the sprite is closer to the receiver.
This study aims to examine the relationship between variations in VLF perturbations and lightning strikes, which have been observed to cause ionospheric disturbances. Specifically, we investigate the scattering characteristics and decay profiles of unusually long-duration recovery events. Long-Wavelength Propagation Capability (LWPC) modeling was applied to a selected set of VLF perturbation events to analyze variations in the D-region’s Wait parameters. Observations were conducted using signals from the NWC (19.8 kHz) and VTX (18.2 kHz) transmitters recorded at the low-latitude station in Dehradun, India. This location was selected for the study period, which spanned from September 2020 to October 2021, coinciding with the active lightning season in the Indian–Pacific region. The objective of this study is to establish an empirical relationship between lightning strikes, as detected by the World Wide Lightning Location Network (WWLLN) [22], and the occurrence rate of VLF events, including early/slow VLF events, VLF LOREs, and step-like VLF LOREs. Additionally, LWPC version 2.1 was employed to calculate the nighttime D-region Wait parameters, specifically the reflection height (H′) in km and the sharpness factor (β) in km−1, associated with the lightning-generated VLF perturbations.

2. Materials and Methods

The amplitude and phase of very-low-frequency (VLF) signals were measured using Minimum Shift Keying (MSK) modulation from the NWC and VTX ground-based navy transmitters. Phase data were recorded using a phase logger to ensure measurement accuracy. The NWC transmitter, located on the North West Cape, Australia (21.82° S, 114.15° E), operates at 19.8 kHz with an approximate transmission power of 1 MW. The VTX transmitter, based in Vijayanarayam, South India (8.43° N, 77.73° E), operates at 18.2 kHz with a power of 500 kW. These transmitters are commonly employed as signal sources in subionospheric propagation studies, particularly in research on ionospheric and lightning-induced perturbations [23,24]. The NWC transmitter is one of the most studied VLF transmitters in the context of ionospheric and magnetospheric wave–particle interactions. Previous studies have shown that NWC signals propagate poleward into the conjugate hemisphere, revealing complex magnetospheric propagation paths influenced by geomagnetic field structures [25]. Moreover, the NWC transmitter has been observed to induce pitch-angle scattering of energetic electrons (30–400 keV), resulting in enhanced precipitation fluxes detected by low-Earth-orbiting satellites such as PROBA-V, demonstrating its active role in radiation belt dynamics [26]. These characteristics make NWC a valuable signal source for diagnosing ionospheric disturbances and wave–particle interactions in the near-Earth space environment. Similarly, the VTX navigational transmitter has been widely studied for its interaction with ionospheric disturbances, such as those induced by solar flares, geomagnetic storms, and lightning discharge, and has served as a key reference transmitter in numerous ionospheric and space weather studies [23,24]. We analyzed 12 months of VLF data from September 2020 to October 2021 for both transmitters and observed the onset and recovery times of early/slow VLF events, VLF LOREs, and step-like VLF LOREs associated with lightning. Data were recorded daily during this period at a 1 s resolution. All early VLF events were visually inspected, and the number of events was recorded along with their onset time, recovery time, and amplitude change (ΔA) in decibels (dB). The amplitude change (ΔA) is defined as the difference between the signal level preceding the event and the peak of the amplitude perturbation. Furthermore, it is very well known that lightning events themselves can also cause disturbances in VLF signals [22]. These signals often coincide exactly with the time of the return stroke and are known to directly couple into the Earth–ionosphere waveguide [2]. Conversely, scattering effects due to D-region electron density changes generally result from processes such as lightning EMP (electromagnetic pulse) interactions or lightning-induced electron precipitation (LEP). These mechanisms lead to more gradual and sustained perturbations in VLF amplitude and/or phase, often lasting from hundreds of milliseconds to several seconds. These VLF events often correspond to transient luminous events (TLEs) like sprites and elves, but may be observed without direct optical confirmation using a combination of signal processing, event characterization, and correlation with lightning data [8].
In order to confirm that observed events are indeed due to D-region scattering, we directly identified these events in the VLF signal based on their specific pattern in the VLF signal (step-like event). Further, the events were also confirmed with WWLLN lightning data such as the location, timing, and energy of the causative lightning events along the TRGCP. We used two transmitter–receiver great circle paths to verify that the observed events were path-specific, confirming a regional ionospheric disturbance. We identified lightning strikes occurring within 1000 km of the transmitter–receiver great circle path (TRGCP) for both transmitters using WWLLN data [22] in order to correlate them with the observed VLF events. The details of the TRGCP between the VLF receiving stations at DDN and the VLF navigational transmitters NWC and VTX are shown in Figure 1.
The WWLLN is a global network of over 80 VLF radio receivers, with a sampling rate of 48,000 samples per second, which detect VLF radiation in the range of 100 Hz to 24 kHz. The WWLLN provides real-time lightning location data, which helps in measuring the VLF radiation from lightning discharges. Currently, the stroke detection efficiency of WWLLN for peak currents exceeding 50 kA has been documented at 88% [22,23,24]. The precision of lightning location and timing is expected to be within approximately 5 km and 10 microseconds, respectively [25].
In addition to lightning location, the WWLLN offers data on lightning stroke far-field energy and the associated measurement uncertainty. The root-mean-square electric field of the detected lightning stroke, determined by a 1.3 ms waveform sampling within the 6–18 kHz range, is used to estimate the lightning stroke energy [23].
The D-region ionospheric profile is described using the Wait parameters, H′ and β. H′ represents the effective reflection height of the D-region ionosphere for very-low-frequency (VLF) radio waves and is measured in km. The sharpness factor, β (km−1), characterizes the rate at which electron density increases with altitude above H′. The Long-Wavelength Propagation Capability (LWPC) was used to observe the reflection height (H′) in kilometers and the sharpness factor (β) in km−1, both related to VLF perturbations. It was developed by the Space and Naval Warfare Systems Center [26] and is a well-established numerical tool for simulating VLF and LF radio wave propagation within the Earth–ionosphere waveguide. It utilizes specified ionospheric and ground conductivity profiles to compute signal characteristics. Changes in the VLF reflection height (H′) and the sharpness factor (β), predicted using LWPC version 2.1, are associated with VLF perturbations. The LWPC has undergone extensive validation [11,26]. The effectiveness of the proposed method has been demonstrated through its successful modeling of long-range VLF signal propagation phenomena

3. Results

We observed NWC and VTX VLF data from September 2020 to October 2021, recorded at a low-altitude station in Dehradun, India. The focal point of our analysis was the amplitude of the NWC and VTX transmitter signals, and we identified a total of 153 perturbing events exhibiting various types of disturbances in both signals. These included 17 events in the VTX signal and 136 in the NWC signal. Perturbations were observed in the signal amplitudes. In most cases, the phase data from both transmitters was insignificant and not particularly useful. A larger number of events were identified in the NWC signal compared to the VTX signal, likely due to the longer TRGCP (transmitter–receiver great circle path) for the NWC–Dehradun path, which also traverses several active lightning regions. One example of each category of perturbed events from both transmitters is presented in Figure 2. Furthermore, a summary of all four types of VLF perturbing events is presented in Table 1, providing the details of onset and recovery time of these events along with the previous literature
For the NWC signal, out of the 136 events in total, 91 were classified as early/slow events, 19 as LOREs, 13 as step-like LOREs, and 13 as unusual events. For the VTX signal, out of 17 events, 13 were early/slow events, 2 were step-like LOREs, and 2 were unusual events. Of the total observed perturbing events (153) across both transmitters, approximately 68% were classified as early/slow VLF events, ~12% as VLF LOREs, and ~10% as step-like VLF LOREs. All VLF events occurred during the nighttime TRGCPs.
Early/slow VLF events are generally identified by a sudden perturbation with an onset duration ranging from 20 ms to 2 s, followed by a slow recovery time (typically 10–180 s) [5]. In the present study, the onset and recovery times for early/slow VLF events ranged from 0.7 to 2.8 s and from 5 to 309 s, respectively. These values fall within the range reported in previous studies [4,5,27]. These events were observed exclusively during nocturnal TRGCPs, with no occurrences during the daytime. This contrasts with findings from other studies that reported such events during both day and night, under conditions of full sunlight and partial darkness [6,27]. Nevertheless, it is important to note that a majority of early/slow VLF events (approximately 82%) have been documented to occur during nighttime periods [27].
The second most common type of VLF perturbation recorded was the VLF LORE, first reported by [6], which is associated with very high peak current flashes exceeding 250 kA [15]. These events are characterized by long recovery times, typically lasting up to 30 min [6]. In our observations, the onset times for LOREs ranged from 0.7 to 4.3 s and the recovery times ranged from 4 to 31 min.
We also recorded a significant number of step-like VLF LOREs (~10% of the total). This category of VLF LORE is typically characterized by very long or indefinite recovery times [8].
Additionally, we observed a number of unfamiliar VLF events, termed unusual events (~10%), with onset and recovery times that differed from those of early/slow events, LOREs, and step-like LOREs. These unusual events had an onset time ranging from 3 to 5 s, while their recovery durations ranged from approximately 25 to 191 s. These range do not fit into the existing range of known events. The classification of unusual VLF perturbing events is limited due to the ambiguity of quantitative criteria. The establishment of specific benchmarks for onset and recovery times is imperative for enhancing our comprehension of these phenomena in the context of ionospheric research. Subsequent studies should endeavor to delineate these parameters, which may yield novel insights into the mechanisms of unusual VLF events. The summary of onset and recovery time characteristics of all three types of event (early/slow events, LOREs, and step-like LOREs) is presented in Table 2.

3.1. Relationship Between Perturbed VLF Events and Lightning Lateral Distance from TRGCPs

We used WWLLN lightning data to identify lightning-induced perturbations in the VLF signal. Figure 1 shows WWLLN lightning activity (red dots) along the TRGCPs. We considered lightning strikes occurring within 1000 km of the TRGCPs. Out of a total of 157 perturbed VLF events, the WWLLN was able to identify 107 causative lightning events—approximately 68% of the total perturbations (consistent with the WWLLN’s detection efficiency of around 70%). This detection rate is significantly higher than that reported by Chand [28], who identified causative lightning for only 36% of events.
The observed very-low-frequency (VLF) perturbations are attributed to the scattering of VLF signals caused by lightning-induced changes in D-region electron density and conductivity. Understanding the nature of this scattering is crucial. In this study, we attempt to characterize the scattering behavior based on the location of lightning discharges responsible for the VLF perturbations.
Inan [29] categorized lightning strikes based on their proximity to TRGCPs and developed a taxonomy of atmospheric electrical phenomena to enhance the understanding and prediction of such events. Lightning discharges occurring within 100 km of the TRGCPs are primarily associated with narrow-angle forward scattering. In contrast, those located between 100 km and 1000 km away are typically linked to wide-angle forward scattering.
In the present study, a total of 107 VLF events were examined. Of these, 15 (14.01%) were found to be linked to narrow-angle scattering, while 92 (85.98%) were attributed to wide-angle scattering. As demonstrated in Figure 3, the lateral distance of lightning strikes from the transmitting station serves as an indicator of their geographical position and the potential for these lightning strikes to contribute to VLF events. From Figure 3, it can be seen that the maximum number of early/slow events is associated with lateral distances in the range of 200–500 km and falls under wide-angle scattering. Similar conditions are also observed for the other two types of event, i.e., LOREs and step-like LOREs.
While LOREs fall within the lateral distance range, our observations indicate that as the distance of lightning strikes from the transmitting station increases, the occurrence of VLF events also increases within the range of 200–500 km. This finding suggests that most VLF perturbing events are associated with wide-angle scattering. Focusing specifically on early/slow events, it was observed that approximately 43% of these were linked to wide-angle scattering, whereas about 67% were associated with narrow-angle scattering. Therefore, the findings of this study indicate that the majority of VLF perturbations caused by lightning occur within 200 to 500 km of the TRGCP and are associated with wide-angle scattering.

3.2. Relationship Between VLF Events and the Energy Contained Within Lightning Strokes

In this section, we investigate the correlation between early VLF events and lightning stroke energy using data from the WWLLN lightning energy database. Figure 4 presents the energy distribution of lightning strokes that resulted in amplitude perturbations associated with both narrow- and wide-angle scattering. The analysis indicate that the majority of early/slow VLF events were associated with lightning strokes with energies in the range of 0.142 kJ to 1 kJ, accounting for approximately 42.58% of the total events. Additionally, around 9% of LOREs and 6.49% of step-like LOREs were also associated with strokes in this energy range.
Furthermore, about 24.46% of early/slow events were linked to lightning strokes with energies between 1 and 2 kJ—significantly higher than the proportions observed for LOREs and step-like LOREs in this energy range.

3.3. Lower Ionospheric Region Parameters Using LWPC Modeling

LWPC version 2.1 was used to estimate variations in lower ionospheric (D-region) parameters [30]. The model includes two key parameters: the reflection height (H′) in kilometers and the exponential sharpness factor (β) in km−1. Estimations were made by modeling VLF perturbing events observed in the NWC and VTX signals. Initially, we ran the LWPC code for a characteristic (unperturbed) day to obtain the baseline amplitude of VLF navy transmitter signals. During early/slow VLF events, VLF LOREs, and step-like VLF LOREs, the LWPC model was used to simulate corresponding amplitude changes. We then calculated the change in amplitude (ΔA) using Minimum Shift Keying (MSK) data for each type of VLF event. These values were integrated into the LWPC amplitude calculation to derive the observed amplitude (A′ = A + ΔA). This method was used to determine the observed amplitudes for early/slow VLF events, VLF LOREs, and step-like VLF LOREs.
A total of 20 VLF perturbation events were analyzed (10 each from NWC and VTX). For the NWC transmitter, the average unperturbed parameters were H′ = 87 km and β = 0.4 km−1, which are typical of quiet nighttime D-region conditions. Under perturbed conditions, early/slow events showed H′p values between 83.0 and 85.8 km and βp values from 0.66 to 0.81 km−1, indicating a moderate lowering of the ionospheric reflection height and a sharpening of the electron density profile. LOREs were associated with H′p values of 81.1 to 85.2 km and βp values of 0.76 to 0.81 km−1, reflecting slightly deeper ionospheric perturbations and steeper density gradients. Step-like LOREs produced the most substantial changes, with H′p values as low as 73.7 km and βp values ranging from 0.55 to 0.79 km−1, suggesting a significant downward displacement of the D-region and a wider range of sharpness, likely due to more energetic lightning strokes. These ranges reveal how each event type distinctly influences the structure and altitude of the ionosphere, with step-like LOREs typically producing the most intense and sustained perturbations. Detailed values are provided in Table 3.
Similarly, for the VTX transmitter, the quiet-time values were H′ = 87 km and β = 0.37 km−1. The mean perturbed values across all events were H′p = 86.55 km and βp = 0.54 km−1. During early/slow events, H′p ranged from 84.1 to 87.5 km, and βp ranged from 0.42 to 0.58 km−1. These relatively small variations suggest that the perturbations were weaker compared to those observed in NWC data, possibly due to differences in path geometry or transmitter–receiver distance. Step-like LOREs showed less dramatic changes, with H′p values between 85.0 and 85.2 km and βp values between 0.50 and 0.56 km−1, indicating a more localized or less intense disturbance in comparison with those associated with the NWC path. Complete perturbed and unperturbed values for the VTX events are presented in Table 4. The results are further discussed in Section 4 (Discussion).

4. Discussion

The present study examines the characteristics of early/slow VLF events, VLF LOREs, and step-like VLF LOREs, as presented in this paper. These events are caused by the scattering of VLF transmitter signals from areas of modified conductivity in the lower ionosphere (D-region). Fluctuations in the conductivity of the D-region may be attributable to either extraction, a process that increases electron density, or electron heating, a process that modifies the electron–neutral collision frequency [31]. The majority of initial VLF occurrences are distinguished by a gradual initiation and subsequent recovery of the amplitude of the received signal, with onset durations ranging from approximately 0.5 to 2.5 s (early/slow) [5]. As summarized in Table 2, in the present case, we observed an onset time of 0.7 to 2.8 s, which is within the range of previously reported cases regarding early/slow event onset time. These types of event are usually observed within 50 km of the TRGCP and are triggered by cloud-to-ground (CG) lightning [32,33,34], but some studies show that causative lightning discharges can produce these types of event up to a few hundred kilometers away from the TRGCP [3]. Previous works also report that early/slow events are triggered due to electromagnetic pulses produced by CG and intracloud lightning [29,35]. Such types of lightning generate events known as transient luminous events (TLEs) [14]. These events are significant for understanding the interactions between lightning and the ionosphere. TLEs are a type of optical phenomenon occurring in the upper atmosphere, typically above thunderstorms. They are caused by electromagnetic pulses (EMPs) generated by lightning strikes. These phenomena include entities such as elves, sprites, and jets, which have been observed to exert a substantial impact on the ionosphere [15]. These events increase the electron density in the D-region altitude enough to disrupt VLF propagation [36]. Haldoupis [14,15] reported a direct correlation between TLEs and early/slow events. However, we are unable to provide any insights regarding the nature of this relationship due to the absence of direct TLE observations and the lack of information regarding the lightning type from the WWLLN.
We found that almost all VLF perturbing events occur at night, which is not entirely consistent with earlier research [4,9,32]. A number of prior studies have documented an occurrence rate of approximately 22.8% for early/slow events during daytime periods [28]. Most previous work supported maximum occurrences during daytime and much lower occurrences during nighttime. It is well known that the daytime D-region is fully controlled by solar radiation, while minor D-region-ionizing sources do not have a significant impact [37], whereas during the nighttime, minor sources such as direct lightning discharges significantly affect the D-region ionosphere [38]. Furthermore, due to differences in the day and nighttime wave attenuation, the propagation of lightning-generated electromagnetic waves is strongly supported during the nighttime. These waves propagate more efficiently during nighttime and are more likely to reach the altitudes where they can interact with VLF transmitter signals. This diurnal modulation of propagation is clearly observed in space-based measurements by the Van Allen Probes [39,40], which demonstrate that whistler-mode waves originating from lightning are significantly more prevalent and less dispersed during nighttime. These findings support our nighttime observations of VLF perturbing events. The other reason for this could be due to regional lightning activity differences (including lightning day/night occurrence frequency over the Indian region compared to other regions, as mentioned in [41]. The geographic location of the TRGCP, its path length, it direction, and patch characteristics, such as its land and sea path, which provide different conductivity conditions, may also have an effect [23]. In the present work, most VLF perturbation events occurred during periods of minimal geomagnetic activity. The Dst index for event days varied from 11 nT to −30 nT, while the Kp index varied from 0 to −3, indicating that geomagnetic influences on the D-region’s ionospheric conditions were negligible. This is consistent with findings from [39,40], who analyzed lightning-generated wave amplitudes using measurements from the Van Allen Probes. Their results showed that the amplitude and propagation characteristics of lightning-generated whistler-mode waves were largely independent of geomagnetic activity levels, particularly during quiet conditions.
The findings of this investigation indicate that the occurrence of VLF LOREs and step-like VLF LOREs can primarily be attributed to a combination of quiescent heating processes and electromagnetic pulse (EMP)-induced ionization during intense cloud-to-ground lightning discharges. In contrast, the predominance of early/slow events is likely due to the effects of quasi-electrostatic fields, which induce fluctuations in electron density in the lower ionosphere [31].
In our study, we discovered that early/slow VLF events occur frequently, accounting for approximately 68% of occurrences, while VLF LOREs account for 12% and step-like VLF LOREs account for 10%. The second largest number of events in our study was observed for LOREs. These events are characterized by a longer recovery time that can extend up to several tens of minutes [6]. As demonstrated in the study by [8], long-recovery early events (LOREs) are associated with intense CG lightning strokes, which result in prolonged ionization within the D-region ionosphere. They further inferred that lightning could be located within 250 km of the TRGCP. In the present work, the majority (>60%) of LOREs were associated with lightning energy of 1 kJ, while the remaining events were characterized by energy as high as 10 kJ. The lightning locations were found to be within 300–700 km of the TRGCP for most events, but a few events were also associated with close lightning (<250 km). Thus, our findings suggest that high-energy lightning discharges cause LOREs, although depending on ionospheric conditions, these events can also be produced by low-energy discharges (<2 kJ). Furthermore, from Table 1, the D-region’s reflection height H′ for LOREs varies in the range of 81–85 km, which matches the LORE observation altitudes reported in previous works [7].
The third type of VLF perturbation event studied in this work is called the “step-like LORE”, in which the signal does not recover to pre-event levels and appears as a step-up or step-down in the VLF signal [6,8]. These events are considered as a subset of LOREs. Previous studies have found that, like LOREs, step-like LOREs are also associated with high-peak-current CG lightning discharges. In the present study, step-like LOREs are found to be associated with energy between 1 and 5 kJ, while the associated lightning is located within 200–700 km of the TRGCP. The results indicate that step-like LOREs are likely generated by a mechanism similar to regular LOREs, generally associated with weaker lightning discharges. The major differences between step-like LOREs and LOREs could be due to variations in complex scattering geometries, waveguide modal propagation conditions, and surrounding ionospheric conditions [10,42]. It is important to note that this hypothesis needs to be further verified through a rigorous analysis of these conditions.
Moreover, an examination of the lightning activity associated with early VLF episodes was conducted by analyzing lightning data obtained from the Worldwide Lightning Location Network (WWLLN). Specifically, the focus was on lightning strikes occurring within a radius of 1000 km from both transmitters’ TRGCPs. The findings of this study contradict earlier investigations that postulated a predominant relationship between early VLF events and narrow-angle scattering [9,17,43,44]. The present research indicates that narrow-angle scattering was associated with only 12.6% of early VLF occurrences, while wide-angle scattering demonstrated a strong correlation with 87.3% of these occurrences. We hypothesize that there could be two major reasons for such a dominance of wide-angle scattering in the present work, namely (1) the uniqueness of the geographical location of the given TRGCP and (2) the possibility of the enhanced lightning detection efficiency of the lightning data. In this study, we considered two transmitter–receiver great circle paths (TRGCPs), DDN–VTX and DDN–NWC, both of which lie within the low-equatorial ionospheric region. This region presents unique ionospheric conditions characterized by high electron density gradients and frequent irregularities arising from phenomena such as the equatorial electrojet and Equatorial Spread-F (ESF) [5,45,46,47]. These processes give rise to plasma bubbles and density irregularities at low latitudes, which can extend into the D-region and cause significant scattering of very-low-frequency (VLF) signals, resulting in broad angular dispersion [47,48].
Furthermore, the Indian subcontinent—located within the low-latitude region—is recognized as one of the three primary global hotspots for lightning activity [41]. This intense lightning and associated thunderstorm activity serves as a strong source of atmospheric gravity waves originating in the lower atmosphere. These gravity waves can propagate into the D-region ionosphere, producing small-scale ionospheric irregularities that further influence VLF signal propagation [49]. Such disturbances cause rapid electron density variations, enhancing signal scattering. Thus, the pronounced wide-angle scattering observed in our data is strongly influenced by the unique ionospheric structure of the studied geographic region. Due to the absence of localized efficiency data, a direct comparison with the detection efficiency of the WWLLN (World Wide Lightning Location Network) in this region remains challenging.
Our results also suggest the possibility of a nonlinear relationship between lightning energy and the resulting scattering angle, although this has not been fully quantified in the present work. As reported in previous works [44,50], higher-energy lightning, with large peak currents or charge moment changes, generates stronger electromagnetic pulses (EMPs) that penetrate deeper into the D-region and produce more extensive ionization over a broader horizontal area, which acts as a larger and more complex scattering region, enabling VLF signals to be reflected or scattered over wider angles [51].
In previous periods, the LWPC code was employed to estimate disruptions to the D-region ionosphere during both daytime and nighttime hours. As indicated by the works of Poulsen and Inan [42], as well as the observations made by Clilverd [52], Thomson [53], and NaitAmor [11], lightning-induced electron precipitation has been demonstrated to be associated with such phenomena as solar eclipses, solar flares, and early VLF perturbations. The values of the atmospheric parameters H′ and β are indicative of the reflection altitude and its potential relationship with the TLEs’ height [54].
In their study, NaitAmor [11] employed the LWPC code and modeled early VLF perturbations observed on 12 December 2009 along the NRK–Tunis VLF path. These perturbations were associated with the presence of a gigantic jet and a sprite halo. The calculation of the nighttime perturbed Hp′ yielded a result of 66.4 km (ΔH′ = 20.6 km), which is 17.07 km lower for NWC and 20.15 km lower for the VTX transmitter compared to our mean values of perturbed Hp′ at 83.47 km and 86.55 km, respectively. For the various types of VLF perturbing events, we observed different ranges for the D-region reflection height (Hp′) and sharpness factor (βp) values. The values vary depending on the specific context. For instance, in the case of early/slow events, the range is from 83 to 87 km, with an average βp of 0.42 km−1, and for step-like LOREs, the range is from 83 to 85 km, with an average βp of 0.5 km−1. For LOREs specifically, the range is from 81 to 83 km, with an average βp of 0.75 km−1. LOREs consistently exhibit significantly lower values of H′ and higher values of β compared to early/slow events. According to Wait and Spies [55], a lower H′ indicates a downward displacement of the D-region’s effective reflection height, while a higher β corresponds to a steeper electron density gradient. The combination of these effects leads to an enhanced D-region electron density profile with a more sharply defined ionospheric transition region. This increase in electron density may result from high-energy, lightning-induced electromagnetic pulses (EMPs), which are more pronounced in LOREs. Furthermore, a lower H′ combined with a higher β implies a more abrupt ionospheric boundary, enhancing the scattering and reflection of VLF waves over a broader angular range, resulting in wide-angle scattering [56,57]. In our analysis, most LOREs (~85%) are associated with wide-angle scattering, supporting this interpretation. Furthermore, the variation in values of Hp′ and βp corresponding to distinct perturbing events also signifies the presence of different ionospheric conditions, which are pivotal in the generation process.

5. Conclusions

The lower ionosphere over Dehradun, India, exhibited very-low-frequency (VLF) perturbations, designated as early VLF events, long-recovery early events (LORE), and step-like LOREs from September 2020 to October 2021. These events were recorded along the NWC and VTX VLF propagation paths. A comprehensive study was conducted to examine the characteristics of these events, including their type (early/slow events, LOREs, and step-like LOREs), occurrence rate, scattering pattern, lightning frequency, energy, and lateral displacement of lightning relative to the transmitter–receiver great circle path (TRGCP). The results indicate that 85.98% of early VLF events exhibited a correlation with wide-angle scattering, resulting from lightning strikes located more than 100 km away from the TRGCP. In contrast, approximately 14.01% of events demonstrated a correlation with narrow-angle scattering, arising from lightning within 100 km of the TRGCP. This finding contradicts the conclusions of previous studies that reported a predominance of narrow-angle scattering in VLF events.
Our observations revealed that approximately 100% of VLF perturbing events occurred during nocturnal hours. This finding does not fully align with previous studies, which indicated a smaller percentage (~22%) of events occurring during daytime hours. We also found that the majority (>60%) of LOREs are associated with lightning energy around 1 kJ, while the remaining events are characterized by energy as high as 10 kJ. Similarly, step-like LOREs were found to be associated with energies between 1 and 5 kJ. Furthermore, the lightning associated with these events was located between 200 and 700 km away from the TRGCPs.
We further investigated alterations in D-region conductivity in relation to early VLF events. By comparing the unperturbed and perturbed values of the NWC and VTX stations, our study reveals significant variations in both height (H′) and conductivity gradient (β) that are influenced by the type and timing of the lightning events. Notably, for the NWC station, the perturbed parameters exhibited a downward shift in altitude (H′) and a marked increase in conductivity gradient (β), with values varying according to event type—early/slow events versus step-like LOREs. Similarly, the VTX station demonstrated comparable trends, although the magnitude of the variations was somewhat lower. These findings suggest that lightning-induced perturbations in the D-region are not uniform, and the specific characteristics of the VLF perturbing events (e.g., early/slow events or step-like-LOREs) play a pivotal role in modulating ionospheric properties. The observed changes in H′ and β provide a better understanding of how lightning impacts the ionosphere, with potential applications in improving ionospheric models and enhancing VLF signal propagation prediction.

Author Contributions

D.B. analyzed the VLF data and wrote the initial draft. R.T. assisted in analyzing the lightning data. A.K.M. conceptualized the study, supervised the analysis, and revised the manuscript. M.N.S., R.S., S.S. and A.D. contributed to writing, reviewing, and editing the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The open-source data are available on their respective websites as mentioned in the manuscript. The narrowband VLF data used in the present study are available at the Zenodo Research Data Repository (https://doi.org/10.5281/zenodo.15291747).

Acknowledgments

The authors acknowledge Robert Holzworth, the Director of the WWLLN (https://wwlln.net/ accessed on 20 January 2025), for providing the data used in the present investigation, including lightning energy. They also thank the NASA Omniweb database team for supplying the geomagnetic data used in this article.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
VLFVery-Low-Frequency
LORELong-Recovery Early Event
WWLLNWorld Wide Lightning Location Network
NWCNorth West Cape

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Figure 1. Distribution of lightning activity from the WWLLN along the great circle path of two transmitters, NWC and VTX, and a receiver in Dehradun, India.
Figure 1. Distribution of lightning activity from the WWLLN along the great circle path of two transmitters, NWC and VTX, and a receiver in Dehradun, India.
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Figure 2. Examples of early/slow VLF events, VLF LOREs, and step-like VLF LOREs, which are recorded on the NWC and VTX transmitter signal. Horizontal arrows in each figure show the recovery time.
Figure 2. Examples of early/slow VLF events, VLF LOREs, and step-like VLF LOREs, which are recorded on the NWC and VTX transmitter signal. Horizontal arrows in each figure show the recovery time.
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Figure 3. Lateral distance of the lighting responsible for the occurrence of very-low-frequency (VLF) events, which affected the VTX and NWC signals.
Figure 3. Lateral distance of the lighting responsible for the occurrence of very-low-frequency (VLF) events, which affected the VTX and NWC signals.
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Figure 4. The present study seeks to establish a correlation between stroke energy and very-low-frequency (VLF) events in the NWC and VTX signals.
Figure 4. The present study seeks to establish a correlation between stroke energy and very-low-frequency (VLF) events in the NWC and VTX signals.
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Table 1. Classification of VLF perturbing events based on their onset and recovery time as found in the previous literature.
Table 1. Classification of VLF perturbing events based on their onset and recovery time as found in the previous literature.
S.No.Event TypeOnset TimeRecovery TimeReference
1Early/fast>20 ms 10–180 s [4]
2Early/slow0.5–2.5 s 10–180 s[5,14]
3LORE>20 ms 5–30 min[6,8]
4Step-like LORE>20 ms (abrupt step) 30 min to 1 h [6,8]
Table 2. Summary of onset and recovery time characteristics of VLF perturbing events for both the NWC-DDN and VTX-DDN TRGCPs.
Table 2. Summary of onset and recovery time characteristics of VLF perturbing events for both the NWC-DDN and VTX-DDN TRGCPs.
For NWC-DDN TRGCP
Event TypeOnset TimeRecovery Time
Early/slow0.72 to 2.88 s9 to 309 s
LORE0.72 to 4.32 s4 to 31 min
Step-like LORE1.08 to 5.76 s5 to 26 min
Unusual3.24 to 5.04 s25 to 191 s
For VTX-DDN TRGCP
Early/slow1.08 to 2.88 s5 to 219 s
Step-like LORE1.8 to 5.0545 s24 to 48 min
Unusual3.96 to 4.68 s59 to 181 s
Table 3. LWPC-obtained D-region parameters of 10 NWC VLF perturbing events.
Table 3. LWPC-obtained D-region parameters of 10 NWC VLF perturbing events.
NWC EventsDateTime (UTC)ΔA(dB)H′βHp′βpEvent TypesDst (nT)Kp
1.20 October 202020:45:54−1.76870.483.20.79Early slow−8−3
2.22 October 202023:01:35−0.69870.485.80.81Early slow−15−1
3.27 March 202113:42:23+0.94870.483.80.79Step like LORE2−2
4.11 April 202119:22:09−1.28870.482.90.79LORE51
5.12 April 202120:08:12+0.89870.485.20.76LORE18−2
6.14 April 202120:35:27−1.57870.4830.78Early slow−2−3
7.19 April 202121:25:36−1.7870.481.10.81LORE3−2
8.30 April 202121:34:36+1.84870.483.80.66Early slow14−1
9.3 May 202113:58:55−1.53870.483.10.76Early slow−110
10.9 June 202118:05:39+3870.483.70.65Step like LORE−51
Table 4. LWPC-obtained D-region parameters modeling of 10 VTX VLF perturbing events.
Table 4. LWPC-obtained D-region parameters modeling of 10 VTX VLF perturbing events.
VTX EventsDateTime (UTC)ΔA(dB)H′βHp′βpEvent TypesDst (nT)Kp
1.12 March 202121:13:37+2.9870.3785.20.75Step like lore−11−3
2.15 March 202116:31:19 −1.78870.3787.20.42Early slow−9−1
3.15 March 202122:47:36+0.72870.3787.40.58Early slow−10+1
4.19 March 202119:22:31+1.09870.3786.90.46Early slow3+1
5.7 April 202114:11:37+2.29870.37850.50Step like LORE2+2
6.30 April 202123:44:53−1.34870.3787.20.45Early slow10−3
7.9 May 202115:01:04+1.22870.3784.70.57Early slow11+2
8.14 July 202123:17:38−0.57870.3787.50.57Early slow−32+1
9.14 July 202123:36:20−0.64870.3787.10.57Early slow−323
10.24 July 202115:50:30+0.7870.3787.20.56Early slow−60
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Bhaskar, D.; Tripathi, R.; Shrivastava, M.N.; Singh, R.; Sasmal, S.; Datta, A.; Maurya, A.K. Lower Ionospheric Perturbations Associated with Lightning Activity over Low and Equatorial Regions. Atmosphere 2025, 16, 832. https://doi.org/10.3390/atmos16070832

AMA Style

Bhaskar D, Tripathi R, Shrivastava MN, Singh R, Sasmal S, Datta A, Maurya AK. Lower Ionospheric Perturbations Associated with Lightning Activity over Low and Equatorial Regions. Atmosphere. 2025; 16(7):832. https://doi.org/10.3390/atmos16070832

Chicago/Turabian Style

Bhaskar, Dayanand, Rajat Tripathi, Mahesh N. Shrivastava, Rajesh Singh, Sudipta Sasmal, Abhirup Datta, and Ajeet Kumar Maurya. 2025. "Lower Ionospheric Perturbations Associated with Lightning Activity over Low and Equatorial Regions" Atmosphere 16, no. 7: 832. https://doi.org/10.3390/atmos16070832

APA Style

Bhaskar, D., Tripathi, R., Shrivastava, M. N., Singh, R., Sasmal, S., Datta, A., & Maurya, A. K. (2025). Lower Ionospheric Perturbations Associated with Lightning Activity over Low and Equatorial Regions. Atmosphere, 16(7), 832. https://doi.org/10.3390/atmos16070832

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