Investigating Mid-Latitude Lower Ionospheric Responses to Energetic Electron Precipitation: A Case Study

Abstract

Localized ionization enhancements (LIEs) in altitude range corresponding to the D-region ionosphere, disrupting Very-Low-Frequency (VLF) signal propagation. This case study focuses on Lightning-induced Electron Precipitation (LEP), analyzing amplitude and phase variations in VLF signals recorded in Belgrade, Serbia, from worldwide transmitters. Due to the localized, transient nature of Energetic Electron Precipitation (EEP) events and the path-dependence of VLF responses, research relies on event-specific case studies to model reflection height and sharpness via numerical simulations. Findings show LIEs are typically under 1000 × 500 km, with varying internal structure. Accumulated case studies and corresponding data across diverse conditions contribute to a broader understanding of ionospheric dynamics and space weather effects. These findings enhance regional modeling, support aerosol–electricity climate research, and underscore the value of VLF-based ionospheric monitoring and collaboration in Europe.

Dataset: https://doi.org/10.5281/zenodo.16212238, accessed on 20 June 2025.

Dataset License: CC BY 4.0

1. Summary

The ionosphere is an extensive and dynamic region of the atmosphere, with its physical and chemical characteristics heavily affected by incoming radiation and localized energetic activity [1,2,3,4]. The lower ionosphere, in particular, plays a crucial role, as variations in its parameters can significantly influence various human-related systems, necessitating ongoing observation and analysis [5,6,7,8]. Given the challenges and expense of direct in situ measurements, this region is primarily studied through remote sensing methods [9,10,11,12,13,14].

High-energy events—whether of terrestrial or extraterrestrial origin, such as flares or lightning—can trigger ionospheric disturbances that alter atmospheric dynamics [15,16,17]. Atmospheric discharges emit radio waves which interact with energetic electrons (30–300 keV), scattering some into the loss cone. These electrons then precipitate into altitude range corresponding to the D-region, producing localized ionization enhancements (LIEs) (e.g., [18,19,20,21]).

Energetic Electron Precipitation (EEP) results from wave–particle interactions between trapped electrons and various plasma waves, including chorus, hiss, and lightning-induced Very-Low-Frequency (VLF) waves [22,23]. EEP intensifies during geomagnetic storms, substorms, solar wind variability, and Lightning-induced Electron Precipitation (LEP) events. The increased ionization affects radio wave propagation, alters atmospheric chemistry (e.g., NOx production), and can influence mesospheric and stratospheric climate processes [24,25].

LEP-driven disturbances in the altitude range corresponding to lower ionosphere cause detectable amplitude and phase perturbations in subionospherically transmitted VLF signals, which propagate within the Earth–ionosphere waveguide—bounded below by Earth’s surface and above by the lower ionosphere [26]. Such perturbations were recorded using the Belgrade VLF receiving system [27]. Signals at 24.0 kHz (NAA, USA) and 22.1 kHz (GVT, UK), etc., were closely analyzed. Numerical simulations based on these recordings were employed to derive propagation parameters and estimate associated electron density enhancements from these transient ionospheric events.

Research on lower ionospheric perturbations is primarily case-study-oriented because EEP events are irregular and localized, VLF responses are path-dependent, and modeling requires event-specific calibration. Each case offers detailed insight that contributes to the cumulative understanding of ionospheric behavior under disturbed conditions. Additionally, the transient and geographically confined nature of EEP effects limits the feasibility of broad statistical approaches [28,29,30]. Case studies also allow integration of high-resolution temporal data with localized modeling to resolve fine-scale ionospheric changes. Over time, multiple case studies across different conditions and locations help build a more generalized picture of ionospheric dynamics and space weather impacts especially related to LEPs.

Moreover, the results are valuable for modeling the region and advancing research on aerosol–electricity interactions in climate science. Monitoring the mid-latitude European ionosphere using VLF signals is crucial, especially since, aside from the Hungarian system [31], this is the only data source available. Given that computational outcomes can vary significantly by method, presenting and comparing new modeling results is essential. These findings also support potential future collaborations on VLF studies in the region.

Although many studies have reported evidence of D-region disturbances through changes in the amplitude and phase of transmitted VLF signals (e.g., Silber & Price [26]), accurately and clearly quantifying changes in electron density within the D-region remains challenging. A major difficulty lies in the lack of direct knowledge about the D-region’s initial, unperturbed state, which is typically estimated or assumed [32].

As a Data Descriptor submission, our primary aim is to provide valuable data that enables scientists to further model the ionosphere.

2. Data Description

The dataset includes new results on amplitude (ΔA) and phase (ΔPh) perturbations, waveguide parameters—sharpness (β) and reflection height (h′)—as well as electron density profiles Ne(h) derived from VLF signal observations during LEP events. Cases of selected LEP events presented in this research are as follows: 5 December 2004 at 02:26UT, 8 September 2005 at 03:36UT, 27 December 2004 at 04:53UT, 30 August 2005 at 03:24UT, 16 November 2004 at 03:37UT and 12 May 2009 at 00:37UT, on monitored VLF signals: NAA/24 kHz (USA), GVT/22.1 kHz (UK), HWU/18.3 kHz (France), DHO/23.4 kHz (Germany) and ICV/20.27 kHz (Italy) (see Figure 1). These events were selected based on the availability of high-quality, well-documented VLF signal data during periods when the receiver was fully operational on the one hand, and due to their clear signatures, favorable propagation conditions, and reliable supporting data, which make them particularly suitable for ionospheric modeling, on the other. The events included in this study were selected through a multi-step process combining VLF signal analysis and lightning data correlation. Continuous recordings of VLF transmitter signals were examined for characteristic perturbations, specifically, sudden and short-duration changes in amplitude and/or phase, indicative of ionospheric disturbances associated with LEP [26]. Candidate events were required to display a clear, transient deviation from the unperturbed baseline, followed by a return to nominal signal levels, consistent with known LEP signatures. To confirm the geophysical origin of these disturbances, each event was cross-referenced with lightning occurrence data from global or regional lightning detection networks, ensuring both temporal and spatial coincidence between the VLF signal anomaly and a causative lightning discharge. Only events with strong signal-to-noise ratios and reliable propagation paths between the VLF transmitter and receiver were retained for further analysis and electron density modeling. Typical cases of LEP related perturbations recorded by Belgrade receiver are presented here. Results are detailed both in this section and in the online material (including data listings related to analyzed signals’ amplitude and phase given in supplementary material S1.dat, electron densities in S2.dat and figures presenting perturbed sections of GCPs, electron density profiles, maximal electron densities during perturbations related to amplitudes’ and phases’ relative changes and amplitude and phase of recorded VLF signals in S3.pdf).

A representative LEP event and VLF data recorded in Belgrade (44.85° N, 20.38° E; Serbia) on 30 August 2005 (Figure 2) shows a characteristic pattern both in amplitude drop of up to few dB and a phase increase of almost ten degrees, lasting approximately few minutes—a typical response to such disturbances.

Table 1. Parameters β (km⁻¹) and h′ (km) for unperturbed and perturbed state for GVT signal during analyzed LEP event on 30 August 2005.

Height	Unperturbed			Perturbed			El. Density Change
h (km)	β (km−1)	h′ (km)	Ne (m−3)	h′p (km)	βp (km−1)	Nep (m−3)	ΔNe (m−3)
83	0.42	87	1.06706 × 107	87	0.42	1.06706 × 107	0
84	0.42	87	1.39781 × 107	86	0.4	2.21423 × 107	8.16425 × 106
85	0.42	87	1.83108 × 107	85	0.34	4.24146 × 107	2.41038 × 107
86	0.42	87	2.39865 × 107	86	0.40	3.65066 × 107	1.252 × 107
87	0.42	87	3.14215 × 107	87	0.42	3.14215 × 107	0

illustrates the evolution of the waveguide parameter dataset during this event.

Electron density Ne(z) data for the GVT signal, calculated over the 60–90 km altitude range, for the LEP event on 30 August 2005 at 03:24 UT are shown in Figure 3 as a representative example, zoomed for an altitude range of 82–88 km, where electron density change occurred. The black line represents the unperturbed ionospheric state, while the red line corresponds to the perturbed state. As shown in Figure 3, the event resulted in measurable increases in electron density. Although the enhancements remained within the same order of magnitude, the Ne dataset confirms the occurrence of localized ionization enhancements (LIEs) associated with the sudden ionospheric changes induced by LEP.

We note that the shape of the altitude dependence of the electron density is similar to that in [30], but the values deviate slightly, with maximal increase in electron density in our modeling estimated as ΔNe = 2.41038 × 10⁷ m⁻³, at an altitude of 85 km and sharpness of 0.34 km⁻¹. The length of the perturbed area along GCP_GVT is estimated to be 773 km.

All datasets are compiled in the online material and are available at: https://doi.org/10.5281/zenodo.16212238. This includes complete results for all analyzed events and parameters.

3. Methods

3.1. Observations

The ionosphere’s composition, altitude structure, and dynamic behaviors are commonly studied via in situ sensors (satellites, rockets, balloons, radars) and remote sensing methods using radio signals. For probing the lowest layer (D-region, 50–90 km), Very-Low-Frequency (VLF; 3–30 kHz) signals are particularly effective, employing hop-wave propagation theory to infer ionospheric changes [26,27]. These techniques have been successful in monitoring responses to diverse drivers—from space weather to terrestrial phenomena (see, e.g., [33]).

In this study, VLF observations were made by the VLF/LF receivers located in Belgrade (44.85° N, 20.38° E), Serbia [27]. Operating in narrow-band/broad-bend mode, stations continuously recorded transmissions from various transmitters (e.g., NAA (24.0 kHz, USA), GVT (22.1 kHz, UK), etc.). A representative LEP event recorded on 30 August 2005 (Figure 2) produced a characteristic amplitude dip (several dB) and phase rise (several degrees) lasting few minutes—typical of such disturbances.

3.2. Modeling

To quantify ionospheric effects, amplitude (ΔA) and phase (ΔPh) perturbations were calculated relative to undisturbed nocturnal baselines. These values served as input for the LWPC (Long Wavelength Propagation Capability) model [34,35], which uses an iterative fitting approach to adjust waveguide parameters—sharpness (β, km⁻¹) and reflection height (h′, km)—via its REXP subroutine until simulated signals matched observations at the Belgrade receiver (see, e.g., [27,36,37]). The errors caused by the employed technique have been carefully analyzed, establishing that the uncertainty in the results ranges from 10% to 20% (see, e.g., papers [33,38,39]), while fits are given in supplementary material S4.xls. More details on the methodology used can be found in, e.g., [40].

Table 1 illustrates the resulting parameter changes during the selected LEP event. In the WS Exponential background model, electron density profiles Ne(h) for the night-time ionosphere between 50 and 90 km were calculated as noted in [19]:

$$\upsilon \left(h\right)=1.86\times {10}^{11}\times e^{-0.15h} \left({\mathrm{s}}^{-1}\right)$$

(1)

$$Ne\left(h\right)=\upsilon \left(h\right)\times 78.57\times e^{\beta \left(h-h^{\prime }\right)} \left({\mathrm{m}}^{-3}\right)$$

(2)

In LEP/EEP scenarios, the total electron density profile is often modeled as

$${Ne}^{total}\left(h\right)={Ne}^{background}\left(h\right)+\delta Ne\left(h\right) \left({\mathrm{m}}^{-3}\right)$$

(3)

where

$${Ne}^{background}\left(h\right)=1.86\times {10}^{11}\times e^{-0.15h}\times 78.57\times e^{\beta \left(h-h^{\prime }\right)} \left({\mathrm{m}}^{-3}\right)$$

(4)

$$\delta Ne\left(h\right)={\delta {Ne}_0}^{\left[-{\left(h-h_0\right)}^{{\displaystyle \frac{2}{{\sigma }^2}}}\right]} \left({\mathrm{m}}^{-3}\right)$$

(5)

The Exponential background model gives the baseline, exponential electron density for undisturbed D-region. The Gaussian perturbation models the localized enhancement due to EEP or LEP. Here δNe₀ is the peak amplitude of the electron density enhancement, h₀ is the altitude at which the electron density enhancement peaks, and σ the width parameter (standard deviation) of the Gaussian profile. This combined model is essential for simulating VLF perturbations in program packages like LWPC.

As shown in Figure 3, LEP led to measurable increases in electron density—confirming localized ionization enhancement (LIE) induced by sudden ionospheric disturbance.

4. User Notes

This dataset provides a valuable, well-annotated resource for studying EEP and its impact on the lower ionosphere, particularly LEP events. Given the transient, path-dependent nature of EEP events in terms of related recorded perturbations and observed VLF signals, the data support event-specific modeling of reflection height and sharpness via numerical simulations. This open-access dataset aids interdisciplinary research in ionospheric physics and space weather, offering practical tools to advance the understanding of atmospheric dynamics, with implications for navigation, telecommunications, and geospace monitoring.

Moreover, the supplied data can be applied in practice in many areas of science in a variety of ways and used by experts such as the following:

• Space Weather Researchers;
• Communications Engineers;
• GNSS and Navigation System Developers;
• Satellite Mission Teams;
• National Defense and Security Agencies;
• Ionospheric Modelers;
• Educators and Students in Earth and Space Physics.

5. Conclusions and Future Work

This study highlights the significant impact of EEP, particularly LEP, on the lower ionosphere, as evidenced by LIEs and their disruption of VLF radio signal propagation. By analyzing amplitude and phase variations in VLF signals recorded in Belgrade from multiple global transmitters, the research confirms that LEP-induced ionospheric disturbances are highly localized (typically <1000 × 500 km) and strongly path-dependent.

Using case-specific modeling, the study successfully derived key ionospheric parameters β, h′, and altitude-resolved electron density profiles Ne(h), for several well-documented LEP events. The resulting open-access dataset, including detailed amplitude and phase changes, provides a strong base for advancing our understanding of D-region ionospheric responses to EEP events.

Beyond scientific insight, the findings have practical applications for VLF-based ionospheric monitoring, regional ionospheric modeling, and climate–aerosol interaction research. The results are especially valuable in the context of Europe’s growing need for collaborative space weather monitoring and communication system resilience.

Future Work:

Building on the current findings, future research should aim at the following:

• Expand the dataset to include additional LEP and EEP events across broader geographic and geomagnetic conditions, enabling statistical analysis and trend identification.
• Integrate real-time or near-real-time VLF monitoring systems to support early detection and characterization of ionospheric disturbances.
• Improve numerical modeling techniques by incorporating machine learning approaches to more accurately estimate D-region parameters from VLF data.
• Investigate the coupling between ionospheric perturbations and atmospheric electrical processes, particularly in relation to climate variability.
• Foster collaboration across scientific disciplines and institutions to enhance the utility of VLF data in satellite mission planning, GNSS error correction, and space weather forecasting.
• Promote educational outreach by developing tools and curricula using this dataset to train the next generation of space physicists and engineers.

As a Data Descriptor contribution, our main objective is to encourage other researchers to build upon the submitted dataset and enhance their own further ionospheric modeling. This work provides both a scientific benchmark and a practical resource for multidisciplinary exploration of ionospheric dynamics and their broader implications.