# Investigating Lightning Generated ELF Whistlers to improve ionospheric models (ILGEW) Deliverable 08: Updated ILGEW Dataset User Manual

Pierdavide Coïsson, G. Hulot, L. Chauvet, Laura Brocco, Rémi Madelon, Pierre Vigneron, V. Truhlik, D. Buresova, Jaroslav Chum, Janusz Mlynarczyk, et al.

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Pierdavide Coïsson, G. Hulot, L. Chauvet, Laura Brocco, Rémi Madelon, et al. Investigating Lightning Generated ELF Whistlers to improve ionospheric models (ILGEW) Deliverable 08: Updated ILGEW Dataset User Manual. Institut de physique du Globe de Paris (IPGP), 1 rue Jussieu, 75005 Paris. 2021. hal-05183397

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# Investigating Lightning Generated
ELF Whistlers to improve ionospheric
models (ILGEW)

Deliverable 08:
Updated ILGEW Dataset User Manual

Université de Paris, Institut de physique du globe de Paris, CNRS (IPGP)

Institute of Atmospheric Physics, Czech Academy of Sciences (IAP)

AGH University, Krakow (AGH)

Prepared:

Pierdavide Coïsson (IPGP)

Date: 2021-08-11
Record of Changes

<table><thead><tr><td>Reason</td><td>Description</td><td>Rev</td><td>Date</td></tr></thead><tbody><tr><td>Initial vers.</td><td>First Version under Doc. no: SW-ILGEW-IPGP-D03</td><td>1</td><td>2020-02-12</td></tr><tr><td>Update with new data and file formats</td><td>Inclusion of new dataset file and update on whistler files version</td><td>2</td><td>2021-08-11</td></tr></tbody></table>

Document contributors

<table><thead><tr><td>Name</td><td>Affiliation</td></tr></thead><tbody><tr><td>Pierdavide Coïsson</td><td>Université de Paris, Institut de physique du globe de Paris, CNRS, France</td></tr><tr><td>Gauthier Hulot</td><td>Université de Paris, Institut de physique du globe de Paris, CNRS, France</td></tr><tr><td>Louis Chauvet</td><td>Université de Paris, Institut de physique du globe de Paris, CNRS, France</td></tr><tr><td>Laura Brocco</td><td>Université de Paris, Institut de physique du globe de Paris, CNRS, France</td></tr><tr><td>Rémi Madelon</td><td>Université de Paris, Institut de physique du globe de Paris, CNRS, France</td></tr><tr><td>Pierre Vigneron</td><td>Université de Paris, Institut de physique du globe de Paris, CNRS, France</td></tr><tr><td>Vladimir Truhlik</td><td>Institute of Atmospheric Physics of the Czech Academy of Sciences, Prague, Czechia</td></tr><tr><td>Dalia Buresova</td><td>Institute of Atmospheric Physics of the Czech Academy of Sciences, Prague, Czechia</td></tr><tr><td>Jaroslav Chum</td><td>Institute of Atmospheric Physics of the Czech Academy of Sciences, Prague, Czechia</td></tr><tr><td>Janusz Mlynarczyk</td><td>AGH University of Science and Technology, Krakow, Poland</td></tr><tr><td>Pawel Rzonca</td><td>AGH University of Science and Technology, Krakow, Poland</td></tr><tr><td>Andrej Kulak</td><td>AGH University of Science and Technology, Krakow, Poland</td></tr></tbody></table>
## Table of Contents

<table><tr><td>1</td><td>Introduction</td><td>6</td></tr><tr><td>2</td><td>Dataset of Swarm burst whistler events in the ELF</td><td>7</td></tr><tr><td>2.1</td><td>Organisation of the dataset</td><td>7</td></tr><tr><td>2.1.1</td><td>Whistlers' daily files</td><td>7</td></tr><tr><td>2.1.2</td><td>Events dataset file</td><td>7</td></tr><tr><td>2.2</td><td>Versions of files</td><td>8</td></tr><tr><td>2.3</td><td>File formats</td><td>8</td></tr><tr><td>2.3.1</td><td>Whistlers' daily files</td><td>8</td></tr><tr><td>2.3.2</td><td>Events dataset file format</td><td>15</td></tr><tr><td>2.4</td><td>Classification of whistler events</td><td>23</td></tr><tr><td>3</td><td>Examples of whistler data visualization</td><td>28</td></tr><tr><td>4</td><td>References</td><td>31</td></tr><tr><td>5</td><td>List of acronyms</td><td>32</td></tr></table>

## Table of Figures

<table><tr><td>Figure 2-1</td><td>Example of standard whistler event.</td><td>24</td></tr><tr><td>Figure 2-2</td><td>Example of high frequencies event.</td><td>24</td></tr><tr><td>Figure 2-3</td><td>Example of low frequencies event.</td><td>25</td></tr><tr><td>Figure 2-4</td><td>Example of medium frequencies event.</td><td>25</td></tr><tr><td>Figure 2-5</td><td>Example of complex event.</td><td>26</td></tr><tr><td>Figure 2-6</td><td>Example of 1- candidate event.</td><td>26</td></tr><tr><td>Figure 2-7</td><td>Example of destroyed event.</td><td>27</td></tr><tr><td>Figure 3-1</td><td>Map of the 137 ILGEW events included in the dataset.</td><td>28</td></tr><tr><td>Figure 3-2</td><td>Example of whistler events detected on 18 August 2019 by Swarm A satellite.</td><td>29</td></tr><tr><td>Figure 3-3</td><td>Example of whistler events detected on 18 August 2019 by Swarm A satellite on the day-side part of the orbit.</td><td>29</td></tr><tr><td>Figure 3-4</td><td>Example of whistler signals contained in the whistler structure.</td><td>30</td></tr></table>

## List of Tables

<table><tr><td>Table 2-1</td><td>Matlab structures contained in a whistler file.</td><td>9</td></tr><tr><td>Table 2-2</td><td>Content of the structure satellite-trajectory.</td><td>9</td></tr><tr><td>Table 2-3</td><td>Content of the whistler structure included in the whistler files.</td><td>11</td></tr><tr><td>Table 2-4</td><td>Content of the whistler characterisation structure included in the whistler files for versions from v1 to v202 and v401 to v402.</td><td>12</td></tr><tr><td>Table 2-5</td><td>Content of the whistler characterisation structure included in the whistler files for versions v301 and v302.</td><td>13</td></tr><tr><td>Table 2-6</td><td>Content of the dataset_description structure of whistler files starting from v301.</td><td>14</td></tr></table>
<table><tr><td>Table 2-7 Matlab structures contained in ILGEW_EventsFile_v0101 file.</td><td>15</td></tr><tr><td>Table 2-8 Content of eventsWhistlers groundELF ionosonde structure in ILGEW_EventsFile_v0101 file.</td><td>16</td></tr><tr><td>Table 2-9 Content of IRTAM substructure of the eventsWhistlers groundELF ionosonde structure in ILGEW_EventsFile_v0101 file.</td><td>16</td></tr><tr><td>Table 2-10 Content of the IRI for digisonde substructure of the eventsWhistlers groundELF ionosonde structure in ILGEW_EventsFile_v0101 file.</td><td>18</td></tr><tr><td>Table 2-11 Content of the digisonde substructure of the eventsWhistlers groundELF ionosonde structure in ILGEW_EventsFile_v0101 file.</td><td>20</td></tr><tr><td>Table 2-12 Content of the lightning substructure of eventsWhistlers groundELF ionosonde structure in ILGEW_EventsFile_v0101 file.</td><td>21</td></tr><tr><td>Table 2-13 Content of dataset_description structure in ILGEW_EventsFile_v0101 file.</td><td>22</td></tr><tr><td>Table 2-14 Content of the Identification substructure of dataset_description structure in ILGEW_EventsFile_v0101 file.</td><td>22</td></tr><tr><td>Table 2-15 Content of the Publication substructure of dataset_description structure in ILGEW_EventsFile_v0101 file.</td><td>22</td></tr><tr><td>Table 2-16 Content of the Processing substructure of dataset_description structure in ILGEW_EventsFile_v0101 file.</td><td>23</td></tr><tr><td>Table 2-17 Classification of lightning-generated whistler events.</td><td>23</td></tr></table>

## Applicable and Reference Documentation

### Applicable Documents

The following documents are applicable to the definitions within this document.

[AD-1] ESA Contract No. 4000126708/10/NL/IA, 2019-04-29

### Reference Documents

The following documents contain supporting and background information to be taken into account during the activities specified within this document.

[RD-1] SW-ILGEW-IPGP-FR, Investigating Lightning Generated ELF Whistlers to improve ionospheric models (ILGEW) Final Report

[RD-2] SW-ASMV-DD-IPGP-0008, Swarm Experimental ASMV Project ASM Burst Product Definition for Operational Release of Data, rev. 2, 2021-02-15

[RD-3] SW-ASMV-DD-IPGP-0009, List of available 0104 ASM Burst Product files, rev. 1A, 2020-01-27

[RD-4] SW-ASMV-DD-IPGP-0010, List of available 0105 ASM Burst Product files, rev. 1, 2020-03-26
[RD-5] SW-ASMV-DD-IPGP-0013, List of available 0201 ASM Burst Product files,
rev. 1, 2021-02-15

[RD-6] SW-ASMV-DD-IPGP-0012, ASM Whistler Product Definition for
Operational Release of Data, rev 1C, 2021-06-08
# 1 Introduction

This document explains how to use the dataset of signals of lightning-generated whistler events and of their related geophysical parameters that have been identified during the development of ILGEW project [RD-1] [AD-1], analysing data from the burst sessions of the Swarm ASM instruments.

ASM burst sessions were first realised during the commissioning phase of the Swarm mission on the three satellites A (Alpha), B (Bravo), C (Charlie) between January and February 2014. After the failure of the AMS onboard Charlie, the burst sessions realised from 2018 onwards have been operated alternatively on A or B satellite. The current length of burst sessions is one week, for a total of two sessions every month, depending on the observational opportunities achievable with the orbital configuration of the satellites.

The content of the dataset files at the time of creation of this document is described in chapter 2. Two different kinds of files have been included in this dataset:

1. **Whistlers' daily files** of whistler data, entirely based on data from the Swarm ASM instruments, one file for each satellite operated in burst mode. These files can hold a different version number according to the data processing performed;

2. **One events dataset file**, which contains a selection of remarkable whistler events in the period between January 2014 and February 2020 for which additional parameters have been included, from ground observations of lightning, ionospheric profiles from ionosondes and models and in-situ electron density from Swarm EFI Langmuir Probe (LP).

The file formats are documented in section 2.3, including all metadata contained in each file.

Examples of data visualisations are also provided in chapter 3.
# 2 Dataset of Swarm burst whistler events in the ELF

The current version of the dataset contains all whistler-like signals identified by analysing the available Swarm ASM burst sessions data, contained in ASM burst Level 1b files (see [RD-1], [RD-2], [RD-3], [RD-4], [RD-5]). Beside many clear strong whistlers that have been studied to identify the originating lightning strike, there are also weak or incomplete events, for which it is more delicate to understand if they are lightning-generated whistler events or other kind of signals. All have been kept in the dataset to allow further studies, but it is important to point out that the selection or rejection of weak events can be arbitrary. Some events are also contaminated by outliers and they have been classified in a group of complex events. Validation with ground observations requires manual operations and could not be realised on all identified events. The complete analyses have been focused on the clearer events.
Each event is recorded in the dataset over a 3 seconds time window, including the original ASM time series and the derived parameters, that are described in this document.

## 2.1 Organisation of the dataset

The dataset of ILGEW project is available on the project server:
https://ilgew.ipgp.fr/
It is reachable from the tab “Dataset”.
Access can be granted upon written request to coisson@ipgp.fr

### 2.1.1 Whistlers' daily files

All daily whistlers' files are contained in the directory:
/dataset/Whistlers/

Each file follows the naming convention: WhistlersSyyyymmdd_vnnn.mat
(e.g. WhistlersA20190516_v1.mat):

* S is the satellite of Swarm constellation: A, B or C;

* yyyy is the year number;

* mm is the month number;

* dd is the day number;

* nnn is the version number of the file, initial ILGEW files have only one digit (1) in the file name.

### 2.1.2 Events dataset file

The events dataset file is contained in the directory:
/dataset/Events/

The events dataset is contained in a single file, following the naming convention:
ILGEW_EventsFile_vnnnn.mat

* nnnn is the version number of the file.

This events dataset file is named ILGEW_EventsFile_v0101.mat
## 2.2 Versions of files

The last part of the daily whistlers' files name indicates a version number. During the development of the project, the data processing algorithms have evolved and some specific analyses were performed only over a limited number of files, thus a completely coherent set of files was not generated over the course of the entire project. Files included in the dataset can therefore have different version numbers. All versions share a common structure and include the basic parameters that allow their analysis with the same processing tools. Some versions include additional parameters, which are detailed in the corresponding tables of this document. These version changes affect the first digit of the string *nnn*.

The last digit of the file version *nnn* was reserved to minor data changes, related to false events rejection, re-classification of whistler events, or bug correction in events time-series selections. Only files that needed corrections have been updated to a new subversion. The distributed dataset includes only the latest version produced for each file.

The events dataset file has been produced for distribution in a unique version at the time of delivery of the ILGEW project.

## 2.3 File formats

### 2.3.1 Whistlers' daily files

The whistlers' daily files are stored in matlab MAT-file format [*MathWorks*, 2021], containing suitable parameters for a self-contained investigation of the whistler events. Each file contains events detected during one UTC day (from 00:00:00 to 23:59:59 UTC), covering the available burst data. On rare occasions ASM burst data covered only a part of one UTC day.

All data are organised in matlab structures, listed in Table 2-1 and including metadata describing the content of each variable. Variables contained in structures are referred in this document using the notation *StructureName.Variable*, where *StructureName* is the name of the structure and *Variable* is the name of the variable. Similarly, some variables are contained in substructures and they are referred using the notation *StructureName.SubstructureName.Variable*, where *SubstructureName* is the name of the substructure.
<table><caption>Table 2-1 Matlab structures contained in a whistler file. Note that the structure `dataset_description` was introduced starting from version v301.</caption><thead><tr><td>Structure name</td><td>Structure description</td></tr></thead><tbody><tr><td>satelliteTrajectory</td><td>Geographical positions of the Swarm satellite along its orbit during the complete UTC day.</td></tr><tr><td>satelliteTrajectory_description</td><td>Description of the contents of satelliteTrajectory structure.</td></tr><tr><td>whistler</td><td>ASM data in a 3 s time window containing a selected event and additional whistler's parameters.</td></tr><tr><td>whistler_description</td><td>Description of the variables contained in the structure whistler.</td></tr><tr><td>dataset_description</td><td>Structure of metadata that describes the whistlers' daily files dataset, and holds information about publication of data, processing of data and identification of data.</td></tr></tbody></table>

### Content of satellite-trajectory structure of a whistler file

The `satellite-trajectory` structure contains the positions (Latitude and Longitude, see Table 2-2) of the Swarm satellite sampled every second over the whole UTC day, with 3 additional seconds included in order to cover all possible 3 s time windows centred on a whistler event over the day. It can be used for visualisation purposes to provide information of the regions crossed by Swarm satellites on that specific day. The complete time series for every burst data record is available only on ASM burst Level 1b files.

<table><caption>Table 2-2 Content of the structure satellite-trajectory.</caption><thead><tr><td>Structure field</td><td>Type</td><td>Units</td><td>Description</td></tr></thead><tbody><tr><td>Latitude</td><td>Array</td><td>deg</td><td>Position in ITRF - Geocentric latitude</td></tr><tr><td>Longitude</td><td>Array</td><td>deg</td><td>Position in ITRF - Geocentric longitude</td></tr></tbody></table>

### Content of a whistler structure of a whistlers file

The `whistler` structure contains one entry per detected event; it includes data and data products obtained from the ASM burst Level 1b files. The complete list of variables is summarized in Table 2-3, Table 2-4 and Table 2-5 for the various versions of files produced during ILGEW project.

For each event, 3 s of different time series are recorded in the `whistler` structure, corresponding to various levels of data processing.

The centre of the time series corresponds to the time of the beginning of the whistler signal. This time is stored in the array `whistlerdateUT`, containing the calendar date and the UTC time. It is the origin time of the arrays of time `t_event`, `t_PSD` and `t`.
The position of the Swarm satellite at the time `whistlerdateUT` is provided in the variables `Latitude`, `Longitude`, `Radius` and the Local Time of the event in the variable `LT`.

It is calculated using:

$$LT = \mod \left( UT + \frac{\text{Longitude}}{15}, 24 \right) \qquad (2-1)$$

`UT` is the Universal Time (hours) obtained from `whistlerdateUT` and the function `mod(x, n)` is the modulus of `x/n`.

Firstly, the original ASM burst time series is provided, including the time stamp in s, `TimestampburstUT`, on the CDF Epoch time scale (counted from the date 1 January 0). Note that this time stamp is obtained from the variables `Timestamp` and `TimeFrac` contained in the corresponding ASM burst Level 1b file, following the guidelines provided in [RD-2]. The corresponding value of the total magnetic field intensity is contained in the variable `F` in nT and is accompanied by the flags: `flag_magnetic`, `flag_heater`, `flag_oulier`, `flag_step`. See [RD-2] for details on these flags, that are an indication of various instrumental artefacts that can be contained in the data. Their value can be 0 in general or 1, when the flag is raised. Note that for the strongest whistler events, incorrect `flag_oulier` can be raised during the whistler event, since it is based on a fixed temporal gradient threshold, that can be crossed during whistler oscillations.

To isolate the whistler events from the main geomagnetic field, a residual `F_filtered`, is obtained from the `F` time series after removal of a 4th order polynomial fit realized over one minute of data. This allowed a high-pass filtering of the data and obtaining the waveform signal of whistlers. The corresponding time stamp `t_event` is provided relative to `whistlerdateUT`.

An algorithm to remove outliers has also been applied, producing the variable `F_filtered_despiked`. This variable might not be reliable for strong events in which the `flag_oulier` is raised during the whistler arrival.

The representation of the whistler in the frequency domain was realised through computation of PSD of the time series `F_filtered`: each FFT was computed over a time window containing 33 points and an overlap of 25 points has been used between successive windows, each centred at the corresponding time contained in `t_PSD`, relative to `whistlerdateUT`. A total of 14 frequencies between 16 and 125 Hz have been saved in the variable `f_PSD` (the lowest frequencies of the FFT have been discarded). These PSD values are contained in the variable `PSD`.
<table><caption>Table 2-3 Content of the *whistler* structure included in the *whistler* files. The description is also contained in the corresponding structure *whistler_description*.</caption><thead><tr><th>Structure field</th><th>Type</th><th>Units</th><th>Description</th></tr></thead><tbody><tr><td>characterization</td><td>Structure</td><td>-</td><td>Whistler parameters. They are described in Table 2-4 and Table 2-5</td></tr><tr><td>whistlerdateUT</td><td>Array</td><td>[year, month, day, hours, minutes, seconds]</td><td>Whistler date and UT time</td></tr><tr><td>sat</td><td>String</td><td>-</td><td>Satellite identifier: A, B or C</td></tr><tr><td>Latitude</td><td>Float</td><td>deg</td><td>Position in ITRF - Geocentric latitude</td></tr><tr><td>Longitude</td><td>Float</td><td>deg</td><td>Position in ITRF - Geocentric longitude</td></tr><tr><td>Radius</td><td>Float</td><td>km</td><td>Position in ITRF - Geocentric radius</td></tr><tr><td>TimestampburstUT</td><td>Array</td><td>s</td><td>High precision timestamp in CDF epoch form</td></tr><tr><td>t_event</td><td>Array</td><td>s</td><td>Timestamp from whistlerdateUT</td></tr><tr><td>t_PSD</td><td>Array</td><td>s</td><td>PSD timestamp from whistlerdateUT</td></tr><tr><td>flag_magnetic</td><td>Array</td><td>-</td><td>Flag for magnetic condition: heater perturbations may occur when this condition is met</td></tr><tr><td>flag_heater</td><td>Array</td><td>-</td><td>Flag for heater activation: heater perturbations may occur when heater is ON</td></tr><tr><td>flag_outlier</td><td>Array</td><td>-</td><td>Flag for outliers: identify all outliers</td></tr><tr><td>flag_step</td><td>Array</td><td>-</td><td>Flag for dynamic steps: correction of field jumps due to the encoding of the data</td></tr><tr><td>F</td><td>Array</td><td>nT</td><td>Magnetic field intensity</td></tr><tr><td>F_filtered</td><td>Array</td><td>nT</td><td>Filtered magnetic field intensity</td></tr><tr><td>F_filtered_despiked</td><td>Array</td><td>nT</td><td>Filtered and despiked magnetic field intensity</td></tr><tr><td>f_PSD</td><td>Array</td><td>Hz</td><td>PSD frequencies</td></tr><tr><td>PSD</td><td>Array</td><td>nT/√Hz</td><td>PSD values</td></tr><tr><td>LT</td><td>Array</td><td>h</td><td>Local time</td></tr></tbody></table>

The computed characteristics of the whistler events are saved in a substructure *characterization*, see Table 2-4 or Table 2-5 according to the file version.

The reported parameters are obtained from the dispersion relation [Eckersley, 1935]:

$$D = \sqrt{f(t)}(t - t_0) \qquad (2-2)$$
D is the whistler's dispersion, *t* the arrival time of a frequency component *f* and *t*₀ the reference time when the whistler signal entered the ionosphere, recorded in the variable *t*₀ UT in this substructure. These parameters were obtained by correlation with the PSD values. See [RD-1] for details.

PSDvalues consists of the PSD points at the intersections with the curve of dispersion, rewritten as:

$$f(t) = \frac{D^2}{(t - t_0)^2} \qquad (2-3)$$

estimated as best correlation with the signal appearing on the PSD over the whole spectral range available. The uncertainty on the time *t*₀ UT, *t*₀ Uncertainty, was obtained from all possible *t*₀ computed from each pair of the selected points (*t*, *f*) and a fixed value of *D*. *t*₀ Uncertainty has been defined as the time range between the first and the last tile of the PSD plot crossed by the dispersion curve.

A rough estimate of the intensity of the event was computed using:

$$I = \sum_{i=1}^{n} \text{PSDvalues}_{i}^{2} \qquad (2-4)$$

where the sum was computed over all the *n* points contained in PSDvalues.

During ILGEW project, whistler events have been classified according to their spectral content: the classification field is described in section 2.4.

<table><caption>Table 2-4 Content of the whistler characterisation structure included in the whistler files for versions from v1 to v202 and v401 to v402. The description of these parameters is also contained in the corresponding structure whistler_description characterisation.</caption><thead><tr><th>Structure field</th><th>Type</th><th>Units</th><th>Description</th></tr></thead><tbody><tr><td>D</td><td>Float</td><td>√s</td><td>Whistler dispersion</td></tr><tr><td>t0_UT</td><td>Array</td><td>[year, month, day, hours, minutes, seconds]</td><td>Estimated UT time when the lightning signal entered the ionosphere from the measured dispersion D</td></tr><tr><td>t0_Uncertainty</td><td>Float</td><td>s</td><td>Computation uncertainty of t0_UT</td></tr><tr><td>intensity</td><td>Float</td><td>pT<sup>2</sup>/Hz</td><td>Whistler intensity</td></tr><tr><td>t</td><td>Array</td><td>s</td><td>Theoretical whistler timestamp</td></tr><tr><td>f</td><td>Array</td><td>Hz</td><td>Theoretical whistler frequencies</td></tr><tr><td>PSDvalues</td><td>Array</td><td>nT/√Hz</td><td>PSD values at intersection with theoretical whistler</td></tr><tr><td>classification</td><td>Integer</td><td>-</td><td>Whistler classification</td></tr></tbody></table>
A second approach for estimating the whistler's parameter was developed during ILGEW project and applied to data acquired during 2020 and on the selected events contained in the events dataset file. A synthetic whistler time series was fitted to the waveform as described in [RD-1]. In this process, the parameters of dispersion D_TS, reference time t0_UT_TS and their corresponding uncertainties D_Uncertainty_TS and t0_Uncertainty_TS were obtained. A different definition for the whistler intensity was also applied:

$$I_{TS} = \frac{1}{t_{\max} - t_{\min}} \int_{t_{\min}}^{t_{\max}} \frac{F_{R}^{2}(t)}{2\mu_{0}} dt \quad (2-5)$$

in which $F_R$ is the whistler's waveform from F_filtered, $\mu_0$ is the vacuum permeability, $t_{min}$ and $t_{max}$ are the initial and final time of the whistler signal. $I_{TS}$ is measured in pT A/m or equivalently in pJ/m³. The integration was computed numerically using the trapezoidal method.

In this processing, the frequency reaching at time $t_{max}$ is stored in the variable $f_{min}$, the minimal frequency observed for that whistler.

This second approach did not allow to characterise all whistlers. In those cases, the parameters D_TS, D_Uncertainty_TS, t0_UT_TS, t0_Uncertainty_TS, intensity_TS and fmin have been set to NaN.

Three metrics have been used to evaluate the quality of the fit: the coefficient of determination ($R^2$), the degree-of-freedom adjusted coefficient of determination and the root mean square error (RMSE). They have been used to determine the quality flag for this second approach: flag_TS. It ranges from 0 to 2 and attests if all these parameters are within the threshold values defined in [RD-1]. When they are all within their thresholds, the quality flag is set to 0. In the case only one of them is outside the threshold or the RMS is within its threshold, the quality flag is set to 1. In all other cases, the flag is set to 2.

<table><caption>Table 2-5 Content of the whistler characterisation structure included in the whistler files for versions v301 and v302. The description of these parameters is also contained in the corresponding structure whistler_description characterisation.</caption><thead><tr><th>Structure field</th><th>Type</th><th>Units</th><th>Description</th></tr></thead><tbody><tr><td>D</td><td>Float</td><td>√s</td><td>Whistler dispersion</td></tr><tr><td>D_TS</td><td>Float</td><td>√s</td><td>Whistler dispersion. This dispersion was obtained by fitting the time signal with a theoretical whistler function</td></tr><tr><td>D_Uncertainty_TS</td><td>Float</td><td>√s</td><td>Computation uncertainty of D_TS</td></tr><tr><td>t0_UT</td><td>Array</td><td>[year, month, day, hours, minutes, seconds]</td><td>Estimated UT time when the lightning signal entered the ionosphere from the measured dispersion D</td></tr><tr><td>t0_Uncertainty</td><td>Float</td><td>s</td><td>Computation uncertainty of t0_UT</td></tr><tr><td>t0_UT_TS</td><td>Array</td><td>[year, month, day, hours]</td><td>Estimated UT time when the lightning signal entered the ionosphere. This value has been</td></tr></tbody></table>
<table><tr><td></td><td></td><td>minutes, seconds]</td><td>obtained by fitting the time signal with a theoretical whistler function</td></tr><tr><td>t0_Uncertainty_TS</td><td>Float</td><td>s</td><td>Computation uncertainty of t0_UT_TS</td></tr><tr><td>intensity</td><td>Float</td><td>pT<sup>2</sup>/Hz</td><td>Whistler intensity</td></tr><tr><td>intensity_TS</td><td>Float</td><td>pT A/m = pJ/m<sup>3</sup></td><td>Estimated density of the magnetic energy carried by the whistler. This energy is computed making the integral of the measured magnetic field (filtered and detrended) over the time duration of the signal</td></tr><tr><td>t</td><td>Array</td><td>s</td><td>Theoretical whistler timestamp</td></tr><tr><td>f</td><td>Array</td><td>Hz</td><td>Theoretical whistler frequencies</td></tr><tr><td>PSDvalues</td><td>Array</td><td>nT/√Hz</td><td>PSD values at intersection with theoretical whistler</td></tr><tr><td>classification</td><td>Integer</td><td>-</td><td>Whistler classification</td></tr><tr><td>fmin</td><td>Float</td><td>Hz</td><td>The frequency below which the whistler signal is completely absorbed by the earth's atmosphere and is no longer detectable</td></tr><tr><td>flag_TS</td><td>Integer</td><td>-</td><td>Flag to estimate the goodness of the TS fit:<br>0 - Good fit<br>1 - Medium fit<br>2 - Bad fit</td></tr></table>

## Content of dataset_description structure of a whistler file

The dataset_description structure contains information on data processing, ILGEW project and data licence. Its content is described in Table 2-6.

<table><caption>Table 2-6 Content of the dataset_description structure of whistler files starting from v301.</caption><thead><tr><td>Structure field</td><td>Field value</td></tr></thead><tbody><tr><td>project_name</td><td>ILGEW</td></tr><tr><td>version</td><td>file version: v301 or v302 or v401 or v402</td></tr><tr><td>date_of_cre</td><td>Date of whistler data processing</td></tr><tr><td>Source</td><td><ul><li>Université de Paris, Institut de Physique du Globe de Paris, CNRS, Paris, France</li><li>Institute of Atmospheric Physics, Prague, Czech Republic</li><li>AGH University of Science and Technology, Krakow, Poland</li></ul></td></tr><tr><td>licence</td><td>CC-BY</td></tr><tr><td>parent_identifiers</td><td><ul><li>Swarm</li><li>GIRO</li><li>WERA</li></ul></td></tr></tbody></table>
<table><tr><td>link</td><td>https://ilgew.ipgp.fr</td></tr><tr><td>reference_doc</td><td>ILGEW Final Report, SW-ILGEW-IPGP-D15<br>Updated ILGEW Dataset User Manual, SW-ILGEW-IPGP-D08</td></tr><tr><td>Processing_chain</td><td>Whistler characterisation chain</td></tr><tr><td>version_of_processing_chain</td><td>3.0</td></tr><tr><td>Input_File_Name</td><td>Name of L1B ASM burst file used to produce the whistler file</td></tr><tr><td>operators</td><td>Name of the operator who processed the whistler file</td></tr></table>

### 2.3.2 Events dataset file format

The file **ILGEW_EventsFile_v0101.mat** includes a selection of 137 whistler classified as standard events, for which a detailed analysis has been performed and additional external parameters are included. They have been acquired between January 2014 and February 2020 during ASM burst sessions of all Swarm satellites. These external parameters are: the parameters of the lightning strike that originated the whistler signal and various ionospheric profiles obtained from IRI-2016 [Bilitza et al., 2017] model, IRTAM [Galkin et al., 2012] and from ionosondes located within 600 km from the Swarm satellite position, if available.

Note that there is one lightning event on 23 February 2014 at 23:21:48.8, that generated two whistlers detected independently by Swarm B and C, thus constituting two different events in the dataset. Only Swarm C was sufficiently close to an ionosonde location, therefore the ionospheric profile is not available for Swarm B.

The ILGEW events file is organised into several structures, as described in Table 2-7.

<table><caption>Table 2-7 Matlab structures contained in **ILGEW_EventsFile_v0101** file.</caption><thead><tr><td>Structure name</td><td>Structure description</td></tr></thead><tbody><tr><td>eventsWhistlers_groundELF_ionosonde</td><td>Numerical structure with 137 items, which correspond to whistler events occurring within 600 km from an ionosonde or some additional events for which the originating lightning signal was identified and characterized.</td></tr><tr><td>eventsWhistlers_groundELF_ionosonde_description</td><td>Description of the contents of eventsWhistlersondeELF_ionosonde structure.</td></tr><tr><td>dataset_description</td><td>Structure of metadata that describes the ILGEW dataset, and holds information about processing, publication and identification of data.</td></tr></tbody></table>

Each of these structures is described in the following paragraphs.
### Content of `eventsWhistlersgroundELF ionosonde structure of the events file`
The `eventsWhistlers groundELF ionosonde` numerical structure contains 137 whistler events, with related information contained in five substructures: `whistler`, `IRTAM`, `IRI_for_digestonde`, `digisonde`, `lightning`, as described in Table 2-8.

<table><caption>Table 2-8 Content of `eventsWhistlers groundELF ionosonde structure in ILGEW EventsFile_v0101 file`.</caption><thead><tr><td>Substructure name</td><td>Substructure description</td></tr></thead><tbody><tr><td>whistler</td><td>Whistler parameters</td></tr><tr><td>IRTAM</td><td>Ionospheric profiles computed using IRTAM and IRI-2016 model and in-situ EFI data at the geographic position of the satellite</td></tr><tr><td>IRI_for_digestonde</td><td>Ionospheric profiles computed using digisonde-derived parameters, IRI-2016 and in-situ EFI at the geographic position of the ionosonde</td></tr><tr><td>digisonde</td><td>Ionospheric profiles obtained from digisonde data</td></tr><tr><td>lightning</td><td>Location and charge moment of the lightning strike that originated the whistler</td></tr></tbody></table>

### Content of `whistler` substructure of the events file

The `whistler` substructure contains the same information as contained in the daily whistler files as described in Table 2-3 and Table 2-5. For all the events contained in this dataset, both algorithms for whistler's characterisation have been applied: the one based on PSD analysis and the one based on waveform analysis. See [RD-1] for details on the algorithms.

### Content of `IRTAM` substructure of the events file

The `IRTAM` substructure contains the ionospheric profile computed from the IRTAM [Galkin et al., 2012] and IRI-2016 [Bilitza et al., 2017] models at the location and time of the whistler event (the Swarm satellite position included in the `whistler` substructure), as described in Table 2-9. Cases for which IRTAM model was not available have been left empty.

<table><caption>Table 2-9 Content of `IRTAM` substructure of the `eventsWhistlers groundELF ionosonde structure in ILGEW EventsFile_v0101 file`. The description of these parameters is also contained in the corresponding structure `eventsWhistlers groundELF ionosonde description`. `IRTAM`.</caption><thead><tr><td>Structure field</td><td>Type</td><td>Units</td><td>Description</td></tr></thead><tbody><tr><td>IRTAMdateUT</td><td>Array</td><td>[year, month, day, hours, minutes, seconds]</td><td>Whistler event date and UT time</td></tr><tr><td>fo_irtam</td><td>Array</td><td>MHz</td><td>Weather IRTAM profile plasma frequency at hi_irtam altitude</td></tr></tbody></table>
<table><tr><td>ni_irtam</td><td>Array</td><td>m<sup>-3</sup></td><td>Weather IRTAM profile electron density at hi_irtam altitude</td></tr><tr><td>hi_irtam</td><td>Array</td><td>km</td><td>Weather IRTAM profile altitudes corresponding to fo_irtam, ni_irtam</td></tr><tr><td>fof2_irtam</td><td>Float</td><td>MHz</td><td>foF2 IRTAM</td></tr><tr><td>hmf2_irtam</td><td>Float</td><td>km</td><td>hmF2 IRTAM</td></tr><tr><td>b0_irtam</td><td>Float</td><td>km</td><td>B0 IRTAM</td></tr><tr><td>b1_irtam</td><td>Float</td><td>-</td><td>B1 IRTAM</td></tr><tr><td>fo_iri</td><td>Array</td><td>MHz</td><td>Climate IRI-2016 profile plasma frequency at hi_iri altitude</td></tr><tr><td>ni_iri</td><td>Array</td><td>m<sup>-3</sup></td><td>Climate IRI-2016 profile electron density at hi_iri altitude</td></tr><tr><td>hi_iri</td><td>Array</td><td>km</td><td>Climate IRI-2016 profile altitudes corresponding to fo_iri, ni_iri, opl_iri, npl_iri, hpl_iri, hepl_iri, opl_iri_corr, npl_iri_corr, hpl_iri_corr and hepl_iri_corr</td></tr><tr><td>fof2_iri</td><td>Float</td><td>MHz</td><td>foF2 IRI</td></tr><tr><td>hmf2_iri</td><td>Float</td><td>km</td><td>hmF2 IRI</td></tr><tr><td>b0_iri</td><td>Float</td><td>km</td><td>B0 IRI</td></tr><tr><td>b1_iri</td><td>Float</td><td>-</td><td>B1 IRI</td></tr><tr><td>opl_iri</td><td>Array</td><td>%</td><td>O<sup>+</sup> percentage from IRI-2016 at hi_iri altitude</td></tr><tr><td>npl_iri</td><td>Array</td><td>%</td><td>N<sup>+</sup> percentage from IRI-2016 at hi_iri altitude</td></tr><tr><td>hpl_iri</td><td>Array</td><td>%</td><td>H<sup>+</sup> percentage from IRI-2016 at hi_iri altitude</td></tr><tr><td>hepl_iri</td><td>Array</td><td>%</td><td>He<sup>+</sup> percentage from IRI-2016 at hi_iri altitude</td></tr><tr><td>opl_iri_corr</td><td>Array</td><td>%</td><td>O<sup>+</sup> percentage from IRI-2016 and corrected for possible discontinuity between bottomside and topside, computed at hi_iri altitude</td></tr><tr><td>npl_iri_corr</td><td>Array</td><td>%</td><td>N<sup>+</sup> percentage from IRI-2016 and corrected for possible discontinuity between bottomside and topside, computed at hi_iri altitude</td></tr><tr><td>hpl_iri_corr</td><td>Array</td><td>%</td><td>H<sup>+</sup> percentage from IRI-2016 and corrected for possible discontinuity between bottomside and topside, computed at hi_iri altitude</td></tr><tr><td>hepl_iri_corr</td><td>Array</td><td>%</td><td>He<sup>+</sup> percentage from IRI-2016 and corrected for possible discontinuity between bottomside and topside, computed at hi_iri altitude</td></tr><tr><td>slt</td><td>Float</td><td>h</td><td>Solar local time (0-24)</td></tr><tr><td>ne_swarm</td><td>Float</td><td>m<sup>-3</sup></td><td>Electron density of Swarm (if EFI LP data available otherwise NaN) - it serves to adjust topside profile</td></tr></table>
<table><tr><td>Gambit_flag</td><td>Integer</td><td>-</td><td>Quality of IRTAM fit nearest ionosonde autoscaled<br/>fof2 data:<br/>0: good fit<br/>2: poor fit<br/>9: no data available</td></tr></table>

The following ionospheric profile parameter are included in the dataset:

* foF2, critical frequency of the maximum of the ionospheric F2 layer

* hmF2, altitude of the maximum of the ionospheric F2 layer

* $B_0$, scale height of the bottomside of the ionospheric F2 layer

* $B_1$, shape parameter of the bottomside of the ionospheric F2 layer

The relation between foF2 (MHz) and the maximum value of electron density, NmF2 (m⁻³) is [Davies, 1990]:

$$ NmF2 = 1.24 \times 10^{10} \text{foF2}^2 \qquad (2-6) $$

The bottomside electron density profile is described by the function [Bilitza, 1990]:

$$ N(h) = NmF2 \frac{\exp(-x^{B_1})}{\cosh x} \qquad (2-7) $$

with

$$ x = \frac{hmF2 - h}{B_0} \qquad (2-8) $$

IRI-2016 parameters represent the climatological ionospheric profile in terms of: electron density, O⁺, N⁺, He⁺, N⁺. Two versions of ions profiles are provided, the _corr profiles are corrected for possible discontinuity between bottomside and topside. The IRTAM parameters represent a weather description of the ionospheric profile, obtained by adjusting the parameters using the global network of digisonde.

## Content of IRI_for_digestonde substructure of the events file

The IRI_for_digestonde substructure contains the IRI profile computed at the location of an ionosonde, using ionosonde-derived profile parameters as input to the model, as described in Table 2-10. In this substructure more than one digisonde profile can be included. The ionospheric soundings occur generally every 15 minutes, the available profiles before and after the whistler event are both recorded in the dataset. In some cases, more than one ionosonde was located within the 600 km radius and all are included as additional entries of the corresponding structure fields. Cases for which digisonde data were not available have been left empty.

<table><caption>Table 2-10 Content of the IRI_for_digestonde substructure of the eventsWhistlers酶groundELF_ionosonde structure in ILGEW_EventsFile_v0101 file. The description of these parameters is also contained in the corresponding structure eventsWhistlers酶groundELF_ionosonde_description. IRI_for_digestonde.</caption><thead><tr><td>Structure field</td><td>Type</td><td>Units</td><td>Description</td></tr></thead><tbody><tr><td>digisondedateUT</td><td>Array</td><td>[year, month, day, hours, minutes, seconds]</td><td>Digisonde date and UT time</td></tr></tbody></table>
<table><tr><td>fo_iri</td><td>Array</td><td>MHz</td><td>Climate IRI-2016 plasma frequency at hi_iri altitude above the digisonde</td></tr><tr><td>ni_iri</td><td>Array</td><td>m<sup>-3</sup></td><td>Climate IRI-2016 electron density at hi_iri altitude above the digisonde</td></tr><tr><td>hi_iri</td><td>Array</td><td>km</td><td>Climate IRI-2016 profile altitudes corresponding to fo_iri, ni_iri, opl_iri, npl_iri, hpl_iri, hepl_iri, fo_iri Adjust, ni_iri Adjust, opl_iri corr, npl_iri corr, hpl_iri corr and hepl_iri corr</td></tr><tr><td>opl_iri</td><td>Array</td><td>%</td><td>O<sup>+</sup> percentage from IRI-2016 at hi_iri altitude</td></tr><tr><td>npl_iri</td><td>Array</td><td>%</td><td>N<sup>+</sup> percentage from IRI-2016 at hi_iri altitude</td></tr><tr><td>hpl_iri</td><td>Array</td><td>%</td><td>H<sup>+</sup> percentage from IRI-2016 at hi_iri altitude</td></tr><tr><td>hepl_iri</td><td>Array</td><td>%</td><td>He<sup>+</sup> percentage from IRI-2016 at hi_iri altitude</td></tr><tr><td>opl_iri corr</td><td>Array</td><td>%</td><td>O<sup>+</sup> percentage from IRI-2016 and corrected for possible discontinuity between bottomside and topside, computed at hi_iri altitude</td></tr><tr><td>npl_iri corr</td><td>Array</td><td>%</td><td>N<sup>+</sup> percentage from IRI-2016 and corrected for possible discontinuity between bottomside and topside, computed at hi_iri altitude</td></tr><tr><td>hpl_iri corr</td><td>Array</td><td>%</td><td>H<sup>+</sup> percentage from IRI-2016 and corrected for possible discontinuity between bottomside and topside, computed at hi_iri altitude</td></tr><tr><td>hepl_iri corr</td><td>Array</td><td>%</td><td>He<sup>+</sup> percentage from IRI-2016 and corrected for possible discontinuity between bottomside and topside, computed at hi_iri altitude</td></tr><tr><td>fo_iri Adjust</td><td>Array</td><td>MHz</td><td>IRI-2016 plasma frequency above the digisonde profile adjusted at digisonde foF2 and hmF2 and Ne Swarm, computed at hi_iri altitude</td></tr><tr><td>ni_iri Adjust</td><td>Array</td><td>m<sup>-3</sup></td><td>IRI-2016 electron density above the digisonde profile adjusted at digisonde foF2 and hmF2 and Ne Swarm, computed at hi_iri altitude</td></tr><tr><td>b0</td><td>Float</td><td>km</td><td>B0 from digisonde or calculated from data</td></tr><tr><td>b1</td><td>Float</td><td>-</td><td>B1 from digisonde or calculated from data</td></tr><tr><td>foF2</td><td>Float</td><td>MHz</td><td>foF2 from digisonde</td></tr><tr><td>hmF2</td><td>Float</td><td>km</td><td>hmF2 from digisonde</td></tr><tr><td>lat</td><td>Float</td><td>deg</td><td>Latitude of the digisonde (-90, 90)</td></tr><tr><td>lon</td><td>Float</td><td>deg</td><td>Longitude of the digisonde (-180,180)</td></tr><tr><td>slt</td><td>Float</td><td>h</td><td>Solar local time (0-24)</td></tr><tr><td>code</td><td>String</td><td>-</td><td>Station URSI code</td></tr></table>
<table><tr><td>dist</td><td>Float</td><td>km</td><td>Distance from the whistler event [km]</td></tr><tr><td>ne_swarm</td><td>Float</td><td>m<sup>-3</sup></td><td>Electron density from Swarm LP at the moment of the whistler event</td></tr></table>

## Content of digisonde substructure of the events file

The digisonde substructure contains the experimental electron density profile obtained from the ionosondes located within 600 km from the location of the satellite at the time of a whistler detection, as described in Table 2-11. Since the ionospheric soundings occur generally every 15 minutes, the available profiles before and after the whistler event are both recorded. In some cases, more than one ionosonde were located within this radius and all available data are presented as additional entries of the corresponding structure fields. Cases for which digisonde data were not available have been left empty.

<table><caption>Table 2-11 Content of the digisonde substructure of the eventsWhistlers_groundELF_ionosonde structure in ILGEW_EventsFile_v0101 file. The description of these parameters is also contained in the corresponding structure eventsWhistlers_groundELF_ionosonde_description.digisonde.</caption><thead><tr><td>Structure field</td><td>Type</td><td>Units</td><td>Description</td></tr></thead><tbody><tr><td>digisondedateUT</td><td>Array</td><td>[year, month, day, hours, minutes, seconds]</td><td>Digisonde date and UT time</td></tr><tr><td>fo</td><td>Array</td><td>MHz</td><td>Plasma frequency at hi altitude, values above hmF2 are extrapolated</td></tr><tr><td>ni</td><td>Array</td><td>m<sup>-3</sup></td><td>Electron density at hi altitude, values above hmF2 are extrapolated</td></tr><tr><td>hi</td><td>Array</td><td>km</td><td>Digisonde profile altitudes corresponding to fo and ni</td></tr><tr><td>b0</td><td>Float</td><td>km</td><td>B0 if available, otherwise 0</td></tr><tr><td>b1</td><td>Float</td><td>-</td><td>B1 if available, otherwise 0</td></tr><tr><td>fof2</td><td>Float</td><td>MHz</td><td>foF2</td></tr><tr><td>hmf2</td><td>Float</td><td>km</td><td>hmF2</td></tr><tr><td>lat</td><td>Float</td><td>deg</td><td>Latitude of the digisonde (-90, 90)</td></tr><tr><td>lon</td><td>Float</td><td>deg</td><td>Longitude of the digisonde (-180,180)</td></tr><tr><td>slt</td><td>Float</td><td>h</td><td>Solar local time (0-24)</td></tr><tr><td>code</td><td>String</td><td>-</td><td>Station URSI code</td></tr><tr><td>dist</td><td>Float</td><td>km</td><td>Distance from the whistler event</td></tr></tbody></table>
## Content of `lightning` substructure of the events file

The `lightning` substructure contains the parameters of the identified lightning that generated the whistler signal. These parameters were obtained from the WERA and WWLLN networks of ground receivers and are described in Table 2-12. The location of the lightning and the distance from the whistler's event are given approximated to the first decimal digit, sufficient for the purposes of this study.

<table><caption>Table 2-12 Content of the `lightning` substructure of `eventsWhistlersgroundELF ionosonde structure in ILGEW EventsFile v0101 file. The description of these parameters is also contained in the corresponding structure `eventsWhistlers groundELF ionosonde description`. `lightning`.</caption><thead><tr><th>Structure field</th><th>Type</th><th>Units</th><th>Description</th></tr></thead><tbody><tr><td>lightningdateUT</td><td>Array</td><td>[year, month, day, hours, minutes, seconds]</td><td>Lightning date and UT time</td></tr><tr><td>dist</td><td>Float</td><td>km</td><td>Distance from Swarm location to lightning location (WWLLN if detected, otherwise ELF location).</td></tr><tr><td>lat</td><td>Float</td><td>deg</td><td>Lightning latitude (-90,90)</td></tr><tr><td>lon</td><td>Float</td><td>deg</td><td>Lightning longitude (-180,180)</td></tr><tr><td>location_system</td><td>String</td><td>-</td><td>Flag on the ground system used for computing lightning location<br>W: dist, approximated lat, lon inferred from WWLLN<br>E: dist, lat, lon inferred from ELF if not available from WWLLN<br>S: dist, lat, lon corresponding to Swarm position<br>U: dist, lat, lon unavailable</td></tr><tr><td>iCMC</td><td>Float</td><td>C km</td><td>Lightning charge moment calculated from the ELF data</td></tr><tr><td>comment</td><td>String</td><td>-</td><td>Analysis comment, if any.</td></tr></tbody></table>

## Content of `eventsWhistlers groundELF ionosonde description` structure of the events file

The `eventsWhistlers groundELF ionosonde description` is a structure of metadata describing the content of the numerical variables as detailed in Table 2-9, Table 2-10, Table 2-11 and Table 2-12 above. It is organized similarly to the `eventsWhistlers groundELF ionosonde` structure, with five main fields corresponding to each of this structure's main fields and as many sub-fields as the corresponding variables and substructures.

## Content of `dataset description` structure of the events file

The `dataset_description` is a structure of metadata describing the ILGEW dataset, and holds information about publication of data, processing of data and identification of data. The content of the `dataset_description` is summarised in Table 2-13.
<table><caption>Table 2-13 Content of dataset_description structure in ILGEW_EventsFile_v0101 file.</caption><thead><tr><td>Structure name</td><td>Structure description</td></tr></thead><tbody><tr><td>Identification</td><td>Project identification metadata such as the name, the version and the date of creation of the dataset</td></tr><tr><td>Publication</td><td>Project publication metadata such as the licence of the dataset, all parent datasets used and reference documents name.</td></tr><tr><td>Processing</td><td>Project processing metadata such as the processing chain name and version and the operators.</td></tr></tbody></table>

Identification of the project, dataset version and date of creation of the dataset are contained in Table 2-14.

<table><caption>Table 2-14 Content of the Identification substructure of dataset_description structure in ILGEW_EventsFile_v0101 file.</caption><thead><tr><td>Structure field</td><td>Field value</td></tr></thead><tbody><tr><td>project_name</td><td>ILGEW</td></tr><tr><td>version</td><td>0101</td></tr><tr><td>date_of_cre</td><td>2021-05-31</td></tr></tbody></table>

Participating institutes, licence on dataset usage, links to the parent datasets from which ILGEW dataset has been constructed, and documents of reference are contained in Table 2-15.

<table><caption>Table 2-15 Content of the Publication substructure of dataset_description structure in ILGEW_EventsFile_v0101 file.</caption><thead><tr><td>Structure field</td><td>Field value</td></tr></thead><tbody><tr><td>source</td><td><ul><li>Université de Paris, Institut de Physique du Globe de Paris, CNRS, Paris, France</li><li>Institute of Atmospheric Physics, Prague, Czech Republic</li><li>AGH University of Science and Technology, Krakow, Poland</li></ul></td></tr><tr><td>licence</td><td>CC-BY</td></tr><tr><td>parent_dataset</td><td><ul><li>https://ilgew.ipgp.fr</li><li>www.oa.uj.edu.pl</li><li>http://giro.uml.edu/</li><li>https://earth.esa.int/eogateway/</li><li>https://earth.esa.int/web/guest/swarm/data-access</li><li>http://irimodel.org</li><li>http://wwlln.net/</li></ul></td></tr><tr><td>reference_doc</td><td>ILGEW Final Report, SW-ILGEW-IPGP-D15<br>Updated ILGEW Dataset User Manual, SW-ILGEW-IPGP-D08</td></tr></tbody></table>
Information on the processing chain used to generate the whistler events, the version of the processing chain and the people who processed the data are contained in Table 2-16.

<table><caption>Table 2-16 Content of the Processing substructure of dataset_description structure in ILGEW_EventsFile_v0101 file.</caption><thead><tr><td>Structure field</td><td>Field value</td></tr></thead><tbody><tr><td>Processing_chain</td><td>Whistler characterisation chain</td></tr><tr><td>version_of_processing_chain</td><td>3.0</td></tr><tr><td>operators</td><td>Laura Brocco, Louis Chauvet</td></tr></tbody></table>

## 2.4 Classification of whistler events

According to the spectral characteristics of the selected events, a classification was defined, ranging from 1 to 7. A summary description is provided in Table 2-17.

<table><caption>Table 2-17 Classification of lightning-generated whistler events.</caption><thead><tr><td>Classification</td><td>Designation</td><td>Description</td></tr></thead><tbody><tr><td>1</td><td>Standard</td><td>Nominal event covering the whole spectrum between 20 to 120 Hz</td></tr><tr><td>2</td><td>High Frequency</td><td>Event seen only the upper part of the spectral band</td></tr><tr><td>3</td><td>Low Frequency</td><td>Event seen only the lower part of the spectral band</td></tr><tr><td>4</td><td>Medium Frequency</td><td>Event seen only the central part of the spectral band</td></tr><tr><td>5</td><td>Complex</td><td>Event associated with outliers in the ASM time series or overlaps of events</td></tr><tr><td>6</td><td>1.</td><td>Possible candidate for 1. whistler</td></tr><tr><td>7</td><td> DESTROYED</td><td>Unclear event presenting some spectral feature similar to whistlers.</td></tr></tbody></table>

An example of whistler for each of these classifications is included. Note that in the 3 s time window more than one whistler event can be present, but the characterisation is for the whistler at the centre of the window. The others are characterised in a separate event.
1. **Standard** event, presenting a clear signal covering all the spectral band between 20 and 125 Hz. Most of the events contained in this category can be assumed to be lightning-generated whistlers. An example is shown in Figure 2-1.

<figure><img src="image_6.png" /><figcaption>Figure 2-1 Example of standard whistler event. Top panel: three seconds of time series, variable *t_event* and *F_filtered*. Bottom panel, PSD plot showing the whistler event at the centre: variables *t_PSD*, *f_PSD* and *PSD*\*1000. The white dots identify the points selected: *characterization.t* and *characterization.f*</figcaption></figure>

2. **High frequencies** event, presenting a clear signal only in the upper part of the spectrum, usually above 50 Hz. An example is shown in Figure 2-2.

<figure><img src="image_7.png" /><figcaption>Figure 2-2 Example of high frequencies event. Same representation as Figure 2-1.</figcaption></figure>
3. **Low frequencies** event, presenting a clear signal only in the lower part of the spectrum, usually below 50 Hz. An example is shown in Figure 2-3.

<figure><img src="image_8.png" /><figcaption>Figure 2-3 Example of low frequencies event. Same representation as Figure 2-1.</figcaption></figure>

4. **Medium frequencies** event, presenting a clear signal covering only in the central part of the spectrum, usually between 50 and 100 Hz. An example is shown in Figure 2-4.

<figure><img src="image_9.png" /><figcaption>Figure 2-4 Example of medium frequencies event. Same representation as Figure 2-1.</figcaption></figure>
5. **Complex** event, when an outlier is present in the ASM time series during or close to the whistler signal. The event is classified as complex, since the derived whistler parameters might be negatively affected by the presence of the outlier. An example is shown in Figure 2-5.

<figure><img src="image_10.png" /><figcaption>Figure 2-5 Example of complex event. Same representation as Figure 2-1.</figcaption></figure>

6. **1. candidate** event, when the dispersion is high compared to the expected dispersion at the location and local time. High dispersion might due to the propagation path reaching the Swarm satellite from above, but **the events included in the version 1 of the dataset have not been validated by a propagation simulation.** An example is shown in Figure 2-6.

<figure><img src="image_11.png" /><figcaption>Figure 2-6 Example of 1. candidate event. Same representation as Figure 2-1.</figcaption></figure>
7. ** DESTROYED ** event, when the spectral signature contains elements indicating a possible presence of parts of a whistler signal, but the intensity and extent are not sufficient to clearly characterise it as a whistler. The estimated dispersion *D* and *t*₀ are affected by high uncertainty. An example is shown in Figure 2-7.

<figure><img src="image_12.png" /><figcaption>Figure 2-7 Example of destroyed event. Same representation as Figure 2-1.</figcaption></figure>
## 3 Examples of whistler data visualization

The whistlers contained in the dataset files, can be visualised using plotting tools available with Matlab software.

In the examples shown here on geographical maps, an additional grid of magnetic inclination isolines has been added. It is not contained in the whistler files, but can be computed using the IGRF model [Thébault et al., 2015].

The global map of the 137 ILGEW events dataset is shown in Figure 3-1. It shows all events as coloured points, all belong to classification 1. These events were all classified in the category 1 of standard events. They were acquired by the various Swarm satellites during the ASM burst mode sessions from January 2014 to February 2020.

In this example the estimated dispersion of all events is colour-coded on a colour scale between 0 and $15\sqrt{s}$.

<figure><img src="image_13.png" /><figcaption>Figure 3-1 Map of the 137 ILGEW events included in the dataset. Each coloured point represents an event, the estimated dispersion follows the colour scale. The maroon isolines indicate the magnetic inclination at the satellite height.</figcaption></figure>

A global map of whistler occurrences over 24 hours on 18 August 2019 is shown in Figure 3-2. It shows all events as coloured points, including all categories of whistler classification.
<figure><img src="image_14.png" /></figure>

Figure 3-2 Example of whistler events detected on 18 August 2019 by Swarm A satellite. Each coloured point represents an event, the estimated dispersion follows the colour scale. The locations covered by the Swarm orbit during the whole day are represented in grey. The maroon isolines indicate the magnetic inclination at the satellite height.

The example shown in Figure 3-3 contains the map of whistler occurrences on the same day, but detected only during the day-side part of the Swarm satellite orbit. The colour scale shows the classification of whistler's signals from 1 to 7 as defined in Table 2-17.

<figure><img src="image_15.png" /></figure>

Figure 3-3 Example of whistler events detected on 18 August 2019 by Swarm A satellite on the day-side part of the orbit. Each coloured point represents an event, the classification of events follows the colour scale. The locations covered by the Swarm during the day-side part of its orbit are represented in grey. The maroon isolines indicate the magnetic inclination at the satellite height.
An example of the data contained in the *whistler* structure is shown in Figure 3-4. The figure title is composed of *whistlerdateUT*, the satellite *sat*, the coordinates *Latitude* and *Longitude* and the local time *LT*.

The top panel displays the ASM burst time series of *F* over the 3 s time window as a function of *TimestampburstUT*. The middle panel contains the *F_filterd* as function of *t_event*, which origin is *whistlerdateUT*. The bottom panel contains the *PSD* represented as function of *t_PSD* and *f_PSD*. On top of the PSD are displayed white dots that correspond to the *characterization.t* and *characterization.f* points. The title of the bottom panel includes the dispersion *characterization.D*, the estimated time when the signal entered the ionosphere *characterization.t0* and the classification of the event *characterization.classification*.

<figure><img src="image_16.png" /><figcaption>Figure 3-4 Example of whistler signals contained in the whistler structure. Top panel: time series of total magnetic field intensity. Middle panel: time series of filtered magnetic field intensity. Bottom panel: PSD with highlight of the pixels used to identify the whistler signal.</figcaption></figure>
## 4 References

Bilitza, D. (1990), International Reference Ionosphere 1990, Tech. Rep. 90-22, NSSDC, Greenbelt, Maryland, USA.

Bilitza, D., D. Altadill, V. Truhlik, V. Shubin, I. Galkin, B. Reinisch, and X. Huang (2017), International Reference Ionosphere 2016: from ionospheric climate to real-time weather predictions, *Space Weather*, **15**, 418-429, doi:10.1002/swe.1593.

Eckersley, T. (1935), Musical atmospherics, *Nature*, **135** (3403), 104–105, doi:10.1038/135104a0.

Galkin, I. A., B. W. Reinisch, X. Huang, and D. Bilitza (2012), Assimilation of GIRO data into a real-time IRI, *Radio Science*, **47**(4), doi:10.1029/2011RS004952.

Davies, K. (1990), Ionospheric Radio, IEE electromagnetic wave series, vol. 31, Peter Peregrinus Ltd., London.

MathWorks (2021), MAT-File Format, https://www.mathworks.com/help/pdf_doc/matlab/matfile_format.pdf

Thébault, E., C. C. Finlay, C. D. Beggan, P. Alken, J. Aubert, O. Barrois, F. Bertrand, T. Bondar, A. Boness, L. Brocco, E. Canet, A. Chambodut, A. Chulliat, P. Coïsson, F. Civet, A. Du, A. Fournier, I. Fratter, N. Gillet, B. Hamilton, M. Hamoudi, G. Hulot, T. Jager, M. Korte, W. Kuang, X. Lalanne, B. Langlais, J.-M. Léger, V. Lesur, F. J. Lowes, S. Macmillan, M. Mandea, C. Manoj, S. Maus, N. Olsen, V. Petrov, V. Ridley, M. Rother, T. J. Sabaka, D. Saturnino, R. Schachtschneider, O. Sirol, A. Tangborn, A. Thomson, L. Tøffner-Clausen, P. Vigneron, I. Wardinski, and T. Zvereva (2015), International Geomagnetic Reference Field: the 12th generation, *Earth, Planets and Space*, **67**(1), 79, doi:10.1186/s40623-015-0228-9.
## 5 List of acronyms

<table><tr><td>ASM</td><td>Absolute Scalar Magnetometer</td></tr><tr><td>CDF</td><td>Common Data Format</td></tr><tr><td>ELF</td><td>Extremely Low Frequency</td></tr><tr><td>FFT</td><td>Fast Fourier Transform</td></tr><tr><td>IGRF</td><td>International Geomagnetic Reference Field</td></tr><tr><td>ILGEW</td><td>Investigating Lightning Generated ELF Whistlers to improve ionospheric models</td></tr><tr><td>IRI</td><td>International Reference Ionosphere</td></tr><tr><td>IRTAM</td><td>IRI Real Time Assimilative Mapping</td></tr><tr><td>LP</td><td>Langmuir Probe</td></tr><tr><td>NaN</td><td>Not a Number</td></tr><tr><td>LT</td><td>Local Time</td></tr><tr><td>PSD</td><td>Power Spectral Density</td></tr><tr><td>UTC</td><td>Universal Time Coordinated</td></tr><tr><td>WERA</td><td>World ELF Radiolocation Array</td></tr><tr><td>WWLLN</td><td>World Wide Lightning Location Network</td></tr></table>