RESEARCH LETTERThe Strongest Negative Lightning Strokes in WinterThunderstorms in Japan

Key Points:

Ting Wu1

10.1029/2021GL095525• N egative return strokes with peakcurrents larger than 150 kA in
winter in Japan produce unusualelectric field change waveforms• These strongest negative returnstrokes are preceded by a fast
downward negative leader with shortduration and large peak current• The preceding downward negative
leader may be associated withdownward terrestrial gamma-ray
flashes
Supporting Information:
Supporting Information may be foundin the online version of this article.
Correspondence to:T. Wu,
tingwu@gifu-u.ac.jp
Citation:
Wu, T., Wang, D., Huang, H., &Takagi, N. (2021). The strongest
negative lightning strokes in winterthunderstorms in Japan. GeophysicalResearch Letters, 48, e2021GL095525.
https://doi.org/10.1029/2021GL095525
Received 31 JUL 2021
Accepted 19 OCT 2021

© 2021. American Geophysical Union.
All Rights Reserved.

WU ET AL.

, Daohong Wang1, Haitao Huang1, and Nobuyuki Takagi1

Department of Electrical, Electronic, and Computer Engineering, Gifu University, Gifu, Japan

1

Abstract Winter thunderstorms in Japan are well known for the frequent production of energetic
positive cloud-to-ground (CG) lightning flashes. By contrast, strong negative CG flashes or negative returnstrokes in winter thunderstorms are largely unknown. In this study, we demonstrate that negative returnstrokes with peak currents larger than 150 kA (absolute value) in winter thunderstorms in Japan mostly
produce electric field change waveforms that are different from those of normal return strokes. Due totheir unusual waveforms, these strongest negative strokes have not been recognized as return strokes by
the lightning research community. We further demonstrate that these strong negative return strokes arepreceded by a fast downward negative leader with a short duration and strong peak current. We suggestthat the fast and powerful downward negative leader may be associated with downward terrestrial
gamma-ray flashes. We also present evidence that these strong negative strokes are likely “superbolts”
observed from space.
Plain Language Summary The negative return stroke is the most well studied lightning

discharge process. In this study, we demonstrate that negative return strokes with peak currents larger
than 150 kA in winter thunderstorms in Japan mostly produce unique electric field change waveforms
that are generally different from those of normal return strokes. Due to their unusual waveforms, thesestrongest negative strokes have not been recognized as return strokes by the lightning research community
and may not be correctly identified by nationwide lightning location systems.

1. Introduction
Lightning discharges in winter thunderstorms in Japan exhibit many unique features. The most well known
feature is the frequent production of positive cloud-to-ground (CG) lightning flashes (Brook et al., 1982;
Takeuti et al., 1978). Exceptionally powerful and energetic positive CG flashes in winter thunderstorms
have been observed and investigated extensively and are known to induce transient luminous events
such as sprites and elves (Adachi, 2005; Hayakawa, 2004; Matsudo et al., 2007; Suzuki et al., 2006; Wanget al., 2021). By contrast, strong negative CG flashes or return strokes in winter thunderstorms are muchless well known.
Strong electric field change (E-change) pulses with the same polarity as those of negative return strokes
in winter thunderstorms in Japan have been reported by a few studies but their physical mechanism is
not yet clear. Ishii and Saito (2009) reported strong electric field change (E-change) waveforms that wereassociated with transmission-line faults in winter along the Japan Sea coast. Some of the waveforms had
the same polarity as those produced by negative return strokes but were quite different from those of typicalreturn strokes. Ishii and Saito (2009) attributed these waveforms to upward lightning. Wu et al. (2014) reported the so-called “large bipolar events” (LBEs) that produced characteristic large and bipolar E-change
waveforms. LBEs were almost always found on land so they were believed to be associated with tall objects.
Wada et al. (2020) observed two downward terrestrial gamma-ray flashes (TGFs) in winter thunderstorms
and found that both TGFs were associated with strong pulses with estimated peak currents of −260 kA
and −197 kA. These strong pulses were quite similar to those reported by Ishii and Saito (2009) and wereinterpreted by Wada et al. (2020) as energetic in-cloud pulses (Lyu et al., 2015), which is a type of intraclouddischarge closely associated with TGFs. Similar strong pulses were also reported by Wu et al. (2020b) totrigger upward negative leaders from tall objects. Wu et al. (2020b) speculated that these strong pulses were
produced by special negative return strokes. These scarce and contradictory reports of strong negative discharges fully reveal how little we know about negative strokes in winter thunderstorms.
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In this study, we will demonstrate that negative return strokes with estimated peak currents larger than
150 kA (absolute value) in winter thunderstorms in Japan mostly produce E-change waveforms that are
generally different from normal return stroke waveforms but are very similar to strong pulses reported by
the above-mentioned studies. Our results also indicate that many, if not all, of the strong pulses reported
by previous studies, were also produced by strong negative return strokes. Due to the unusual waveforms,
the existence of these strongest negative return strokes is not yet recognized by the lightning research community, and at least some of these strokes are apparently not identified by nationwide lightning location
systems.

2. Observation and Data
2.1. Winter Observation With the FALMA
A fast antenna lightning mapping array (FALMA) system including 14 observation sites were set up forthe winter observation in the Hokuriku region of Japan from December 2018 to March 2019. Locationsof these 14 sites are shown in Figure S1 in Supporting Information S1 and can also be found in our previous studies (e.g., Wu et al., 2020b). The FALMA is a low-frequency lightning mapping system using fastantennas as the sensors (Wu et al., 2018). Every site records E-change waveforms in the frequency rangeof 500 Hz to 500 kHz with a sampling rate of 10 MHz. The FALMA is capable of high-quality 3-D lightning mapping. However, during the winter observation, due to the fact that winter lightning discharges areclose to the ground and are usually very complicated, location results of source height are not reliable (Wuet al., 2020a), so we will only use 2-D location results in this study.
The atmospheric electricity sign convention is used in this study, so a negative return stroke produces apositive E-change waveform.

2.2. Data Selection
In this study, we will analyze negative strokes with estimated peak currents larger than 150 kA (absolutevalue). Peak currents are estimated by correlating normalized E-change magnitudes with peak currents
reported by the Japanese Lightning Detection Network (JLDN; Matsui et al., 2019) as described by Wu
et al. (2021). We identified all pulses having the same polarity as negative return strokes and having estimated peak currents larger than 150 kA in𝐴𝐴a 150𝐴𝐴
× 150 km2 area shown in Figure S1 in Supporting Information S1. A total of 104 pulses were identified. We will demonstrate that these strong pulses are produced bynegative return strokes. Locations of these strokes are also shown in Figure S1 in Supporting Information S1.
Among the identified 104 strokes, 6 strokes saturated all FALMA sites. These six strokes are not includedin the analyses of this study but their E-change waveforms are shown in Figure S2 in Supporting Information S1. E-change waveform figures of the remaining 98 strokes are provided in the data repository.

2.3. LAPOS Observation of a Strong Stroke
A stroke with a JLDN-reported peak current of −335 kA was observed in December of 2020 by the FALMA
as well as a high-speed optical system called Lightning Attachment Process Observation System (LAPOS;
Wang et al., 2013) and is analyzed separately in Section 3.3. The LAPOS was set up at the same location
as the FALMA site represented by a red square in Figure S1 in Supporting Information S1. A detailed
description of the LAPOS used for this study can be found in Wang et al. (2013). The LAPOS contains 14
photodiodes including 7 photodiodes with high sensitivity and 7 with low sensitivity. Photodiodes withhigh and low sensitivities are aligned alternatively in the vertical direction, and each photodiode is configured to measure the light intensity at a certain elevation angle. Light intensity waveforms from 14 photodiodes along with the GPS signal are recorded by an oscilloscope with a sampling rate of 10 MHz. Detailsof the location and the field of view of the LAPOS with respect to the stroke can be found in Figure S3 in
Supporting Information S1.

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Figure 1. (a–j) E-change waveforms of the 10 strongest negative strokes. The red arrow indicates the first identified
𝐴𝐴 of 𝐴𝐴 represents the distance of the stroke to the site recording the plotted waveform.
pulse before each stroke. The value(k) Typical waveform of the first return stroke in a negative cloud-to-ground flash.

3. Results
3.1. E-Change Waveform
The most prominent feature of strong negative strokes in winter in Japan is that they produce E-change
waveforms that are different from those produced by normal negative return strokes. Figures 1a–1j shows
E-change waveforms of the 10 strongest negative strokes. As a direct comparison, Figure 1k shows the waveform of a typical first return stroke in a negative CG flash. The shape of stroke pulses in Figures 1a–1j are
generally different from those of normal negative return strokes. Some common waveform characteristics
of typical first return strokes in negative CG flashes, including much larger fall time than rising time, finestructure in the falling portion, and relatively small preceding leader pulses, cannot be seen in the strong
strokes in Figures 1a–1j. More importantly, these strong strokes are preceded by discharges with very short

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𝐴𝐴 500 𝐴𝐴s. Red arrows in Figures 1a–1j indurations, generally shorter thandicate the first identified pulses before these strokes. Details about theidentification of the first pulses can be found in Wu et al. (2021).

In fact, almost all negative strokes with peak currents larger than 150 kA
have similar features to those in Figures 1a–1j. Figure 2a shows the distribution of durations of preceding discharges (defined as the time difference between the first identified pulse in the flash and the peak of thestroke pulse). Three strokes with exceptionally long preceding discharges
are not included here and will be described in more detail later. Excluding these three special cases, Figure 2a shows that all strokes with peakcurrents larger than 150 kA are preceded by discharges with durations
shorter than𝐴𝐴 700 𝐴𝐴s. The minimum and maximum durations in Figure 2aare 65.3 and 𝐴𝐴
681.4 𝐴𝐴s, respectively, with a mean value of 𝐴𝐴
252.8 𝐴𝐴s. Suchshort durations are due to fast downward negative leaders (velocity of one
event is analyzed in Section 3.3) preceding the return stroke and the close
proximity of winter thunderclouds to the ground. Furthermore, discharges preceding the first return stroke in normal negative CG flashes consist
of PB and stepped leader processes. Pulses before these strong strokes
may be somewhat similar to PB pulses but they cannot be divided into
two clearly different stages corresponding to the PB and stepped leader processes. Enlarged waveforms of discharges preceding the strokes in
Figures 1a–1j are shown in Figure S4 in Supporting Information S1.
Figures 2b–2e show statistics of pulse rise time, fall time, pulse width,
and the ratio of fall time to rise time for these strokes. Definitions of theseparameters are shown in Figure S5 in Supporting Information S1. Compared with parameters of E-change waveforms of first strokes in negative
CG flashes reported in the literature (Rakov & Uman, 2003), it is clearthat these strong strokes in winter have a larger rise time but smaller
pulse width. As a result, the ratio of the fall time to rise time is relatively
small, resulting in somewhat symmetric stroke pulses. However, it is important to note that the fall time, pulse width, and the ratio of fall time
to rise time are dependent on the duration of preceding discharges as
demonstrated in Figures 2c–2e. Similar results were also reported for special strokes in winter with durations of preceding discharges shorter thanFigure 2. E-change waveform parameters of strong negative strokes
1 ms (Wu et al., 2021). We can see that as the duration of preceding disand preceding discharges. (a) The distribution of durations of preceding
charges increases, the fall time, the pulse width, and the ratio of fall timedischarges. Scatterplots of durations of preceding discharges versus (b)Rise time, (c) Fall time, (d) Pulse width, (e) The ratio of fall time to rise
to rise time all increase, getting closer to waveform characteristics of firsttime, and (d) Peak currents of stroke pulses. Blue bars and points represent
strokes in normal negative CG flashes. High correlations between durastrokes on the sea and black ones on land. Red diamonds in panel (f)tion of preceding discharges and waveform parameters of stroke pulses
𝐴𝐴 100-𝐴𝐴s range.
represent mean values of peak currents in eachalso indicate that both the stroke and preceding discharges are associatedwith the same channel, and a reasonable explanation is that preceding
discharges are leader processes creating a channel in the virgin air andthe stroke pulse is due to the return stroke traversing the same channel. Figure 2f shows that peak currentsof strokes are also associated with durations of preceding discharges, which indicates that the strongest
strokes usually have preceding discharges with durations of 200–300
𝐴𝐴𝐴𝐴s, as can also be seen in Figures 1a–1j.
Three special cases have durations of preceding discharges of 188, 27, and 16 ms and peak currents of 187,
184, and 180 kA. Details of these three special cases are provided in Figures S6–S8 in Supporting Information S1. It is interesting to note that although durations of preceding discharges of these cases are orders ofmagnitudes larger than other cases, the stroke pulses are generally similar to those in Figures 1a–1j. Since
these cases are uncommon and their peak currents are not exceptionally strong, we will not analyze these
special cases in this study.

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Waveforms of all 98 strokes with peak currents larger than 150 kA are provided in the data repository (seeAcknowledgment). We can see that almost all waveforms are somewhat different from those of normal
return strokes, although it is rather subjective to determine what kind of waveforms are “normal.” Thestrongest stroke we found that looks very like a normal return stroke and was preceded by discharges withobvious two stages presumably corresponding to the PB and stepped leader has a peak current of −148 kA.
Its E-change waveform is shown in Figure S9 in Supporting Information S1.

3.2. Current Reflection in Stroke Channels
An interesting feature of waveforms of these strong strokes is that a small pulse sometimes can be foundsuperimposed on the negative cycle of the stroke pulse. Examples can be found in Figures 1a, 1c, 1hand 1i. Two strong pulses reported to be associated with downward TGFs in winter thunderstorms (Wadaet al., 2020) also had such features. The origin of the small pulses is not yet clear. Here we demonstrate thatthe small pulses are due to the current reflection in return stroke channels.
First, it should be noted that features of the small pulse are varied; they are not always as pronounced
as those reported by Wada et al. (2020) and are not always right at the negative cycle of the stroke pulse.
Figures 3a–3d show some examples of small pulses with different features. The small pulses indicated bypurple arrows are treated as the same phenomenon in the following analysis.
We found 62 strokes with recognizable small pulses such as those in Figures 3a–3d. These small pulses are
identified as the most significant pulse between the main peak and the end of the negative cycle. Figures of
their E-change waveforms are provided in the data repository. We calculated the time difference between
the small pulse and the onset of the stroke pulse. The definition of the time difference, denoted
𝐴𝐴 as Δ𝑡𝑡, is𝐴𝐴Δ𝑡𝑡 and the duration of preceding disillustrated in Figure 3a. The correlation between the time difference
charges is shown in Figure 3e. We can see that the two parameters show a very strong correlation with acorrelation coefficient of 0.81.
Our previous study demonstrated that when the duration of preceding discharges is very small, the preceding discharges are likely produced by a downward negative leader developing directly to the ground, andthus the duration is related to the stroke channel length (Wu et al., 2021). Therefore, the strong correlationin Figure 3e indicates that the time difference𝐴𝐴
Δ𝑡𝑡 is also related to the stroke channel length, and a reasonable explanation is that the small pulse is produced by the current reflection in the stroke channel.
Assuming that the reflected pulse is produced when the return stroke wave is reflected from the upper endof the channel, the slope of the linear regression line (which is 0.052) in Figure 3e equals the ratio of theleader velocity to the return stroke velocity. For the velocity of the downward negative leader, we have estimated that the velocity is about 3 ×106 m/s for cases with durations of preceding discharges smaller than𝐴𝐴 200 𝐴𝐴s (Wu et al., 2021). The leader velocity of a strong stroke observed directly by the optical system LAPOS
analyzed in Section 3.3 is about 5 ×106 m/s. If we use these values as the leader velocity, the return stroke
velocity would be 5.8 to 9.6 ×107 m/s, which is slightly smaller than return stroke velocities reported in theliterature (Rakov, 2007). However, considering the fact that the velocity decreases as the return stroke wave
propagates and that most previous measurements mainly focused on velocities at the channel bottom, ourresult can be considered as a reasonable result. Moreover, the above estimation assumes a fixed upper end of
the return stroke channel. In reality, the upper end may extend upward during the downward propagationof the negative leader, resulting in an underestimation of the return stroke velocity.
The above estimation demonstrates that the current reflection is a plausible explanation for the small pulses. It also provides supporting evidence that these special strokes are return strokes. The estimation is also
consistent with the assumption of a downward negative leader with the velocity on the order
𝐴𝐴 of 106 m/s
right before these strong strokes. It is also an expected result that strong negative return strokes are usually
preceded by fast negative leaders (Nag & Cummins, 2017; Shi et al., 2019; Zhu et al., 2015).

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Figure 3. (a)–(d) Stroke pulses with different types of reflected pulses. Purple arrows indicate reflection pulses. The red
horizontal line in panel (a) represents the definition of the time difference between the reflection pulse and the onset of𝐴𝐴𝐴𝐴
Δ𝑡𝑡 and the duration of preceding discharges.
the return stroke(Δ𝑡𝑡) (e) Correlation between the time difference

3.3. A Strong Stroke Observed by the LAPOSA negative stroke with a JLDN-reported peak current of −335 kA was observed by the high-speed opticalsystem LAPOS, enabling us to directly estimate the speed of the return stroke and the preceding downward
leader.
E-change waveforms recorded by two sites of the FALMA and light intensity waveforms recorded by the
LAPOS are shown in Figure 4. Time zero corresponds to the first detected pulse indicated by the red arrow
in Figure 4b. All FALMA sites were saturated by this stroke, and Figure 4a shows the least saturated waveform. We can see that the E-change waveform of this stroke is generally similar to those in Figures 1a–1j.
Figure 4b shows the E-change waveform recorded by a site 6 km away. Due to the close distance, we canclearly identify the first pulse. The duration of preceding discharges is about
𝐴𝐴 357 𝐴𝐴s.

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Figure 4. A stroke with a peak current of −335 kA observed by the lightning attachment process observation system(LAPOS) and the fast antenna lightning mapping array (FALMA). E-change waveforms observed by FALMA sites at
distances of (a) 56 km and (b) 6 km. Waveforms of light intensity observed by (c) High-sensitivity photodiodes and (d)Low-sensitivity photodiodes of LAPOS. The red arrow in panel (b) indicates the first identified pulse before the stroke.
𝐴𝐴 of 𝐴𝐴𝐴 and 𝐴𝐴 represent the height and onset time of light pulses observed by the LAPOS.
ValuesFigure 4c shows the light intensity waveform recorded by the high-sensitivity photodiodes of the LAPOS.
Similar waveforms from negative CG strokes were reported by Wang et al. (2015). We can see that the firstclear light signal was detected by channel 6 at the height of 1,205 m and at the time of 𝐴𝐴156.1 𝐴𝐴s. Note thatthe time corresponds to the onset of the pulse and is calculated as the time of 20% pulse peak (Huang
et al., 2020). It is likely that before the time of𝐴𝐴156.1 𝐴𝐴s, the leader was propagating inside the thundercloud
and thus could not be detected by the LAPOS. The second light pulse was detected by channel 4 at theheight of 912 m at the time of 𝐴𝐴228.1 𝐴𝐴s and the third by channel 2 at the height of 624 m and at the timeof 𝐴𝐴
281.7 𝐴𝐴s. Variations of the height and time of these channels indicate a leader propagating downward,
and no other light signals detected before these signals indicate that this flash started from inside the cloud.
From channels 6 and 4, we can estimate that the speed of the downward leader is 4.1 ×106 m/s, and fromchannels 4 and 2, the estimated speed is 5.4 ×106 m/s. These values are one order of magnitude larger than
the typical speed of stepped leaders before first return strokes in negative CG flashes but are similar tothe speed of downward negative leaders preceding compact return strokes in winter thunderstorms (Wuet al., 2021).
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Figure 4d shows the light intensity waveform recorded by the low-sensitivity photodiodes of the LAPOS.
These light signals mainly correspond to the return stroke process. Two large pulses were detected by channels 1 and 5, from which we can estimate that the speed of the return stroke was 1.5 ×108 m/s, a typicalreturn stroke speed. Channel 7 also detected a pulse but it was relatively weak and had large noise, probably
because the return stroke wave already developed into the cloud. That channel 3 did not have any response
may be due to a malfunction of the channel.
Assuming the light intensity recorded by the LAPOS is proportional to the peak current (Idone & Orville, 1985; Wang et al., 2005), we can roughly estimate the peak currents of the stepping processes of thedownward negative leader. In Figures 4a and 4d, a leader pulse is observed by Channel 1 indicated by the
red dashed line. The ratio of the light intensity of the leader pulse peak to the return stroke pulse peakmeasured by the same channel is 0.12. Neglecting current attenuation with height, we can estimate thatthe peak current of the leader pulse is about −40 kA. Peak currents of other leader pulses can be estimatedfrom E-change magnitudes in Figure 4a by calculating the ratio to the E-change magnitude of the −40-kA
leader pulse indicated by the red dashed line, assuming that the peak current of the leader pulse is proportional to the E-change magnitude of its radiation electric field. Peak currents of four pulses of leader steps
are estimated as shown in Figure 4a. The peak currents range from −5 to −40 kA. These values are one totwo orders of magnitude larger than peak currents of downward stepped leaders inferred from magnetic
or electric field observations (Shen et al., 2019; Thomson et al., 1985; Williams & Brook, 1963) and upward
negative leaders in triggered lightning or from tall objects (Miki et al., 2014; Pu et al., 2017; Watanabe
et al., 2019; Zhou et al., 2012). In fact, strong strokes analyzed in this study are almost always preceded byclear leader pulses as shown in Figures 1a–1j, indicating strong peak currents of stepped leaders. A strong
negative stroke reported by Lyu et al. (2015) was also preceded by large leader pulses. We speculate thatstrong negative strokes may be usually preceded by strong leader pulses.
The above analysis demonstrated unambiguously that the strong stroke in Figure 4 was preceded by a fast
downward negative leader with large currents. The negative leader connected to the ground and initiatedthe return stroke, which produced the exceptionally strong pulse.

4. Discussion and Conclusion
In this study, we presented conclusive evidence that strong negative return strokes with peak currents larger
than 150 kA in winter thunderstorms in Japan produce unusual E-change waveforms that are generally
different from those produced by first return strokes in normal negative CG flashes. As lightning location
systems such as NLDN in the US (e.g., Cummins & Murphy, 2009) and JLDN in Japan classify CG flashes
based on waveform characteristics of return strokes, it is possible that some of these strongest negative return strokes in winter are not identified by these systems. In fact, the two strong strokes with peak currentsof −260 kA and −197 kA reported to be associated with downward TGFs were identified as intracloud discharges by JLDN (Wada et al., 2020). The fact that few studies have investigated strong negative CG flashesor return strokes in winter may be simply due to the uncertainty in identifying strong negative strokes in
winter.
The finding that the unusual pulses such as those in Figures 1a–1j are produced by return strokes makes
it doubtful that whether they are directly associated with TGFs (Wada et al., 2020). It is well known that
TGFs are usually associated with upward negative leaders in intracloud flashes (Cummer et al., 2015; Lu
et al., 2010). Pu et al. (2020) also reported a downward TGF produced during a downward negative leader
preceding a negative return stroke. We have demonstrated that these strong negative return strokes arepreceded by a fast and powerful downward negative leader. Therefore, it is possible that it is actually thestrong leader pulses right before negative return strokes, rather than the return strokes themselves, that areassociated with TGFs.
The duration of the preceding leader pulses is closely related to the properties of the stroke pulse as demonstrated in Figure 2. Wu et al. (2021) recently reported that strokes with durations of preceding discharges
shorter than𝐴𝐴 200 𝐴𝐴s mostly occurred in mountain areas and were inferred to have very short (on average
300 m) channels and were named “compact strokes.” Wu et al. (2021) demonstrated that LBEs reported
by Wu et al. (2014) were also compact strokes. Lyu et al. (2021) recently reported a type of strong pulses
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that were located in the high mountains of the western US in summer. Those pulses share the same feature as pulses of strong strokes analyzed in this study: some small pulses with durations of a few hundred
microseconds followed by a large pulse. Lyu et al. (2021) speculated that those pulses were produced byterrain-initiated upward positive leaders, which might be wrong if the pulses had the same nature as thoseanalyzed in this study.
Finally, we point out that these strong negative strokes are likely “superbolts” observed by the WWLLN(Holzworth et al., 2019) and from the space (Ripoll et al., 2021). Holzworth et al. (2019) reported that mostsuperbolts observed by the WWLLN occurred in winter, and the sea of Japan is one of the hotspots. Further,
Holzworth et al. (2019) examined superbolts (with energy larger𝐴𝐴than 106 J) that were also observed by the
ENTLN (Mallick et al., 2015), and they found that out of 18 matches, 14 were negative strokes with peakcurrents larger than 100 kA. Ripoll et al. (2021) reported that only 15% superbolts were positive CG flashes,compared with 28% of positive CG flashes during their observation. Ripoll et al. (2021) also found that compared with normal strokes, waveforms produced by superbolts have larger rise time and smaller fall time,“leading to a more symmetric ground wave.” These characteristics are all consistent with the characteristics
of strong negative strokes analyzed in this study. Lyu et al. (2021) also suggested that similar pulses detected
in high mountains in the US may be related to superbolts.
Strong negative strokes reported in this study are largely unknown in the lightning research community.
Now that we have demonstrated that they are undoubtedly returned strokes in negative CG flashes, more
efforts should be made to clarify their physical mechanisms, conditions for their productions, and relationship with other lightning discharges and high-energy phenomena. It is also necessary to modify criteria
employed by lightning location systems to classify return strokes.

Data Availability Statement
Related data can be found at https://doi.org/10.5281/zenodo.5150405.

Acknowledgments
This study was supported by the Ministry of Education, Culture, Sports, Science, and Technology of Japan (Grants
20H02129 and 21K03681).

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