NH1.6

The atmospheric electric potential gradient (PG, the reverse of the atmospheric vertical electric field) is commonly measured near the ground. The PG plays a pivotal role in studying the global electric circuit (GEC) which comprises all large scale quasi-static electrical processes occurring in between the Earth's surface and the lower ionosphere [1]. Therefore, long-term, coherent PG measurements are of high importance in atmospheric electricity research. Nevertheless, it is a challenging task to use PG as a reliable diagnostic tool for investigating global changes in Earth’s electromagnetic environment because of its high variability.

There are few PG datasets around the globe which are long enough and have been recorded continuously for decades. One of the datasets that fulfil these requirements has been recorded in the Széchenyi István Geophysical Observatory, Nagycenk, Hungary, Central Europe (NCK, 47°38’ N, 16°43’ E). A necessary correction of the recorded PG time series due to the time-dependent shielding effect of nearby trees at NCK was introduced earlier [2,3]. In this study, the corrected long-term (1962-2009) variation of PG at NCK is exhibited and discussed.

In the present study, the behaviour of annual minima, maxima, means, and summer and winter means of the PG at NCK are investigated. As these PG time-series exhibited quite different characteristics, the joint analysis of these data is required. The long-term variation of these PG time series can be divided into three periods: the first period (1962-1985) is characterized by a rather steep increase and is mostly driven by the wintertime data. The increase continues with a moderate magnitude and less significantly in the second period (1986-1997) where summertime data dominate the change, whereas there is a pronounced reduction of the PG in the third period (1997-2009) with almost equal magnitude in both the winter- and summertime records. These observed trends are confirmed by independent PG observations made at other measuring sites (e.g., the Swider Observatory, Poland).

The PG at NCK is generally greater in winter than in summer, which is a well-known phenomenon at northern hemisphere continental stations [4]. The annual minima, however, do not comply with this trend in every year. The month with the lowest average PG is in late spring (May) in most years of the examined epoch at NCK but minimum values occur in autumn and winter months as well.

References:

[1] Rycroft, M. J., Israelsson, S., and Price, C.: The global atmospheric electric circuit, solar activity and climate change, J. Atmos. Sol-Terr. Phy., 62, 1563–1576, 2000.

[2] Buzás, A., Barta, V., Steinbach, P., and Bór, J.: Impact of local environmental conditions on atmospheric electrical potential gradient measurements, Geophysical Research Abstracts, 19, EGU2017-1193-1, 2017.

[3] Buzás, A., Horváth, T., Barta, V., and Bór, J.: Revisiting the decreasing trend of atmospheric electrical potential gradient measured in Central Europe at Nagycenk, Hungary, Geophysical Research Abstracts, 20, EGU2018-6723, 2018.

[4] Chalmers, J. A.: Atmospheric Electricity, second edition, Pergamon Press, London, pp. 168-169 1967.

How to cite: Buzas, A., Barta, V., Bór, J., and Horváth, T.: Investigating the long-term variation of atmospheric electric potential gradient at Nagycenk, Hungary, Central Europe, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5761, https://doi.org/10.5194/egusphere-egu21-5761, 2021.

The response of tall structures such as towers to the electrical atmosphere is well known, but much has to be learned about how the rotation of wind turbine blades affects the electrical response of wind turbines. To better understand current induction and the appearance of point/corona discharge from wind turbine blades, a series of experiments lifting vertical wires with drones under fair weather conditions have been conducted. During the experiments, the length of the wire (vertical) and its vertical velocity were recorded using the drone’s telemetry. Additionally, the wire was grounded through a pico-ammeter to measure current induction and a corona discharge detector, based on a wideband current measurement coil, was placed close to the tip of the lifting wire to detect possible point/corona discharge appearance at the wire tip.

Preliminary tests included testing the sensor in the laboratory by measuring artificially generated corona pulses, to verify that pulses from this sensor registered on the field could be attributed to point/corona discharge phenomena. Measured amplitude for this induced current was on the order of hundreds of nano-amps.

For these experiments, an insulated copper wire with  0.14Ω/m resistance and with the top tip exposed to the environment was deployed using two different tips, a rounded tip of 1mm radius and a sharp needle tip of 0.1mm radius. The electric field at the ground level was measured using an electric field mill. All flights were performed during the morning and the ground electric field amplitude ranged from 50V/m to 200V/m.

When using rounded tips, corona discharge was not detected by the coil, but an induced current proportional to the vertical speed of the wire was measured. This component of the current is interpreted as a change of potential in time, and the amplitude of these induced currents is on the order of tens of nano-amps.

Results when using the sharp tip showed two clear sources of induced currents on the wire, vertical speed (as in the rounded case) and corona discharge. When using the sharp tip, corona discharge was detected when the wire reached around 50 m and induced current amplitude increased with altitude. A pulsating current was measured by the coil sensor indicating the existence of corona discharge on the wire.

The rate of decrease of the measured currents after reaching steady positions of the wires might be attributed to the screening effect of the released charge.

These experiments proved that key factors for the current induction on wind turbine blades include the change in height at a certain speed, along with the occurrence of point/corona discharges with the radius of curvature of the blade tips. Under the effects of electrified thunderclouds, the magnitudes of the currents could reach several orders of magnitude.

How to cite: Fontanes Molina, P., Arcanjo, M., Montanyà Puig, J., and Guerra-Garcia, C.: Deploying vertical wires with drones to study wind turbine electrification under fair weather, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4556, https://doi.org/10.5194/egusphere-egu21-4556, 2021.

We present here the results of a study on the changes of PM 2.5 and atmospheric electric field (Potential Gradient, PG) during COVID-19 measures implemented at Xanthi in comparison with the 2019 measurements according to 10 classes of circulation weather types (CWT). There are two study periods. The first period was from February to May of both 2019 (no lockdown measures were implemented) and 2020 (under lockdown), and the second period was from September to December of both 2019 (no lockdown) and 2020 (lockdown). For both study periods of 2020, Xanthi was subjected to additional measures, such as curfew. Specifically, from 01/04/2020 to 27/04/2020 from 20:00 to 08:00 and from 13/11/2020 to 31/12/2020 from 21:00 to 05:00. These periods were selected according the two lockdown periods of 2020 at Xanthi and the same periods were selected for the previous year. PM 2.5 was measured in two different locations, one in the city center of Xanthi and the other is located at a semirural location approximately 2 kilometers from the city center, where also PG was measured. We present results in comparison with mean PM 2.5 and mean PG per circulation weather type on no lockdown and lockdown periods of 2019 and 2020 respectively, at Xanthi. There were changes on mean PM 2.5 and mean PG per circulation weather type between the two years. A moderate decrease of PM 2.5 per CWT between the two periods of lockdown on 2020 due to COVID-19 measures and the same periods for 2019 is observed when there was neither lockdown nor curfew. Fluctuations and a variability on mean PG per CWT are also observed between the two years. We acknowledge support of this work by the project “PANhellenic infrastructure for Atmospheric Composition and climatE change” (MIS 5021516) which is implemented under the Action “Reinforcement of the Research and Innovation Infrastructure”, funded by the Operational Programme "Competitiveness, Entrepreneurship and Innovation" (NSRF 2014-2020) and co-financed by Greece and the European Union (European Regional Development Fund).

How to cite: Karagioras, A., Nita, A., and Stavroulas, I.: Changes on atmospheric electric field and PM 2.5 during the COVID-19 measures at Xanthi on 2020 compared to the 2019 measurements and depending on the circulation weather types, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8930, https://doi.org/10.5194/egusphere-egu21-8930, 2021.

Results of the study of the impact of sporadic sources of disturbances on the state of the atmospheric electric field at the high-mountain Tien Shan station (3340 m above sea level, 20 km from Almaty) are presented. The absence of unitary variation (Carnegie curve) is the characteristic feature of diurnal changes in the atmospheric electric field under good weather conditions.

The most geoeffective sporadic sources of disturbances in the near-Earth space and the Earth's atmosphere are giant coronal mass ejections (CME), accompanied by Forbush effects in the intensity of galactic cosmic rays and by geomagnetic storms. Our studies were carried out taking into account the peculiarities of CME manifestations in the atmosphere and magnetosphere of the Earth. It was found that large magnetic storms affect the average level of the atmospheric electric field (increasing or decreasing it due to a change in the rigidity of the geomagnetic cutoff Rc), and also cause its fluctuations in the minute range (10^-3 ÷ 10^-2) Hz. A significant decrease in the atmospheric electric field after CME is due to large Forbush effects during weak geomagnetic disturbances.

Anomalous changes in the atmospheric electric field on the eve and during earthquakes were recorded, which are unambiguously associated with the activation of seismic processes. Since the city of Almaty is surrounded by a number of potential sources of strong earthquakes, the problem of their prediction is actual for the city and its environs.

How to cite: Antonova, V., Kryukov, S., Lutsenko, V., and Malimbayev, A.: Impact sporadic sources of disturbances on the atmospheric electric field оn Tien- Shan, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7250, https://doi.org/10.5194/egusphere-egu21-7250, 2021.

High energy cosmic rays of galactic and solar origin, natural radioactivity, lighting in thunderstorms and electrified shower clouds, produce ion clusters and charge the whole atmosphere causing a ubiquitous potential difference between the ionosphere and the surface. This Global Electric Circuit (GEC) allows the flow of charges to the surface in the fair-weather regions of the globe. Here, we simulate the effect of highly energetic particle radiation, in particular the 774 AD solar proton event, on the GEC with the aid of the global circulation model EMAC/MESSy. The simulations assume pre-industrial atmospheric conditions and the coupling of aerosol and atmospheric electricity schemes allows for ion-ion and ion-aerosol capture reactions. We discuss effects in fair weather current and atmospheric conductivity at different latitudinal bands.

How to cite: Misios, S., F. Knudsen, M., and Karoff, C.: Simulating effects of the 774 AD solar proton event on atmospheric electricity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13496, https://doi.org/10.5194/egusphere-egu21-13496, 2021.

During the formation of thunderclouds, simultaneous macrophysical and microphysical processes cause the separation of charges inside the cloud, forming the electrical structure of storm clouds. As a result of that, the electric field at the ground level can change significantly. Irregularities on the surfaces of grounded structures can provide conditions for corona discharges that generate ions and form a space charge layer at ground level.

In this work, we investigate the features of corona point discharges from grounded conductive rods installed in three different sites. In all of them, we measured current along the grounded rod under high background electric field conditions or during its fast changes caused by lightning strikes. The current signals reveal pulses with a fast rise time (tens of nanoseconds) and slow decay (hundreds of nanoseconds), with polarity compatible with the background electric field. Comparing laboratory experiments with the results in the field, we observed that positive discharges required a lower electric field threshold than negative discharges. Their pulse frequency is also equivalent to one-tenth of the pulse frequency of negative discharges, for a similar electric field level.

In one of the sites, one current sensor coupled to a grounded rod, 1.5 m above a roof, was installed in a site located at an altitude of 2525 m, near a ski-station. We observed a large number of events, and we were able to correlate the frequency of the pulses with the electric field, as well as evaluate the effect of the wind on the discharges. In the other two sites, the rods were placed near the ground and on the roof of a conventional building. Pulses were registered on some occasions when there was lightning activity nearby, either before or after lightning events. Previous works on this topic correlate the electric field with the average current flow, and on this work, we evaluate the pulse frequency and electric field. This investigation is relevant for understanding the production of corona and space charges from high structures.

How to cite: Arcanjo, M., Montanyà, J., Lorenzo, V., and Pineda, N.: Corona point discharges from grounded rods under high background electric field, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9622, https://doi.org/10.5194/egusphere-egu21-9622, 2021.

Lightning interaction with aircraft becomes an increasingly important research topic due to the recent advancements in aviation industry and pressure put on it from climate regulations. A flying aircraft can interact with three known types of discharges, (i) aircraft-intercepted lightning, (ii) aircraft-triggered lightning, and (iii) electrostatic discharges.

Aircraft-intercepted lightning is a remotely initiated lightning discharge that attaches to the aircraft. Such events are relatively rare and constitute only a few percent of all lightning strikes to aircraft.

Aircraft-triggered discharge is a lightning that is initiated from the aircraft. These events happen when the aircraft polarizes and enhances the ambient electric field to the magnitude sufficiently high for triggering a bi-directional leader from opposite sharp extremities. More than 95% of all lightning strikes to aircraft are of this type.

Electrostatic (EST) discharges can be considered as another type of aircraft discharges but excluded from the above statistics because they can happen without presence of a thundercloud, lightning or strong ambient electric field. They happen when the aircraft collects significant charge on its surface by collisions with ice particles. Such discharges are usually associated with a noise in analog radiocommunication. It is not completely clear if the surface charge is necessary or the EST discharges can be initiated only by polarization of the aircraft in ambient electric field. Remarkably, EST discharges have been reported in association with positron annihilation signatures inside a thundercloud [1].

            In this work we report yet another type of discharge that was recently observed developing from an airplane. These discharges start from the aircraft in response to the electric field change caused by a nearby lightning flash. The remote lightning flash can redistribute the ambient electric field in such a way that the local electric field near the aircraft exceeds the breakdown threshold. We call them a “lightning-triggered discharge”.  Similar initiation mechanism is proposed for the high-altitude sprite discharges.

The lightning-triggered discharges from aircraft have not been studied and characterized before. They were observed and reported in [2] but were not identified as a separate type of aircraft discharges. As will be demonstrated, they are very likely to be underreported by pilots due to their often attachment to wings and relatively low current.

            The Remote Lightning Damage Assessment System (www.reldas.no) has been developed with the purpose to identify and systematically collect data on lightning strikes to aircraft. Besides other discharges, the system collected records of the aircraft-triggered discharges and their characteristics. Examples of such discharges will be shown with the photographs and current measurements.

References

1. Kochkin, P., et al. "In‐flight observation of positron annihilation by ILDAS." Journal of Geophysical Research: Atmospheres123.15 (2018): 8074-8090

2. Kochkin, P., et al. "In‐flight observation of gamma ray glows by ILDAS." Journal of Geophysical Research: Atmospheres122.23 (2017): 12-801.

How to cite: Kochkin, P.: New type of electric discharges from aircraft., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10916, https://doi.org/10.5194/egusphere-egu21-10916, 2021.

Recently, Hare et al. 2020 found that individual leaders steps could be imaged in the VHF band, and for leaders below 5 km altitude, the radio emission from each step is mostly consistent with a point-source. We will report on new observations of negative leaders above 7 km altitude that behave significantly differently than lower altitude leaders. These higher- altitude leaders are a few 100 meters wide and have step lengths a few 100 meters long, as opposed to lower altitude leaders that are at most 10 meters wide with 10 meter stepping lengths. Furthermore, unlike lower altitude leaders, the radio emission from individual steps of higher altitude shows extensive structure. Each step shows a burst of radio radiation, followed by the growth of multiple filamentary structures. The nature of these filaments is presently unclear, but they could be long streamers or leader branches. We have observed one leader that clearly starts at low altitude and propagates to higher altitude. This leader shows that the transition from the low altitude mode of propagation to the higher altitude mode does not occur smoothly as one may expect, but occurs abruptly at around 6 km altitude within only one kilometer, somewhat similarly to a phase change.

Previous work has measured 100 m long stepping lengths of higher altitude leaders, and it is often assumed that this is a simple pressure scaling effect. However, our data shows that the stepping process at lower altitudes and higher altitudes appears very differently in VHF, and that the transition between the two modes occurs rapidly. This implies higher and lower altitude leaders actually have different propagation modes, and are not merely pressure-scaled versions of each other.

We will also present new detailed VHF measurements of needle activity. We will show that needle twinkles have a wide range of propagation speeds, from 10⁵ to 10⁷ m/s, and that needle twinkles sometimes show stepping behavior, which strongly implies that needle twinkles can propagate similar to stepped leaders or dart leaders depending on the conductivity of the needle. We will also show that recoil leaders can quench needle activity, which leads to a cycle of increasing needle activity followed by quenching by a recoil leader, as originally predicted by Hare et al. 2019.

How to cite: Hare, B., Scholten, O., Dwyer, J., Ningyu, L., and Strepka, C. and the LOFAR CR KSP: Needle properties and a new higher altitude negative leader structure; observations by the LOFAR radio telescope, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14435, https://doi.org/10.5194/egusphere-egu21-14435, 2021.

We report on recent observations made by the LOFAR radio-telescope of a fast propagation mode in negative leaders we named Rapid Negative Leader (RNL).
The RNL has a variety of properties that make them clearly distinct from negative leaders or dart leaders, such as -- fast propagation, -- emission of strong broad-band pulses, -- emission of very high VHF power, -- a reduced density of located sources, and -- terminating with the spawning of a large number of negative leaders in a small area. RNLs are almost always observed in the initial stage of a lightning flash, but may also occur much later. They may occur repeatedly in a certain part of the cloud.

We interpret a RNL as negative leader developing in strong electric field due to a relatively small highly-charged cloud, probably created by a local turbulence, with a typical size of order 5 km².  The strong field will lead to a larger than usual charge at the leader tip resulting in an increased propagation velocity as well as a strongly enhanced emission of VHF power.
Since for the initiation of a lightning flash strong ambient electric fields are required, it is thus no surprise that the initial leader is in fact a RNL. 

How to cite: Scholten, O. and the LOFAR Lightning and Cosmic ray group: Observations by the LOFAR radio telescope of a fast negative leader propagation mode, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10876, https://doi.org/10.5194/egusphere-egu21-10876, 2021.

We present recent progress in pulsed discharge modeling in Amsterdam that is motivated by high voltage and plasma engineering and by lightning.

We perform streamer simulations with adaptive mesh refinement in 2D and 3D using PIC particle models and fluid models, where we now can include complex electrode shapes and dielectric boundaries. For the longer time evolution, we also have added Ohmic heating, gas expansion, and the relevant ­­­plasma chemistry for air and methane-air mixtures.

Results relevant for lightning physics include

• Validation and verification of streamer propagation models (with S. Dijcks and S. Nijdam for the experimental counterpart)
• Simulations of streamer branching and comparison with experiments
• Parameter studies for long non-branching streamers that can accelerate or decelerate, and vary largely in velocity, radius and inner electron density, depending on the electric field
• Different stagnation behavior of positive and negative streamers in low electric fields
• Positive streamers in air that can continue to propagate as isolated patches of positive charge, without a conducting channel behind the streamer head
• Repetitive discharges, heating, and plasma-chemistry

How to cite: Ebert, U., Bouwman, D., Francisco, H., Guo, B., Li, X., Malla, H., Martinez, A., and Teunissen, J.: Progress in modeling streamer and leader discharges­­­, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15358, https://doi.org/10.5194/egusphere-egu21-15358, 2021.

We report results from imaging the initiation region of lightning via 3D interferometric beamforming on data collected by the Netherlands-based core of the Low Frequency Array of Antennas (LOFAR). LOFAR achieves 1 nanosecond timing accuracy and meter-scale spatial precision in lightning imaging on pulses observed in the 30-80 MHz band via the 38 Dutch-based stations. This project complements and enhances the previous work of the LOFAR lightning group of Groningen [Hare, B.M., et al., Nature 568, 360363 (2019)], and [Scholten, O., et al., ESSOAr 10503153] in order to improve image detail in regions with weak sources. This project incorporates beamforming techniques to improve upon previously employed methods with the result of improving both spatial and time resolution of lightning sources. In doing so, we have located and imaged the first non-impulsive sources in lightning flashes. These sources are believed to be caused by a streamer-cascade-like initiation event leading to the formation of the first leader in two separate lightning flashes. The initiation starts from essentially background and within a tens of microseconds ramps up a few orders of magnitude before the first impulsive sources connected with lightning leaders are observed. The events are likely an analog of fast breakdown in narrow bipolar events, and here we report their ramp-up rate, propagation speed, and trajectories.

How to cite: Sterpka, C., Dwyer, J., Liu, N., Hare, B., and Scholten, O.: The Impulsive Nature of Lightning Initiation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13711, https://doi.org/10.5194/egusphere-egu21-13711, 2021.

The accurate determination of parameters of electric streamer propagation in air, such as their velocity, transverse size (radius) and the maximum field at the tip, is extremely important, e.g., for the studies of further lightning development and acceleration of electrons at the tip, which may lead to generation of x-rays. Relations between these parameters produce a family of streamer-shaped solutions, while the radius remains undetermined. We hypothesize that all these solutions are, in fact, valid solutions of hydrodynamic equations, but the physical radius emerges when one solution is selected by the condition of being maximally unstable, i.e., having the highest velocity.

Direct verification of this hypothesis by hydrodynamic simulations is complicated by the fact that the streamer length is one of the background conditions which determine its parameters, and in a propagating streamer the length is constantly changing. To circumvent this, we simulate a `steady-state' streamer, such that its length is kept constant by synchronizing the motion of the electrode to which it is attached. We show that the predicted maximally-unstable selected solution does, in fact, emerge in the infinite time limit of the simulation. We note, however, that we were yet unable to test the first part of the hypothesis, i.e. to produce the non-selected solutions in the predicted family, as they are quickly replaced by the selected one.

We present the calculated streamer parameter dependence on external uniform field and streamer length for an isolated streamer and streamers propagating parallel to each other. In the latter case, the field of neighboring streamers makes the streamer propagation independent of its length when it exceeds the inter-streamer distance. We draw parallels of this situation to the selected solution for a viscous Saffman-Taylor finger of infinite length in a narrow channel [Luque et al, 2008, doi:10.1103/PhysRevE.78.016206].

The practical interest of this work lies in reducing the complexity of streamer propagation modeling, by avoiding detailed simulation of the streamer head, if we can calculate the parameters by a simpler algorithm.

How to cite: Lehtinen, N.: Emergence of transverse size in electric streamers, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5792, https://doi.org/10.5194/egusphere-egu21-5792, 2021.

Q-bursts are signatures of exceptionally powerful lightning strokes which produce intense radio waves typically in the extremely low frequency band (ELF, 3Hz-3kHz). Due to the finite conductivity of the Earth’s surface, radio waves in this frequency range can be also detected in greater depths. While the penetration of electromagnetic (EM) waves in a conducting half space has been investigated and utilized, e.g., under water for submarine radio communication, very few field measurements consider the subsurface detection of ELF waves in the continental crust.

In this work, Q-bursts recorded in near surface and corresponding underground ELF band observations are compared in order to characterize the frequency dependent effect of the upper section of the Earth’s crust on the spectrum of the Q-burst signals.

Practically co-located, but not simultaneous quasi-surface and underground temporal ELF band magnetic field measurements were made near Mátraszentimre, in the Mátra Mountains, Hungary. The underground measurement was carried out inside a mine shaft in the Matra Gravitational and Geophysical Laboratory (MGGL) at a depth of 140 m. ELF observations from two permanent recording stations in the Széchenyi István Geophysical Observatory (NCK, Hungary) and in Hylaty (HYL, Poland), less than 250 km away from MGGL, were involved in the analysis to deduce the transfer function between the unsynchronized quasi-surface and underground measurements in the Mátra.

The set of Q-bursts, which were parallelly detected at all three locations, was identified using GPS synchronized time stamps. Natural origin of the signals was confirmed by identifying the parent lightning strokes in the database of the World Wide Lightning Location Network (WWLLN) via matching the detection times and the corresponding source directions calculated at NCK station.

The good agreement of the results from independent Matra-NCK (5-30 Hz) and Matra-HYL (5-140 Hz) station-pairwise analyses confirm that the frequency dependence of the wave attenuation due to overlying rocks is exponential. The deduced integrated local conductivity, 30-40 S/m, of the upper section of the Earth’s crust suggests that probably the soil has prominent role in attenuating ground penetrating EM waves in the ELF band.

How to cite: Bór, J., Lemperger, I., Szabóné André, K., Bozóki, T., Mlynarczyk, J., Steinbach, P., Lévai, P., and Ván, P.: Comparison of Q-bursts detected near the Earth’s surface and 140 m below ground level, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4443, https://doi.org/10.5194/egusphere-egu21-4443, 2021.

Below 100 Hz, in the lowest part of the extremely low frequency (ELF, 3 Hz - 3 kHz) band lightning-radiated electromagnetic waves propagate with extremely low attenuation (roughly below 1 dB/Mm) within the Earth-ionosphere waveguide which makes possible the formation of global electromagnetic resonances, known as Schumann resonances (SRs). The most commonly used description of this resonance field assumes a uniform Earth-ionosphere cavity, i.e. that the propagation conditions for ELF waves are practically the same on the dayside and nightside hemispheres, which is the most vulnerable simplification of these models. 

In this work we present two different forward models for SRs that take into consideration the day-night asymmetry of the Earth-ionosphere cavity and are based on the analytical and numerical solutions of the two-dimensional telegraph equation (TDTE). We present numerical tests showing that the two models produce practically the same output, i.e. the relative difference between them is less than 0.4%. The conspicuous conformity between the outputs establishes not only the correctness of the formalisms but the correctness of the implementations (the coding) as well. To the best of the authors’ knowledge this is the first work that verifies this conformity between the two independent solutions. 

We also compare our stationary models with time-dependent solutions of the TDTE as the stationarity of the resonance field may represent the next most vulnerable simplification that needs to be dismissed to approach a more realistic theoretical description of SRs. All these steps in model development serve our aim to infer global lightning activity based on multi-station ELF measurements by applying a sophisticated inversion algorithm.

How to cite: Bozoki, T., Pracser, E., Satori, G., Kulak, A., Mlynarczyk, J., and Williams, E.: Modeling ELF waves in the non-uniform Earth-ionosphere cavity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3973, https://doi.org/10.5194/egusphere-egu21-3973, 2021.

Although lightning discharge is not the only source or only physical phenomenon that affects the Schumann resonances, they have the highest contribution to the Schumann resonances oscillating between the ground the ionosphere. Schumann resonances are predicted through several different numerical models such as the transmission-line matrix model or partially uniform knee model. This contribution reports a different prediction method for Schumann resonances derived from the first principle of fundamental physics combining both particle radiation patterns and the mathematical concept of the Golden ratio. This prediction allows the physical understanding of where Schumann resonances originate from radiation emitted by a particle that involves many frequencies that are not related to Schumann resonances. In addition, this method allows predicting the wave propagation direction of each frequency value in the Schumann frequency spectrum. Particles accelerated by lightning leader tip electric fields are capable of contributing most of the Schumann resonances. The radiation pattern of a single particle consists of many frequencies. There are only specific ones within this pattern that contribute to the Schumann radiation. The vast majority of Schumann resonances distribute quite nicely obeying the Golden ratio interval. This property, used in conjunction with the full single-particle radiation patterns, also revealed that high-frequency forward-backward peaking radiation patterns, as well as low-frequency radiation patterns, can contribute to Schumann resonances. This method allows to locate them on the full radiation pattern. A theoretical analysis using the Golden ratio spiral, predict that there are more Schumann resonances in the high-frequency forward-backward peaking radiation pattern of a relativistic particle than low-frequency dipole radiation pattern. Extending the idea to an octave that identifies the identical sounding notes with different frequencies in standing waves. By knowing the value of the initial Schumann resonant frequency, this method allows us to predict the magnitude of other Schumann resonances on the radiation pattern of a single accelerated charged particle conveniently. In addition, it also allows us to find and match Schumann resonances that are on the same radiation lobe, which is named electromagnetic Schumann octaves. Furthermore, it is important to find Schumann octaves as they propagate in the same direction and have a higher likelihood of wave interference.

How to cite: Yucemoz, M.: Locating Schumann Resonant Frequencies on a Single Particle Radiation Patterns Using Golden Ratio Spiral and Octave Relationship of Schumann Points., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9927, https://doi.org/10.5194/egusphere-egu21-9927, 2021.

Lightning location networks commonly use the time stamps of the received radio waveforms from lightning flashes (sferics) at numerous stations to determine a single lightning flash location. In order to extract more information from the recorded lightning waveforms, a complex analysis of the received electromagnetic waves is investigated.  

The long term aim of this study is to use the complex radio waveforms from lightning flashes (complex sferics) to develop an interferometry technique that uses complex parameters inferred from each individual sample (Liu et al., 2018). In this work, the coherency of complex lightning sferics is investigated, which is a measurement of the phase information. Complex lightning sferics are not well understood such that an analysis strategy needs to be developed. We adapt here the idea of using a waveform bank (Said et al., 2010), which is composed of lightning sferics at predefined distances. Before generating the waveform bank, a rigorous quality check process is carried out to ensure a high data reliability for the selected lightning events. Both, the amplitude waveform bank and the coherency waveform bank are generated. The novelty of this work is that the coherency measurement is shown to be as valid as the amplitude measurement towards the characterisation of the received lightning waveforms at various distances. In particular, the coherency has the capability to detect more skywaves than the amplitude alone.  

The potential impact of this research for lightning detection and location networks is that this novel method is able to locate a single event in an area defined by the coherency distribution map when different combinations of the waveform time stamps are used (Füllekrug et al., 2016). In future work, the coherency distribution map of a single lightning event could be calculated, and a dynamic coherency map can be built on top of that. The dynamic coherency map is capable to reveal formation on propagation track of each storm, and may possibly be used for lightning forecasting.

References:

Füllekrug, M., Liu, Z., Koh, K., Mezentsev, A., Pedeboy, S., Soula, S., Enno, S.E., Sugier, J., and Rycroft, M.J. (2016), Mapping lightning in the sky with a mini array, Geophys. Res. Lett., 43, 10,448–10,454, doi:10.1002/2016GL070737.

Liu, Z., Koh, K.L., Mezentsev, A., Enno, S.E., Sugier, J., and Fullekrug, M. (2018), 

Lightning sferics: Analysis of the instantaneous phase and frequency inferred from complex waveforms. Radio Science, 53, 448– 457, doi:10.1002/2017RS006451.

Said, R.K., Inan, U.S., and Cummins, K.L. (2010), Long‐range lightning geolocation using a VLF radio atmospheric waveform bank, J. Geophys. Res., 115, D23108, doi:10.1029/2010JD013863.

How to cite: Bai, X. and Fullekrug, M.: Lightning Sferics for Lightning Location – Complex Analysis, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5547, https://doi.org/10.5194/egusphere-egu21-5547, 2021.

Tropical cyclones have been observed in recent years to be increasing in intensity due to global warming, and projections for the future are for further shifts to stronger tropical cyclones, while the changes in the number of storms is less certain in the future.  These storms have been shown to exhibit strong lightning activity in the eyewall and rainbands, and some studies (Price et al., 2009) showed that the lightning activity peaks before the maximum intensity of the tropical cyclones.  Now we have investigated the impact of these tropical storms on the upper tropospheric water vapor (UTWV) content.  Using the ERA5 reanalysis product from the ECMWF center, together with lightning data from the ENTLN network, we show that the lightning activity in tropical cyclones is closely linked to the increase in UTWV above these storms.  We find the maximum enhancement in UTWV occurs between the 100-300 mb pressure levels, with a lag of 0-2 days after the peak of the storm intensity (measured by the maximum sustained winds in the eyewall).  The lightning activity peaks before the storm reaches its maximum intensity, as found in previous studies.  The interest in UTWV concentrations is due to the strong positive feedback that exists between the amounts of UTWV and surface global warming.  Water Vapor is a strong greenhouse gas which is most efficient in trapping in longwave radiation emitted from the Earth in the upper troposphere.  Small changes in UTWV over time can result in strong surface warming.  If tropical cyclones increase in intensity in the future, this will likely result in increases in UTWV, reducing the natural cooling ability of the Earth.  Lightning may be a useful tool to monitor these changes.

How to cite: Price, C., Plotnik, T., Guha, A., and Saha`, J.: Tropical cyclone eyewall thunderstorms as a driver of upper tropospheric water vapor increases, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4269, https://doi.org/10.5194/egusphere-egu21-4269, 2021.

The aim of this study is to enhance our understanding about the microphysical structure of convective cloud systems and its relationships to the ambient electrical field, and to assess the capability of a model to capture the cloud electrical properties. This study relies on the EXAEDRE (EXploiting new Atmospheric Electricity Data for Research and the Environment) aircraft campaign that took place from 13 September to 8 October 2018 in Corsica Island. Eight electrified convective systems were successfully sampled during the campaign by the French Falcon 20 aircraft (e.g. RASTA Doppler cloud radar, microphysics probes, electric field mills) and ground-based platforms (Lightning Mapping Array network, Météorage operational lightning locating system and Météo-France weather radars). In this study, a multi-cell thunderstorm which developed over the complex topography of Corsica Island on 17 September 2018 was selected to investigate and to understand the physical processes linking lightning occurrence, electrification efficiency, cloud microphysics and dynamics. The detailed analysis results using the unprecedented airborne and ground-based dataset and their comparison to the numerical simulation results with a horizontal grid spacing of 1 km comprising the explicit electrical scheme CELLS (Cloud Electrification and Lightning Scheme) implemented in the cloud resolving model Meso-NH has been conducted. The key results will be presented at the conference.

How to cite: Lee, K., Defer, E., Combarnous, P., Pinty, J.-P., Buguet, M., Caumont, O., Delanoë, J., Jaffeux, L., Pedeboy, S., Prieur, S., Richard, E., and Schwarzenboeck, A.: Thunderstorms in Corsica Island measured during the EXAEDRE aircraft campaign, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10494, https://doi.org/10.5194/egusphere-egu21-10494, 2021.

From September 13th to October 12th 2018, the EXAEDRE field campaign took place in Corsica, dedicated to the characterisation of thunderstorm clouds and electrical activity. Among a wide range of observation instruments, an array of 4 microphones, arranged on a  30-m wide triangle located near the island eastern coast, recorded over the frequency band 1 to 80 Hz the acoustical signal, or thunder, associated to lightning flashes. The search for  coherent signals between the four sensors within prescribed frequency bands allows to determine the thunder apparent sound velocity and azimuth (PMCC algorithm [1]). Knowing the flash emission time provided by Meteorage low frequency (1-350 kHz) electromagnetic ground lightning locating system, as well as the local speed of sound, it is possible to reconstruct the various positions of coherent sound sources within a single lightning flash. Co-localisation of acoustic sources with in-cloud detections provided by SAETTA high-frequency (60-66 MHz) electromagnetic lightning locating system, and with ground impacts provided by Meteorage, ensures the efficiency and precision of the method. This one was already used successfully in a previous field campaign (HyMeX-SOP1) in Cévennes in 2012 [2,3,4]. The detection algorithm PMCC also provides the various recorded signal intensities. Assuming each sound point source radiates a spherical wave, the different propagation distances between the sources and the recording array can be compensated, so that the thunder source energies can also be localised within the flash with their relative levels. For EXAEDRE, two storms have been studied from an acoustical point of view, one with a low electrical activity on October 2nd mainly over the Mediterranean sea, and one with an intense activity on 17th September mainly overland. A significant number of flash events has been analysed, reconstructed and their energy distribution determined. For the 17th of September,  acoustical events of large amplitudes are well correlated to (mostly negative) Cloud to Ground flash events. Energy localisation indicates a strong heterogeneity of its distribution within the flash, with intense sound sources concentrated inside the return strokes, mostly within the two first kilometres above the ground. Intracloud parts of the flashes appear much less energetic from an acoustical point of view. For the 2nd October, overversea events turn out quite different. [The authors acknowledge the EXAEDRE program, lead by E. Defer, for supplying the data. Present results have been obtained within the frame of the LETMA Contractual Research Laboratory between CEA, CNRS, Ecole Centrale Lyon, C-Innov, and Sorbonne Université.]

[1] Y. Cansi, Geophys. Res. Lett., 22, 1021-1024, 1995

[2] L.J. Gallin, Th. Farges, R. Marchiano, F. Coulouvrat, E. Defer, W. Rison, W. Schulz, M. Nuret, J. Geophys. Res. Atmos., 121, 3929-3953, 2016

[3] A. Lacroix, Th. Farges, R. Marchiano, F. Coulouvrat, J. Geophys. Res. Atmos., 123, 12040-12065, 2018

[4] A. Lacroix, F. Coulouvrat, R. Marchiano, Th. Farges, J.-F. Ripoll, Geophys. Res. Lett., 46, 11479-11489, 2019

How to cite: Bestard, D., Farges, T., and Coulouvrat, F.: Acoustical reconstruction and thunder energy localisation in lightning flashes measured over Corsica during EXAEDRE field campaign, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7390, https://doi.org/10.5194/egusphere-egu21-7390, 2021.

In the lightning channel pressures can be of the order of 100 atm and hence in the produced thunder, sound pressure levels (SPL) can be very high. Additionally, the thunder frequency spectra have peaks for peal and claps at around 100 Hz and around 50 Hz for rumble sounds, with intracloud lightning having peaks at even fewer Hz. These low frequencies are ideal for acoustically induced orthokinetic agglomeration of droplets. Thunder occurs in cloud environments where not only large numbers of droplets are present, but additionally the shockwave front expands at supersonic velocities in excess of 60 km/s and hence could cause also modulations of droplet size distributions through e.g. vibrational breakup. We present calculations for the two mechanisms above (orthokinetic agglomeration and vibrational breakup) for typical cloud droplet sizes and concentrations. In thunderstorm conditions, it is found that acoustic orthokinetic agglomeration of droplets can be very effective and can produce very rapidly changes in the mean cloud droplet diameter. Also, it is found that the critical Weber number, over which breakup occurs, is easily exceeded in thunderstorm environments and may lead to droplet and ice nuclei breakup. We note that these processes need further study to assess how they could interfere with the lightning generation process itself, through charge redistribution in the modified droplet size distribution spectra. 

How to cite: Kourtidis, K. and Stathopoulos, S.: On the impact of thunder on droplets, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-471, https://doi.org/10.5194/egusphere-egu21-471, 2021.

The north-western Mediterranean basin often experiences thunderstorms with heavy precipitation, strong wind, lightning activity and sometimes waterspouts/tornadoes. One of the objectives of the EXAEDRE (EXploiting new Atmospheric Electricity Data for Research and the Environment) project is to better monitor the thunderstorms in this area through a better understanding of the physical processes that drive the dynamics, the microphysics and the electrical activity of the convective systems. Characteristics of the electrical activity such as flash rate, charge layer distribution or flash polarity are good proxies for thunderstorm monitoring and good evidences of the storm severity.

The 29^th October 2018, an intense trough developed over Mediterranean Sea between Balearic Islands and Corsica. This storm, called ADRIAN, produced several hazards (heavy precipitation, strong winds, intense lightning activity and hailstorm) in Corsica. Two tornadoes and one waterspout were observed in the morning at Porto Vecchio (EF2 tornado and waterspout) and Aleria (EF1 tornado), causing significant damages.

In this study, we take a look at electrical and microphysical characteristics of the two tornadic cells. For that, observations of the LMA (Lightning Mapping Array) SAETTA network, deployed in Corsica, are used to document in 3D the total lightning activity. Complementary 2D lightning observations recorded by the French national lightning detection network METEORAGE are also investigated. We also use weather radar data from the Météo France network. A clustering algorithm is applied on both the lightning and radar data to identify and track the cells to document the evolution of several lightning-related and microphysical characteristics during the lifetime of each cell. We also applied a new method based on lightning leader velocity to automatically infer the vertical and horizontal structure of the electrical charge regions within each electrical cell.

We first introduce the different observations and methodologies applied here. Then the main electrical properties of the tornadic cells (e.g. flash duration, vertical flash extension, charge layer, flash type and polarity) and microphysical characteristics as well as their temporal evolution are presented. Overall, the studied electrical cells produced few cloud-to-ground lightning flashes predominantly of negative polarity. The peaks of electrical activity occurred when tornadoes hit the land and these storms presented all an anomalous charge structure.

How to cite: Houel, R., Defer, E., Combarnous, P., Prieur, S., Lambert, D., and Pédeboy, S.: Observational study of tornadic cells that hit Corsica during the ADRIAN storm on the 29th October 2018, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9621, https://doi.org/10.5194/egusphere-egu21-9621, 2021.

Lightning activity over oceans is normally greatly suppressed in comparison with continents.  The most conspicuous region of enhanced lightning activity over open ocean is found in the equatorial Pacific (150 W) in many global lightning climatologies (OTD, LIS, WWLLN, GLD360, RHESSI, Schumann resonance Q-bursts) and is associated with the South Pacific Convergence Zone (SPCZ).  This oceanic lightning anomaly completes the zonal wavenumber-4 structure of continent-based lightning maxima (with nominal 90-degree longitudinal separation between sources), and so is appropriately named “the fourth chimney”.  This region is now under continuous surveillance by the Geostationary Lightning Mapper (GLM) on the GOES-17 satellite (at 137 W).  This total lightning activity is compared with Convective Available Potential Energy (CAPE) from ERA-5 reanalysis.  These CAPE values are correlated with values extracted from thermodynamic soundings at proximal stations Atuona, Rikitea and Tahiti.  The shape of the regional climatology of CAPE resembles that of the SPCZ and is oblique to the equator.  The total lightning flash rate is positively correlated with CAPE, and lightning locations are found preferentially in regions of elevated CAPE on individual days.  The diurnal variation of total lightning for January exceeds a factor-of-two and shows a phase at odds with the usual behavior of oceanic lightning near continents.

How to cite: Williams, E., Enore, D., Mattos, E., and Wu, Y.-J. J.: Thermodynamic Contribution to Lightning Activity in the Fourth Chimney, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13955, https://doi.org/10.5194/egusphere-egu21-13955, 2021.

As many severe weather events, such as torrential rainfall, tropical cyclones, tornados, and downbursts, are closely related to lightning activities, a continuous monitoring of thunderstorms is a key component for the prediction of the severe weather intensity development and for mitigating the natural disasters caused by these severe weather events.  The integration of lightning data has the high potential contributing to short term forecasts of thunderstorms, further meteorological studies, and supplement disaster risk response strategies.  This presents the activities and current status of the Understanding Lightning and Thunderstorm (ULAT) project, which is led by Hokkaido University and other Japanese institutes and Advanced Science and Technology Institute (ASTI), Department of Science and Technology (DOST) in the Philippines supported by the Japan International Cooperation Agency (JICA) and Japan Science and Technology Agency (JST).  The ULAT Project is aimed at the following: a) establishment of a dense network of lightning and weather detectors in Metro Manila and nearby municipalities in order to provide thunderstorm “now-casting” and supplement weather-related research and disaster response studies and strategies; b) establishment of a ground receiving station for the direct reception of the satellite imagery and utilization of existing ground receiving facilities in order to develop effective observation methods by comparing 3D structures of thunderclouds from satellite images with lightning/precipitation data; c) establishment of a methodology for short term forecasts; and d) development of software for sharing information on short term forecast weather to concerned agencies.  Especially for the purpose a), we have developed new lightning and weather observation systems, called as P-POTEKA and V-POTEKA.  These systems can be automatically operated without any daily maintenance.  So far, we have installed 35 P-POTEKA systems in Metro Manila and 7 and 4 V-POTEKA systems in the Philippines and in Indonesia, Palau, Guam, and Okinawa in Japan, respectively.  At the presentation, we will show the updated status of this project and will show the initial results derived from the cross correlation analyses between lightning activities monitored by V-POTEKA systems and the intensity developments of tropical cyclones.

(This research is supported by Science and Technology Research Partnership for Sustainable Development (SATREPS), funded by Japan Science and Technology Agency (JST) / Japan International Cooperation Agency (JICA).)

How to cite: Sato, M., Takahashi, Y., Kubota, H., Noda, A., Hamada, J., and Lopez, G. V. C.: Quasi-Real Time Monitoring of Lightning and Weather in the Philippines and Western North Pacific for the Severe Weather Intensity Prediction, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13950, https://doi.org/10.5194/egusphere-egu21-13950, 2021.

The distribution of cloud-to-ground lightning energies is well established, and its most extreme values appear only in extremely rare flashes (< 0.0001%), defined as lightning "super-bolts". There are varying definitions of the specific energy values of super-bolts, depending on the detector or mode of observation. When using optical energy as viewed from a satellite, one usually refers to the brightest flashes (10³ times brighter than average), while when relating to the electromagnetic radiation received by lightning detection networks, the definition revolves around the strongest signals in the VLF or ELF range, or the largest peak-current or charge-moment-change (CMC) inferred from the signal. These are all different metrics for evaluating the lightning's intensity, and they are inter-related and exhibit mutual dependence (e.g. extreme values of peak current positively correlate with extreme VLF amplitudes). The global distribution of these extremely powerful lightning is remarkably different from that of normal lightning, which are concentrated in the 3 convective "chimneys" of tropical Africa, South-America and the maritime continent in South-east Asia. Superbolts are found mostly over the oceans and near coastlines, such as Sea of Japan, the North Sea and in the Andes mountains (Holzworth et al., 2019). They are also discovered in maritime winter storms over the Mediterranean Sea which is one of the most prolific regions, especially in the months November-January. We present the climatology of east-Mediterranean super-bolts (peak current > 200 kA), and compare data obtained by various lightning detection networks (ENTLN, WWLLN and ILDN). Some storms exhibit a larger percentage of superbolts compared with the global average, up to 0.65% of total flashes. While the physical mechanisms producing these powerful flashes remains unknown, we suggest that such flashes are more common when large amounts of desert dust aerosols, coming from the Sahara Desert, are ingested into maritime winter storms and contribute to convective invigoration, enhanced freezing and efficient charge separation. Initial modelling results will be discussed.

How to cite: Yair, Y., Price, C., Namia-Cohen, Y., Lynn, B., Shpund, J., and Yaffe, M.: Why are lightning super-bolts more frequent in East Mediterranean winter thunderstorms?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1474, https://doi.org/10.5194/egusphere-egu21-1474, 2021.

A lightning model was developed (Sato et al. 2019, 2021) and implemented into a community meteorological model in Japan (SCALE: Nishizawa et al. 2015, Sato et al. 2015). The lightning model coupled with SCALE was validated through the comparison with the ground base lightning measurement (LIghtning DEtection Network system: LIDEN) operated by Japan Meteorological Agency. For the validation, we conducted downscale simulations targeting on two heavy rain events, which occurred on July, 2017 and July, 2018. The heavy rainfall in both events were triggered by Baiu front system on July in Japan and cumulative precipitation exceeded 800 mm/48 hours, but lightning frequency in the 2017 case was much higher than that of the 2018 case.

Our results indicated that the model successfully reproduced the difference of the lightning frequency between the two heavy rain events. Our analyses elucidated that the difference in the lightning frequency was originated from the difference in the vertical distribution of the charged graupel, and as consequence, the vertical structure of the charge separation rate and the charge density.

How to cite: Sato, Y., Hayashi, S., and Hashimoto, A.: Development of a lightning model and implementation into a meteorological model developed in Japan~ Validation through the comparison with the ground base measurement, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6984, https://doi.org/10.5194/egusphere-egu21-6984, 2021.

Winter storm Filomena battered the Iberian Peninsula on the 9-10^th January 2021, covering the eastern half of Spain with a huge amount of snow. Apart from the historical snowfall (e.g. Madrid 40-50 cm), lightning has been recorded during this winter episode. Most of the lighting was oversea, associated with the surface low in southern Spain. Still, some scattered lightning was also recorded in other regions of the Iberian Peninsula like Galicia, Asturias, Extremadura, Valencia and Catalonia.

This study focuses on the just over a dozen of stokes that hit southern Catalonia. Interestingly, inland lightning took place on the evening of the 9^th January although NWP models showed no convection conditions over land, the sounding was stable and CAPE was found only far away over sea.

A closer look at the lightning spots showed wind turbines in the close vicinity of all CG stokes. To check the veracity of these winter lightning, data has been gathered from two independent Lightning Location Systems.

By means of data from different meteorological systems from the Meteorological Service of Catalonia (weather radar, automatic weather stations), the meteorological conditions during the lightning occurrence are analysed.

Since lightning only occurred on wind turbines, the effect of rotation may be a key factor on the triggering of lightning from wind-turbines, because the rotation might enhance the electric field at the tips of the blades because they are less shielded by the space charge produced by themselves.

How to cite: Pineda, N., Montanyà, J., Romero, D., van der Velde, O. A., Soler, X., López, J. A., and Solà, G.: Lightning to wind-turbines during snowstorm Filomena over Catalonia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15005, https://doi.org/10.5194/egusphere-egu21-15005, 2021.

Rainfall by thunderstorms and typhoons causes a large-scale disaster, especially in Southeast Asia and other tropical regions. Damage caused by disasters could be minimized by monitoring and predicting in real-time. It is known that typhoon shows the maximum wind speed 1-2 days after the peak of lightning frequency. There is a strong correlation between lightning activity and torrential rainfall. If we could monitor the lightning activity quantitatively, it must be useful to monitor and predict strong rainfall. Lightning is an electrical phenomenon, and the magnitude of its significance is usually represented by its peak current and charge moment change before and after the stroke. However, the energy dissipation by lightning, which might be a good indicator of atmospheric convection, cannot be estimated only from the electromagnetic field measurement since it is impossible to measure the conductivity in the discharge path. Here we focus on infrasound below 20 Hz, which may be a good proxy of energy dissipation caused by lightning stroke. In order to estimate the dissipated energy by lightning stroke, we need to know the quantitative relationship between the dissipated energy and the intensity of infrasound in another way.

In the present research, we try to calibrate the quantitative relationship between infrasound intensity measured at a known distance and dissipated energy in the atmosphere, using two kinds of fireworks displays. At a building of Hokkaido University we measured infrasound pressure of fireworks for some cases which occurred at the range of 5 km. We also carried out similar measurement in lakeside of Lake Toya in Hokkaido in distance range of 0.3 - 4 km. The maximum dissipated energies of the fireworks are in ~10^6 J, which is approximately 1,000-5,000 times smaller than that of typical lightning, namely. Based on these measurements, we determined the constant to calculate the dissipated energy from infrasound pressure measurement. On the other hand, this constant is not very stable for different cases probably due to the variations in sound spectrum, height of explosion, temperature profile of the atmosphere near surface. We need to consider such conditions when we estimate the dissipated energy of lightning, adding to the effect of line source of the sound in lightning path while the fireworks has a point source.

This research was supported by Science and Technology Research Partnership for Sustainable Development (SATREPS), funded by Japan Science and Technology Agency (JST) / Japan International Cooperation Agency (JICA)."

How to cite: Watabe, N., Takahashi, Y., Sato, M., and Kubota, H.: Estimation of dissipated lightning energy by infrasound measurement, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16237, https://doi.org/10.5194/egusphere-egu21-16237, 2021.

Bursts of gamma rays observed on the Earth’s surface – so called Thunderstorm Ground Enhancements (TGE) were detected by a plastic scintillator (disassembled from the particle detector SEVAN) located in the observatory building on the Milešovka peak (50.6N, 13.9E, altitude 837 m) in Czechia. The TGEs observed during two thunderstorms on 23 April 2018 respectively lasted 65 and 15 minutes and exceeded the background radiation levels by 30 and 40 percent.

The first storm was a part of an evolving squall line which crossed the Milešovka peak. The second storm was probably a supercell, which moved near Milešovka but did not hit its top. Both storms caused heavy precipitation and strong wind gusts. The onset of the TGEs preceded the onset of precipitation by approximately 8 minutes. During the increases of TGE radiation, the European lightning detection network EUCLID detected numerous predominantly negative intracloud lightning discharges at distances closer than 5 km from the particle detector.

To understand the conditions for the TGE observation we investigated the data collected during the enhancements by a Ka-band cloud radar, an electric field mill, and a broadband electromagnetic receiver installed in the Milešovka peak observatory. Using the cloud radar measurements, we estimated the vertical extent of the thunderclouds. The cloud base was found at about 500 m above the observatory. Estimated heights of the cloud tops for the two storms were 12 and 8 km, respectively, indicating that the storm center of the second storm was not directly above the cloud radar. The updraft velocities reached 10 m/s.  A composition of hydrometeors suggested good conditions for cloud electrification.

We have found that the increases of TGE radiation corresponded to the large negative electric fields (up to – 20 kV/m) measured by the electric field mill rather than to individual discharges. We also identified numerous microsecond-scale pulses in the broadband magnetic field records, which can be attributed to corona-type discharges occurring near the receiving antenna in high local electric fields below the thunderstorm.

Based on our analysis we assume that observed TGEs corresponded to the bremsstrahlung generated during collisions of electrons accelerated in the thunderstorm electric field with the air molecules. Because of a very small number of cloud-to-ground lighting discharges we hypothesize that the electrons might have been accelerated by a strong lower positive charge center at the bottom of the thundercloud. As the TGE radiation increases were unusually long, we speculate that their later part might have been assigned to the radon progeny which was lifted to the atmosphere by a near-surface electric field and returned back to the ground with the rain precipitation.

How to cite: Kolmašová, I., Santolík, O., Ploc, O., Langer, R., Popová, J., Sokol, Z., Zacharov, P., Šlegl, J., Diendorfer, G., Strhárský, I., Lán, R., and Kákona, M.: First observation of significant long-lasting Thunderstorm Ground Enhancements on the Milešovka peak (altitude 837 m) in Czechia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3141, https://doi.org/10.5194/egusphere-egu21-3141, 2021.

Sprites and halos are transient luminous events occurring above thunderclouds. They can be observed simultaneously or they can also appear individually. Circumstances leading to initiation of these events are still not completely understood. In order to clarify the role of lightning channels of causative lightning return strokes and the corresponding thundercloud charge structure, we have developed a new model of electric field amplitudes at halo/sprite altitudes. It consists of electrostatic and inductive components of the electromagnetic field generated by the lightning channel in free space at a height of 15 km. Above this altitude we solve Maxwell’s equations self-consistently including the nonlinear effects of heating and ionization/attachment of the electrons. At the same time, we investigate the role of a development of the thundercloud charge structure and related induced charges above the thundercloud. We show how these charges lead to the different distributions of the electric field at the initiation heights of the halos and sprites. We adjust free parameters of the model using observations of halos and sprites at the Nydek TLE observatory and using measurements of luminosity curves of the corresponding return strokes measured by an array of fast photometers. The latter measurements are also used to set the boundary conditions of the model.

How to cite: Kaspar, P., Kolmasova, I., Santolik, O., Popek, M., Spurny, P., and Borovicka, J.: Simulations of electric field at the initiation altitudes of sprites and halos generated by the thundercloud charge structure and lightning channels of causative return strokes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8667, https://doi.org/10.5194/egusphere-egu21-8667, 2021.

Sprites are bright and sudden events occuring above thunderstorms between 40 and 90 km altitude. These phenomena are usually observed using ground-based cameras and from spacecrafts. The Imager of Sprites and Upper Atmospheric Lightning (ISUAL), a payload on the FORMOSAT-2 satellite, recorded several sprite events during its mission. Contrary to JEM-GLIMS (JAXA) or ASIM (ESA), which are  space missions dedicated to the observation of TLEs from a nadir-viewing geometry, ISUAL used a limb-viewing geometry. This configuration offers the possibility to directly estimate the altitude of the event from its camera.
The challenge consists in estimating the altitude and the electric field from spectrophotometer measurements. The method of the spectrophotometric ratios consists to use ratios computed from different band systems to estimate the altitude and the electric field. It is the one of the most encouraging to achieve this goal.
In this work, we propose a method to estimate the electric field and the altitude from an observation made by the ISUAL instrument using the following ratios LBH/1PN2, 2PN2/1PN2 and LBH/1NN2+. We show that some spectroscopic ratios are more useful then others and point out some limitations of this approach that will need to be widen to nadir-viewing geometry observations.

How to cite: Garnung, M., Celestin, S., and Farges, T.: Estimation of the electric field and the altitude from  spectrophotometric observations in limb-viewing geometry, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16407, https://doi.org/10.5194/egusphere-egu21-16407, 2021.

This study is a multi-instrumental analysis of a ~20-hour duration northwestern Mediterranean storm on September 21, 2019 that produced 21 sprites recorded with a video camera, of which 19 (90 %) were dancing sprites. A dancing sprite is a phenomenon in which sequences of sprites appear in succession with time intervals of no more than a few hundred milliseconds. For the most part, the individual sprites are a consequence of discrete strokes from one extended lightning flash. In this case, we find that 87.5% of the sprite sequences were triggered by distinct positive cloud-to-ground (+CG) strokes. The time between successive sprite parent (SP)+CG strokes within the same dancing sprite was between 40 and 516 ms, and the distance ranged between 2 and 87 km. The storm size and vertical development were analyzed from the infrared radiometer onboard Meteosat Second Generation satellite and the lightning activity was documented with several lightning location systems (LLS): the French LF network (Météorage), the GLD360 network operated by Vaisala company, the VHF SAETTA Lightning Mapping Array (LMA) system located in Corsica. Additionally, the vertical electric field at the time of the dancing sprites was measured with a broadband ELF vertical dipole whip antenna ~700 km away from the storm. The SAETTA LMA allows to map the SP+CG flashes in their both full extent and temporal evolution, and to infer the charge structure of the parent storm. We show that the SP+CG flashes followed a common propagation: they originated from the convective and very electrically active regions of the storm, and then escaped and extended horizontally far (tens of km) into the stratiform cloud region. Most of the sprites were triggered by +CG strokes in the stratiform region often following flash development resembling cutoff of a long negative leader. Additionally, we present a detailed analysis of two dancing sprite events in which the SP+CGs triggered new bidirectional breakdown with fast moving leaders that extended into the stratiform cloud region and resulted in new SP+CG strokes. In both events, we find in both LLS and ELF vertical electric field records, that the last sprite sequence was triggered by three almost simultaneous +CG strokes.

How to cite: Tomicic, M., Soula, S., Farges, T., Prieur, S., and Defer, E.: Dancing Sprites Above a Lightning Mapping Array - an analysis of the storm and flash/sprite developments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1695, https://doi.org/10.5194/egusphere-egu21-1695, 2021.

ASIM has now observed several hundreds of TGFs since the launch in 2018. Highlights and new science from the first ten months of observations were presented in Østgaard et al. (2019) paper. In this presentation we will present observational highlights from the last 1.5 year, when the relative timing accuracy between the TGF observations and the optical measurements is +/- 5 us (compared to +/- 80 us before march 2019). This includes many more simultaneous TGF and Elve observations, high flux TGFs, double TGFs simultaneous with double optical pulses and many TGFs with good radio measurements.  ASIM has also observed several Gamma Ray Bursts. 

How to cite: Ostgaard, N., Mezentsev, A., Marisaldi, M., Kochkin, P., Neubert, T., Reglero, V., Chanrion, O., Lehtinen, N., Sarria, D., Skeie, C., Bjørge-Engeland, I., Lindanger, A., Christiansen, F., Ullaland, K., Genov, G., and Budzt-Jørgensen, C.: New highlights of gamma-ray observations by ASIM, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4600, https://doi.org/10.5194/egusphere-egu21-4600, 2021.

Terrestrial Gamma-ray Flashes (TGFs) are short flashes of high energy photons, produced by thunderstorms. When interacting with the atmosphere, they produce relativistic electrons and positrons, and a part gets bounded to geomagnetic field lines and travels large distances in space. This phenomenon is called a Terrestrial Electron Beam (TEB). The Atmosphere-Space Interactions Monitor (ASIM) mounted on-board the International Space Station detected a new TEB event on March 24, 2019, originating from a tropical cyclone, Johanina. Using ASIM's low energy detector, the TEB energy spectrum is resolved down to 50 keV. We provide a new method to contrain the TGF source spectrum based on the detected TEB spectrum. Applied to this event, it shows that only fully developed RREA spectrums are compatible with the observation. More specifically, assuming a TGF spectrum proportional to 1/E exp(-E/ε), the compatible models have ε ≥ 6.5 MeV. We could not exclude models with ε of 8 and 10 MeV. This is the first time the source energy spectrum of a TGF is contrained based on the detection of the associated TEB.

How to cite: Sarria, D., Østgaard, N., Kochkin, P., Lehtinen, N., Mezentsev, A., Marisaldi, M., Lindanger, A., Maiorana, C., Carlson, B. E., Neubert, T., Reglero, V., Ullaland, K., Yang, S., Genov, G., Qureshi, B. H., Budtz-Jørgensen, C., Kuvvetli, I., Christiansen, F., Chanrion, O., and Navarro-Gonzalez, J. and the Atmosphere-Space Interactions Monitor (ASIM) team: Constraining spectral models of a terrestrial gamma-ray flash from a terrestrial electron beam observation by the Atmosphere-Space Interactions Monitor, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4785, https://doi.org/10.5194/egusphere-egu21-4785, 2021.

The Atmospheric Space Interactions Monitor (ASIM) was launched in 2018, and has since then observed Terrestrial Gamma-ray Flashes (TGFs) and Transient Luminous Events (TLEs). ASIM consists of the Modular X- and Gamma-ray Sensor (MXGS) and the Modular Multispectral Imaging Array (MMIA). Using data from both MXGS and MMIA, we investigate observations of TGFs (detected by MXGS) with accompanying elves (detected by MMIA). We study the optical signatures of the elves detected by a photometer of MMIA operating in the 180-230 nm band. Lightning sferics associated with these events have been detected by WWLLN and GLD360. Several TGFs have associated lightning sferics outside the field of view of MMIA, but due to the expanding rings of the elves we can still observe optical signatures from accompanying elves. Using GLD360 data we also study properties of the lightning strokes.

How to cite: Bjørge-Engeland, I., Østgaard, N., Mezentsev, A., Neubert, T., Skeie, C. A., Marisaldi, M., Ullaland, K., Genov, G., Chanrion, O., and Reglero, V.: ASIM TGFs with accompanying elves, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5878, https://doi.org/10.5194/egusphere-egu21-5878, 2021.

The Atmosphere-Space Interactions Monitor (ASIM) has been installed on board of the International Space Station in April 2018, successfully providing science data for 2.5 years. The Modular X- and Gamma-ray Sensor (MXGS) of ASIM is designed to detect Terrestrial Gamma-ray Flashes (TGFs) (short intense bursts of gamma-ray photons), produced during the initial breakdown phase of the +IC lightning discharges.

In this contribution we report and summarize the results on the ASIM TFGs associated with high peak current lightning detections (detected by GLD and WWLLN networks). High peak current detections tend to be associated with short duration TGFs and do not exhibit a tendency to correlate with the fluence of the TGF.

How to cite: Mezentsev, A., Østgaard, N., Neubert, T., and Reglero, V.: ASIM TGFs associated with high peak current strokes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7573, https://doi.org/10.5194/egusphere-egu21-7573, 2021.

This study reports on simultaneous optical and radio observations of a possible blue starter that took place in north-west Texas in the United States in 2018.  The optical observations come from the Atmospheric-Space Interactions Monitor (ASIM) onboard the International Space Station [Neubert et al., 2019, doi: https://doi.org/10.1007/s11214-019-0592-z] and the radio observations were from the National Lightning Detection Network (NLDN) [Cummins and Murphy, 2009, doi: 10.1109/TEMC.2009.2023450] and the West Texas Lightning Mapping Array (WTLMA) [Chmielewski and Bruning, 2016, doi: https://doi.org/10.1002/2016JD025159]. It was identified by the ASIM CHU1 337 nm imager and shows a diffuse, conical emission shape reaching approximately 7 km above cloud top, characteristic of blue starters.  The ASIM CHU2 777.4 nm imager shows a simple point-source of emissions, highly contrasting the 337 nm imager observations.  The 337 and 777.4 nm photometers show four distinct pulses, the first two of which were dominated by the 337 nm emissions and also showed clear UV (180-230 nm) photometer peaks.  From the WTLMA data, which clearly mapped the negative and positive leaders (or negative recoil events) even at low altitudes, the parent storm cell exhibited what appears to be a classic tri-polar charge structure, with upper and lower positive and middle negative charge.  The blue starter occurs during what appears to be an initial ascending negative leader into the upper positive charge region, which continues to develop into a positive intracloud (IC) flash between the upper positive and middle negative charge region.  During this time, there are several small NLDN positive cloud pulses (+IC), consistent with a traditional IC flash, but these are followed by two moderately high peak current (40-50 kA) negative cloud-to-ground strokes, which appear to be misclassified by the NLDN as there were no WTLMA VHF source points at low altitudes during this time. The misclassified negative strokes are concurrent with the first blue peak from the ASIM 337 nm photometer.  We conjecture that these misclassified negative CG strokes were actually electromagnetic pulses from in-cloud (or near-cloud-top) sources, which were perhaps directly associated with the blue starter. 

How to cite: Boggs, L., Neubert, T., Chanrion, O., Reglero, V., Østgaard, N., Heumesser, M., Nag, A., and Bruning, E.: Optical and Radio emissions of a possible blue starter observed by ASIM and the West Texas LMA, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13649, https://doi.org/10.5194/egusphere-egu21-13649, 2021.

A possible mechanism responsible for Terrestrial Gamma-ray Flashes (TGFs) is feedback in the relativistic runaway electron avalanches (RREA) dynamics. In this research, a new way of RREAs self-sustaining is suggested. This self-sustaining feedback can be described in the following way. Let the thundercloud consist of two regions with the electric field so that runaway electrons accelerated in one region move in the direction of another one and vice versa. For instance, such an electric field structure might appear with one positive charge layer situated between two negative charge layers. In this system, the following feedback mechanism occurs. An RREA developing in one region will produce bremsstrahlung gamma-rays. These gamma-rays will propagate into another region and produce RREAs within it. These RREAs will develop backward and radiate gamma-rays, which will penetrate the first region, generating secondary RREAs. In this way, the primary avalanche reproduced itself by the gamma-ray exchange between two sideways oriented areas with the electric field. In this work, it is shown that the electric field values required for TGF generation by this mechanism are lower than values required in Relativistic Feedback Discharge Model.

How to cite: Stadnichuk, E., Zemlianskay, D., and Efremova, V.: Simple "Reactor model" of relativistic runaway electron avalanches dynamics, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13395, https://doi.org/10.5194/egusphere-egu21-13395, 2021.

We investigate and determine the sequence of the Terrestrial Gamma-ray Flashes (TGFs) and the observed optical emissions associated with lightning flashes, as well as the connection between the duration of TGFs and the time between the onset of the TGFs and the observed main optical pulses. Over 200 observations from the instruments of the Atmosphere-Space Interactions Monitor (ASIM) on board the International Space Station (ISS) are used, together with data from the lightning detection networks GLD360 and WWLLN. The ASIM data consist of two separate recordings: High energy measurements from the Modular X- and Gamma-ray Sensor (MXGS), and optical measurements from the Modular Multi-Spectral Imaging Array (MMIA). The optical measurements are from photometers operating in the 337 and 777.4 nm bands, and the temporal uncertainty between the two instruments of ASIM is +- 5 µs.

How to cite: Skeie, C. A., Østgaard, N., Mezentsev, A., Bjørge-Engeland, I., Sarria, D., Lindanger, A., Neubert, T., and Reglero, V.: Investigating the relationship between TGF durations and the onset times of the optical pulses and the TGFs, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14757, https://doi.org/10.5194/egusphere-egu21-14757, 2021.