NH1.5

Analysis of the variation of the potential gradient (PG) at ground level is important to monitor the global electric circuit and the different solar and geophysical phenomena affecting it. However, this is challenging since several factors (e.g., meteorological) produce perturbations in the potential gradient. For this reason, timeseries and spectral analysis of PG at several stations are required. In this work, for the first time we performe the spectral analysis of the potential gradient recorded at several sites located at Vostok, Concordia, Halley and Casleo (South Hemisphere), and Sodankyla and Reading (North Hemisphere). In order to find the main periodicities and how the amplitude of those periods change as a function of time we use the Lomb-Scargle Periodogram and the Wavelet Transform, respectively. For all PG sites the periodicities of 0.5, 1, ~180 and 365-day were found. It was also found evidence of the ~27- and ~45-day periods. Further analysis using the cross-wavelet transform for PG versus cosmic rays, PG versus Madden-Julian Oscillation index, and PG versus meteorological parameters, shows that the 27- and 45-day periods are likely related to the solar rotation and Madden- Julian Oscillation, respectively. Moreover, for the 27-day period we found that the relationship is stronger during the occurrences of co-rotating interaction regions.

How to cite: Tacza, J., Nicoll, K., and Macotela, E.: Periodicities in fair weather potential gradient at ground level from different latitudes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-97, https://doi.org/10.5194/egusphere-egu22-97, 2022.

Nowadays, there is a great need for the preservation of historical data in earth sciences as time series covering a long time period are of extreme importance in studying long-term variations of the Earth’s environment. This is the case in the field of atmospheric electricity research, too. In this work, we focus on one of the most frequently recorded parameters of the discipline, the atmospheric electric potential gradient (PG).

The PG is the reverse of the vertical atmospheric electric field, a quasi-DC quantity measured in Vm-1 units usually near the ground most often at 1–3 m heights [1]. The PG has been measured quasi-continuously at the Széchenyi István Geophysical Observatory near Nagycenk, Hungary (NCK, 47°38’ N, 16°43’ E) since 1962 [2]. Between 1962 and 2011, the PG was recorded on photo papers which were evaluated manually and the hourly averaged PG values were archived. Nevertheless, the original photopapers, too, were kept.

In this contribution, we present a recently developed image processing algorithm to digitize the analogue PG records on the old photo papers semi-automatically. By means of this algorithm, PG averages can be obtained with a temporal resolution as high as 30 s. In order to validate the digitized data, they have been compared to the archived hourly PG averages between 1999 and 2009. The long-term, seasonal, and diurnal variations of the PG at NCK between 1999 and 2009 based on the digitized and the archived data are also presented.

[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] Bór, J., Sátori, G., Barta, V., Szabóné-André, K., Szendrői, J., Wesztergom, V., Bozóki, T., Buzás, A., and Koronczay, D.: Measurements of atmospheric electricity in the Széchenyi István Geophysical Observatory, Hungary, Hist. Geo Space. Sci., 11, 53–70, https://doi.org/10.5194/hgss-11-53-2020, 2020.

How to cite: Magos, L., Bozóki, T., Bozsó, I., Bór, J., Horváth, A., Kuslits, L., Timkó, M., and Buzás, A.: Digitizing archive atmospheric electric potential gradient data for scientific research, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5228, https://doi.org/10.5194/egusphere-egu22-5228, 2022.

Charge influences the properties of liquid droplets, such as their evaporation rates, hydrodynamic stability and sticking probabilities in droplet-droplet collisions. Introducing additional charge into an assembly of droplets therefore provides a possible method of influencing the droplet properties, and, in the case of a fog or cloud, a route to weather modification. The effect of charging on natural droplets has been investigated by releasing additional unipolar and bipolar ions into a natural surface fog, from both within the fog at the surface and above the fog, using an Uncrewed Aerial Vehicle (UAV). Droplet properties were monitored before and after the ion release using optical methods, together with the atmospheric electric field using a field mill. The atmospheric electric field measurements robustly demonstrate the present of the additional ions. Further, changes in the fog properties apparent when the additional ions are introduced are discussed.

How to cite: Harrison, G., Marlton, G., Ambaum, M., and Nicoll, K.: Releasing corona ions into natural fog, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8665, https://doi.org/10.5194/egusphere-egu22-8665, 2022.

SOFT-IO-LI is a new tool merging space and ground based lightning observations and aircraft NOx measurements to provide a database of new parameters characterizing lightning-NOx air masses observed at the global scale, to the scientific community.

The tool takes lightning-NOx air masses measured by IAGOS (In-service Aircraft for a Global Observing System) a European Research Infrastructure for global observations of atmospheric composition using commercial aircraft. FLEXPART the Lagrangian transport and dispersion model is used, together with ERA5 reanalysis weather data from ECMWF, to perform a backward trajectory of these air masses, resulting in hourly mass residence times for lightning-NOx particles for the days prior to the flight measurements.

We will present, for IAGOS transatlantic flights, the comparison of these mass residence times with ground based NLDN (National Lightning Detection Network) and space based GLM (Global Lightning Mapper) lightning observations, as well as with ABI (Advanced Baseline Imager) cloud data. We will show that SOFT-IO-LI is capable of relating lightning events and chemical species observed in situ, in order to determine characteristics of the different lightning-related air masses.

How to cite: Mackay, C., Sauvage, B., Wolff, P., Defer, E., Mahnke, C., Petzold, A., Bundke, U., and Kennert, M.: SOFT-IO-LI: a new tool merging space and ground based lightning observations and aircraft NOx measurements., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7794, https://doi.org/10.5194/egusphere-egu22-7794, 2022.

Lightning in winter (December, January, February, DJF) is rare compared to lightning in summer (June, July, August, JJA) in central Europe. The conventional explanation attributes the scarcity of winter lightning to seasonally low values of variables that create favorable conditions in summer. Here we systematically examine whether different meteorological processes are at play in winter. We use cluster analysis and principal component analysis and find physically meaningful groups in ERA5 atmospheric reanalysis data and lightning data for northern Germany. Two sets of conditions emerged: Wind-field dominated and mass-field (temperature) dominated lightning conditions. Wind-field type lightning is characterized by increased wind speeds, high cloud shear, large dissipation of kinetic energy in the boundary layer, and moderate temperatures. Clouds are close to the ground and a relatively large fraction of the clouds is warmer than −10 degree Celsius. Mass-field type lightning is characterized by increased convective available potential energy (CAPE), the presence of convective inhibition (CIN), high temperatures, and accompanying large amounts of water vapor. Large amounts of cloud-physics variables related to charge separation such as ice particles and solid hydrometeors further differentiate both mass-field and wind-field lightning. Winter lightning is wind-field driven whereas in summer lightning is mostly mass-field driven with a small fraction of cases being wind-field driven. Consequently, typical weather situations for wind-field lightning in the study area in northern Germany are strong westerlies with embedded cyclones. For mass-field lightning, the area is typically on the anticyclonic side of a southwesterly jet.

Keywords: ERA5, cold-season thunderstorm, k-means clustering, winter lightning.

How to cite: Morgenstern, D., Stucke, I., Simon, T., Mayr, G. J., and Zeileis, A.: Differentiating lightning in winter and summer with characteristics of wind field and mass field, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-824, https://doi.org/10.5194/egusphere-egu22-824, 2022.

We used the World Wide Lightning Location Network and cyclones tracks from the International Best Track Archive for Climate Stewardship to study properties of lightning strokes occurring in tropical cyclones. We studied 429 cyclones occurring from 2012 to 2017 in both hemispheres with more than 11 million lightning strokes found within a distance of 600 km from the cyclone eye. For purposes of our study, we divided the cyclones into 6 basins: Indian Ocean, North Atlantic, Northeast Pacific and Northwest Pacific in the northern hemisphere and Indian Ocean and Southern Pacific in the southern hemisphere. We found differences in the numbers, energies and multiplicities of lightning strokes occurring in the cyclones in the northern and southern hemispheres. We calculated the median stroke energy for each cyclone. We used Saffir-Simpson scale for classifying the intensity of tropical cyclones and found a tendency of decreasing median stroke energies with an increasing cyclone intensity. We compared the evolution of lightning activity accompanying the cyclones with the evolution of their central pressure and wind speed to examine the possibility of using the lightning activity for prediction of cyclone intensity changes. In the northern hemisphere, there was on average about 28 thousands of strokes per cyclone with a median energy of 1.7 kJ, while in the southern hemisphere, there was on average 24 thousands of strokes per cyclone with a median energy of 2.7 kJ. The difference in multiplicity is not really noticeable with an average of 1.39 strokes per flash in the northern hemisphere and 1.34 strokes per flash in the southern hemisphere. In our dataset, we found 28 strokes with an energy over 1 MJ (superbolts), which occurred in a short period during the winter 2013-14, which was the winter exhibiting the largest SOI (Southern oscillation index).

How to cite: Rosická, K., Kolmašová, I., and Santolík, O.: Lightning activity accompanying tropical cyclones, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4164, https://doi.org/10.5194/egusphere-egu22-4164, 2022.

Pre-monsoon thunderstorms are a common source of natural destruction over eastern India, commonly known as Nor'westors. Meteorologists studied these Nor'westers for more than a century over India. Various studies highlighted that the Chota Nagpur plateau, situated in Jharkhand state, acts as a triggering source for initiating these thunderstorms. The present study attempts to evaluate the topographical variations of the Chota Nagpur plateau for initiating the Nor'westors. The current research simulated ten thunderstorm events over the Kolkata region, West Bengal, by changing the Chota Nagpur plateau's topography (increasing and decreasing along with natural topography). The study uses the Weather Research and Forecasting model (WRF ARW 3.9.1) with a triple nested domain. The innermost domain has a resolution of 3 km across eastern India. The simulated model variables are validated against vertical profiles and surface observations of point locations obtained from the India Meteorological Department's radiosonde and automatic weather station data sets. The model simulations significantly capture the observational (surface and vertical profile) characteristics. Thermodynamic indices obtained from simulations revealed that the plateau's changed (increased/decreased) topography alters the values considerably below/above thresholds for thunderstorms over the region.

Keywords: Thunderstorms; Topography; Numerical Simulation; Thermodynamic Indices

How to cite: Sahu, R. K., Singh, K. S., Nayak, H. P., and Tyagi, B.: Evaluating the Impact of Topography on Initiation of Nor’westers over Eastern India, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-558, https://doi.org/10.5194/egusphere-egu22-558, 2022.

All evolution stages of cloud-to-ground (CG) lightning flashes, both positive (+CG) and negative (-CG), generate electromagnetic radiation, which can be used for their investigation. We focus on the electromagnetic activity immediately following the first return stroke (RS). We combine measurements of the broadband receiver BLESKA and the lightning mapping array (LMA) network SAETTA, capable of detecting sources of narrowband very high frequency (VHF) radiation. The French lightning location system Météorage provided us with the information about 2D location, polarity and peak currents for studied CG and intracloud (IC) discharges. From our data collected in the northwestern Mediterranean region from September to December 2015, we have selected and investigated the electromagnetic activity following 16 +CG and 38 -CG flashes.

Using the data from individual SAETTA stations we found that 36 -CG flashes exhibited a fast decrease in the counts and in the power of VHF radiation sources immediately after the RS pulse. The maximum count of 2000 VHF radiation sources was detected by the closest SAETTA station at an average time delay of 66 μs after the RS pulse peak. At a delay of 1.85 ms after the RS pulse peak or sooner, the VHF radiation rate decreased below 1500 VHF radiation sources, with the median value of this time equal to 195 μs, and kept decreasing.

In the case of all inspected +CG flashes, we observed an unexpectedly fast increase in the counts of the VHF radiation sources and their power after the RS pulse. Up to 161.95 ms after the RS pulse, the VHF radiation rate decreased below 1500 VHF radiation sources, with the median value of this time equal to 34.53 ms, much longer than in case of –CGs. At the same time, we observed a visible sequence of bipolar pulses lasting up to 50 ms in the magnetic-field waveforms recorded by BLESKA, with the amplitude of the biggest pulse varying from 2 to 10 nT.

This observed longer presence of VHF radiation after +CG flashes may be caused by a potential difference between the end of neutralized RS channel and the positive charge layer in the thundercloud in case of +CGs, which might result in a new electrical breakdown. Then a stepwise propagation of a new negative leader inside the thundercloud is possible, emitting electromagnetic radiation in a wide range of frequencies. This radiation can be detected by narrowband LMA stations in the form of VHF radiation sources, same as by a broadband receiver in the form of pulses.

How to cite: Kolínská, A., Kolmašová, I., Santolík, O., Defer, E., Pedeboy, S., and Lán, R.: Electromagnetic radiation following the first return strokes of negative and positive cloud-to-ground lightning flashes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9076, https://doi.org/10.5194/egusphere-egu22-9076, 2022.

Our study aims at initial stage of energetic negative cloud-to-ground (CG) winter lightning flashes. We analyze broadband magnetic-field measurements recorded in the West Mediterranean region in winter 2014/2015. By combining our data with information provided by the French national lightning locating system MÉTÉORAGE, we were able to select 200 waveform captures, which contained return stroke (RS) pulses emitted by negative CG discharges with peak currents exceeding 100 kA. The frequency band of our instrumentation (5 kHz-90 MHz) allowed us to investigate fine details of recorded waveforms. We found that the winter pre-stroke processes were very short, lasting on average only 1.7 ms from the first bipolar preliminary breakdown (PB) pulse to the following return stroke pulse.  The amplitudes of the strongest PB pulses reached on average only 25 % of the corresponding RS pulse. We investigate the evolution of peak amplitudes and inter-pulse intervals of PB pulses within individual PB trains. We found that in some trains the amplitudes of pulses were nearly monotonically increasing with time, they reached a maximum in a few hundreds of microseconds, and then decreased again being relatively regularly distributed in time. Within other PB trains, the pulses were chaotically spaced and their peak amplitudes did not show any trend. We assume that the short duration of the pre-stroke process indicate strong electric fields inside winter thunderclouds and hypothesize that the time evolution of PB pulse amplitudes and interpulse intervals reflect the spatial arrangement of the negative charge region.

How to cite: Kolmašová, I., Santolík, O., Pedeboy, S., Kolínská, A., Amrich, S., Lán, R., and Uhlíř, L.: Characteristics of pulse trains observed during the initial stage of high peak current winter flashes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3752, https://doi.org/10.5194/egusphere-egu22-3752, 2022.

We have developed a new electrodynamic return stroke (RS) model, which is based on solving the full set of Maxwell’s equation together with the electrostatic Poission’s equation for a realistic thundercloud charge structure. The evolution of the line conductivity of the RS channel is characterized by a nonlinear resistance model. The RS channel consists of a vertical channel connecting the ground with the thundercloud and a horizontal in-cloud channel. The RS processes are initiated by adding a zero potential element into the bottom end of the vertical channel. The channel-base current does not have a predefined form, but results from our model. We show the comparison of the modeled magnetic field waveforms with the observations at distances of tens of kilometers from their source lightning discharge. We also verify the simulated electric and magnetic field RS waveforms at shorter distances and compare them with their typical shapes found in the literature. The line charge density and the electric potential prior to and after the RS initiation are also investigated from the point of view of the bidirectional leader concept.

How to cite: Kaspar, P., Kolmasova, I., and Santolik, O.: The electrodynamic model of the return stroke processes involving a bipolar leader scheme, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4157, https://doi.org/10.5194/egusphere-egu22-4157, 2022.

We have recently developed a new 3D beamforming algorithm, using data from the LOw Frequency ARray (LOFAR) radio telescope (in the 30-80 MHz band), that is capable of resolving even the most complex lightning phenomena with meter and nanosecond scale accuracy. Because it operates in full 3D, this algorithm inherently extracts and accounts for the 3D polarization of the VHF sources. Here we demonstrate the full power of this technique by extracting the full 3D polarization of multiple sections of recoil leaders. We confirm previous work that showed that recoil leaders have significant polarization perpendicular to the lightning channel, likely due to charge flow between the lightning channel core and corona sheath. However, we also show that recoil leaders can also have significant polarization parallel to the channel as well. In addition, we show that the ratio of parallel-to-perpendicular polarization is strongly correlated with faster and more intensely emitting recoil leaders. We will argue that this could be due to faster recoils causing the electric field parallel to the channel to change more rapidly, and if the electric field changes rapidly enough than the recoil leader can create streamers parallel to the lightning channel which are much more strongly emitting than streamers perpendicular to the lightning channel.

How to cite: Hare, B., Scholten, O., Dwyer, J., Liu, N., Strepka, C., Buitink, S., and ter Veen, S.: The 3D Polarization of Recoil Leaders, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7791, https://doi.org/10.5194/egusphere-egu22-7791, 2022.

In continuation of the study reported last year, we report additional results from imaging lightning initiation via interferometric beamforming of data collected by the Dutch LOw Frequency ARray (LOFAR).  Significant improvements have been incorporated into the analysis, including: more accurate antenna characterization, improvements to location accuracy, and the inclusion of a polarization model [Scholten, O., et al., PRD DD12993].  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] and elucidates regions in which there are a high number of sources within a short duration of time.  Interferometric beamforming techniques enhance both spatial and temporal resolution of lightning sources and as a result, locates and images the first non-impulsive sources in lightning flashes.  These sources are believed to be caused by a streamer-cascade-like initiation event which leads to the formation of the first leader.  Previously observed initiation events start from essentially background and within tens of microseconds ramp up a few orders of magnitude before the first impulsive sources connected with lightning leaders are observed.  The new techniques build upon those previously reported [Sterpka, C., et al., Geophysical Research Letters 48 (2021)] and [Sterpka, C., et al., EGU General Assembly EGU21-13711 (2021)], uncovering new detail in the lightning initiation region and characterization of additional flashes.  This new data includes a slow-propagating initiation discharge, starting 60 ms before the formation of the corresponding lightning leader.  The discharge is within 50 m of the initiation of the lightning leader and propagation speed of this discharge is about 700 ± 30  m/s, comparable to the ion drift speed.  This discharge continues for 30 ms before ceasing, and is likely a failed initiation attempt.

How to cite: Sterpka, C., Dwyer, J., Liu, N., Demers, N., Hare, B., and Scholten, O.: Uncharacteristically Slow Discharge Process Observed Preceding Lightning Initiation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6066, https://doi.org/10.5194/egusphere-egu22-6066, 2022.

We have developed a time-resolved interferometric imaging in 3D (TRI-D) method for LOFAR data where the signals of 400 individual antennas are added coherently. This allows us to reach an even better resolution in 3D than with our original impulsive imager (based on a time-of-arrival-difference method and reaching a meter scale resolution) and still have a time resolution close to the impulse-response time of our system (25 ns).

After a short outline of the TRI-D technique we show that with this new imaging technique we can resolve the fine dynamics in the different negative leader propagations modes, varying from normal negative leaders with a stepping distance of the order of a few tens of meters to negative leaders at altitudes above 7 km that propagate with steps of a few hundred meter to Intensely Radiating Negative Leaders that propagate as a broad front with an area of up to km^2  over distances of a few kilometers.  

How to cite: Scholten, O., Hare, B., Dwyer, J., Liu, N., Sterpka, C., Buitink, S., and ter Veen, S.: A range of different negative leader propagation modes as imaged with LOFAR, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5965, https://doi.org/10.5194/egusphere-egu22-5965, 2022.

Lightning is inherently a spatio-temporal process with individual lightning strikes represented by their time of occurrence and spatial coordinates. In this paper, we characterise and model lightning strikes in time and space from single thunderstorms, considering each set of lightning strikes to be a set of point events. This allows for real-world datasets to characterise lightning strikes and their physical properties. We select two case studies of severe thunderstorm systems over the UK, based on their synoptic analysis information as available in the published literature. This information allows us to separate the lightning strike dataset into subsets representing individual thunderstorms producing these strikes. We first identify three supercell thunderstorms with 7955, 11988 and 5655 lightning strikes from the larger storm system that crossed the English Midlands on 28 June 2012. A second set of three structurally different severe thunderstorms with 4218, 455 and 1926 lightning strikes was selected from a severe storm system across northern England on 1 to 2 July 2015. The six lightning strike datasets are representative of individual thunderstorms and each examined with regards to three physical properties: storm movement speed, lightning inter-event time distribution and lightning spatial spread distribution about the storm track. We use a least-squares plane fit in the spatio-temporal domain to estimate a range of representative movement speed values, finding 46-52 km/h for the first storm system and 67-105 km/h for the second. For inter-event time distribution, we find that values range from 0.01 to 100 s with all thunderstorms showing two peaks in density values around 0.1 s and between 1 and 10 s. To identify temporal structure in the inter-event time series, we perform autocorrelation analysis in natural time, which returns statistically significant autocorrelation values for all thunderstorms with some storms exhibiting short-range and others long-range autocorrelation. For estimating the storm track about which the orthogonal distances are calculated between the storm track and the lightning strikes, we consider orthogonal distance regression in the two-dimensional space domain. The analysis is done similarly to inter-event times for these orthogonal distances. We find a typical range of spatial spread values to be up to 50 km in magnitude, with one thunderstorm having exceptionally high values of up to 150 km. Autocorrelation analysis of these orthogonal distance values in natural time also return significant results that vary between individual thunderstorms. Finally, we present a synthetic lightning strike model where we can freely select the number of individual storms, their starting points, direction and movement speeds. For each storm, the point events after the starting point are produced about the storm track with inter-event times and orthogonal distance values taken from synthetic time series based on the analysis done during the characterisation. The characterisation in this paper of lightning strikes in time and space is representative of real-world severe thunderstorms and can inform statistical models to simulate lightning strike events.

How to cite: Zandovskis, U., Malamud, B. D., and Pigoli, D.: Characterisation and modelling of lightning strikes in time and space, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10295, https://doi.org/10.5194/egusphere-egu22-10295, 2022.

Streamers are an important stage of lightning, taking place before the formation of a leader discharge. The goal of the novel Streamer Parameter Model (SPM) is to explain the mechanism that determines the parameters of a streamer, such as its radius and propagation velocity. We demonstrate that SPM predictions agree well with the published hydrodynamic simulation (HDS) results [1]. The discrepancies between SPM and HDS were of the same order of magnitude as the discrepancies between different HDS codes. The comparison was performed for two different streamer propagation mechanisms: photoionization and background ionization. The largest discrepancies were for the case of low background ionization, which was also challenging for HDS. Electron diffusion did not change the streamer parameters significantly. We propose that SPM, despite the crudeness of the model, provides a computationally simple way to reliably assess streamer properties.

How to cite: Lehtinen, N. and Marskar, R.: Verification of the Streamer Parameter Model by comparing to Hydrodynamic Simulations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6211, https://doi.org/10.5194/egusphere-egu22-6211, 2022.

Thunderstorm ground enhancements (TGEs) registered on Aragats research station (530 events during  2008-2021) are frequently interrupted by the nearby lightning flashes. We are monitoring charged and neutral particle fluxes, near-surface electric field, distance to lightning flash, and numerous meteorological parameters 24/7. Our datasets [1,2] contains 165 TGEs interrupted mostly by the negative cloud-to-ground discharges (-CGs: 50%, inverted intracloud (IC) flashes followed by –CGs: 21%, inverted ICs: 18%, normal ICs:11%). The mean distance to the lightning flash estimated by EFM-100 electric mill is 5.8 +/- 3.1 km (based on 130 TGEs). Mean distance, estimated by a smaller subsample (18 TGEs) of this dataset, which contains also an estimate made by the worldwide lightning located network (WWLLN) is 6.6 +/- 5.6 km, by EFM – 4.5 +/- 2.6 km. The times of lightning occurrences measured by Aragats facilities and by the WWLLN coincide within a few microseconds. TGEs were interrupted by lightning during positive and negative near-surface electric fields. The TGEs which started at the negative (positive) near-surface electric field and terminated during the positive (negative) electric field were also observed.

How to cite: Chilingarian, A. and Soghomonyan, S.: Thunderstorm ground enhancements abruptly terminated by a Lightning flash, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-331, https://doi.org/10.5194/egusphere-egu22-331, 2022.

One of the possible sources of Terrestrial Gamma-ray Flashes (TGF) is Relativistic Runaway Electron Avalanches (RREA) accelerated in large-scale thunderstorm electric fields. In order to produce short and bright gamma-ray flash, a huge number of RREAs must exist simultaneously. This can be obtained through positive feedback mechanisms in RREA dynamics. At quasi-uniform thunderstorm electric fields, relativistic feedback provides RREAs multiplication via positrons and reversed gamma-rays. A significant disadvantage of relativistic feedback is that it requires high electric field strength in order to produce a TGF.

In complex thunderstorm electric structures, an additional feedback mechanism appears, the reactor feedback. Reactor feedback emerges if a thunderstorm consists of several RREA-producing regions, cells. A RREA developed in a cell radiates bremsstrahlung gamma-rays. Gamma-rays have high penetrative power and propagate through semi-critical electric field regions, where runaway electrons can stop, reaching other cells. There gamma-rays interact with air molecules, producing RREAs. Therefore, cells amplify each other by the gamma-ray exchange. The amplification rate can be strong enough to make RREAs self-sustainable, that is infinite reactor feedback. Infinite reactor feedback requires lower electric field strength compared to infinite relativistic feedback. Moreover, such RREA multiplication can cause a TGF.

In this report, a theoretical technique is developed to describe relativistic runaway electron avalanches dynamics in complex electric structures. Cells' interaction via high-energy particles exchange can be described with the Feedback Matrix, which is a matrix consisting of feedback operators. A feedback matrix action on RREA starting point distribution in i-th feedback generation creates RREAs starting point distribution in the next (i+1)-th generation. Matrix elements depend on thunderstorm electric field parameters and include RREA development physics and gamma-ray propagation physics. Diagonal matrix elements describe the self-action of cells, which is the relativistic feedback. The proposed approach includes all the feedback mechanisms and reduces the problem of avalanche dynamics to finding eigenvalues and eigenfunctions of the feedback matrix, as the eigenvalues are feedback coefficients.

With the feedback matrix solutions for RREA dynamics in different electric field geometries are obtained. Thunderstorm conditions required for TGF development are found. It is shown that the more complex the electric structure is the lower the electric field strength is required to produce a TGF.

How to cite: Stadnichuk, E.: Terrestrial Gamma-ray Flashes produced by complex thunderstorm electric structures, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-496, https://doi.org/10.5194/egusphere-egu22-496, 2022.

Atmosphere Space Interaction Monitor (ASIM) has now observed more than 1000 Terrestrila Gamma-ray falshes (TGFs) since the launch in 2018. ASIM has two payloads, the Modular X- and Gamma-ray Sensor (MXGS) and the Modular Multi-Spectral Imaging Assembly (MMIA). MXGS consists of two detector layers, one pixelated detector in the low energy range (50 keV to 400 keV) and another in the high energy range (300 keV to >30 MeV), with temporal resolution of 1µs and 28 ns, respectively.  MMIA has three photometers (337 nm, 180-230 nm, 777 nm) and two cameras (337 nm and 777 nm). During nighttime we observe both the TGFs and the lightning that produced them. Multiple and double TGFs  separated by 1-2 ms have frequently been observed by ASIM. In this paper we present three events of double TGFs. All of them are associated with  optical pulses from a hot leader (777 nm), and the first and second pulses come from the same location, indicating that the double TGFs are produced by the same leader as it propagates upward. 

How to cite: Ostgaard, N., Mezentsev, A., Marisaldi, M., Sarria, D., Ullaland, K., Yang, S., Genov, G., Neubert, T., Chanrion, O., Christiansen, F., Cummer, S., Lu, G., Reglero, V., and Luque, A.: Double TGFs and optical pulses observed by ASIM, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3382, https://doi.org/10.5194/egusphere-egu22-3382, 2022.

TGFs are short-duration bursts of high-energy photons shot from Earth’s atmosphere to space. They are produced during the initial upward propagation of +IC lightning leaders and are often associated with LF radio sferics. The Atmosphere-Space Interactions Monitor (ASIM) instrument provides X- and gamma-ray measurements synchronous with optical recordings in 180-240 nm, 337 nm and 777.4 nm wavelengths, allowing simultaneous detection of TGFs and the lightning processes associated with them.

ASIM observations show that TGFs are accompanied by a prominent optical pulse that marks the beginning of a lightning flash. TGFs tend to precede the pulse slightly, but the short duration of TGFs, together with the delay of the optical pulses from photon scattering in cloud particles, does not allow to resolve the correct sequence of events with confidence.

The same problem is present in measurements of radio waves, where the waves emitted by the TGF currents usually are mixed with those of the lightning currents because of the temporal proximity of the processes.

Here we report a remarkable TGF, with a high fluence of 360 counts in the energy range 0.4 - 20 MeV and a relatively long duration of 580 µs. The associated optical pulse is clearly following the TGF, which leads us to conclude that the current surge inside the leader channel is not generating the TGF, as has been proposed by models, but instead that the TGF process conditions the current surge that follows.

How to cite: Mezentsev, A., Østgaard, N., Marisaldi, M., Neubert, T., Chanrion, O., and Reglero, V.: Discerning TGF and leader current pulse in ASIM observation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4240, https://doi.org/10.5194/egusphere-egu22-4240, 2022.

On January 10th 2022 the ASIM mission to the ISS was moved to the SDN mount point on the Columbus module for observations towards the earth rim and the cosmos. This enabled the ASIM/MXGS imager spectrometer to observe TGFs in a small ISS nadir FOV covering 35-65 degrees off-axis, and GRBs in the cosmos which covers most of its FOV. We present here initial results of imaging locations for off-axis TGFs with a low spectral hardness ratio and a much larger event count due to the reduction in the "flux cosine effect", and how this might facilitate estimates of mean TGF altitude. We also present the first MXGS location images of second long GRBs and their associated lightcurves and spectra.

How to cite: Connell, P., Reglero, V., Navarro, J., and Eyles, C.: Imaging TGFs and GRBs from the earth rim mounting of the ASIM/MXGS imager-spectrometer on the ISS, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3037, https://doi.org/10.5194/egusphere-egu22-3037, 2022.

ASIM TGF catalog presents more than one thousand TGF from June 2018 till the end of 2021. Using this dataset, the detection rate of TGF by ASIM was about one TGF per day. The TGF detection is a stochastic process (each TGF is not related with the next) assuming this, the time-difference distribution between one detection and the next should fit an exponential distribution. This time between TGF events distribution fits the exponential with a significant deviation. We see an excess in the number of TGF separated by less than 5 min. From the expected value of less than 1% of the events, we have an 8% of the events in this range. We call that TGF population with a time separation of fewer than 5 minutes “Brother TGF”. Large storm areas could explain this deviation, because of the storm size or also the propagation effects of the TGF inside the storm as an effective mechanism to increase the TGF production in this region. This work we present is a detailed study of the most relevant “Brothers” in the ASIM TGF imaging list. With this, we can locate where these Brothers TGF are located, and add some clues about this Brother TGF production mechanism.

How to cite: Navarro-González, J., Connell, P., Eyles, C., Reglero, V., López, J. A., Montanyà, J., Marisaldi, M., Mezentzev, A., Kochkin, P., Lindanger, A., Sarria, D., Østgaard, N., Chanrion, O., Christiansen, F., and Neubert, T.: Brother TGF, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8201, https://doi.org/10.5194/egusphere-egu22-8201, 2022.

Relativistic electrons in strong large-scale thunderstorm electric fields can obtain more energy from acceleration by the electric field than they on average lose on interactions with air molecules. Such accelerating electrons are called runaway electrons. Runaway electrons can produce additional runaway electrons by Moller scattering on air molecules. In this way, runaway electrons multiply and form a relativistic runaway electron avalanche (RREA).

In strong electric fields, RREA can multiply by relativistic feedback. Infinite relativistic feedback makes avalanches self-sustaining and could hypothetically trigger a terrestrial gamma-ray burst (TGF). This report presents the results of modeling the simplest reactor - the model of the appearance of a TGF, consisting of two cells looking at each other, their comparison with theoretical calculations and previous models.  It was found that the considered model predicts lower requirements for the electric field for the appearance of TGF than the others.

How to cite: Zemlianskaya, D. and Stadnichuk, E.: Simulation of the simplest reactor model of the dynamics of runaway electron avalanches in thunderclouds, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-563, https://doi.org/10.5194/egusphere-egu22-563, 2022.

Terrestrial Gamma-ray Flashes (TGFs) are short flashes of high-energy photons produced by thunderstorms. They are intense phenomena that may have high photon fluxes with energies up to 40 MeV when observed from detectors in orbit. All instruments in space have suffered instrumental saturation during bright events, including CGRO-BATSE, RHESSI, Fermi-GBM, AGILE-MCAL and ASIM-MXGS. The effects include dead-time and pulse pile-up, which lead to an underestimation of the TGF fluences and, in some cases, incorrect photon energies. 
    A key asset of ASIM is that it has two detectors on the same platform: the High Energy Detector (HED, 300 keV to ~40 MeV) and the Low Energy Detector (LED, 50 keV to 400 keV). LED is only weakly affected, which makes it possible to estimate corrections to the HED measurements for even the brightest TGFs. With the method we propose, we estimate the loss of photons by combining the LED and HED measurements with GEANT4 Monte-Carlo simulations of the detector responses. 
    We applied the method to three TGF events. The first, TGF-200728, has about 0.15 counts per microsecond  per unit, and is not expected to experience saturation and is used as a sanity check for the method. The other events, TGF-181102 (1.5 counts per microsecond per unit) and TGF-181025 (2.8 counts per microsecond per unit), indicate that the HED misses at least 50% of the photon counts for the brightest TGF events.

How to cite: Sarria, D., Østgaard, N., Marisaldi, M., Lindanger, A., Mezentsev, A., Lehtinen, N., Neubert, T., Christiansen, F., and Reglero, V.: Investigating the fluence of bright TGF events detected by the Atmosphere-Space Interactions Monitor, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7408, https://doi.org/10.5194/egusphere-egu22-7408, 2022.

Terrestrial gamma-ray flashes (TGFs) are short and highly energetic bursts of photons, produced in association with lightning in thunderstorms. Elves are rapidly expanding rings of optical emissions, with radii of several hundred kilometers, produced when electromagnetic pulses from lightning hit the base of the ionosphere. The Atmosphere-Space Interactions Monitor (ASIM) detects both TGFs and Elves, sometimes simultaneously. Here, we present a study of observations where TGFs are accompanied by Elves. The optical signatures from Elves are identified from measurements by the ASIM UV photometer. Using ground-based lightning location networks, we find associated sferic detections to these events, placing them mainly over oceans and in coastal regions. Using sferic detections by GLD360, we compare the peak currents of the lightning associated with the TGF-Elve pairs to peak currents associated with other TGFs detected by ASIM, as well as with lightning in general. We show that the TGFs accompanied by Elves are among the shorter TGFs detected by ASIM, and they are associated with very high peak currents of typically several hundred kA.

How to cite: Bjørge-Engeland, I., Østgaard, N., Mezentsev, A., Skeie, C. A., Sarria, D., Lapierre, J., Lindanger, A., Neubert, T., Marisaldi, M., Lehtinen, N., Chanrion, O., Ullaland, K., Yang, S., Genov, G., Christiansen, F., and Reglero, V.: Observations by ASIM of Terrestrial Gamma-ray Flashes accompanied by Elves, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9085, https://doi.org/10.5194/egusphere-egu22-9085, 2022.

Narrow Bipolar Events (NBEs) are powerful radio emissions from thunderstorms, which sometimes occur isolated from lightning and at other times appear to initiate lightning. They are recently associated with Blue LUminous Events (BLUEs) on cloud tops and attributed to extensive streamer electrical discharges named fast breakdown, but their physics is not fully understood. Here, we analyse simultaneous observations of NBEs detected by radio receivers on the ground with their optical emissions observed by the Atmosphere-Space Interactions Monitor (ASIM) on the International Space Station (ISS). In this study, we focus on the multiple-pulse BLUEs that include one primary BLUE pulse and one or several subsequent BLUE pulses with a few millisecond intervals, as detected by a photometer at 337 nm. The observations indicate that the initial streamer discharge of an NBE is followed within a few milliseconds of horizontally oriented secondary streamer discharges at similar or higher altitudes but without triggering a leader process.

How to cite: Li, D., Luque, A., Lehtinen, N. G., Gordillo-Vázquez, F. J., Neubert, T., Lu, G., Chanrion, O., Zhang, H., Østgaard, N., and Reglero, V.: Multiple-pulse blue luminous events detected by ASIM, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9138, https://doi.org/10.5194/egusphere-egu22-9138, 2022.

How to cite: Bai, X., Fullekrug, M., Chanrion, O., Soula, S., Peverell, A., Mashao, D., Kosch, M., and Neubert, T.: Height Determination of a Blue Discharge Observed by ASIM/MMIA on the International Space Station, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5994, https://doi.org/10.5194/egusphere-egu22-5994, 2022.

Elves are rapidly expanding rings of optical emissions in the lower ionosphere. Narrow bipolar events (NBEs) are signatures in radio signals from intra-cloud discharges. They are thought to be fast streamer breakdown that may trigger the onset of lightning and blue jets. However, there is a lack of experimental evidence on whether the streamer discharges of NBEs carry sufficient currents to generate elves in the lower ionosphere. Here, we report the first simultaneous observation of NBEs and elves that confirm this hypothesis. The NBEs are observed simultaneously from the ground by an array of wave receivers located in China and from space by spectral measurements by the Atmosphere-Space Interactions Monitor (ASIM) on the International Space Station (ISS). We observe thirteen negative and six positive NBEs produced in four thunderclouds penetrating into the stratosphere. Five NBEs are accompanied by elve emissions observed in the near-ultraviolet of the Lyman–Birge–Hopfield (LBH) band.  They were at ~18 km altitude, and their peak currents, estimated by a ground-based lightning detection network, were larger than 135 kA. The observations show that the impulse currents of the streamers are of sufficient magnitude to power elves, thereby adding to the new pathways that thunderstorms affect the lower ionosphere. 

How to cite: Liu, F., Neubert, T., Chanrion, O., Zhu, B., Lu, G., Lyu, F., Dimitriadou, K., Lei, J., Østgaard, N., and Reglero, V.: Ionospheric elves powered by negative Narrow Bipolar Events in overshooting thunderclouds, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7949, https://doi.org/10.5194/egusphere-egu22-7949, 2022.