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We analyze the lightning activity over central Europe from 2017 to 2022 using the optical data from the Lightning Imaging Sensor (LIS) on board the International Space Station (ISS). The area of interest covers a central European region limited by 54.5° N, 7.5° E and 44.5° N, 22.5° E. A total number of 68192 lightning flashes was detected during 1805 ISS orbital overpasses. This study compares the lightning activity in central Europe to the global lightning activity and investigates the impact of the COVID-19 pandemic. While there is a global reduction of the lightning activity during the lockdowns in 2020, no significant decrease is observed in central Europe.On the territory of Czechia, the highest density of flashes was detected in the northwestern part of the country. We combine the ISS-LIS data with measurements of the Shielded Loop Antenna with Versatile Integrated Amplifier (SLAVIA) detectors located in this region. The measurements of the ISS-LIS and SLAVIA detectors are combined with data from the World Wide Lightning Location Network (WWLLN) or Global Lightning Dataset (GLD360) in order to understand the correlation between electromagnetic radiation from selected lightning flashes and their optical characteristics observed from space.

A presence of regular sequences of microsecond-scale pulses has been occasionally reported in the lightning literature for more than forty years. Due to a fine time resolution of modern electromagnetic receivers, the properties of these pulse trains are now well described. Nevertheless, the conditions for their occurrence are still not understood, and the information needed for their proper modelling is not sufficient. To contribute to this effort, we report for the first time properties of negative recoil stepped leaders accompanied by regular trains of microsecond-scale pulses simultaneously seen by the broadband magnetic loop antenna SLAVIA (Shielded Loop Antenna with a Versatile Integrated Amplifier; 5 kHz-90 MHz), and the radio telescope LOFAR (Low Frequency Array; 30-80MHz). We investigate four pulse trains that occurred during complicated intracloud flashes on 18 June 2021, when heavy thunderstorms hit Netherlands.The pulses within the trains are unipolar, a few microseconds wide with an inter-pulse interval of about ten microseconds. The pulse trains last from 100 µs to 800 µs. After a careful time alignment of both magnetic field and LOFAR time series, we found that the broadband pulses perfectly match with regularly distributed and relatively isolated bursts of VHF sources localized by the LOFAR impulsive imager. All trains were generated by negative recoil stepped leaders propagating downward (two events) or upward (two events) at altitudes between 5.5 km and 8.5 km. Their tracks were formed by positive leaders occurring within the same flash several hundreds of milliseconds previously. The peak powers of VHF sources seen by the LOFAR electric antennas closest to the investigated discharges were about one order of magnitude higher than the power of signals emitted by normal negative leaders. These stepped recoil leaders propagate at a relatively low speed of about 2-5x10^6 m/s, when similar recoil leaders often reach speeds of 10^7 m/s. The velocity and inter-pulse intervals decrease towards the end of trains.We show that observed pulse trains are due to stepping recoil leaders. However, we consider this strong pulsing nature of the examined recoil leaders to be quite unusual. The physical mechanism giving rise to the energetic VHF bursts and accompanying regular microsecond-scale pulses remains unclear.

We study evolution of lightning activity accompanying rapid intensity changes of tropical cyclones worldwide. We use a dataset of 400 tropical cyclones occurring between 2012 and 2017. We use the cyclones tracks from the International Best Track Archive for Clime Stewardship. The lightning data are provided by the World Wide Lightning Location Network (WWLLN). We inspect the lightning activity and median stroke energies accompanying rapid intensifications (RI) of cyclones, defined as increases of the wind speed by more than 30 kt in 24 hours, and their rapid weakenings (RW), defined as decreases of the wind speed by more than 40 kt in 24 hours.In an area of radial wind maximum (RWM), we observe a stroke density of 15.1 strokes/(100 km)2/hour for RI and 21.8 strokes/(100 km)2/hour for RW, respectively, which is much higher than average RWM density 7.9 strokes/(100 km)2/hour over the duration of the cyclone. A median stroke energy is 0.3 kJ during RI and 0.7 kJ during RW. It means that during rapid intensification of cyclones, there are less strokes with slightly higher energies and during rapid weakening there are more strokes with slightly lower energies. When analyzing the cyclones in both hemispheres separately, we obtain 0.3 kJ for RI and 0.6 kJ for RW in the northern hemisphere, and 0.8 kJ for RI and 0.9 kJ for RW in the southern hemisphere. The difference in the stroke density during RI and RW was observed larger in the northern hemisphere (19.7 vs 34.1 strokes/(100 km)2/hour), when in the southern hemisphere the stroke density is much lower and differs less (4.4 strokes/(100 km)2/hour for RI and 5.1 strokes/(100 km)2/hour for RW).

Electromagnetic pulses (EMPs) and quasi‐static electric fields (QE) from powerful lightning heat and ionize the lower ionosphere. The EMP disturbance may appear as an elve at ∼80–95 km altitude, and the QE field as a halo or a sprite at ∼60–80 km altitude. Both are thought to perturb crossing radio signals because of changes to the electrical conductivity of the regions. Here we present an analysis of 63 elves and corresponding radio signal perturbations from an almost stationary thunderstorm system that allows us to untangle some of the dependencies of perturbations on the lightning characteristics. The amplitude perturbations of a VLF‐transmitter signal are characterized as either long‐recovery, early events (LOREs) or as early events. We find that LOREs are related to lightning with high peak currents and bright elves, and that their sign (amplitude increase or decrease) depends on the relative locations of the transmitter, disturbance and receiver. Based on a subset of strokes, lightning with elves has on average ∼3 times the impulse charge‐moment‐change and power in broadband as lightning of similar peak currents without elves. The early events occur without observed elves, sprites or halos. They recover in ∼10–100 s and are observed for both polarities of cloud‐to‐ground lightning and for intracloud flashes. It is proposed that these observations may relate to regions of reduced conductivity caused by an electron attachment/detachment process at lower heights, or by electron enhancements associated with TLEs that are too dim to be detected by the camera.

Two long-lasting thunderstorm ground enhancement (TGE) events were registered at the Milešovka meteorological observatory in Czechia (50.55∘ N, 13.93∘ E; 837 m altitude) on 23 April 2018, during linearly organized thunderstorms. Two intervals of increased photon counts were detected by a plastic scintillator, respectively lasting 70 and 25 min and reaching 31 % and 48 % above the background radiation levels. Using numerical simulations, we verified that the observed increases in count rates are consistent with the energy spectrum of previously observed TGEs. We investigated the relevant data from a suite of meteorological instruments, a Ka-band cloud radar, an electric field mill, and a broadband electromagnetic receiver, all placed at the Milešovka observatory, in order to analyse the context in which these unique continental TGEs occurred at an exceptionally low altitude. The onset of the TGEs preceded the onset of precipitation by 10 and 3 min, respectively, for the two events. Both this delayed rain arrival and an energy threshold of 6.5 MeV for registered particles clearly exclude the detection the decay products of the radon progeny washout during the TGE intervals. At the same time, the European lightning detection network EUCLID detected numerous predominantly negative intracloud lightning discharges at distances closer than 5 km from the particle detector, while the occurrence of cloud-to-ground discharges was suppressed. The cloud radar recorded presence of graupel below the melting level, and the composition of hydrometeors suggested good conditions for cloud electrification. The observed variations in the near-surface electric field were unusual, with very brief negative-electric-field excursions reaching −20 kV in a quick succession. At the same time, sub-microsecond unipolar pulses emitted by close corona discharges saturated the broadband magnetic loop antenna. All these measurements indicate that a strong lower positive-charge region was present inside the thundercloud. The bottom thundercloud dipole was probably responsible for acceleration of the seed electrons in the air. These seed electrons might originate in the secondary cosmic ray particles but could also come from a high concentration of radon in the air collected during the propagation of the convective system above the uranium-rich soils before the thunderstorms overpassed the Milešovka observatory.

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.

Our case study aims at contributing to the discussion on sources of plasmaspheric hiss, which is known for its interactions with the Earth radiation belts. We analyze multi-point measurements of electromagnetic field waves by the Wide Band Data instruments onboard the four Cluster spacecraft in order to find sources of hiss observed close to the geomagnetic equator in the dayside outer plasmasphere. We find hiss to be triggered from whistlers of different spectral properties. Whistlers with the lowest observed dispersion arrive to different spacecraft with time delays indicating their origin in the northern hemisphere. Positions of source lightning discharges are then found using the time coincidences with the Word Wide Lightning Location Network data from three active thunderstorm regions in Europe. We find that subionospheric propagation of lightning atmospherics is necessary to explain the observations. Geographic locations of their ionospheric exit points then determine spectral properties of resulting unducted whistlers and triggered hiss. By this well documented chain of events starting with a lightning discharge in the atmosphere we confirm that magnetospherically reflecting whistlers and hiss triggered from them are among possible sources of plasmaspheric hiss.

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).

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.

A broad-band analyzer of the IME-HF instrument (“Instrument de Mesure du champ Electrique Haute Frequence”) is prepared for the TARANIS (Tool for Analysis of RAdiation from lightNIng and Sprites) micro-satellite of the French space agency CNES. The spacecraft is based on the MYRIADE series platform. It will be launched on a Sun synchronous polar orbit at 700 km altitude. TARANIS will carry a complex payload of six scientific instruments to study radiation from lightning and optical phenomena (Transient Luminous Events) observed at altitudes between 20 and 100 km (blue jets, red sprites, halos, elves). The scientific instruments onboard TARANIS will detect electromagnetic radiation from very low frequencies up to 37 MHz, optical radiation, X rays (with an aim to study the Terrestrial "Gamma-ray" Flashes), and energetic electrons.The IME-HF instrument will record waveform measurements of fluctuating electric fields in the frequency range from a few kHz up to 37 MHz, with the following scientific aims: (i) Identification of possible wave signatures associated with transient luminous phenomena during storms; (ii) Characterization of lightning flashes from their HF electromagnetic signatures; (iii) Identification of possible HF electromagnetic or/and electrostatic signatures of precipitated and accelerated particles; (iv) Determination of characteristic frequencies of the medium using natural waves properties; (v) Global mapping of the natural and artificial waves in the HF frequency range, with an emphasis on the transient events. The instrument will be also able to trigger and record interesting intervals of data using a flexible event detection algorithm.

VLF (VeryLow Frequency) spectrograms registered at Kannuslehto ground station, after cleaning them from strong sferics, reveal VLF noise suppression by whistlers and whistler echo trains, which consists in significant reduction in the noise spectral power after a strong whistler event. We have found similar effect in the VLF data from Van Allen Probe B taken in the equatorial region on L‐shell ~3. Detailed analysis of the data shows that the whistler echo train and the VLF noise have small wave normal angles. Based on this observation, we limit our analysis to parallel (ducted) whistler wave propagation. The persistence of whistler echo train, as well as the VLF noise, suggests that in the events under discussion, plasma is unstable in the frequency range corresponding to the observed VLF noise band. In an attempt to explain the effect of VLF noise suppression, we follow up the long‐standing idea that relates this effect to the reduction of free energy in the unstable plasma distribution by whistler echo train. To develop this idea into qualitative model, we have studied the motion of energetic electrons, responsible for the noise generation, in the field of ducted whistler echo train. We show that energetic electrons that make the main contribution to the growth rate of VLF noise, during their bounce oscillations in the magnetosphere, are subject to multiple resonant impacts from the whistler echo train. These lead to energetic electron diffusion in the phase space and the corresponding reduction in free energy of the unstable distribution.