A Comprehensive analysis of lightning Initiation with LOFAR

Characteristics of a Negative Cloud-to-Ground Lightning Discharge Based on Locations of VHF Radiation Sources

Abstract. We investigate properties of the electromagnetic activity following the first lightning return stroke (RS), using concurrent observations from the SLAVIA (Shielded Loop Antenna with a Versatile Integrated Amplifier) sensor, the SAETTA (Suivi de l'Activité Electrique Tridimensionnelle Totale de l'Atmosphère) Lightning Mapping Array (LMA) and the METEORAGE LF (low-frequency) network in the Corsica region. From the data collected between September and December 2015, we selected 66 negative cloud-to-ground (−CG) and 26 positive cloud-to-ground (+CG) lightning flashes in the north-western Mediterranean region. In the SAETTA data, we observe a decrease in the very high frequency (VHF) radiation rate and the VHF power, as recorded within a typical 80 µs time window at the LMA stations, immediately after the RS pulse in 59 −CG flashes. Contrastingly, we show that all examined +CG flashes exhibit a rapid increase in the VHF radiation rate and the VHF power immediately after the RS. We suggest a possible explanation of this phenomenon by considering the step-like propagation of a negative part of the bidirectional leader starting at the top end of the positive lightning channel inside the thundercloud, emitting electromagnetic radiation across a broad frequency spectrum.

The paper presents the results of the first measurements devoted to the experimental verification of the capability of space object detection by passive radar system using astronomical LOFAR (LOw-Frequency ARray) radio telescopes as receivers and DAB+ commercial transmitters as illuminators of opportunity. Two LOFAR stations were used in the measurements, one station as a surveillance receiver and the other as a reference receiver. The obtained results confirmed the earlier expectations about the possibility of passive space target detection employing the LOFAR radio telescopes or dedicated phased antenna arrays with dimensions and complexity comparable to the LOF AR radio telescopes.

The discharge process is often discussed in the lightning physics. VHF (very high frequency) observations play an important role in studying lightning discharge processes, because they show where air breakdown occurs. Triggered lightning flashes are ideal research objects, because they are generated in a fixed time and place. Regular experiments of triggering lightning have been conducted by Chinese Academy of Meteorological Sciences since 2006, and 189 lightning flashes are triggered successfully. Besides regular observations, such as fast/slow antenna, channel-based current and high-speed video, the continuous interferometer (CINTF) is developed and deployed at the site for triggered lightning after 2016 in order to observe detailed discharge processes. The CINTF can provide high-precision location with time resolution of around 1 μs.A multi-stroke triggered lightning with 8 return strokes is observed by a self-developed lightning continuous interferometer during Guangdong Comprehensive Observation Experiment on Lightning Discharge on 11 June 2019. The whole discharge processes, including precursor current pulse, initial precursor current pulse, initial continuous current and return strokes, last for about 1600 ms. The channel-based current, electric field change and radiation source positions are obtained. CINTF results show that the radiation signal generated from discrete precursor current pulse with a minimum value of about 8 A can be detected. The average transfer charge of initial precursor current pulse is twice as much as that of the precursor current pulse. During the initial continuous current stage after continuously developing of the upward positive leader, the obviously forward channel extension is the main discharge form at the beginning, and then the extension can be accompanied by discontinuously backward propagation, and recoil leader often occurs in the following stage. M-component in the ICC stage is caused by two discharge processes:Forward positive streamer (or leader) or recoil leader generated in the existing channel. Multi recoil leaders occur during the whole M process, which maintain M discharge. The main discharge form in return stroke stage is recoiling discharge. There are multi recoil leaders before return strokes, however, most of them cannot develop to ground, keep as an attempted leader until the last leader-stroke occurs. Adjacent two strokes with a short duration time of 4.5 ms have the same optical channel in optical images, but they are obviously different in development paths, which lead to a very short RS interval.

A basic broadband digital interferometer was developed, which is capable of locating Very High Frequency (VHF) radiation sources in two spatial dimensions and time. Three antennas sensed the time series of broadband electromagnetic (EM) signals and digitized with 4 ns sampling interval for a duration of several milliseconds. A technique based on cross-correlations has been implemented for mapping lightning source locations. A map of the first return stroke (RS) and the preceding stepped leader was mapped successfully, using the system with a time resolution of few milliseconds. The result was compared with the visible events of the ground flash to validate the system.

Studies of physical conditions in active galactic nuclei and cosmological studies require low-frequency observations of compact radio sources with high sensitivity and resolution. To advance considerably in this research, observations in the meter wavelength range with a sensitivity of about 1 mJy are needed. The sensitivity of low-frequency observations of extragalactic radio sources is limited by the confusion effect. To suppress it to a 1 mJy level, one needs antennas with apertures of 300–400 km. Currently, new-generation SKA and LOFAR radio telescopes are developed, which are aperture-synthesis systems based on small antennas which have dimensions of several hundreds of kilometers. Such measuring complexes will allow one to achieve a sensitivity of 1 mJy. In this work, we propose to make an interferometer with baseline exceeding 5000 km on the basis of in-phase kilometer antenna arrays. It is shown that such a system will have a sensitivity of about 1 mJy at a frequency of 100 MHz. Along with the high sensitivity, the proposed interferometer based on diffraction gratings will make it possible to form a multilobe directional pattern that will cover the entire observable hemisphere in terms of inclination. This opens up new opportunities for fast surveys of weak small-size radio sources.

Characteristics of a Negative Cloud-to-Ground Lightning Discharge Based on Locations of VHF Radiation Sources

An investigation into the characteristic features of lightning flashes to ground in Port Moresby, Papua New Guinea (latitude 9°S, longitude 147°E), has been carried out. A scheme is presented for the identification of discharge features from simultaneous records of optical radiation, electric field change, and HF radiation. Data were obtained from records of 80 identified ground flashes comprising 282 strokes, 202 interstroke intervals, 223 continuing currents, and 53 stepped leaders. Flash durations are log normally distributed with a geometrical mean of 604 ms and a standard deviation of 0.25 decade. Sixty‐eight per cent of pre‐first‐stroke processes had durations exceeding 100 ms, the mean being 240 ms. Of 59 flashes outside the reversal distance electric field, records of 80% indicated that negative charge was raised before the first return stroke. The distribution of stepped leader durations is normal with a mean of 21 ms and standard deviation of 8 ms. Interstroke intervals are approximately log normally distributed, with a geometrical mean of 61 ms and a standard deviation of 0.36 decade. The geometrical mean value of continuing current duration is 5.5 ms and the standard deviation 0.70 decade, but the distribution is neither normal nor log normal. Of 34 multiple stroke flashes, 47% have at least one long continuing current. The form of the frequency distribution of continuing current duration may reflect a fundamental distinction between long and short continuing currents: however, it is probably the result of differences between the 1976 and 1977 distributions.

Stochastic equations of the hydrodynamic theory of plasma are presented in relation to strong external fields. It is shown that the use of these stochastic equations makes it possible to obtain new theoretical solutions for plasma as a result of its heating in a strong external electric field. Theoretical solutions for the conductivity of turbulent plasma when heated in an external electric field of 100 V/cm are considered. Calculated values for the electron drift velocity, electron mobility, electron collision frequency, and the Coulomb logarithm in the region of strong electric fields are obtained. Here we consider experiments on turbulent heating of hydrogen plasma in the range of electric field strength of 100 < E < 1000. The calculated dependences of plasma conductivity are in satisfactory agreement with experimental data for heating plasma in a strong electric field. It is shown that the plasma turbulence in the region of strong electric fields E ~1000 V/cm is close to 100%. For the first time, it is confirmed that the derived dependences for collision frequency, drift velocity, and other values include the degree of turbulence of plasma, which makes it possible to correctly describe experimental data for heating plasma even with strong electric fields. In addition, it was determined that the scatter of experimental data may be associated with the variability of the function in the expression for the heat flux density. For the first time, it is shown theoretically that the experimentally determined fact of the possibility of the existence of an approximate constancy of plasma conductivity in the region E = 100–1000 V/cm can occur with an error of ~30%. The results show significant advantages of the stochastic hydrodynamic plasma theory over other methods that are not yet able to satisfactorily as well as qualitatively and quantitatively predict long-known experimental data while taking into account the degree of turbulence.

The characteristics of two natural positive cloud‐to‐ground flashes are analyzed in detail with the broadband very high frequency (VHF) mapping system. The bidirectional leader processes at the lightning initiation are observed, with negative leaders propagating upward and positive leaders propagating downward. The downward positive leaders can be divided into two stages, with scattered and slowly extending sources in the first stage and more concentrated and faster sources in the second stage. The overall extension speeds of the first stages are 1.8 × 104 and 2.7 × 104 m/s for the two flashes, and the speeds of the second stages are 7.8 × 105 and 1.0 × 106 m/s, respectively. The received peak power of the positive leaders is substantially lower than that for the negative leaders. The frequency characteristics show that the second stage emits more high‐frequency radiation than the first stage, while there is no significant difference in low‐frequency radiation content.

This chapter contains sections titled: Introduction Previous Treatments of the Nonlinear Dielectric and Birefringence Relaxation Orientational Relaxation Processes in the Presence of Strong Electric Fields: The Rotational Diffusion Model Nonlinear Transients in Dielectric Relaxation and Dynamic Kerr Effect Arising from Sudden Changes of a Strong dc Electric Field Steady-State Response Arising fom a Weak ac Field Superimposed on a Strong dc Bias field Nonlinear stationary Respones in Strong ac and dc Electric Fields Effects Due to Hyperpolarizabilities of Molecules on Kerr Effect Relaction Intertial Effects in Dielectric and Birefringence Relaction in a Strong sc ElectricField: Linear response Application to Related Relaxation Phenomena Conclusion

Experimental observations of the optical and electromagnetic fields generated by lightning flashes during the last 50 years have significantly advanced our knowledge concerning the mechanism of the lightning flash. Nevertheless, this knowledge is not as exhaustive as that of long laboratory sparks due to our inability to observe lightning flashes under controlled conditions. Thus, the mathematical description of the mechanism of the lightning flash is relatively poor at present even though the main features of the lightning flashes themselves are well known. The main goal of this chapter is to provide the reader with the important features of the mechanism of the lightning flash. No attempt is made to provide an exhaustive list of the literature since this can be found elsewhere. The chapter is organised as follows. First a basic description of the mechanism of lightning flashes is given to introduce the reader to the terminology used in lightning research. After that, each event associated with the lightning flash is described in detail with particular attention being paid to the electromagnetic fields generated by these events. Nomenclature: In this chapter a positive discharge is defined in such a way that the direction of motion of electrons in such a discharge is opposite to that of the discharge itself; a negative discharge is defined as one in the opposite sense. According to this definition a 'negative return stroke' is a positive discharge and a 'positive return stroke' is a negative discharge. A positive field change is defined to be in the sense of negative charge being lowered to ground or positive charge being raised. According to this definition a lightning flash that transports negative charge to ground gives rise to a positive field change.

A positive streamer in a strong electric field in air at 1-atm pressure is investigated with particle-in-cell/Monte Carlo collision method including a photoionization model. The simulation results of streamer discharge in parallel plate air gap with and without the photoionization model are compared. It is shown that the seed electrons produced by photoionization generate secondary avalanches which lead to the formation of the streamer. No streamer is observed without photoionization model.

The results of a linear analysis for the stability of a homogeneous dielectric liquid with respect to density perturbations in a uniform electric field are presented. The electric field increases the instability increment for the stratification along the field and decreases this increment in the transverse direction. Thus, a strong electric field can induce anisotropic decay into liquid and vapor phases for a fluid that is initially both in a labile state and in a metastable or stable state. Theoretical calculations are confirmed by the results of numerical modeling of the fluid dynamics. The new regions of a low-density phase have the form of narrow cylindrical channels oriented along the field. This new mechanism of gas phase formation in strong local electric fields probably plays a key role in the inception and ultra-fast propagation of streamers during the breakdown in dielectric liquids in a nanosecond range.

Obtaining lensing time delay measurements requires long-term monitoring campaigns with a high enough resolution (< 1 arcsec) to separate the multiple images. In the radio, a limited number of high-resolution interferometer arrays make these observations difficult to schedule. To overcome this problem, we propose a technique for measuring gravitational time delays which relies on monitoring the total flux density with low-resolution but high-sensitivity radio telescopes to follow the variation of the brighter image. This is then used to trigger high-resolution observations in optimal numbers which then reveal the variation in the fainter image. We present simulations to assess the efficiency of this method together with a pilot project observing radio lens systems with the Westerbork Synthesis Radio Telescope (WSRT) to trigger Very Large Array (VLA) observations. This new method is promising for measuring time delays because it uses relatively small amounts of time on high-resolution telescopes. This will be important because instruments that have high sensitivity but limited resolution, together with an optimum usage of followup high-resolution observations from appropriate radio telescopes may in the future be useful for gravitational lensing time delay measurements by means of this new method.