Recently, a new mechanism of the lightning discharge initiation in a thundercloud has been discussed. It is based on a noise-induced kinetic transition, which consists of an increase in the concentration of atmospheric ions in the intracloud environment under the impact of the stochastic electric field of charged and polarized hydrometeors (drops, snowflakes, graupel, hail). The source of noise is electric field bursts that occur during collisions or near collisions of hydrometeors and are accompanied by the streamerless corona discharge ignition. The key point in this scenario is the relay process, in which new ionization centers, when the corona discharge is ignited, arise against the background of spots of negative ion charge remaining from the spreading old centers. This leads to a gradual increase in the concentration of negative ions, which could serve as a source of free electrons with a new increase in the electric field. In this work, we theoretically study the possibility of electron detachment from negative ions formed in a streamerless negative corona near hydrometeors under thundercloud conditions. It is shown that in this case the dominant negative ions near corona hydrometeors are O_2^ - (H_2O)_k and O_4^ - cluster ions. It follows from the calculations that when a high electric field is applied, electrons are released not via direct detachment from cluster ions, but in multi-steps in a sequence reverse to that observed without a field during the formation of cluster ions. As a result, to detach electrons from ions like O_2^ - (H_2O)_k , fairly moderate pre-breakdown reduced electric fields are required at a level of 65 Td.

Lightning is a self-developing charge transport system that exhibits fundamental manifestations of the macroscale polarity asymmetry. This asymmetry is tightly connected with peculiarities of the reversal point (the point of zero leader sheath charge or, alternatively, a site where the lightning channel potential coincides with the intracloud one) movement along the leader growth direction. The study presents a stochastic lightning discharge model that is used to analyze the reversal point dynamics. It is shown that the reversal point shifts towards the direction of growth of the dominant leader, i.e. the leader with prevailing peripheral current. The cases of upward and downward directions of the positive leader propagation are considered to study how the difference in altitudes of positive and negative lightning poles influences the mode of reversal point drift. The model predicts that, for the case of an intracloud lightning, the amplitudes of oscillation of reversal point altitude and voltage are of the order of 1 km and 100 MV, respectively, with the reversal point voltage change rate being of the order of 10 MV/ms. The results allow to conclude that the reversal point meandering is a fundamental manifestation of lightning evolution.

The article presents the sociological study results on the preferred forms of medical students’ academic work in the era of digitalization. The study was carried out among students of the Volga Research Medical University using an online survey. The study purpose is to identify the attitude of various typological groups of medical students to various forms and methods of scientific and methodological support of the educational process, including the use of digital technologies. The work showed that behind the variety of medical students’ attitudes on the effectiveness of different forms of education, there is a willingness of young people to move away from traditional methods of obtaining professional knowledge and skills, and they clearly have a complex request for new forms of teaching, including using the Internet and electronic resources. The assessment of various forms and teaching methods has noticeable differences depending on the affiliation of students to various faculties. The article analyzes the students’ assessments of the various forms’ effectiveness of education using multidimensional data analysis, which made it possible to identify four types of student attitudes: 1) orientation to online learning; 2) live contact with the teacher; 3) electronic library; 4) work with summaries of educational materials. Each of these types contains its own set of preferred and rejected items

The bipolar lightning development model was used to study the dependence of the potential that is transported to the earth by the downward leader channel. It was shown that this parameter strongly depends on the starting position of the lightning and on the trajectories of formation of its bipolar leaders. It was shown that the main reason for the change in potential is not the loss of voltage in the lightning channel with a finite conductivity but its polarization in the electric field of the storm cloud. An estimate was made of the range of potential variation in the channel with ideal conductivity depending on the starting position and trajectory of the lightning at a constant charge in the thunderstorm cell. It was shown that, for the variation of the lighting current within two orders of magnitude, a mere twofold change in the charge of the thunderstorm cell is sufficient. The preferable starting position is found for the lightning whose upward leader can penetrate into the upper layers of the troposphere, turning into a blue jet.

It is widely known that there is an asymmetry in development modes of positive and negative leaders. Negative leaders in air always propagate in a step-wise manner, while positive ones can grow either continuously (at least optically) or via steps (restrikes). The step-formation mechanism in negative and especially positive leaders is poorly understood. In particular, it is still unknown how space stems, precursors of space leaders, form inside and/or at the boundary of the streamer corona, where an expected electric field is sufficiently smaller than the dielectric strength of air. The study presents numerical simulation of electrostatic conditions inside streamer zones of positive and negative lightning leaders. It is shown that, regardless of polarity, streamer coronas are characterized by highly nonuniform electric field distribution. Moreover, it appears that they always contain the areas of locally increased electric fields sometimes exceeding the breakdown level that can be the potential places of space stems origin. Due to the well-known asymmetry between propagation threshold fields of positive and negative streamers, the number of such zones and the magnitudes of electric fields inside them are sufficiently bigger for negative leaders. A significant factor influencing the level of electric field amplification is found to be the share of streamer zone channels that deviate from an expected direction of growth. The role of the streamer corona fractal dimension and the leader channel sheath line charge density is also analyzed. The model predicts that the space stem formation conditions worsen with increasing altitude above the mean sea level and get better with rising humidity which is in line with observations. The study is helpful for further development of the theory of propagation mechanisms of positive and negative leaders.

We review the basic issues of the initiation and development of the lightning discharge, which top the list of the most important and yet unsolved problems of atmospheric electricity. The main challenges of creating theoretical models are due to the fact that the value of the electric strength of atmospheric air exceeds the peak electric fields measured in thunderstorm clouds by approximately an order of magnitude. Among several concepts proposed to explain the process of initiation of a lightning discharge at different times, two ideas stand apart, namely, the hypothesis of the lightning birth caused by the initiation of a positive streamer from the surface of a hydrometeor and the hypothesis of lightning initiation due to the development of a runaway electron breakdown. However, none of these approaches has become universally recognized or dominant due to various difficulties. A fundamentally new mechanism of lightning discharge initiation, which has been proposed recently, is based on the noise-induced kinetic transition occurring in the stochastic field of charged hydrometeors. The proposed approach looks like a sequence of discharge activity transitions from small spatial scales to increasingly larger ones. One of the main features of the proposed hypothesis is that the generation of streamers is determined by the level of small- and mesoscale fluctuations of the electric field in the thunderstorm and is essentially independent of the large-scale field. In this case, the role of the large-scale field is to ensure interaction of the initiated streamers, when they start developing mainly in the direction specified by this field. In the final section of the review, we discuss the fundamental role of the polarity asymmetry in the processes of the initiation and further development of lightning discharges.

The main intrigue of lightning initiation is that electric fields measured in thunderclouds have peak values about an order of magnitude lower than the dielectric strength of air and do not even exceed the minimum field needed for positive streamers propagation. It was recently established that pronounced decimeter-scale electric field fluctuations appear against the background of a relatively weak thundercloud large-scale electric field owing to turbulent mixing and polarization of hydrometeors. In the course of thundercloud development these field enhancements reach a level sufficient for initiation of positive streamers and their propagation over the distances of the order of decimeters. Streamers initiate, develop, decay, and merge with each other, thereby providing the appearance of an extensive hierarchical system of interacting plasma channels at different stages of development with embedded and growing hot segments. In this study the renormalization group approach is employed to calculate the value of thundercloud electric field that provides hot plasma channel network explosive formation. The calculated threshold field is considerably less than the minimum field required for positive streamers propagation and may be viewed as the true lightning initiation field.

A new mechanism of charge transport inside a thundercloud is suggested and numerically investigated. The considered mechanism can be called “relay” because it is provided by a dynamical network of a relatively small amount of continuously decaying and arising conducting plasma formations. It manifests itself in two consecutive modes corresponding to pre-streamer and streamer/leader stages of thundercloud development. The first one is provided by dynamics of conducting ionic spots recently described by Iudin et al.1 that prepare conditions for initiation of positive streamers. The second mode relies on dynamical network of streamer/leader discharges and finally results in the formation of a compact well-conducting structure that bridges an area of strong electric field inside a thundercloud and can be associated with a lightning “seed”. The effectiveness of relay charge transport strongly depends on the relative proportion of conductive elements (plasma formations) and drastically increases in the field-dependent case.

The problem of diffusion through the fluctuating medium, where processes of substance disintegration and reproduction are possible, was posed at the end of the last century. It was established that the action of multiplicative external noise on a system can result in qualitative reorganization of its dynamical behavior. When such reorganization leads to the appearance of a new stationary dynamic mode, it is customary to speak about a noise-induced phase or kinetic transition. In this paper the noise-induced kinetic transition in two-component environment where the interacting components have contrasting lifetimes and diffusion coefficients is considered. It is shown that the presence of an additional long-lived component can lead to a dramatic decrease in the system generation threshold. We called this effect the depository reproduction. Analytical consideration of the diffusion process in a fluctuating medium causes enormous difficulties even for a single component substance. Meanwhile, in some cases of practical interest, the problem consideration can be conducted using stochastic geometry and percolation theory in particular. In the present work the noise-induced kinetic transition in two-component distributed systems is studied by the tools of directed percolation. To present the depository reproduction effect more vividly we use a new numeral grossone that allows to express different infinitesimal and infinite numerals. It was shown that the reverse conversion of the long-lived component to the short-lived one ensures the survival of the system at significantly lower concentrations of production centers.

It's a common knowledge for spark discharge researches that negative leaders in air always propagate in a step-wise manner. A new step forms when a space leader attaches to the primary negative one. Space leaders originate from small fusiform plasma formations which are called space stems. For now, there is no any established idea about how space stems form in conditions when the background electric field inside the negative corona volume is about three times less than the dielectric strength of air. In this study, we propose a new mechanism of space stem precursors formation which is based on the joint action of ionization and electron drifting processes occurring at the negative corona streamer burst boundary in the presence of strongly inhomogeneous stochastic electric field. It is supposed that electric field fluctuations are formed by chaotically positioned clusters of negative charge which can be considered as basic elements of spatio-temporal noise and are transported into the negative corona volume by negative streamer heads. The last are emanated from the newly-formed leader tip during the development of the negative corona streamer burst finishing the step-formation process. The model is applied to specify conditions under which space stem precursor genesis is possible and to analyze the peculiarities of proposed mechanism at different altitudes above sea level.

Продолжение статьи ЗиВ №1, 2021, стр. 46–58.

Несмотря на многочисленные достижения современной науки, до сих пор остается нераскрытой проблема зарождения молниевого разряда в безэлектродном грозовом облаке, максимальная напряженность электрического поля в котором примерно на порядок меньше диэлектрической прочности воздуха.Хотя не вызывает сомнений тот факт, что развитие разряда начинается с появления в облаке положительных стримеров, развитие которых становится возможным при примерно вдвое меньших значениях электрического поля по сравнению с отрицательными, на настоящий момент остается неизученным вопрос о том, каким образом холодные слабопроводящие стримерные системы объединяются в горячий хорошо проводящий лидерный канал, способный к самостоятельному распространению за счет эффективной поляризации в относительно слабом внешнем поле.В данной работе представлена самоорганизующаяся транспортная модель, реализованная на примере формирования фрактального древа электрического разряда в грозовом облаке и направленная на численное моделирование процесса начальной стадии развития молниевого разряда.Среди инновационных особенностей нашего подхода, отсутствующих в других численных моделях развития молнии, можно выделить отсутствие привязки элементов проводящей структуры графа к узлам пространственной решетки, высокое пространственно-временное разрешение и учет временной эволюции электрических параметров транспортных каналов.Кроме того, модель учитывает известную из многочисленных экспериментов асимметрию полей развития положительных и отрицательных стримеров.В рамках используемого подхода результирующий хорошо проводящий лидерный канал формируется за счет коллективного эффекта объединения токов десятков тысяч взаимодействующих между собой стримеров, каждый из которых изначально обладает пренебрежимо малой проводимостью и температурой, не отличающейся от температуры

Recently for a number of thunderstorm systems in different regions of the planet there have been reports of observations of high frequency radio emission with duration varying from several to more than one hundred of milliseconds which may both be isolated or precede the lightning initiation. There is a consensus that this radiation originates from continuous sequences of short-duration discharges that randomly fill the areas of strong electric field between the main negative and the upper positive charge regions. But the concrete physical mechanism of discharge activity evolution and its contribution to the lightning leader formation remain unclear. To answer these questions several hypotheses were proposed, among which the runaway breakdown and fast positive discharge can be accentuated. In this study, we present another explanation which is based on comparison of experimental data of thundercloud radio activity obtained in the Nizhny Novgorod region of the Russian Federation on May 15, 2019 with the use of a wide band radio interferometer with the results of our numerical simulation of the lightning leader formation in the active part of a thundercloud. It is shown that the observed intense electric field bursts can originate from mergers of two or more streamer systems or bipolar leaders with lengths ranging from several to several tens of meters. In the framework of our approach, the sequence of such mergers can, under certain conditions, result in the self-sustaining leader channel formation finishing the lightning initiation process.

In this work, we represent the lightning initiation scenario as a sequence of two transitions of discharge activity to progressively larger spatial scales: the first one is from small-scale avalanches to intermediate-scale streamers; and the second one is from streamers to the lightning seed. We postulate the existence of ion production centers in the cloud, whose occurrence is caused by electric field bursts accompanying hydrometeor collisions (or near collisions) in the turbulent thundercloud environment. When a new ion production center is created inside (fully or partially) the residual ion spot left behind by a previously established center, there is a cumulative effect in the increasing of ion concentration. As a result, the essentially non-conducting thundercloud becomes seeded by elevated ion-conductivity regions (EICRs) with spatial extent of 0.1–1 m and a lifetime of 1–10 s. The electric field on the surface of an EICR (due to its conductivity being at least 4 orders of magnitude higher than ambient) is a factor of 3 or more higher than ambient. For a maximum ambient electric field of 100 kV/m typically measured in thunderclouds, such field enhancement is sufficient for initiation of positive streamers and their propagation over distances of the order of decimeters, and this will be happening naturally, without any external agents (e.g., superenergetic cosmic ray particles) or extraordinary in-cloud conditions, such as very high potential differences or very large hydrometeors. Provided that each EICR generates at least one streamer during its lifetime, the streamers will form a 3D network, some parts of which will contain hot channel segments created via the cumulative heating and/or thermal-ionizational instability. These hot channel segments will polarize, interact with each other, and cluster, forming longer conducting structures in the cloud. When the ambient potential difference bridged by such a conducting structure exceeds 3 MV, we assume that the lightning seed, capable of self-sustained bidirectional extension, is formed.

В статье представлены основы современных представлений о физике атмосферного электричества в целом и многообразии форм молниевых разрядов в частности. Освещаются основные проблемы, методы исследований и прикладная значимость данной области науки. Кратко обсуждаются возможные причины, отвечающие за многообразие типов атмосферных разрядов.

A numerical model with physical timing and grid spacing of 3 m is applied to studying the progression (including stepping and branching) of negative lightning stepped leader. The asymmetry between positive and negative streamers is taken into account via using polarity‐dependent initiation and propagation field thresholds. The stepped nature of negative leader is confirmed to be caused by this asymmetry. The step formation process of the negative leader is modeled to begin with the appearance of space stems inside and in the immediate vicinity of its streamer zone (corona streamer burst completing the preceding step). Some of those space stems evolve into space leaders, which can connect to the primary leader channel, thereby facilitating its extension. Model‐predicted morphology and dynamics of negative leaders are in good agreement with the recent recordings of lightning stepped and dart‐stepped leaders obtained using high‐speed video cameras, and their electrical parameters are in line with the current knowledge on negative lightning leaders.

We propose a scenario in which elevated ionic conductivity regions (EICRs) with dimensions of the order of 0.1–1 m are formed in the turbulent thundercloud environment. The starting point in this scenario is the occurrence of electron avalanches in the vicinity of colliding hydrometeors, leading to the formation of ion production centers. Their dimensions are of the order of $$1{0}^{-3}-1{0}^{-2}$$ m, and their lifetime is of the order of $$1{0}^{-4}-1{0}^{-3}$$ s. When a new ion production center is created inside the decimeter-scale residual ion concentration spot left behind by a previously established center, the local ion concentration steadily increases, which leads to the formation of decimeter-scale EICRs whose lifetime is measured in seconds. The relatively high conductivity of EICRs (up to $$1{0}^{-9}$$ S/m or so) relative to the background conductivity ($$1{0}^{-14}$$ S/m or less) ensures their polarization in external electric field within a few milliseconds or so. The EICR formation mechanism requires only one condition: the rate of occurrence of ion production centers per unit time in a unit volume should exceed the percolation-theory-based critical level of $$1{0}^{-1}$$ m$${}^{-3}$$ s$${}^{-1}$$. Hydrometeor collision rates three and even four orders of magnitude higher than this value have been reported from observations. Presence of EICRs in the cloud provides local electric field enhancements and pre-ionization levels that will lead to the formation of additional ion production centers and may be sufficient for the initiation and development of streamers and, eventually, lightning.