On ULF Signatures of Lightning Discharges

Recent works on magnetic signatures due to distant lightning discharges are reviewed. Emphasis is laid on magnetic signatures in the ULF range (in the old definition from less than 1 mHz up to 1 Hz), that is in the frequency range below the Schumann resonance. These signatures are known to be of importance for the excitation of the ionospheric Alfvén resonator (IAR) which works only at night time conditions. This emphasizes the difference between night and day time ULF signatures of lightning. The IAR forms a link between the atmosphere and magnetosphere. Similarities and differences of this link in the VLF (Trimpi effect) and ULF range are worked out. A search for a unique signature of sprite-associated positive cloud-to-ground (⁺CG) lightning discharges ended with a negative result. In this context, however, a new model of lightning-associated induced mesospheric currents was built. Depending on mesospheric condition it can produce magnetic signatures in the entire frequency range from VLF, ELF to ULF. In the latter case it can explain signatures known as the Ultra Slow Tail of ⁺CG lightning discharges. A current problem on the magnetic background noise intensity has been solved by taking more seriously the contribution of ⁺CG lightning discharges to the overall background noise. Their low occurrence rate is more than compensated by their large and long lasting continuing currents. By superposed epoch analysis it could be shown that the ULF response to ^?CG is one to two orders smaller that in case of ⁺CG with similar peak current values of the return stroke.     

Sprite-Producing Lightning-Ionosphere Coupling and Associated Low-Frequency Phenomena

Theoretical and experimental evidence is collected for additional ionization regions in the mesosphere above lightning discharges and their connection with transient luminous events (TLE). New insights are reported into the different appearances in the Very Low Frequency (VLF) link traces as affected by the mesospheric ionization regions. Based on these findings, physical processes going on in the ionization regions and their coupling to the primary lightning discharge process are conceptualized. Thereafter, some diagnostic potential is outlined. Finally, the ionization regions are considered as a transmitter of secondary low-frequency waves. The distinction is made between the primary electromagnetic waves emitted by the lightning and the secondary waves. As a consequence the Extremely Low Frequency (ELF) and Ultra Low Frequency (ULF) transient signatures observed on the ground are understood as a composite of both wave types. In addition a novel method is introduced to extract the charge moment change of a sprite producing lightning discharge.     

Coupling at the Atmosphere-Ionosphere-Magnetosphere Interface and Resonant Phenomena in the ULF Range

We discuss the electromagnetic processes in the ULF range which are important for the coupling between the atmosphere, ionosphere, and magnetosphere (AIM). The main attention is given to the Pc1–2 frequency ranges (f≈0.1–10 Hz) where some natural resonances in the AIM system are located. In particular, we consider the resonant structures in the spectra of the magnetic background noise related to the Alfvén resonances in the ionosphere as a possible diagnostic tool for studies of the ionospheric parameters. We also discuss the self-excitation of Alfvén waves in the ionosphere due to the AIM coupling and the role of such waves in the acceleration of electrons in the upper ionosphere and magnetosphere. Precipitation of magnetospheric ions due to their interaction with the ion-cyclotron waves is analyzed in relation to the ionospheric current systems, formation of partial ring current, and the influence of the ionosphere-magnetosphere feedback on the generation of such waves.     

An Overview of Earth’s Global Electric Circuit and Atmospheric Conductivity

The Earth’s global atmospheric electric circuit depends on the upper and lower atmospheric boundaries formed by the ionosphere and the planetary surface. Thunderstorms and electrified rain clouds drive a DC current (～1 kA) around the circuit, with the current carried by molecular cluster ions; lightning phenomena drive the AC global circuit. The Earth’s near-surface conductivity ranges from 10^?7 S?m^?1 (for poorly conducting rocks) to 10^?2 S?m^?1 (for clay or wet limestone), with a mean value of 3.2 S?m^?1 for the ocean. Air conductivity inside a thundercloud, and in fair weather regions, depends on location (especially geomagnetic latitude), aerosol pollution and height, and varies from ～10^?14 S?m^?1 just above the surface to 10^?7 S?m^?1 in the ionosphere at ～80 km altitude. Ionospheric conductivity is a tensor quantity due to the geomagnetic field, and is determined by parameters such as electron density and electron–neutral particle collision frequency. In the current source regions, point discharge (coronal) currents play an important role below electrified clouds; the solar wind-magnetosphere dynamo and the unipolar dynamo due to the terrestrial rotating dipole moment also apply atmospheric potential differences. Detailed measurements made near the Earth’s surface show that Ohm’s law relates the vertical electric field and current density to air conductivity. Stratospheric balloon measurements launched from Antarctica confirm that the downward current density is ～1 pA m^?2 under fair weather conditions. Fortuitously, a Solar Energetic Particle (SEP) event arrived at Earth during one such balloon flight, changing the observed atmospheric conductivity and electric fields markedly. Recent modelling considers lightning discharge effects on the ionosphere’s electric potential (～+250 kV with respect to the Earth’s surface) and hence on the fair weather potential gradient (typically ～130 V?m^?1 close to the Earth’s surface. We conclude that cloud-to-ground (CG) lightning discharges make only a small contribution to the ionospheric potential, and that sprites (namely, upward lightning above energetic thunderstorms) only affect the global circuit in a miniscule way. We also investigate the effects of mesoscale convective systems on the global circuit.     

A Review of Low Frequency Electromagnetic Wave Phenomena Related to Tropospheric-Ionospheric Coupling Mechanisms

Investigation of coupling mechanisms between the troposphere and the ionosphere requires a multidisciplinary approach involving several branches of atmospheric sciences, from meteorology, atmospheric chemistry, and fulminology to aeronomy, plasma physics, and space weather. In this work, we review low frequency electromagnetic wave observations in the Earth-ionosphere cavity from a troposphere-ionosphere coupling perspective. We discuss electromagnetic wave generation, propagation, and resonance phenomena, considering atmospheric, ionospheric and magnetospheric sources, from lightning and transient luminous events at low altitude to Alfvén waves and particle precipitation related to solar and magnetospheric processes. We review ionospheric processes as well as surface and space weather phenomena that drive the coupling between the troposphere and the ionosphere. Effects of aerosols, water vapor distribution, thermodynamic parameters, and cloud charge separation and electrification processes on atmospheric electricity and electromagnetic waves are reviewed. Regarding the role of the lower boundary of the cavity, we review transient surface phenomena, including seismic activity, earthquakes, volcanic processes and dust electrification. The role of surface perturbations and atmospheric gravity waves in ionospheric dynamics is also briefly addressed. We summarize analytical and numerical tools and techniques to model low frequency electromagnetic wave propagation and to solve inverse problems and outline in a final section a few challenging subjects that are important to advance our understanding of tropospheric-ionospheric coupling.     

复合材料层合板雷击烧蚀损伤模拟

 为解决复合材料层合板的雷击烧蚀问题,通过复合材料层合板雷电流烧蚀的热-电-结构耦合分析,建立复合材料层合板烧蚀的三维有限元模型。利用删除单元法模拟复合材料层合板在不同脉冲波形雷电流作用下的冲击响应,进行复合材料层合板雷击损伤机理和损伤模式分析,得出了复合材料层合板在不同脉冲波形和峰值雷电流作用下的瞬态热传递和热烧蚀规律。分析了不同雷电流参数对烧蚀结果的影响。结果表明峰值电流、放电量和比能对复合材料层合板的烧蚀尺寸和内部损伤产生很大的影响。     

Thermal damage analysis of aircraft composite laminate suffered from lightning swept stroke and arc propagation

Lightning strike is a complicated process involving multi-field coupling. In order to investigate the thermal damage mechanism of carbon fiber reinforced composites subject to lightning swept stroke, a complete numerical method is presented. Numerical model of lightning discharge is established based on Magneto Hydro Dynamics (MHD) and calculated by FLUENT secondary development technology. Considering aerodynamic flow effect, channel formation and evolution process during lightning discharge is analyzed for lightning current waveform A. Thermal-electric Coupling model is presented according to Radial Basis Function (RBF) interpolation theory, which is implemented by compiling program to make lightning current and heat energy inject into composite laminate. Consequently, damage mechanism of composite laminate under lightning swept stroke is studied based on the coupled numerical model and element deletion method. Ablation damage morphology of composite laminate is analyzed to understand plasma expansion and reattachment in arc root. The results show that aerodynamic flow makes the lightning channel move fast and composite laminate is deteriorated due to thermal damage.     

MHD Flow Control of Oblique Shock Waves Around Ramps in Low-temperature Supersonic Flows

This article is devoted to experimental study on the control of the oblique shock wave around the ramp in a low-temperature supersonic flow by means of the magnetohydrodynamic(MHD) flow control technique. The purpose of the experiments is to take advantage of MHD interaction to weaken the oblique shock wave strength by changing the boundary flow characteristics around the ramp. Plasma columns are generated by pulsed direct current(DC) discharge, the magnetic fields are generated by Nd-Fe-B rare-earth permanent magnets and the oblique shock waves in supersonic flow are generated by the ramp. The Lorentz body force effect of MHD interaction on the plasma-induced airflow velocity is verified through particle image velocimetry(PIV) measurements. The experimental results from the supersonic wind tunnel indicate that the MHD flow control can drastically change the flow characteristics of the airflow around the ramp and decrease the ratio of the Pitot pressure after shock wave to that before it by up to 19. 66%, which leads to the decline in oblique shock wave strength. The oblique shock waves in front of the ramp move upstream by the action of the Lorentz body force. The discharge characteristics are analyzed and the MHD interaction time and consumed energy are determined with the help of the pulsed DC discharge images. The interaction parameter corresponding to the boundary layer velocity can reach 1. 3 from the momentum conservation equation. The velocity of the plasma column in the magnetic field is much faster than that in the absence of magnetic field force. The plasma can strike the neutral gas molecules to transfer momentum and accelerate the flow around the ramp.     

Atmospheric Electricity at Saturn

The Cassini mission provides a great opportunity to enlarge our knowledge of atmospheric electricity at the gas giant Saturn. Following Voyager studies, the RPWS (Radio and Plasma Wave Science) instrument has measured again the so-called SEDs (Saturn Electrostatic Discharges) which are the radio signature of lightning flashes. Observations by Cassini/ISS (Imaging Science Subsystem) have shown cloud features in Saturn’s atmosphere whose occurrence, longitudinal drift rate, and brightness were strongly related to the SEDs. In this paper we will review the main physical parameters of the SEDs. Lightning does not only give us clues about the dynamics of the atmosphere, but also serves as a natural tool to investigate properties of Saturn’s ionosphere. We will also discuss other lightning related phenomena and compare Saturn lightning with terrestrial and Jovian lightning.     

Plasma waves at Venus

Many significant wave phenomena have been discovered at Venus with the plasma wave instrument flow on the Pioneer Venus Orbiter. It has been shown that whistler-mode waves in the magnetosheath of the planet may be an important source of energy for the topside ionosphere. Plasma waves are also associated with thickening of the ionopause current layer. Current-generated waves in plasma clouds may provide anomalous resistance resulting in electron acceleration, possibly producing aurora. Ion-acoustic waves are observed in the bow shock, and appear to be a feature of the magnetotail boundary. Lastly plasma waves have been cited as evidence for lightning on Venus.     

脉冲等离子体推力器烧蚀建模与仿真

为了深入地揭示平板型脉冲等离子体推力器（PPT）工作机理,通过对PPT在放电过程中的欧姆加热能量分配分析,建立了基于能量守恒原理的改进烧蚀模型,并对磁流体动力学方程中的能量方程进行了修正。结合LES-6对PPT工作过程进行了一维磁流体动力学数值仿真,获得了PPT的速度分布、放电烧蚀质量和元冲量等参数的变化过程。仿真结果表明,改进模型能正确地反映脉冲等离子体推力器工作过程,仿真得到的出口速度、元冲量与实验值吻合较好,而放电烧蚀质量则相对误差较大;当电容容量变化时,元冲量随放电能量增大呈近似的线性增长关系,放电烧蚀质量随放电能量的增大单调增加且呈非线性关系。     

VLF Studies During TLE Occurrences in Europe: A Summary of New Findings

The paper reviews the past few years’ European efforts for characterising the effects of TLEs, in particular sprites and elves, on the lower ionosphere. A mostly experimental approach was applied for the analysis of data collected during the EuroSprite campaigns by optical cameras, very low frequency (VLF, 3–30?kHz) receivers and lightning detection systems. The new findings of these multi-instrumental studies can be summarised as follows: 1) A close relationship between sprites and early VLF perturbations was established which constitutes evidence of upper D-region electron density changes in association with sprites. 2) VLF backscatter from the sprite-affected regions exists but it occurs rarely. 3) Long-delayed sprites were present in a large percentage, contrary to previous reports; they occurred in relation to long-lasting continuing currents that contribute to the build-up of sprite-causative quasi-electrostatic fields. 4) Intracloud lightning was found to be the key-factor which determines the sprite morphological features. 5) A new subcategory of VLF events was discovered, termed early/slow, characterised by long onset durations from 100?ms up to ～2?s. The slow onsets, which were attributed to a gradual ionisation build-up, are driven by a dense sequence of intracloud electromagnetic pulses that accompany the sprite-causative discharge. 6) A D-region chemical model was applied to simulate the measured recovery phases of the early VLF perturbations. This led to estimates about the mean altitude and electron density enhancements of the sprite-related ionospheric perturbations. 7) Early VLF events were identified for the first time to occur in association with elves, providing evidence that corroborates theoretical predictions on lower-ionospheric ionisation production by lightning-emitted electromagnetic pulses.     

飞机复合材料表面导电层的雷击仿真研究

复合材料在飞机上的应用越来越广泛,与金属相比复合材料导电性较差,因此处于雷击区域的复合材料部件表面要设计导电防护层以承受雷击时的瞬间放电,避免复合材料部件的烧蚀损伤。本研究利用数值仿真的方法建立了数学模型,计算了复合材料表面导电层通过雷击电流脉冲后的电场和温度场分布,并对该导电层的雷击防护能力及可靠性进行了评价。模拟结果表明,厚度为0.20 mm的铝导电层最适宜做飞机表面的雷击防护层。     

Fluctuating magnetic fields in the magnetosphere

The study of ULF waves in space has been in progress for about 12 years. However, because of numerous observational difficulties the properties of the waves in this frequency band (10^-3 to 1 Hz) are poorly known. These difficulties include the nature of satellite orbits, telemetry limitations on magnetometer frequency response and compromises between dynamic range and resolution. Despite the paucity of information, there is increasing recognition of the importance of these measurements in magnetospheric processes. A number of recent theoretical papers point out the roles such waves play in the dynamic behavior of radiation belt particles.At the present time the existing satellite observations of ULF waves suggest that the level of geomagnetic activity controls the types of waves which occur within the magnetosphere. Consequently, we consider separately quiet times, times of magnetospheric substorms and times of magnetic storms. Within each of these categories there are distinctly different wave modes distinguished by their polarization: either transverse or parallel to the ambient field. In addition, these wave phenomena occur in distinct frequency bands. In terms of the standard nomenclature of ground micropulsation studies ULF wave types observed in the magnetosphere include quiet time transverse — Pc 1, Pc 3, Pc 4, Pc 5 quiet time compressional — Pc 1 and Pi 1; substorm compressional Pi 1 and Pi 2; storm transverse — Pc 1; storm compressional Pc 4, 5. The satellite observations are not yet sufficient to determine whether the various bands identified in the ground data are equally appropriate in space.Publication No. 982. Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Calif. 90024.     

Dayside Proton Aurora: Comparisons between Global MHD Simulations and IMAGE Observations

The IMAGE mission provides a unique opportunity to evaluate the accuracy of current global models of the solar wind interaction with the Earth's magnetosphere. In particular, images of proton auroras from the Far Ultraviolet Instrument (FUV) onboard the IMAGE spacecraft are well suited to support investigations of the response of the Earth's magnetosphere to interplanetary disturbances. Accordingly, we have modeled two events that occurred on June 8 and July 28, 2000, using plasma and magnetic field parameters measured upstream of the bow shock as input to three-dimensional magnetohydrodynamic (MHD) simulations. This paper begins with a discussion of images of proton auroras from the FUV SI-12 instrument in comparison with the simulation results. The comparison showed a very good agreement between intensifications in the auroral emissions measured by FUV SI-12 and the enhancement of plasma flows into the dayside ionosphere predicted by the global simulations. Subsequently, the IMAGE observations are analyzed in the context of the dayside magnetosphere's topological changes in magnetic field and plasma flows inferred from the simulation results. Finding include that the global dynamics of the auroral proton precipitation patterns observed by IMAGE are consistent with magnetic field reconnection occurring as a continuous process while the IMF changes in direction and the solar wind dynamic pressure varies. The global simulations also indicate that some of the transient patterns observed by IMAGE are consistent with sporadic reconnection processes. Global merging patterns found in the simulations agree with the antiparallel merging model, though locally component merging might broaden the merging region, especially in the region where shocked solar wind discontinuities first reach the magnetopause. Finally, the simulations predict the accretion of plasma near the bow shock in the regions threaded by newly open field lines on which plasma flows into the dayside ionosphere are enhanced. Overall the results of these initial comparisons between global MHD simulation results and IMAGE observations emphasize the interplay between reconnection and dynamic pressure processes at the dayside magnetopause, as well as the intricate connection between the bow shock and the auroral region.     

Observations of Vertical Reflections from the Topside Martian Ionosphere

The Martian ionosphere has for the first time been probed by a low frequency topside radio wave sounder experiment (MARSIS) (Gurnett et al., 2005). The density profiles in the Martian ionosphere have for the first time been observed for solar zenith angles less than 48 degrees. The sounder spectrograms typically have a single trace of echoes, which are controlled by reflections from the ionosphere in the direction of nadir. With the local density at the spacecraft derived from the sounder measurements and using the lamination technique the spectrograms are inverted to electron density profiles. The measurements yield electron density profiles from the sub-solar region to past the terminator. The maximum density varies in time with the solar rotation period, indicating control of the densities by solar ionizing radiation. Electron density increases associated with solar flares were observed. The maximum electron density varies with solar zenith angle as predicted by theory. The altitude profile of electron densities between the maximum density and about 170m altitude is well approximated by a classic Chapman layer. The neutral scale height is close to 10 to 13 km. At altitudes above 180 km the densities deviate from and are larger than inferred by the Chapman layer. At altitudes above the exobase the density decrease was approximated by an exponential function with scale heights between 24 and 65 km. The densities in the top side ionosphere above the exobase tends to be larger than the densities extrapolated from the Chapman layer fitted to the measurements at lower altitudes, implying more efficient upward diffusion above the collision dominated photo equilibrium region.     

Natural and man-made noise in the Earth-ionosphere cavity at extremely low frequencies (Schumann resonances and man-made ‘interference’)

The equations for the Earth-ionosphere cavity resonance fields are given and some of the approximations used in their derivation are indicated. Typical electric and magnetic 5 to 20 Hz Schumann resonance field intensities are listed and compared with the level of other natural and man-made electromagnetic noise. Applications of Schumann resonances to thunderstorm location and measurement of global lightning activity are reviewed briefly. Ionospheric conductivity profiles appropriate for this frequency range are discussed and the importance of atmospheric conductivity below 60 km is pointed out.     

Venus lightning

Although it is not unanimously accepted, many independent observations lead to the conclusion that lightning is prevalent on Venus. The electromagnetic signals detected by all 4 Venera landers are most readily explained as generation by lightning. The Venera 9 spectrometer appears to have observed a lightning storm on one occasion. The Pioneer Venus plasma wave instrument detects waves both below the electron gyrofrequency that may be due to lightning and signals above the electron gyrofrequency but at very low altitudes that may be due to the near field of the lightning. The VLF observations suggest that Venus lightning must be an intra-cloud phenomenon which is most frequent in the afternoon and evening sector. The occurrence rate is likely to be greater than on Earth.