| Lightning is a serious natural disaster.Study of the propagation characteristics of the lightning electromagnetic waves in multiple frequency bands over different distances is important in many academic researches and applications,such as the long-range lightning locating,the remote sensing of the lower ionosphere,and the study of Transient Luminous Events(TLEs).In the waveguide formed by the Earth and the ionosphere,the lightning-radiated electromagnetic(EM)field(also called LEMP)can not only propagate along the Earth’s surface in the form of groundwave,it also can be reflected by the lower ionosphere and the ground and propagate in the form of skywave.Considering the complex propagation path along ground surface and the anisotropic ionosphere,a two-dimensional time-domain finite-difference(2-D FDTD)method for the simulation of LEMP propagation in the Earth-Ionospheric Waveguide(EIWG)is developed.Based on the FDTD method,this paper studies the propagation characteristics of LEMP in the low frequency(LF)band,the very low frequency(VLF)band and the extremely low frequency(ELF)band for observation distances within thousands of kilometers,especially studies the influence of EIWG on the propagation of the groundwave and skywave in multiple frequency bands over different distances.The main conclusions are summarized as follows:(1)The effect of mountains and the Earth’s curvature on the LEMP are investigated systematically.Results show that,the far vertical electric field and azimuthal magnetic field will be enhanced due to the reflection process in the transition between the mountain terrain and the flat ground,and a sharper or higher mountain causes a larger enhancement.For a mountain located on the propagation path,the attenuation effect of the mountain on the field peak can be ignored for most practical purposes when its height is lower than 1 km.The propagation effect introduced by the Earth’s curvature can be ignored when the observation distance is within 200 km.The ratio of the far-field peak considering both the propagation effect of finite conductivity and Earth’s curvature to the far-field peak over a perfect conducting and flat ground decreases exponentially with the increase in the observation distance.When the LEMP propagates along the coastal surface,with a increase of 100 km in the observation distance,the wave arrival time and peak time will be delayed by 0.1 μs and 0.64 μs on average,respectively;the corresponding values are about 0.36 μs and 0.96 μs when LEMP propagates along the land surface.(2)The influence of the Earth’s geomagnetic field and the electron density profile in the lower ionosphere on the propagation of LEMP in the EIWG are studied.Due to the anisotropic ionosphere which is caused by the geomagnetic field,the skywave waveforms are different when LEMP propagates in different directions under nighttime conditions.Under typical nighttime conditions,the cut-off frequency of the first-order mode in the EIWG is about 1.8kHz.Besides the first-order mode,higher-order modes below the fifth-order mode have a greater contribution to the azimuthal magnetic field within 1000 km.While under daytime conditions,the first-order mode with a cut-off frequency of about 2.3 kHz is dominated.Therefore,the interference effect of LEMP within the EIWG under nighttime conditions is more obvious than that under daytime conditions.(3)The horizontal and vertical components of the induced current density produced by the LEMP in the ionosphere are evaluated.Their different contributions to the first skywave at different observation distances are analyzed in detail.Furthermore,a physical explanation for the polarity reversal in the time domain is proposed.It is found that,for relatively short observation distances(within ～200 km),the first skywave is dominated by the component generated by the horizontal equivalent current in the Fresnel zone,while for longer observation distances(larger than ～300 km),the first skywave is dominated by the component generated by the vertical equivalent current in the Fresnel zone.Since the polarities of the skywave components generated by the vertical current source and horizontal current source are opposite,the polarity of the skywave will reverse when increasing the observation distance.Under nighttime conditions,when the LEMP propagates westward,the horizontal current density at heights of about 90 km enhances.At observation distances larger than 300 km,both of the horizontal and vertical equivalent currents in the ionosphere contribute significantly to the first skywave,the superposition of these two components produce a double-peak skywave.(4)Using the 2-D FDTD method,the propagation impulse response of the EIWG system for the lightning-produced quasi-transverse electromagnetic waves(QTEM waves)below 1kHz are studied.Results show that,the geomagnetic field has a significant effect on the propagation impulse response waveforms under both nighttime and daytime conditions.For observation distances within 2000 km,the maximum error of the amplitude of the impulse response when ignoring the geomagnetic field can reach about +30%.Under nighttime conditions,the lightning-produced QTEM wave can penetrate into the E region of the ionosphere;while under a daytime condition,the QTEM energy is mainly reflected by the D region of the ionosphere.The geomagnetic inclination at different latitudes mainly influence the response waveform after the zero-crossing point,while almost don’t have influence on the initial part of the propagation impulse response waveform.The reference height(h’)of the D region has a relative small effect on the amplitude of the propagation impulse response(the difference is within about 15%),while the electron density profile sharpness(β)in D region can strongly influence the propagation impulse response.For observation distances within 2000 km,with the increase of β,the amplitude of the impulse response can largely increase by about 30%and 40%,respectively,under nighttime and daytime conditions. |
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