We suggest a possible explanation of the influence of pre-seismic activity on the registration rate of natural ELF(extremely low frequency)/VLF(very low frequency) pulses and the changes of their characteristics. The ...We suggest a possible explanation of the influence of pre-seismic activity on the registration rate of natural ELF(extremely low frequency)/VLF(very low frequency) pulses and the changes of their characteristics. The main idea is as follows. The distribution of the electric field around a thundercloud depends on the conductivity profile of the atmosphere. Quasi-static electric fields of a thundercloud decrease in those tropospheric regions where an increase of air conductivity is generated by pre-seismic activities due to emanation of radioactive gas and water into the lower atmosphere. The electric field becomes reduced in the lower troposphere, and the probability decreases of the cloud-to-ground (CG) strokes in such “contaminated” areas. Simultaneously, the electric field grows inside and above the thunderclouds, and hence, we anticipate a growth in the number of horizontal and tilted inter-cloud (or intra-cloud) (both termed as IC discharges) strokes. Spatial orientation of lightning strokes reduces vertical projection of their individual amplitudes, while the rate (median number strokes per a unit time) of discharges grows. We demonstrate that channel tilt of strokes modifies the spectral content of ELF/VLF radio noise and changes the rate of detected pulses during the earthquake preparation phase.展开更多
The recently developed high-quality WHU ELF/VLF receiver system has been deployed in Suizhou, China(geomagnetic latitude 21.81°N, longitude 174.44°E, L=1.16) to detect low latitude extremely-low-frequency(EL...The recently developed high-quality WHU ELF/VLF receiver system has been deployed in Suizhou, China(geomagnetic latitude 21.81°N, longitude 174.44°E, L=1.16) to detect low latitude extremely-low-frequency(ELF: 0.3-3 k Hz) and very-low-frequency(VLF: 3-30 k Hz) emissions originating from either natural or artificial sources since February 2016. During the first-month operation of the receiver system, a total of 3039 clear whistlers have been recorded at this low latitude station with the majority(97.0%) occurring on 28 February and 1 March 2016. Observed whistlers manifest various types including single one-hop, echo train, multi-flash, and multi-path. They tend to intensify after local midnight, reach the peak around 04-05 LT, and then weaken quickly. Both features of lower cutoff frequencies of most whistlers below ~1.6 k Hz and almost uniform dispersion for many successive multi-flash whistlers suggest that these whistlers propagate along the geomagnetic field lines in the duct mode. The computed dispersion varies between ~15 s^(1/2) and 23 s^(1/2) for observed one-hop whistlers and is greater than 50 s^(1/2) for three-hop echo train whistlers, indicating that the whistlers observed at the Suizhou station are low latitude whistlers.展开更多
基于高电离层质动力非线性加热理论,引入差频双波束概念,建立双频双波束加热电离层激发甚低频/极低频(VLF/ELF)辐射理论仿真模型,通过对已有实验参数进行仿真计算,验证了模型的正确性.据此模型,全面分析了不同纬度、有效辐射功率、加热...基于高电离层质动力非线性加热理论,引入差频双波束概念,建立双频双波束加热电离层激发甚低频/极低频(VLF/ELF)辐射理论仿真模型,通过对已有实验参数进行仿真计算,验证了模型的正确性.据此模型,全面分析了不同纬度、有效辐射功率、加热频率、极化模式、频率差、实验时段等对激发VLF/ELF辐射强度的影响,并对比分析了中低纬度地区双频双波束和幅度调制两种方法激发VLF/ELF信号的差异.分析结果表明:VLF/ELF辐射效果随着地磁倾角的增加而增强,随着系统有效辐射功率的增大而增强;X波模式优于O波模式;实验时段冬季最好,夜晚优于白天;加热频率和频率差存在最优值选取问题.对于背景自然电流较弱的中低纬度地区,相对现有幅度调制方法,利用双频双波束方法激发VLF/ELF辐射更加有效,两者相差10 d B以上.展开更多
文摘We suggest a possible explanation of the influence of pre-seismic activity on the registration rate of natural ELF(extremely low frequency)/VLF(very low frequency) pulses and the changes of their characteristics. The main idea is as follows. The distribution of the electric field around a thundercloud depends on the conductivity profile of the atmosphere. Quasi-static electric fields of a thundercloud decrease in those tropospheric regions where an increase of air conductivity is generated by pre-seismic activities due to emanation of radioactive gas and water into the lower atmosphere. The electric field becomes reduced in the lower troposphere, and the probability decreases of the cloud-to-ground (CG) strokes in such “contaminated” areas. Simultaneously, the electric field grows inside and above the thunderclouds, and hence, we anticipate a growth in the number of horizontal and tilted inter-cloud (or intra-cloud) (both termed as IC discharges) strokes. Spatial orientation of lightning strokes reduces vertical projection of their individual amplitudes, while the rate (median number strokes per a unit time) of discharges grows. We demonstrate that channel tilt of strokes modifies the spectral content of ELF/VLF radio noise and changes the rate of detected pulses during the earthquake preparation phase.
基金supported by the National Natural Science Foundation of China(Grants Nos.41204120,41474141,41304127,41304130,and 41574160)the Projects funded by China Postdoctoral Science Foundation(Grants Nos.2013M542051,2014T70732)the 985 funded project of School of Electronic information,Wuhan University
文摘The recently developed high-quality WHU ELF/VLF receiver system has been deployed in Suizhou, China(geomagnetic latitude 21.81°N, longitude 174.44°E, L=1.16) to detect low latitude extremely-low-frequency(ELF: 0.3-3 k Hz) and very-low-frequency(VLF: 3-30 k Hz) emissions originating from either natural or artificial sources since February 2016. During the first-month operation of the receiver system, a total of 3039 clear whistlers have been recorded at this low latitude station with the majority(97.0%) occurring on 28 February and 1 March 2016. Observed whistlers manifest various types including single one-hop, echo train, multi-flash, and multi-path. They tend to intensify after local midnight, reach the peak around 04-05 LT, and then weaken quickly. Both features of lower cutoff frequencies of most whistlers below ~1.6 k Hz and almost uniform dispersion for many successive multi-flash whistlers suggest that these whistlers propagate along the geomagnetic field lines in the duct mode. The computed dispersion varies between ~15 s^(1/2) and 23 s^(1/2) for observed one-hop whistlers and is greater than 50 s^(1/2) for three-hop echo train whistlers, indicating that the whistlers observed at the Suizhou station are low latitude whistlers.
文摘基于高电离层质动力非线性加热理论,引入差频双波束概念,建立双频双波束加热电离层激发甚低频/极低频(VLF/ELF)辐射理论仿真模型,通过对已有实验参数进行仿真计算,验证了模型的正确性.据此模型,全面分析了不同纬度、有效辐射功率、加热频率、极化模式、频率差、实验时段等对激发VLF/ELF辐射强度的影响,并对比分析了中低纬度地区双频双波束和幅度调制两种方法激发VLF/ELF信号的差异.分析结果表明:VLF/ELF辐射效果随着地磁倾角的增加而增强,随着系统有效辐射功率的增大而增强;X波模式优于O波模式;实验时段冬季最好,夜晚优于白天;加热频率和频率差存在最优值选取问题.对于背景自然电流较弱的中低纬度地区,相对现有幅度调制方法,利用双频双波束方法激发VLF/ELF辐射更加有效,两者相差10 d B以上.
