甚低纬多跳哨声的非导管传播

实验发现,在磁纬20°以下区域地面可重复接收到多跳哨声.本文着重考察了低纬多跳哨声非导管传播的可能性.计算结果证明,在给定的电离层状态下,该地区可以形成多跳哨声的非导管传播路径.射线追踪得到的多跳路径特征能满意地解释文中所列实验结果.因此,作为一种可能的传播机制,低纬多跳哨声的非导管传播方式值得重视.     

甚低纬多跳哨声的非导管传播

实验发现,在磁纬20°以下区域地面可重复接收到多跳哨声.本文着重考察了低纬多跳哨声非导管传播的可能性.计算结果证明,在给定的电离层状态下,该地区可以形成多跳哨声的非导管传播路径.射线追踪得到的多跳路径特征能满意地解释文中所列实验结果.因此,作为一种可能的传播机制,低纬多跳哨声的非导管传播方式值得重视.     

低纬导管哨声

本文利用在我国海南岛三亚地区（18.24°N,109.5°E;磁纬7.04°N）观测到的哨声和吱声资料,研究了低纬哨声的物理性质。观测资料和理论分析的结果表明,在低纬仍能接收到沿着按场排列的导管传播的导管哨声。结果间接地表明,在电离层中也存在哨声导管,导管哨声的临界截止纬度应当位于比磁纬7.04°N更低的地区。     

在榆林港地面（磁纬=6.8°N）收录到哨声

地区的磁纬愈低,磁倾角也愈小,从而甚低频的电磁波也愈难被沿地磁力线分布的导管捕获并以哨声模式传播。因此,在低纬地区哨声出现率小、强度弱,地面接收相当困难。但由于低纬哨声的特征及其传播机制对研究磁层和电离层有重要意义,所以近年来人们越来越重视对低纬哨声的观测和研究。我们在1980年冬季于广东湛江首次收录到哨声信号的基础上,又于1982年1月8日至17日在海南岛榆林港（磁纬（?）6.8°N）和湛江（磁纬（?）9.8°N）同时进行观测,结果两地均收录到短哨信号,如图1（a）、（b）     

电离密度分布对中低纬非导管哨声波传播的影响

本文在偶极子地磁模型和各种电离密度分布模型中,对中低纬非导管哨声波的传播特性进行了射线追踪研究。结果表明,地磁场位形及其引起的磁层等离子体的各向异性是决定哨声射线几何特征及速度结构的主要因素,而电离密度仅在一定程度上改变上述特征;电离层是形成哨声射线聚焦的主要原因;赤道异常和电离层显著地影响哨声波的到达纬度;哨声群时延和色散值主要决定于电离密度;中低纬非导管哨声近似符合Eckersley定律;电离层是决定波法线角能否满足透射条件的主要因素。     

低纬哨声的路径纬度和临界截止纬度

本文采用国际地磁参考场（1980 IGRF,n=8）低纬磁力线经验公式和模式电离层,结合海南岛三亚（18.24°N,109.5°E;磁纬7.04°N）观测到的哨声资料,对磁纬10°N以下哨声的路径纬度和临界截止纬度进行了分析和讨论.     

湛江地区哨声观测的初步分析

本文介绍了1980年冬和1981年冬在广东省湛江地区（磁纬9.8°N）观测哨声的结果。观测记录表明,不仅可以收录到大量的短哨及其回波,而且还观测到一些少见的现象;分析结果表明,它们似乎具有导管传播的特征,地面哨声的“截止纬度”有可能低于磁纬9.8°N。     

初始波法线角对中低纬非导管哨声波传播的影响

本文对初始波法线角δ_s不同的中低纬非导管哨声进行了射线追踪研究,结果表明,当δ_s≤0°时,哨声波可到达较大的L值,在另一半球较高纬度处形成“聚焦区”,这组哨声色散值较大;当δ>0°时,随以的增加,哨声射线内移,δ>10°的哨声射线汇聚于另一半球较低纬度处,这组哨声色散值较小。据此提出解释中低纬哨声色散值连续变化的一种可能机制:电离层电子浓度水平梯度的连续变化引起初始波法线角以的连续变化,从而导致射线路径、群时延和色散值的变化。     

磁纬20°以下地区哨声多台宽带定向观测及其初步结果

武汉大学哨声组与日本名古屋大学空电研究所合作于1988年1月在我国南方进行了哨声多台宽带定向测量。利用近两年研制的宽带数字化频率追踪定向分析系统,首次获得了磁纬20°以下地区哨声波出口区与偏振状态的实验结果。初步分析发现,在湛江附近（磁纬约10°N）存在一个比较稳定的哨声路径出口区,三台同时接收到的哨声大多从这同一出口区透射出来;有时存在两个出口区,一个仍位于湛江附近,另一个位于桂林和武昌之间;沿两条不同路径传播的同源哨声具有几乎相同的色散。本文所做的非导管射线追踪计算能比较满意地解释定向实验结果。另外,此次观测中还发现一些新的有意义的现象,如记录到近百例两跳、三跳及五跳回波等,这在低纬地区是非常少见的。     

磁纬20°以下地区哨声多台宽带定向观测及其初步结果

武汉大学哨声组与日本名古屋大学空电研究所合作于1988年1月在我国南方进行了哨声多台宽带定向测量。利用近两年研制的宽带数字化频率追踪定向分析系统,首次获得了磁纬20°以下地区哨声波出口区与偏振状态的实验结果。初步分析发现,在湛江附近(磁纬约10°N)存在一个比较稳定的哨声路径出口区,三台同时接收到的哨声大多从这同一出口区透射出来;有时存在两个出口区,一个仍位于湛江附近,另一个位于桂林和武昌之间;沿两条不同路径传播的同源哨声具有几乎相同的色散。本文所做的非导管射线追踪计算能比较满意地解释定向实验结果。另外,此次观测中还发现一些新的有意义的现象,如记录到近百例两跳、三跳及五跳回波等,这在低纬地区是非常少见的。     

Application of matched filtering and parameter estimation technique to low latitude whistlers

The Matched Filtering and Parameter Estimation (MFPE) technique developed for the analysis of mid/high latitude whistlers has been extended to analyze whistlers recorded at low latitude ground station Varanasi, India (geomagnetic latitude 14° 55′ N, longitude 153° 59′ E, L=1.07). Some of the whistlers recorded at Varanasi are found to have propagated along higher L-values (L>2). It has been argued that these whistlers after exiting the ionosphere have propagated towards the equator in the Earth-ionosphere waveguide. Trace splitting is observed below the nose frequency and above 2.0 kHz, a result in agreement with mid/high latitude whistlers. The trace splitting structure revealed by MFPE demonstrates the complexities of whistler wave propagation and is quite helpful in deriving information about high resolution features of the duct structure. The banded features observed in the dynamic spectrum are clearly seen in the output of the matched filter. The observed banded features may arise due to interference between the wavelets propagating in the duct/waveguide.     

低纬哨声通过低电离层传播的透射特征——全波解

本文利用全波解方法研究了低纬哨声通过低电离层传播的过程。文中主要计算了哨声波的透射损失和反射系数随投射角与方位角的变化,并结合哨声多站同时观测结果进行了简要的讨论。全波解数值计算结果表明,透射损失随投射角的变化具有南北不对称性,当波矢沿磁力线方向时透射损失最小,且这种不对称性随磁倾角的减小而明显加剧。计算结果还表明,下行哨声的内反射系数基本上不随投射角而变化;但随波频率的升高而减小,并伴有数个相间的极大和极小值,这可能导致哨声回波频谱畸变。     

81型哨声定向观测系统及其观测结果

本文系统地叙述了哨声定向观测的原理、观测设备和观测结果。我们于1982年3月11日在北京怀柔成功地实现了群哨的定向观测,平均入射角为13°,方位角为32°,平均场强为270微伏/米。     

On the observations of unique low latitude whistler-triggered VLF/ELF emissions

A detailed analysis of the VLF/ELF wave data obtained during a whistler campaign under All India Coordinated Program of Ionosphere Thermosphere Studies (AICPITS) at our low latitude Indian ground station Jammu (geomag. lat. = 22° 26′ N, L = 1.17) has yielded two types of unusual and unique whistler-triggered VLF/ELF emissions. These include (1) whistler-triggered hook emissions and (2) whistler-triggered long enduring discrete chorus riser emissions in VLF/ELF frequency range during night time. Such types of whistler-triggered emissions have not been reported earlier from any of the ground observations at low latitudes. In the present study, the observed characteristics of these emissions are described and interpreted. Dispersion analysis of these emissions show that the whistlers as well as emissions have propagated along a higher geomagnetic field line path with L-values lying ∼L = 4, suggesting that these triggered emissions are to be regarded as mid-latitude emissions. These waves could have propagated along the geomagnetic field lines either in a ducted mode or in a pro-longitudinal (PL) mode. The measured intensity of the triggered emissions is almost equal to that of the source waves and does not vary throughout the period of observation on that day. It is speculated that these emissions may have been generated through a process of resonant interaction of the whistler waves with energetic electrons. Parameters related to this interaction are computed for different values of L and wave amplitude. The proposed mechanism explains some aspects of the dynamic spectra.     

Some unusual discrete VLF emissions observed at a low-latitude ground station at Agra

A detailed analysis of the VLF emissions data obtained during occasional whistler campaigns at the low-latitude ground station Agra (geomagnetic latitude 17°1 N, L = 1.15) has yielded some unusual discrete VLF emissions of the rising type. These include (1) emissions occurring at time intervals increasing in geommetrical progression, (2) emissions occuring simultaneously in different frequency ranges and (3) emissions observed during daytime. In the present study, the observed characteristics of these emissions are described and interpreted. It is shown that the increasing time delay between different components of the emissions match closely with the propagation time delays between different hops of a whistler of dispersion 19 s^1/2, the unusual occurrence of the emissions in two different frequency ranges approximately at the same time may possibly be linked with their generation at two different locations, and the occurrence of emissions during daytime may be due to propagation under the influence of equatorial anomaly.     

Connections between whistlers and pulsation activity

Simultaneous whistler records of one station and geomagnetic pulsation (Pc3) records at three stations were compared. In a previous study correlation was found between occurrence and L value of propagation/excitation for the two phenomena. The recently investigated simultaneous records have shown that the correlation is better on longer time scales (days) than on shorter ones (minutes), but the L values of the propagation of whistlers/excitation of pulsations are correlated, i.e. if whistlers propagate in higher latitude ducts, pulsations have periods longer than in the case when whistlers propagate in lower latitude ducts.     

中国北方地区冬季哨声的传播特征

本文对漠河、哈尔滨、北京三个台站1983年冬季的哨声观测资料进行了统计分析。结果表明,这三个地区的哨声活动在日落和后半夜都同时有高峰出现。在日落期间,哨声的平均色散值较高,出现有较高纬度的哨声穿出电离层后沿地球-电离层波导传播到较低纬度地区的现象;而在午夜后,哨声的平均色散值较低,有较低纬度的哨声穿出电离层后传播到较高纬度的现象。从哨声形态学特征判断,漠河和哈尔滨属于中纬度台站,而北京则属于低纬度台站的范畴。