海口上空中高层大气强烈的突发钠层观测

海南激光雷达站（20°N,110°E）2011年11月2日观测到强烈的突发钠层（SSL）.该SSL峰值密度达37087cm-3,半峰全宽仅为0.9km.对该SSL发生前后的钠层峰值密度和高度特性进行分析.统计激光雷达2010年5月至2013年12月共计377天观测到的SSL事件222次,其中仅有1次SSL峰值密度超过30000cm-3.对比距离最近的海南儋州（19.5°N,109.1°E）测高仪和VHF雷达观测到的突发E层（Es）事件,分析事件的相关性.Es最低高度与SSL峰值高度差均在5km以内,约有75%的Es与SSL时间差在±30min之内.实验中使用continuum的Nd:YAG激光器和泵浦染料激光器产生589nm激光,能量为45mJ,使用直径为1000mm的望远镜接收钠层的光子回波.      

电离层赤道异常峰区的微分多普勒跟踪

本文介绍在武汉(30.5°N,114.4°E)地区利用微分多普勒方法对电离层赤道异常峰进行跟踪的结果.由观测资料的分析表明,东经115°线附近,电离层赤道异常峰区在我国长沙(28.1°N,113.0°E)以南和广州(23.2°N,113.3°E)以北地段,其北‘驼峰’顶位于衡阳(26.8°N,112.5°E)至英德(24.2°N,113.5°E)一带上空.对异常峰活动特征的研究也获得了一些有益结果.     

2012年7月海口与北京大气钠层观测

2012年7月海南激光雷达站（20°N,110°E）和北京激光雷达站（40°N,116°E）同时进行观测,对比低纬和中纬度中高层大气钠层分布及密度变化特性.两地使用相同的Nd:YAG激光器和泵浦染料激光器产生589nm激光,设定相同的激光参数,使用相同的直径1000mm的望远镜接收钠层的光子回波.两地均观测到突发钠层和双钠层事件.其中:海口观测到突发钠层事件9个,双钠层事件1个;北京观测到突发钠层事件1个,双钠层事件4个.分析海口和北京同一天夜间观测的大气钠层特性,对比两地同时观测到的钠层高度与密度的不同变化,发现当天夜间低纬度地区海口市上空钠层出现双钠层结构,持续时间123min,但是北京上空钠层没有发现该结构.      

武汉上空偶发E层与流星雨的联系

利用全天空流星雷达和数字测高仪联合观测数据,初步分析了武汉(30°32′N,114°22′E)上空2002年3次大的流星雨(英仙座、狮子座、双子座)及2003年象限座流星雨爆发期间Es的出现率变化趋势及其与流星观测量的关系.结果表明,在流星雨爆发时,雷达观测到的流星数目会显著增加;流星雨峰值过后,Es出现率也会增大.流星数和Es出现率间的平均互相关函数有两个峰值,第一个峰值平均出现在流星雨爆发后的第2天前后,第二个峰值平均出现在流星雨后第6天前后.我们认为,第一个峰值对应于流星雨期间的大量流星雨电离产生的Es,第二个峰值对应于流星雨后沉积的金属离子在风剪切作用下形成的Es.     

电离层离子丰度对哨声传播经度效应的影响

Iss-b卫星探测结果表明,|电离层离子丰度呈现经度变化.本文选取较真实的电离层模型参数,对130°E和114°E两个子午面内传播的低纬哨声进行了射线跟踪.我们发现,由于[O~+],[He~+]和[H~+]决定的电子浓度垂直梯度的经度变化,导致哨声传播特性的经度效应,使得130°E比114°E内传播的低纬哨声更好地满足透射条件:|μsinδ|≤1.与电子浓度的水平梯度的影响比较,离子丰度决定的电子浓度垂直梯度随经度的变化使哨声传播产生的经度效应明显得多.武昌(地理30.5°N,114.6°E;地磁19.3°N).和鹿儿岛(31.5°N,130.8°E;20.5°N)两台站观测到的哨声出现率的经度效应证实了本文的理论预言.     

廊坊中频雷达流星观测及初步结果

中频雷达用来开展夜间100km高度以上的流星观测,获得流星随时间、高度、方位的分布情况及流星体速度、流星辐射点、流星余迹径向速度等参数,其探测数据可用于流星天文学、中层大气动力学等领域的研究.利用2017年11月16日12:00UT-22:00UT期间廊坊观测站（39.4°N,116.7°E）的中频雷达数据,首次开展了中国中纬度地区夜间流星观测实验,共检测到94个流星回波信号,集中分布在97～115km高度范围内,平均高度为106.5km,计算得到了流星回波的双极扩散系数、方位分布等相关参数,并与国外中频雷达流星探测结果进行了初步比较.      

中低纬电离层不同季节地磁暴响应的事例分析

利用中国中低纬台站漠河（53.5°N,122.3°E）、北京（40.3°N,116.2°E）、武汉（30.5°N,114.2°E）和三亚（18.3°N,109.6°E）的电离层观测数据,对比分析了4个台站电离层参数在2015年不同季节4个地磁扰动事件期间的变化特征.结果表明,4个磁暴事件期间电离层的响应特征并不完全一致,有着明显的季节特征,春季、夏季和秋季电离层以负相扰动为主,冬季以正相扰动为主.分析发现,中性成分O/N₂的降低与电离层负相扰动有关,但三亚地区的负相扰动还与扰动发电机电场相关.正相扰动的机制在不同事件中并不相同,穿透电场可能是引起春季磁暴事件期间电离层短时正暴效应的原因,而冬季长时间的正暴效应则是扰动电场和中性风共同作用的结果.      

电离层中小尺度扰动的多频多普勒快速探测与分析

电离层反射回波的高频多普勒频移测量是研究电离层扰动的重要方法之一.利用基于单频信号相位变化率测量的实时探测方法,获取连续高精度多频多普勒频高图,以实现中小尺度电离层扰动的快速探测研究.该方法被应用于敏捷式高性能电离层无线电多频探测系统样机平台.经验证在该平台上获取的多普勒频移分辨率可达0.039Hz,频高图探测周期最短小于1min.基于这种快速探测方法和平台在武汉观测站进行了较长时间的观测和数据采集,获得高精度多普勒频移并反演出电子浓度等值面法向运动速度,得到电离层反射寻常波的多普勒频高图和垂直扰动速度等信息,进而推演出电离层扰动随时间和空间的实时变化特征.对这些时域信号进行频谱分析,初步结果显示这些扰动主要是由极区活动激发的中国中部地区冬季出现率较高的中尺度TID.此外,对三种常用的电离层扰动反演分析方法进行了对比研究,结果显示电离层扰动的变化趋势基本一致,说明观测数据和研究方法可靠稳定,为多频多点电离层扰动的传播特性观测研究提供了基础.      

华南地区电离层闪烁与TEC耗空的时间和空间分布统计分析

利用位于赤道异常区的深圳站（22.59°N,113.97°E）2011年1月至2012年12月及2015年1月至2015年12月监测到的GPS-TEC数据,统计分析华南地区电离层闪烁与TEC耗空同时出现、电离层闪烁单独出现和TEC耗空单独出现3种现象的时间和空间分布特性.结果表明:这3种现象均主要发生在春秋季节;闪烁与TEC耗空同时出现、闪烁单独出现和TEC耗空单独出现分别主要发生在纬度为19°-23°N,21°-24°N和24°-26°N的空间区域.探测到闪烁和TEC耗空同时出现、闪烁单独出现和TEC耗空单独出现的时间分别主要分布在20:00LT-22:00LT,21:00LT-23:00LT和22:30LT-23:30LT.闪烁与TEC耗空同时出现、闪烁单独出现和TEC耗空单独出现3种现象的时间和空间分布特性对应了华南地区不规则体和赤道等离子体泡（EPBs）从产生到消失的演变过程.      

广州地区上空电离层F_2层异常

广州电离层观测站位于磁赤道附近(地磁经纬度:183.0°E,11.5°N).根据对18个站电离层资料的分析,广州站位于f_0F_2极大值双驼峰现象北侧的位置;在太阳活动高年期间,f_0F_2极大值在夜间仍然保持存在;另外,电离层骚扰发生的次数多于国内各电离层观测站发生的次数.这些特性都是F_2层赤道异常现象.     

A statistical study of the F2 layer stratification at the northern equatorial ionization anomaly

Ionograms recorded at Puer station (PUR, 22.7°N, 101.05°E, Dip Latitude 12.9°N) in the Southwest of China from January 2015 to December 2016 were used to study characteristics of the F2 layer stratification at the northern equatorial ionization anomaly. Ionosonde observations show that the development of the F2 layer stratification is different under different conditions. Both the upward and downward movement of the F2 layer stratification could be observed. The F2 layer stratification could occur both at daytime and nighttime. The new cusp could originate from different positions on ionograms. Moreover, statistical results indicate that the F2 layer stratification occurred later in the winter than in other seasons at daytime, it occurred frequently in the local spring, and most of ionograms with the F2 layer stratification at post-midnight occurred in March and April. Our results also show that the F2 layer stratification has a correlation with solar activity.     

Daytime VHF amplitude scintillations recorded at an Indian low-latitude station,Waltair (17.7°N, 83.3°E) during 1997–2003

In this research, it is presented the daytime amplitude scintillations recorded at VHF frequency (244 MHz) at an Indian low-latitude station, Waltair (17.7°N, 83.3°E) during seven continuous years (1997–2003). Contrary to the nighttime scintillation seasonal trends, the occurrence of daytime scintillations maximizes during summer followed by winter and the equinox seasons. The fade depths, scintillation indices and the patch durations of daytime scintillations are meager when compared with their nighttime counterparts. A co-located digital high frequency (HF) ionosonde radar confirms the presence of sporadic (Es) layers when daytime scintillations are observed. The presence of daytime scintillations is evident when the critical frequency of the Es-layer (foEs) is ≥4 MHz and Es-layers are characterized by a highly diffuse range spread Es echoes as can be seen on ionograms. It is surmised that the gradient drift instability (GDI) seems to be the possible mechanism for the generation of these daytime scintillations. It is quite likely that the spread Es-F-layer coupling is done through polarization electric fields (Ep) that develop inside the destabilized patches of sporadic E layers, which are mapped up to the F region along the field lines as to initiate the daytime scintillations through the GDI mechanism. Further, the presence of additional stratification of ionosphere F-layer, popularly known as the F3-layer, is observed on ionograms once the Es-layers and daytime scintillations are ceased.     

Investigation of Satellite Trace (ST) and Multi-reflected Echo (MRE) ionogram signatures and its possible correlation to nighttime spread F development from Cyprus over the solar mini-max (2009–2016)

Multi-reflected echoes (MREs) and satellite traces (STs) are referred in literature as ionogram signatures of Travelling Ionospheric Disturbances (TIDs) which is a phenomenon that apparently drives spread F development mainly at nighttime mid-latitude ionosphere. A long-term statistical study has been undertaken to investigate the morphological aspect of these signatures over the lower midlatitude European station of Nicosia, Cyprus (35.19°N, 33.38°E geographic; magnetic dip. 29.38°N) by inspecting all ionograms recorded by the DPS-4D digisonde in the interval 2009–2016. The results underline the systematic manifestation of these TID signatures over Cyprus with a possible (although not quite clear) solar activity dependence and a distinctive seasonal and diurnal occurrence rate with a seasonal peak of STs during summer and of MREs during January to April. Based on the experimental results of the present study, the seasonal occurrence rate of MREs and STs is found to increase by 75% and 56% during high solar activity periods. Satellite traces are well known ionogram signatures of TIDs and mostly correlated to the nighttime spread F formation. The occurrence of mid-latitude spread Fs over European longitude sector normally increases during summer. The occurrences of TIDs are also prominent at this interval of the year over nighttime mid-latitude ionosphere. The presence of MREs as an ionogram signature of TIDs over mid-latitude ionosphere is unique in nature.     

Ionospheric disturbances at low and mid-low latitudes of the South American sector during the March 2015 great storm

This paper analyzes the response of the near equatorial and low latitude ionosphere of the South American sector to the geomagnetic storm occurred on 17 March 2015. Ionosonde data from Ramey (18.5° N, 292.9° E), Jicamarca (12.0° S, 283.2° E), Boa Vista (2.8° N, 299.3° E), Sao Luis (2.6° S, 315.8° E), Fortaleza (3.9° S, 321.6° E) and Cachoeira Paulista (22.7° S, 315.0° E) are used for the study. The results show negative disturbances in foF2 at low latitudes during the main phase of the storm, which were attributed to prompt penetration electric fields. Thus, the Equatorial Anomaly (EA) started to reduce their structure in this sector since on 17 March. During the recovery phase (on 18 March), positive disturbances were observed at low, mid-low latitudes (in the post-midnight – predawn hours), which can be mainly attributed to enhanced storm-time neutral winds and composition changes (i.e., increase in the O/N2 ratio). Disturbance dynamo electric fields would also contribute in modulating the electron density of the EA during this storm period.     

A method for automatic detection of equatorial spread-F in ionograms

A method is presented for automatic detection of spread-F. The method is based on an image recognition technique and is applied to ionograms recorded at the ionospheric station of Tucumán (26.9°S, 294.6°E). The performance achieved is statistically evaluated and demonstrated with significant examples. The proposed method improves Autoscala's ability to reject ionograms with insufficient information, including those featuring Spread-F. Automatic identification of cases of spread-F is of additional interest in Space Weather applications, when it helps detect degraded radio propagation conditions.The present data analysis is a retrospective study but forms the basis for real-time application as an extension of Autoscala’s capabilities.     

Intercosmos-19 observations of an additional topside ionization layer: the F3 layer

Spatial properties of an additional ionization layer in the topside ionosphere were investigated using Intercosmos-19 satellite ionograms. The data under analysis were choosen for equinoctial conditions of the high solar activity period (1979–1981). The F3 layer was detected in a narrow longitude sectors (about 60°) between the equatorial anomaly crests. Its intensity has a maximum just above the equator and decreases poleward within ±10° dip. A nighttime F3 layer was observed as well as the daytime events.     

A comparative study of ionospheric spread-F and scintillation at low- and mid-latitudes in China during the 24th solar cycle

The spread-F echo of ionograms and scintillation of satellite signal propagation along the Earth-space path are two typical phenomena induced by ionospheric irregularities. In this study, we obtained spread-F data from HF (high frequency) digital ionosonde and scintillation index (S4) data from L-band and UHF receivers at low- and mid-latitudes in China during the 24th solar cycle. These four sites were located at Haikou (HK) (20°N, 110.34°E), Kunming (KM) (25.64°N, 103.72°E), Qingdao (QD) (36.24°N, 120.42°E), and Manzhouli (MZL) (49.56°N, 117.52°E). We used these data to investigate spread-F and scintillation occurrence percentages and variations with local time, season, latitude and solar activity. A comparative study of spread-F and scintillation occurrence rates has been made. The main conclusions are as follows: (a) FSF occurred mostly during post-midnight, while RSF and scintillation appeared mainly during pre-midnight at HK and KM; (b) FSF occurrence rates were larger at QD and MZL than expected; (c) the FSF occurrence percentages were anti-correlated with solar activity at HK and KM; meanwhile RSF and scintillation occurrence rates increased with the increase of solar activity at this two sites; (d) the highest FSF occurrence rates mostly appeared during the summer months, while RSF and scintillation occurred mostly in the equinoctial months at HK and KM; (e) the scintillation occurrence was usually associated with the appearance of RSF, probably due to a different physical mechanism comparing with FSF. Some of these results verified the conclusions of previous papers, whereas some show slight difference. These results are important in understanding ionospheric irregularities variations characteristic at low- and mid-latitudes in China.     

Collocated ionosonde and dense GPS/GLONASS network measurements of midlatitude MSTIDs

To analyze midlatitude medium-scale travelling ionospheric disturbances (MSTIDs) over Kazan (55.5°N, 49°E), Russia, the sufficiently dense network of GNSS receivers (more than 150 ground-based stations) were used. For the first time, daytime MSTIDs in the form of their main signature (band structure) on high-resolution two-dimensional maps of the total electron content perturbation (TEC maps) are compared with ionosonde data with a high temporal resolution. For a pair of events, a relationship between southwestward TEC perturbations and evolution of F2 layer traces was established. So F2 peak frequency varied in antiphase to TEC perturbations. The ionograms show that during the movement of plasma depletion band (overhead ionosonde) the F2 peak frequency is the highest, and vice versa, for the plasma enhancement band, the F2 peak frequency is the lowest. One possible explanation may be a greater inclination of the radio beam from the vertical during the placement of a plasma enhancement band above the ionosonde, as evidenced by the absence of multiple reflections and the increased occurrence rate of additional cusp trace. Another possible explanation may be the redistribution of the electron content in the topside ionosphere with a small decrease in the F peak concentration of the layer with a small increase in TEC along the line-of-sight. Analysis of F2 peak frequency variation shows that observed peak-to-peak values of TEC perturbation equal to 0.4 and 1 TECU correspond to the values of ΔN/N equal to 13% and 28%. The need for further research is evident.     

Seasonal variations in vertical distribution of meteor decay time as observed from meteor radars at 8.5°N and 80°N

The decay times of meteor radar echoes have been used for decades to investigate characteristics of the mesosphere and lower thermosphere (MLT) region. As the meteor echo decay time depends on background atmospheric parameters, in the present communication, we examine the seasonal variation of the vertical distributions of underdense meteor echo decay times with respect to echo strength. Observations from two similar radars located at two distinct geographical locations, Thumba (8.5°N, 77°E) and Eureka (80°N, 85.8°W) were used for the present study. Here, the radar received signal power is categorized into strong and weak echoes and vertical profiles of their decay times are constructed. It has been noticed that the monthly mean decay time vertical profile turning altitude (i.e., inflection point) varies in the range of 80–87?km of altitude depending on latitude. The turning altitude is observed at relatively lower heights in the winter than in summer at both the latitudes. The present analysis shows that the meteor decay time below the mean turning altitude follows a decreasing trend with decreasing altitude, which is quite distinct to the behaviour of ambipolar diffusion. It is also observed that there is a difference in mean decay time of strong and weak echoes below 90?km of altitude, which is very prominently seen at lower altitudes. This difference shows a seasonal pattern at high latitude, but does not show any seasonal variation at low latitude. The present results are discussed in light of current understanding of the meteor decay time.