声重波激发的赤道Spread—F的拓扑特性研究

本文对声重波激发的Spread-F扰动的非线性制约方程的拓扑特性进行了定性讨论和数值计算．研究了Spread-F的混沌特征及参数范围．结果表明声重波在不同条件时对Spread-F的激发作用有本质的不同．     

声重波激发的赤道 Spread—F 不稳定性

本文导出了声重波扰动与赤道电离层扩展F层相互作用的基本方程,并对数值模拟结果进行了讨论。结果表明当声重波扰动与Kavleigh-Taylor不稳定性共同作用于赤道F层时,将导致大尺度波状结构的Spread-F.其特征为以一定速度漂移的准周期不规则性,发展到后期伴有羽柱形状的bubble(电子密度低区)产生。这种现状的bubble可上升至F层顶部并发生倾斜。声重波波矢量的方向决定着羽柱的倾斜方向。这些结果与实验观测特征相符,说明声重波扰动不仅是大尺度Saread-F的触发源,而且对其整个形态发展过程起着重要的作用。      

Aura/MLS与TIMED/SABER观测全球重力波特性

大气重力波是临近空间环境主要大气波动之一,对全球环流具有重要影响。卫星上搭载的临边探测器能够探测临近空间大气温度,可用于临近空间大气重力波研究。利用2012－2014年Aura的微波临边探测器（MLS）和TIMED的红外临边探测器（SABER）的探测数据,对20~50 km高度的大气重力波扰动分布特征开展了分析研究,两种观测重力波活动基本一致,重力波随季节、纬度及高度的变化显著。冬季半球高纬度重力波扰动较强,赤道和夏季半球近赤道地区上空也存在明显重力波活动区域,夏季半球高纬度重力波扰动最弱。重力波扰动强度随高度增加。TIMED/SABER重力波扰动强度数值比 Aura/MLS略强。      

中高层大气对低层大气脉冲扰动的响应

从基本的大气运动方程出发,采用二维全隐欧拉格式(FICE)建立极坐标下二维声重波传播的数值模式.对小振幅高斯形态声重波传播过程的模拟结果表明,该数值模式能够再现声重波波包向上的稳定传播过程.应用此数值模式模拟了在没有背景风场和存在背景风场条件下中高层大气对10km高空处近似脉冲点源扰动的响应过程,获得了中高层大气对低层大气脉冲扰动响应的图像.结果表明,电离层高度对重力波扰动的响应不但出现在激发源激发之后,还与激发源具有较大距离,扰动的幅度随高度的增大而增大;有背景风时,顺风情况下重力波波动传播路径较逆风情况下平缓,且传播的水平距离比逆风情况下远,波动的振幅较逆风情况下弱,逆向背景风场能够加速重力波向上传播.同时,模拟还表明脉冲扰动正上方可以激发产生一种以6 min为主要周期,具有声学分支特性的快速波动.本文模拟的扰动结果与2004年12月26日苏门答腊-安达曼地震引起的电离层TEC扰动具有类似的图像.      

低热层臭氧含量振荡和温度起伏间的耦合关系

本文求出了声重波传播过程中,低热层臭氧含量振荡和温度场起伏间的耦合关系。首次揭示出其正负相关性的内在原因和判据——动力扰动尺度。在中小尺度声重波扰动范围内,臭氧含量振荡与温度场起伏是正相关的,且相关幅度可达或超过7;而在大尺度或行星尺度波范围内,则一般为负相关。此发现可解释低热层中臭氧含量快速变化及臭氧含量长时间纬圈平均值变化与温度场成负相关的观测事实。本文还在声重波尺度范围内比较了线性和非线性理论的结果,结果表明两者没有太大的差异。     

1980年中层大气温度波空间特征的初步分析

利用1980年Nimbus-7卫星网络点资料(温度场)对中层大气行星波的空间结构进行诊断和分析后发现,行星波扰动主要集中于冬半球,夏半球及赤道地区上空的扰动则相对较弱,但也不可忽视.冬半球行星波扰动中的瞬变波部分可以跨过赤道向夏半球传播,且传播主要集中于20kin和70km两个高度层附近.波数1冬季以准定常行里波为主,夏季瞬变行星波与准定常行星波波幅相当.行星波扰动的波幅从冬到夏的衰减主要表现在波数1和波数2上,波数3变化不大.      

赤道地区和中纬度地区扩展F的特点和比较

通过数值模拟研究了赤道地区和中纬度地区扩展F的触发机制间的联系,发现相同的触发机制在不同纬度的条件下所演化出的结构形态不相同,因此观测上会有所不同;研究了西向电场在赤道地区的作用,发现在适合的电离层参数组合下会产生观测到的电离层F区顶部扰动出现率高于底部扰动出现率的状况;在数值模拟的基础上提出了一个中纬度地区电波散射的模型,用以解释观测到的中纬度地区电波散射的出现率高于线性理论预期的现象.      

大尺度行星波跨赤道传播的E—P通量诊断

利用Nimbus－7卫星温度探测资料，计算了平流层和中间层的风场、位势高度扰动场，进行了行星波E－P通量分析．结果表明，中层大气中的准定常行星波，其定常分量不能跨过赤道上空的零风线，由于其幅度的起伏和相位的变化，激发的瞬变行星波分量不受赤道零风线的限制，可以从冬半球向夏半球传播，也可以从夏半球向冬半球传播．这种跨赤道传播为夏半球行星波的能量来源提供了一种解释．     

低纬地区TEC观测的初步分析

1985年6、7月间在低纬地区进行了TEC观测。发现赤道异常的北峰在广州江门一带,峰的位置有波动,低纬地区本地产生的声重波周期低于20—48分钟且随纬度而异;极地产生的大尺度声重波可以传播到广州、香港一带,难于传到海南岛;低纬地区的不均匀结构的出现,集中在2000—0400之间,其尺度不大,漂移没有明显的东向优势。本文还给出了由视速求真实速度的方法,在接近太阳活动低年的1985年夏季仅在三亚看到电离层汽泡。      

石英晶体和石英谐振器的非线性特性述评

石英晶体的非线性特性几乎是使谐振器和振荡器产生频率不稳定的总根源。虽然固有的和感应的非线性具有相同的量级,但还是有可能将两者加以区别。非线性引起两种不同的现象:一个有限幅度的波在非线性介质中的传播,或一个小幅度的波在非线性应变介质中的传播。谐波产生、幅频效应、相互调制是与第一类型有关的现象。它们取决于晶体的各向异性和几何形状,也取决于波的结构。对外部或内部扰动的灵敏性,是高频波和由扰动感应产生的准静态变形之间非线性耦合的结果。对温度、力、压力、加速度和电场的灵敏性都在进行研究,以便能应用于高稳定振荡器和传感器。描述晶体振荡的另一种方法是应用点阵波和声子的概念。这样一个微观模型,使我们能够借助于晶体非谐性引起的声子相互作用(有限的导热性、热膨胀、声衰减、速度变化等)进行单一的描述。     

Effect of the seed perturbation amplitude on the equatorial spread F initiation during solar minimum

The contribution of gravity wave (GW) to the initiation/development of spread F during a solar minimum year was investigated through the comparison of the observed precursory parameters and characteristics of the corresponding equatorial spread F (ESF) events. The ionospheric parameters were recorded at the magnetic equatorial station Sao Luis (2.3°S, 44°W, dip latitude 2°S) during March and October 2010. These data were used to estimate the influence of the relative gravity wave amplitude and the ambient ionospheric condition on the diurnal variation of the spread F initiation. The vertical velocity drift indicated a clear control and defines the threshold for the seasonal variability of the ESF occurrence. However, it was insufficient to solely determine or predict the day to day variation of ESF occurrence. Thus, few days with contrasting ambient ionospheric condition and magnitude of GW amplitude were analysed in order to investigate the role of the different precursory factors in the observed diurnal variation of the plasma irregularity development. The density scale length and gravity wave amplitude were shown to immensely contribute to the linear instability growth rate, especially during the days with a low post-sunset rise. Thus, the experimental observations have demonstrated the strong inter-dependence between the precursory factors and they have also highlighted the probable control of the ESF morphology.     

The variation of equatorial spread-F occurrences observed by ionosondes at Thailand longitude sector

The equatorial spread-F (ESF) is a phenomenon of ionopheric irregularities which are mainly generated by the generalized Rayleigh–Taylor (R–T) instability mechanism in conjunction with the other physical mechanisms, originated at the bottom side of the F-layer in the equatorial region after sunset. It degrades the quality of signals that propagate through these irregularities, especially in the navigation satellite system, which requires the high integrity signals. In this work, we analyze the ESF statistics obtained from the FM/CW ionosonde stations over Thailand longitude sector. One is at Chumphon (10.72°N, 99.37°E, dip latitude 3.0°), located near the geomagnetic equator, and the other station is located at Chiangmai (18.76°N, 98.93°E, dip latitude 12.7°). Both stations are as part of the South-East Asia Low Latitude Ionospheric Network (SEALION) project. The ionograms are obtained at every 15 min from September 2004 to August 2005, which has the monthly mean of solar 10.7 cm flux (F10.7) from ∼80 to ∼110. In addition, we compare the diurnal patterns between the ESF occurrences and the variation of virtual height of the F-layer bottom side (h’F) of these two stations. The results show that the ESF occurrences at Chumphon stations are higher than Chiangmai station in all seasons. The high ESF occurrences of both stations mostly occur in equinoctial months corresponded with the rapid rising of the monthly mean h’F in the post-sunset. However, some inconsistent results are still observed, implying the role of other factors such as gravity waves and planetary waves to ESF occurrences.     

ESF强散射理论初探

本文研究了高频寻常波在均匀等离子体中垂直均匀背景磁场传播时,由于在传播方向上有限区域内存在等幅密度扰动驻波引起共振折反射的理论.认为这种共振反射理论可能解释ESF条件下VHF/UHF雷达强散射和卫星与地面通讯中断等现象.      

Equatorial ionosphere–thermosphere system: Electrodynamics and irregularities

The equatorial ionosphere and thermosphere constitute a coupled system, with its electro dynamical and plasma physical processes being responsible for a variety of ionospheric phenomena peculiar to the equatorial region. The most important of these phenomena are: the equatorial electrojet (EEJ) current system and its instabilities, the equatorial ionization anomaly (EIA), and the plasma instabilities/irregularities of the night ionosphere (associated with the plasma bubble events – ESF). They constitute the major topics of investigations having both scientific and practical objectives. The tidal wind interaction with the geomagnetic field is responsible for the atmospheric dynamo electric fields, that together with the wind system, drives the major phenomena, under quiet conditions. Drastic modifications of these phenomena can occur due to magnetospheric forcing under solar-, interplanetary- and magnetospheric disturbances. They can also undergo significant modifications due to forcing by atmospheric waves (such as planetary- and atmospheric gravity waves) propagating upward or from extra tropics. This article will focus on the ambient conditions of the ionosphere–thermosphere system and the electro dynamics and plasma instability processes that govern the plasma irregularity generation. Major emphasis is given to problems related to the structuring of the equatorial night ionosphere through plasma bubble/ESF irregularity processes. Specific topics to be covered will include: equatorial electric fields, thermospheric winds, sunset electrodynamic processes, plasma drifts, EEJ plasma instability/irregularity generation, nighttime/post sunset plasma bubble irregularity generation, and very briefly, disturbance electric fields and winds and their effect on the ionization anomaly, the TEC and ESF/plasma bubble irregularities.     

Characteristics of equatorial nighttime spread F – An analysis on season-longitude,solar activity and triggering causes

To understand global variability and triggering mechanisms of ionospheric nighttime equatorial spread F (ESF), we analyzed measurements from satellite and a ground-based GPS station for the years between 2010 and 2017. In this study we present seasonal-longitudinal as well as monthly variability of ESF occurrence for solar minimum and yearly variations of ESF occurrence for solar maximum and minimum periods. One of the long standing open questions in the study of ESF is what exactly initiates the Rayleigh-Taylor (RT) plasma instability growth. This question is the focus of the present work. Zonal background eastward electric field and E × B upward plasma drift speed patterns are found to be critically important in understanding plasma irregularity formation. In addition to particular patterns observed on these parameters, the background plasma density in the local evening hours just before the onset of ESF occurrence is very important. Stronger plasma densities just before the onset of irregularities resulted in stronger plasma irregularities, while relatively less dense plasma just before the onset of irregularities resulted in relatively lower plasma irregularities. Seasonal variations in ESF activity between March and September equinox seasons with comparable plasma densities can be defined in terms of the rate of change of solar flux F10.7 (dF10.7/day) index. Strongest ESF occurrence and strongest dF10.7/day are measured in the same month out of all other months in 2016 and 2017. Longitudinal variations of ESF activity in our measurements are related to longitudinal variations of plasma densities. We also found that ESF occurrence is better correlated with rate of change of F10.7 index for months in equinox seasons than for months in solstice seasons for the years between 2013 and 2016.     

Statistical characteristics of locally generated ESF during equinoctial months over Sanya

Understanding the local generation rate of equatorial spread-F (ESF) is important for forecasting ionospheric scintillation. Using the GPS ionospheric scintillation/TEC and VHF radar data during March-April and September-October from 2010 to 2014, the occurrence of ionospheric scintillation, TEC fast fluctuation, and backscatter plume were studied. Through analyzing the simultaneous occurrence of ionospheric scintillation, TEC fast fluctuation and backscatter plume, the local generation rate of ESF over Sanya was investigated. The results show that the monthly generation rate varies between 0% and 68%. A significant equinoctial asymmetry of local generation rate of ESF can be found in 2010, 2013 and 2014. The local generation rate of ESF increases from 2010 to 2014 during March-April, while it does not have similar trend during September-October. The plasma vertical drift influenced by solar activity has a significant impact on the monthly generation rate. The equinoctial asymmetry of plasma vertical drift may contribute a lot to the equinoctial asymmetry of the generation rate of ESF.     

Solar activity variations of equatorial spread F occurrence and sustenance during different seasons over Indian longitudes: Empirical model and causative mechanisms

A comprehensive analysis using nearly two decades of ionosonde data is carried out on the seasonal and solar cycle variations of Equatorial Spread F (ESF) irregularities over magnetic equatorial location Trivandrum (8.5°N, 77°E). The corresponding Rayleigh Taylor (RT) instability growth rates (γ) are also estimated. A seasonal pattern of ESF occurrence and the corresponding γ is established for low solar (LSA), medium solar (MSA) and high solar (HSA) activity periods. For LSA, it is seen that the γ maximizes during post sunset time with comparable magnitudes for autumnal equinox (AE), vernal equinox (VE) and winter solstice (WS), while for summer solstice (SS) it maximizes in the post-midnight period. Concurrent responses are seen in the ESF occurrence pattern. For MSA, γ maximizes during post-sunset for VE followed by WS and AE while SS maximises during post-midnight period. The ESF occurrence for MSA is highest for VE (80%), followed by AE (70%), WS (60%) and SS (50%). In case of HSA, maximum γ occurs for VE followed by AE, WS and SS. The concurrent ESF occurrence maximizes for VE and AE (90%), WS and SS at 70%.The solar cycle variation of γ is examined. γ shows a linear variation with F10.7?cm flux. Further, ESF percentage occurrence and duration show an exponential and linear variation respectively with γ. An empirical model on the solar activity dependence of ESF occurrence and sustenance time over Indian longitudes is arrived at using the database spanning two solar cycles for the first time.     

ESF的Transition区谱分布特性

本文提出了研究Transition区谱分布的静电模型,并按等离子体弱耦强湍理论对其重正化得到了谱方程.对谱方程作适当近似后,证明了该近似谱方程存在扰动密度谱为k~(-5),扰动电场谱为k~(-3)分布的解,从而定量解释了Transition区的实验谱分布结果.     

Propagation of plasma bubbles observed in Brazil from GPS and airglow data

Equatorial spread-F is a common occurrence in the equatorial ionosphere that is associated with large variations in plasma density that often cause scintillation and interference in communication signals. These events are known to result from Rayleigh–Taylor instability, but the day-to-day variability of their occurrence is not well understood. The triggering mechanism of plasma depletions is still a matter of debate, but may be linked to gravity waves that under favorable conditions propagate to the middle atmosphere. Understanding the triggering of ESF was the focus of the SpreadFEx campaign near Brasilia, Brazil in 2005. The campaign provided co-located airglow and GPS observations to study the onset of plasma depletions and their evolution as they traversed the region. Comparisons between the 630.0 nm airglow data and GPS data demonstrate the ability of the compact dual frequency GPS array to detect the plasma bubbles and retrieve reliable propagation characteristics of the depletions. In this case study, a plasma depletion was detected and moved over the array at velocities of 85–110 m/s, slowing as it moved towards the east. Correlation of consecutive airglow images gives consistent estimates of the eastward drift over the same time period. Mapping the airglow data to the GPS line-of-sight geometry allows direct comparison and reveals a resolvable westward tilt of the plasma depletion that may be due to vertical shear. The uniqueness of this study is the ability to resolve locally the characteristics of the plasma depletion without relying on assumptions about the mapping of the depletion along magnetic field lines to large latitudinal distances. It presents new information for understanding ESF development and the development of depletions strong enough to produce scintillation.     

Precursors of Equatorial Spread F observing GPS TEC data from different GAGAN stations in India

Occurrence of Spread F is more or less a daily phenomenon in the equatorial and low latitudinal stations during high to moderate sunspot number years. In this paper efforts have been made to identify possible precursors of Equatorial Spread F (ESF) using the Total Electron Content (TEC) data of seven GAGAN (GPS Aided Geo Augmented Navigation) stations in India during the two equinoxes of moderate sunspot number year 2004. Large Scale Periodic Structures found prior to TEC bite out can be taken as possible precursors to ESF. A threshold value of the peak to peak amplitude of this wave structure is chosen 2.6 TEC unit above which there is a possibility of ESF or TEC bite out with S4 > 0.26.