大气重力波产生大尺度赤道扩展F的理论

本文提出了大气重力波触发Ｒａｙｌｅｉｇｈ－Ｔａｙｌｏｒ不稳定性并导致大尺度赤道扩展Ｆ的理论．重力波作为外部扰动派触发等离子体扰动，这种扰动在Ｒａｙｌｅｉｇｈ－Ｔａｙｌｏｒ不稳定性的作用下继续增长，经过４００ｓ扰动幅度就能增长到５０％，经过７００ｓ后幅度趋近１００％，即为完全的等离子体泡．由于Ｅ区的影响，赤道扩展Ｆ主要出现于晚上．本文的理论阐明了重力波与Ｒａｙｌｅｉｇｈ－Ｔａｙｌｏｒ不稳定性相互作用的性质，揭示了大尺度赤道扩展Ｆ的产生机制．     

大气重力波产生的大尺度赤道电离层扰动

本文研究了大气重力波产生的大尺度赤道电离层扰动的性质．当重力波的传播方向与磁场方向倾斜相交时，重力波在Ｆ区产生行进电离层扰动．当重力波垂直于磁场传播时，能触发等离子体Ｒａｙｌｅｉｇｈ－Ｔａｙｌｏｒ不稳定性，形成大尺度赤道扩展Ｆ不均匀体．重力波引起的扩展Ｆ主要出现于晚上，行进电离层扰动则可能出现于任何时间．本文建立了行进电离层扰动和大尺度赤道扩展Ｆ的统一理论模型，深入全面地揭示了电离层扰动的性质．     

大气重力波触发的Rayleigh－Taylor不稳定性的时空演变

对大气重力波触发的赤道电离层Ｒａｙｌｅｉｇｈ－Ｔａｙｌｏｒ不稳定性的时空演变进行了数值模拟．结果表明，重力波能在Ｆ区底部触发Ｒａｙｌｅｉｇｈ－Ｔａｙｌｏｒ不稳定性，这种等离子体不稳定扰动穿过Ｆ峰，到达Ｆ区顶部，形成等离子体泡结构．等离子体泡出现向西倾斜和分岔等特征．当夜Ｆ区较高时，产生等离子体泡的时间仅１９００ｓ．数值模拟结果证实了重力波触发．Ｒａｙｌｅｉｇｈ－Ｔａｙｌｏｒ不稳定性的理论，解释了大量电离层观测现象．     

大气重力波触发的Rayleigh－Taylor不稳定性的时空演变

对大气重力波触发的赤道电离层Ｒａｙｌｅｉｇｈ－Ｔａｙｌｏｒ不稳定性的时空演变进行了数值模拟．结果表明，重力波能在Ｆ区底部触发Ｒａｙｌｅｉｇｈ－Ｔａｙｌｏｒ不稳定性，这种等离子体不稳定扰动穿过Ｆ峰，到达Ｆ区顶部，形成等离子体泡结构．等离子体泡出现向西倾斜和分岔等特征．当夜Ｆ区较高时，产生等离子体泡的时间仅１９００ｓ．数值模拟结果证实了重力波触发．Ｒａｙｌｅｉｇｈ－Ｔａｙｌｏｒ不稳定性的理论，解释了大量电离层观测现象．     

对流电场、场向电流和极光区电集流变化的地磁响应

对流电场、场向电流和极光区电集流是磁层一电离层耦合的主要物理过程．它们的演化发展时间分别为几分钟至半小时的量级．本文用１００°Ｅ和３００°Ｅ的两个地磁经度链附近各１１个台站的１ｍｉｎ均值地磁Ｈ和Ｚ分量资料,分析了１９９４年４月１６-１７日磁暴期间磁层耦合过程对极光区和中低纬区电离层扰动的地磁特征．强磁暴开始时,台站所处的地方时位置不同,则观测到的电离层和地磁响应也完全不同．这是磁层对流和一、二区场向电流共同作用的结果．一般说,扰时极光区的西向电集流变化更为强烈．随着耦合的发展,极光区范围会向南北扩展,电集流中心带则向低纬侧移动．在中低纬区,二区场向电流的建立能屏蔽一区场向电流所产生的扰动,并引起反向的电流及地磁变化．由此,中低纬区夜间有可能出现短时间的东向电场,又可通过ＥＸＢ的垂直向上漂移作用抬升Ｆ层等离子体,并发生同一经度链附近的多站电离层ｈ＇Ｆ同时突增现象．另一方面,磁赤道附近的台站则更多地受内磁层赤道环电流和电离层赤道电集流的影响．     

基于GNSS观测的全球赤道等离子体泡对太阳和地磁活动及季节依赖特征

赤道等离子体泡是在磁赤道和低纬地区一种通常发生在夜间的大尺度电离层不规则结构.本文利用2010—2021年全球低纬地区GNSS TEC监测仪长期观测数据,分析了赤道等离子体泡对太阳和地磁活动及季节的依赖特征.结果表明,全球每个经度区赤道等离子体泡发生率与太阳活动都显著正相关,赤道等离子体泡的日发生率与F₁₀₇指数相关系数接近0.70.经度-60°～-30°在赤道等离子体泡高发月份的日落后发生率与F₁₀₇的相关关系有明显的饱和效应.在季节变化方面,经度-60°～-30°赤道等离子体泡主要发生在1—3月和10—12月,其他经度主要发生在分季.不同经度区赤道等离子体泡日落后发生率季节不对称性存在显著区别,在太阳活动上升期,经度-60°～-30°的10—12月发生率明显高于1—3月;在太阳活动下降期,0°～180°经度区3月分季高于9月分季.此外,地磁扰动对产生赤道等离子体泡主要是抑制作用,特别是在太阳活动高年、分季.     

中纬电离层大尺度扰动的数值模拟

利用计算机模拟研究了不同条件下中纬电离层F区大尺度扰动的时空演变.当存在幅度约为5％的初始扰动时,Perkins不稳定性能发展成为大尺度电离层不均匀体.大气重力波能触发Perkins不稳定性,在更短时间内增长成为大幅度电离层振荡.非均匀局部电场结构也能产生局部电离层的大幅度扰动.本义的模拟表明,中纬电离层大尺度扰动主要表现为磁通管内等离子体沿磁力线的振荡.模拟结果与观测现象符合得很好.     

东亚赤道异常区电离层CT诊断──实验及初步结果

国际上第一个低纬电离层层析（ＣＴ）探测台链于１９９３年１１月建成，并开始数据采集．该ＣＴ台链沿１２０°Ｅ子午圈由６个卫星信标差分多普勒接收站组成，北起我国上海，南至菲律宾的马尼拉，纬度覆盖范围约２０°．该台链专为电高层赤道异常区电子密度大尺度结构ＣＴ成像而设计，其目的是监测研究赤道异常形态与动力学、电离层等离子体泡、行优以及暴时磁层－电离层－热层耦合过程等重要的赤道与低纬区电离层现象．本文介绍该低纬电离层ＣＴ实验的有关技术问题，包括接收台站布设、实验设备、数据采集软件和数据预处理方法．文中还给出利用ＣＴ技术，由实测斜电子含量数据反演电离层电子密度二维分布图像的几个实例．结果表明，利用低纬电离层ＣＴ台链可以有效地监测电离层赤道异常及其它低纬电高层电子密度大尺度结构的时空变化．     

日侧极隙纬度低频波及其与地磁扰动的关系

利用挪威斯瓦尔巴德地区的ＥＬＦ／ＶＬＦ观测资料和世界数据中心公布的Ｋｐ指数，分析了日侧极隙纬度低频波不同季节里的周日变化和１９８４年５月－１９８６年４月的年度变化及这些波与地磁扰动的某些关系．主要结果有：１．夏季ＥＬＦ波出现率极大值最大，而冬季的最小，前者约为后者的３倍，而且此极大值无论哪个季节都出现在磁正午时段．２．ＶＬＦ波出现率在夏季较小冬季较大；其极大值出现在磁午后至磁子夜前的２－４ｈ时段里．３．夏季ＥＬＦ波指数∑Ａ与地磁指数∑ＫＰ日变化较相吻合，在太阳自转周内两者相关系数约为０．６．４．冬季ＶＬＦ波指数∑Ａ与∑Ｋｐ日变化相吻合，在太阳自转周内两者相关系数可达０．８．这些结果表明日侧极隙纬度ＥＬＦ波和ＶＬＦ波各有不同的产生机制和源区．     

基于LAIC电场渗透和SAMI2模拟的地震-电离层扰动现象研究

地震会造成电离层电子密度扰动。 基于三维电场渗透模型和SAMI2模型, 模拟地震发生前产生的异常电场对电离层电子密度的影响。 模拟结果表明, 地震前形成的附加电流会引起电离层产生异常电场, 且异常电场值与地震震级和地震发生时间有着明显的相关。 同时异常电场会使电子产生E×B漂移, 造成电子密度扰动。 随着时间的推移, 电子密度扰动逐渐减小且扰动区域会向着靠近磁赤道方向或远离磁赤道方向漂移。     

Some aspects of electrostatic coupling between E and F regions relevant to plasma irregularities: A review based on recent observations

This paper provides a review on some of the electrostatic coupling effects relevant for generating/modifying plasma irregularities during nighttime in the low latitude ionosphere based on recent observations. Emphasis is given to the role of large polarization electric field associated with an unstable region affecting another region remotely located. Recent radar observations on valley region and E region irregularities from low latitudes show convincing evidence in support of effective electrostatic field coupling along the magnetic field line for their manifestation. Interestingly, the low latitude observations clearly show the ineffectiveness of plasma bubble related fringe fields in generating low latitude valley region irregularities unlike that over the dip equator. Velocity perturbations associated with the unstable low latitude E region relevant for studying the seeding of equatorial spread F are also shown. These new observations have been critically examined in the light of existing experimental knowledge and current understanding of the electrostatic coupling effects for the generation/modification of plasma irregularities in a remote region.     

Response of equatorial ionosphere to episodes of asymmetric ring current activity

We present the characteristics of the response of equatorial ionospheric zonal electric field and F-region plasma density to the asymmetric ring current intensifications that occurred in succession on 16 December 1991, corresponding to the STEP/EITS-2 campaign period. The study is based on high-time-resolution (1-min) data of asymmetic ring current indices, ASY(H/D) and F-region vertical plasma drift, Vz measurements at Kodaikanal (10.25°N; 77.5°E; dip 4°), India and quarter-hourly ionosonde data of For-taleza (4°S; 322°E; dip -9°), Brazil. It is shown that short-lived disturbances in F-layer vertical plasma drift, Vz and height (hF/hpF2) indicative of westward and eastward electric fields prevail simultaneously in the dusk (18–21 LT) and predawn (02–05 LT) sectors, respectively, in association with the decay phase of asymmetic ring current events. Electric fields of opposite polarity do also seem to manifest at these local times, particularly in the early-morning sector in conjunction with the intensification of the asymmetric ring current. At a given location, electric field disturbances associated with individual asymmetric ring current events are thus bipolar in nature, with fields of opposite polarity during the growth and decay phases. The nature and polarity structure of the observed electric field disturbances are in agreement with the theoretical/model predictions of prompt penetration of high-latitude electric fields to the equatorial region.On sabbatical leave from Indian Institute of Astrophysics, Bangalore, India     

Longitudinal statistics of plasma bubbles observed as He+ density depletions at altitudes of the topside ionosphere

This work presents a new examination of the hypothesis regarding the equatorial origin of low He⁺ density plasma depletions (or subtroughs). For this purpose, we have conducted a detailed comparative analysis of longitudinal variations in the occurrence probabilities of subtroughs in both hemispheres and variations in the occurrence probabilities of equatorial F-region irregularities (EFIs), equatorial spread F (RFS and ESF), and equatorial plasma bubbles (EPBs). Taking into consideration the seasonal dependence and some peculiarities of magnetic field variations in different hemispheres, a conclusion has been reached regarding the similarity between longitudinal statistical occurrences of subtroughs and equatorial ionospheric F-region irregularities. In addition, another piece of evidence in favor of the similarity of the nature of the above-mentioned phenomena has been obtained. We have got a confirmation once again that low He⁺ density depletions (or subtroughs) can be rightfully considered as equatorial plasma “bubbles,” which can be observed at altitudes of the topside ionosphere as depletions in the He⁺ density.     

Equatorial plasma bubbles at altitudes of the topside ionosphere

A consistent patter, indicating that subtroughs in the He⁺ density and plasma bubbles can be considered as phenomena of the same origin, has been obtained within the scope of the existent model of equatorial plasma bubbles. The study has been performed based on the measurements of the ISS-b satellite, which flew during the period of high solar activity. The conclusion has been made based on a comparative analysis of the characteristics of subtroughs with the parameters of the known equatorial phenomena. (1) The similarity of the LT variations in the latitude of the minimums of subtroughs in the He⁺ density has been revealed. (2) It has been displayed that the variations in the averaged depth of subtroughs change from season to season similarly to the LT variations in the average velocity of the equatorial vertical plasma drift. (3) Good correlation (R = 0.67) between the occurrence probability of subtroughs and equatorial spread F statistics, constructed as the functions of LT and month, has been obtained. (4) The obtained velocity of the possible rise of plasma irregularities (observed as regions depleted in He⁺) is in good agreement with the ionosonde, satellite, and radar measurements of the equatorial plasma bubble velocities of the same period. (5) It has been indicated that plasma irregularities, reaching the altitudes of the topside ionosphere in the low-latitude and midlatitude regions during high solar activity, are most observable as depleted regions (subtroughs) of He⁺ density.     

Midday reversal of equatorial ionospheric electric field

A comparative study of the geomagnetic and ionospheric data at equatorial and low-latitude stations in India over the 20 year period 1956–1975 is described. The reversal of the electric field in the ionosphere over the magnetic equator during the midday hours indicated by the disappearance of the equatorial sporadic E region echoes on the ionograms is a rare phenomenon occurring on about 1% of time. Most of these events are associated with geomagnetically active periods. By comparing the simultaneous geomagnetic H field at Kodaikanal and at Alibag during the geomagnetic storms it is shown that ring current decreases are observed at both stations. However, an additional westward electric field is superimposed in the ionosphere during the main phase of the storm which can be strong enough to temporarily reverse the normally eastward electric field in the dayside ionosphere. It is suggested that these electric fields associated with the V × Bz electric fields originate at the magnetopause due to the interaction of the solar wind and the interplanetary magnetic field.     

Role of overshielding electric field on the development of pre-midnight plume event: Simulation results

An investigation involving nonlinear numerical simulation has been undertaken based on the observations of two events involving the reversal of nighttime zonal electric field to eastward direction over equatorial region due to the overshielding effect associated with interplanetary electric field. In one of the events, the ionospheric alterations brought forth by the prompt penetration event lead to the triggering of an equatorial spread F (ESF) event around 2040 IST and a plume structure during pre-midnight hours due to overshielding effect. In another observation, the ESF was found to be absent. The two-dimensional modeling investigation revealed that the storm-induced eastward electric field during nighttime over equatorial region is a necessary but not a sufficient condition for the development of the pre-midnight plume structure in the lower F-region altitude. The large scale size (240 km) perturbation amplitude of 5% is found to be insufficient for the development of late night plume event within 2000 s. A pre-seed in varying degrees in a localized altitude region depending upon the background ionospheric conditions, is found to be required for the development of the pre-midnight plume event. The confined ESF irregularities developed in the post-evening hours in a limited altitude range are suggested to provide such seed perturbation. The importance of the pre-seeded structure for the development of pre-midnight plume event is brought out from this investigation. The roles of the electron density scale length and the peak height of the F layer in deciding the required amplitudes of perturbation are also evaluated. This, in turn, can throw light on the night-to-night variability of storm-time ESF when a typical eastward perturbation electric field is operative during pre-midnight hours. The role of off-equatorial E region conductivity is also discussed.