中低纬电离层电子密度的日变化性研究

Diurnal variation of F-layer electron density is complicated and sometimes significantly deviates from the anticipation of the Chapman theory owing to the effects of ionospheric dynamics and electrodynamics processes. Daytime bite-out is an important anomalous phenomenon of ionospheric diurnal variation; its temporal and spatial variations have not been systematically revealed, thus, should be further investigated. It is generally agreed that ionospheric variation primarily depends on the increment of photoionization production rate at sunrise; however, this has not been statistically verified from observations. By far, the classification and distribution of the diurnal variation patterns of ionospheric electron density have not been investigated systematically. In this project, we plan to investigate the diurnal variation of the mid- and low-latitude ionosphere based on multiple ionospheric observations that have a good spatial coverage, such as COSMIC measurements for electron density. We plan to investigate the spatial distribution of daytime bite-out and its dependence on seasons and solar activity levels, and also analyze the physical mechanisms of daytime bite-out. We will investigate the relationship between ionospheric electron density variation and solar zenith at sunrise and also will investigate to what extent the variation of the ionosphere is dominated by the increment of photoionization production rate at sunrise. We also will try to summarize the primary diurnal variation patterns of electron density and to investigate the distributions of these primary patterns as well as their relations to geomagnetic configuration. It is anticipated that the understanding for ionospheric diurnal variation characteristics and the corresponding physical mechanisms as well as the effects of ionospheric dynamics and electrodynamics processes on ionospheric variations will be promoted via the study of this project.

受电离层动力学等因素的影响，F层电子密度的日变化颇为复杂，有时显著偏离Chapman理论的预期。日间“咬失”是电离层日变化的一个重要异常现象，对其时空分布仍缺乏系统认识；一般认为日出时段电离层的变化主要受光电离产生率增长的控制，但缺乏具有统计意义的观测证据；目前对电子密度不同的日变化类型的分布规律尚无系统研究。本项目计划利用COSMIC等覆盖全球的电离层综合观测资料，研究中低纬电离层日间“咬失”的空间分布及其对季节、太阳活动的依赖，揭示不同纬度日间“咬失”的形成机制；研究日出时段电子密度的变化与太阳天顶角的关系，辨明日出时段电离层的变化在多大程度上受光电离产生率增长的控制；尝试进行电子密度日变化类型的分类研究并揭示主要日变化类型的分布规律、与地磁场构型的关联。项目研究有望推进对电离层日变化性和相关物理机制的认识，增进对电离层动力学、电动力学过程如何影响电离层变化的理解。

电离层F2层的日变化受到动力学过程显著调制，是研究和理解电离层动力学过程很好的切入点。在本项目中，主要围绕中低纬电离层密度及其结构的日变化、地方时演化特征与形成机理开展研究。研究了NmF2的日间咬失现象，发现在赤道南北两侧日间咬失的季节变化有明显的南北不对称，揭示低纬至中纬的日间咬失有显著的纬度、季节、太阳活动依赖性。在低纬，喷泉效应和早晨时段中性风驱动的季节性的越赤道输运过程显著调制日间咬失变化形态；在中纬，中性风引起的hmF2降低和向上的热扩散都对日间咬失有贡献，而中性成分和光电离的日变化幅度是决定咬失程度的重要因素。揭示低纬动力学过程对日出时段电离层的增强有显著影响；东向电场引起的向上等离子体运动导致赤道附近NmF2随日出时段太阳辐射增强较快地增长，而中性风驱动的跨赤道输运过程使得赤道附近冬季半球一侧的NmF2增长更快。通过对逐个夜间增强事件的统计分析，表明冬季夜间增强期间hmF2有的增加有的减小，而hmF2的变化对夜间增强无显著影响，并进一步确认了跨半球耦合与等离子体层/顶部电离层向下输运过程在中纬冬季夜间增强中的主导作用。揭示中纬电离层通过等离子体层跨半球耦合使得太阳活动低年夏季日落后840公里高度中纬离子密度的经度变化存在南北共轭效应，结果表明中性风驱动的电离层输运过程能够影响中纬电离层向等离子体层的物质耦合。揭示在分点季节的傍晚时段，地磁偏角大的经度600公里高度离子密度南北不对称性随纬向风的转向出现反转，表明顶部电离层对风场的变化有较快的响应，而中性风引起的垂直输运可显著影响顶部电离层离子成分。