中文摘要：

利用DMSP F13卫星1996—2005年共10年的观测数据,研究地磁中低纬地区黄昏时段（18：00LT）顶部电离层总离子密度经度变化的季节、地磁纬度和太阳活动变化特征.结果表明总的经度变化在低纬地区与中纬地区具有明显不同特征.不同经度结构的季节变化均以年变化为主,但纬度分布具有明显差异.一波结构主要集中在中纬地区,且南半球明显强于北半球;二波结构南北半球不对称性非常明显;三波结构和四波结构均为低纬地区明显强于中纬地区.通过分析不同波结构对总经度变化的贡献发现,一波结构在南半球中纬地区贡献最大,二波结构在12月前后的15°N附近贡献较大,三波结构和四波结构仅在低纬地区有较强贡献.在不同太阳活动条件下,不同波结构的贡献率有明显变化.

英文摘要：

Based on DMSP F13 Satellite observations from 1996 to 2005, the seasonal, geomagnetic latitude and solar cycle variations of the total ion density in the sunset topside ionosphere at middle and low latitudes are investigated. Results indicate that the longitudinal variation of the total ion density is obviously different between the low latitudes and the middle latitudes. Annual components of longitude structures seasonal variations are dominated at most latitudes, and these longitudinal structures show latitudinal dependence： relative strength of mid-latitude wavenumber-one structure in the southern hemisphere are much greater than that in the northern hemisphere; the hemispheric asymmetry of wavenumber-two structure is remarkable at middle and low latitudes; wavenumber- three structure and wavenumber-four structure are both much more dominant in the low latitudes than in the middle latitudes. Besides, contributions of different wave structures to total ion den- sity are also examined： wavenumber-one structure is dominant at southern middle latitudes, and in the northern winter, the region around 15°N is mainly controlled by wavenumber-two structure. Wavenumber-three structure and wavenumber-four structure only prevail at low latitudes. Contri- butions of different wave structures change with solar cycle variations.