北半球电离层中纬槽发生率及其槽极小位置变化的统计研究

本文利用2014年9月到2017年8月全球高时空分辨率TEC数据对北半球四个经度带电离层中纬槽的发生率和槽极小位置的变化进行了统计研究.基于Kp指数,我们引入了一个包含地磁活动变化历史效应的地磁指数(Kp 9)来分析中纬槽位置变化与地磁活动水平的关系.通过与其他地磁活动指数的对比,发现槽极小纬度与Kp 9指数的相关性最好.此外,本文重点分析了中纬槽发生率及槽极小纬度的经度差异、季节变化、地方时变化以及与地磁活动强度等的关系.结果表明,中纬槽的发生率与经度关系不大,主要受到季节、地方时与地磁活动的影响.午夜中纬槽发生率在夏季较低,其随地方时的变化则呈现出负偏态分布的特点,在后半夜发生率更高,而地磁活动增强对中纬槽的发生具有明显的促进作用.对于槽极小纬度,其在四个经度带的分布差异不大,但月变化各不相同,其中-120°经度带呈单峰分布,在夏季槽极小纬度更高,而0°经度带夏季槽极小纬度更低.槽极小的位置显著依赖于地磁活动、地方时以及季节变化.一般说来,地磁活动越强,中纬槽纬度越低.中纬槽位置随地方时的变化有明显的季节差异,冬季昏侧槽极小纬度随地方时变化较快,弱地磁活动条件下22∶00 LT前即达到最低纬度,其后位置几乎保持不变,而两分季槽极小纬度从昏侧至午夜都在降低,夏季槽极小纬度从昏侧连续下降至03∶00 LT左右.

Statistical study on the occurrence of the ionospheric mid-latitude trough and the variation of trough minimum location over northern hemisphere

Using global TEC data with high spatial and temporal resolution from Sep., 2014 to Aug., 2017, the occurrence rate of ionospheric mid-latitude trough and the variation of trough minimum location over Northern Hemisphere was studied statistically. Considering the geomagnetic "historical effects" on the evolution of mid-latitude trough, we introduced one index (here defined as Kp9) calculated from 3-hour Kp index. Through comparing with several other geomagnetic indices, it was found that the linear correlation between the latitude of trough minimum and Kp9 index was the best. And then, the longitudinal difference, seasonal variation, local time variation of the trough occurrence rate and the latitudinal location of mid-latitude trough, as well as the relationship between the location of mid-latitude trough and Kp9 index were analyzed. The results showed that the occurrence of mid-latitude trough is little related to longitude and depend primarily on the season, local time and geomagnetic activity. The occurrence rate of mid-latitude trough in summer is lower than other seasons at midnight and shows a negatively skewed distribution, which there was higher occurrence rate after midnight, and the geomagnetic activity promotes the occurrence of mid-latitude trough. As for the latitudinal distribution of trough minimum, the difference in four longitude bands is not obvious, but monthly variations are different in four longitude bands, of which there was unimodal distribution in 120°W with higher latitude in summer months but lower latitude in winter, and on the contrary in 0° longitudinal band the latitude of trough minimum is lower in summer. In addition, the location of mid-latitude trough is significantly dependent on geomagnetic activity, local time and season. In general, the higher the geomagnetic activity, the lower the latitude of the mid-latitude trough. And there was seasonal difference in the variation of trough minimum with local time. In winter, the location of the trough moves rapidly at dusk with the local time and reaches the lowest latitude before 22:00 LT under the condition of weak geomagnetic activity, and after 22:00 LT the trough minimum location changes little. Comparatively, the minimum location in equinox months decreases from dusk to midnight, and the minimum location in summer months decreases continuously from dusk even to about 03:00 LT.