顶部电离层离子密度经度结构的特征及其随季节、太阳活动和倾角的变化

本文利用DMSP卫星测量数据和傅里叶分解和重构方法,研究了地磁平静期顶部电离层总离子密度(Ni)经度结构的多重波数特征及波数4的年变化、逐年变化、地方时差异和随倾角的变化.傅里叶分解和重构的结果表明,顶部电离层平均Ni的经度结构中同时含有以波数1至波数4为主的多重分量,不同波数分量的幅度和相位各不相同.对波数4分量的分析表明,波数4的幅度在春秋季最强,北半球夏季高于冬季;随太阳活动水平增强,波数4分量的幅度增高,至太阳活动高年幅度达到最高,此后随太阳活动水平降低而减小,与F10.7呈正相关;春秋季和北半球夏季波数4分量在傍晚最强,晚上和上午次之,黎明最弱,从09LT到21LT,波数4的相位依次滞后,暗示向东移动.分析还发现,日落期间波数4幅度依赖倾角,春秋季随倾角的变化呈双峰结构,两个极大出现在倾角±18°附近,暗示赤道等离子体喷泉效应对顶部电离层经度结构的控制作用.

Longitudinal structure of plasma density and its variations with season,solar activity and dip in the topside ionosphere

Recent satellite-based observations have revealed the existence of the ionospheric wave number-4 (WN-4) longitudinal structure. It has been known that this structure has relationship with the diurnal non-migrating tides from the lower atmosphere, which modulates the eastward electric field in the E region, and the modulated electric field map into the F region along with the magnetic field line, where it can cause the WN-4 structure. Many studies show that the WN-4 structure also exists in the topside ionosphere in several ionosphere parameters. In this paper, using the data measured by the DMSP F13 and F15 satellites and the method of Fourier decomposition and reconstruction, we investigate longitudinal varieties of the ion total density (Ni) in the equatorial and mid-low latitudinal topside ionosphere at four local times. Our objective is to extract the harmonic-wave components with different wave numbers from the complex longitudinal varieties of the Ni to comprehend which harmonic-wave components there are in longitudinal varieties of Ni. Besides, we examine the dependence of the amplitude of the WN-4 component on the month, year, local time and dip.To investigate the annual variation, the data from DMSP were separated by two manners. In the first manner, the data were separated into three 4-month long seasonal bins: Spring-Autumn (March, April, September and October), the northern summer (May to August) and the northern winter (January, February, November, December). According to the second manner, the data were separated into twelve a-month-long bins. In order to confine our attention to quiet conditions, the observations taken during periods of high magnetic activity marked by Kp>3 were removed from the data set. Then, to get the longitudinal structure of Ni in the region with different dips, the data were binned for every 10 degrees in longitude into 36 geographical longitude bins and for every 5 degrees in dip. The bins were all 30 degrees wide in the zone and 4 degrees wide in the meridian. Finally, for each bin the mean values of Ni were calculated. The method of Fourier harmonic-wave decomposition was used to extract the harmonic-waves with the different wave numbers from the complex longitudinal variations of Ni, and the method of Fourier reconstruction was used to fit the measured data.By using the DMSP data and Fourier analysis, we quantitatively investigated the multiple wave number's features in the longitudinal variety of the topside ionospheric Ni. The results show that the longitudinal variety of the topside ionospheric Ni reconstructed by Fourier series with 0-order to 6-order harmonic waves is very consistent with the longitudinal variety from the measured data. Further, we analyzed the WN-4's annual variation, year-to-year variation, the distinction in different local times and the variation with dips. Main results include: (1) The Ni's longitudinal variety contains the WN-0 and the WN-1 to WN-4 mainly in the topside ionosphere, which have different amplitudes and phases. (2) The WN-4 in the dip equator is most intense in spring and autumn, very weak in winter which is similar to the annual variation of non-migrating tides. (3) The WN-4 is more intense when the level of solar activity is higher and the former is weaker when the latter is lower. (4) The WN-4 in spring and autumn and the northern summer is most intense at the dusk, weaker after the sunset and the morning, and the weakest at the dawn, while it is the most intense in winter after sunset relative to other local times. (5) The WN-4 during the dusk has strong dependence on the dip: the double peak pattern occurs in spring and autumn, the flat top pattern appears in the northern summer and the obvious asymmetry pattern is present between two hemispheres in northern winter.This study provides a new view of the longitudinal variety of the topside ionospheric Ni. It is found that the harmonic-wave components with the different wave numbers simultaneously exist in the longitudinal variety of the topside ionospheric Ni. The year-to-year variation of the amplitude of the WN-4 reveals the notable positive correlation with the year-to-year variation of F_10.7 index. The results are valuable for understanding the feature of the longitudinal variety of the topside ionosphere and its modeling.