昆明MF雷达及初步探测结果

MF雷达是中层大气风场及低电离层电子密度观测的重要手段. 简要介绍了新建昆明MF雷达设备的工作原理、结构及工作模式, 并对观测结果进行初步分析. 对2009年1月观测数据的分析表明, 该月纬向风场最大可达80 m/s, 经向风场则较小, 一般不超过40 m/s, 且二者均呈现一定波动性. 相应的LS谱及谐波拟合分析表明,周日潮汐是80~100 km高度大气风场的主要扰动成分, 其振幅随高度改变, 相位向下传播, 且周日潮汐经向分量相位超前于纬向分量相位. 此外, 分析了MF雷达白天的电子密度观测结果, 并与IRI2000进行比较, 发现两者在变化趋势上有非常好的一致性, 但雷达观测结果小于IRI2000给出的参考值.      

利用COSMIC掩星数据监测电离层的异常变化

分析了COSMIC掩星数据反演电子密度的方法,利用实例研究反演方法的特点,并采用ISR非相干散射雷达获取的电子密度数据进行验证,进而反演了长三角区域SHAO（IGS）站上空在日全食和太阳风暴期间的电子密度廓线图. 通过与平静日期间电离层电子密度进行比较,发现日全食及太阳风暴导致电离层发生的异常变化,从而提出COSMIC掩星数据反演电子密度在监测电离层变化时所具有的优势.      

强震前异常电场激发声重波对电离层的影响

震前地震孕育期地表异常增强的电场,通过大气电导率传输到电离层高度.该异常电场通过非稳态局部加热,可以在电离层高度激发声重波.基于该理论,利用一维时变中纬电离层物理模型,模拟了该扰动源对电离层电子密度的影响.结果表明,重力波引起的中性风速度扰动对电离层电子密度分布影响甚微,该机理无法解释震前电离层异常扰动现象.      

电离层F层中舒曼共振现象的检测

利用Ａｕｒｅｏｌ－３卫星在电离层Ｆ层高度上（６５０ｋｍ左右）对电离层电场扰动与电子密度扰动的观测进行分析处理,在高纬电离层Ｆ层高度上可以检测到舒曼共振现象。根据等离子体参数,可以认为,发生在这个高度上的舒曼共振现象与Ｌ模式的波传播有关,舒曼共振的基频是与电子密度的大尺度不规则性有关,谐波分量与正向密度梯度相关。     

磁暴期间中纬度电离层剖面结构变化的数值模拟

利用电离层理论模型模拟了磁暴期间热层大气温度、成分、中性风和电场扰动对电离层电子密度剖面结构，特别是峰值密度和峰值高度变化的影响，结果表明，热层大气温度变化所引起光化反应系数的改变对电离层剖面结构影响不大；热层大气成分特别是Ｎ２／Ｏ的变化能有效地引起密度剖面变化，Ｎ２增加足以使峰值密度产生所观测到的负相暴；由中性风和电场引起等离子体漂移是峰值高度ｈｍＦ２变化的主要原因，但对电子密度的影响不足以抵消     

Millstone地区垂直等效风场的气候学变化

基于1976---2006年美国Millstone非相干散射雷达的电离层观测数据, 分析了美国Millstone地区不同太阳活动条件下, 包括中性风场和电场漂移共同贡献的垂直等效风场的变化特征. 结果表明, Millstone地区的垂直等效风场表现出比较明显的周日、太阳活动和季节变化特征. 晚间垂直向上的等效风较强, 白天等效风较弱, 甚至接近于零. 在不同太阳活动和季节变化条件下, Millstone地区的等效风场都表现出类似的周日变化特征, 低太阳活动条件下, 晚间表现出较大的向上漂移. 这种周日变化和太阳活动变化特征与Millstone地区受到极区热源驱动大气循环的调制以及离子曳力的增减有关. 春季和秋季有相似的幅度和相位变化趋势, 表现出分点对称性; 冬季晚间向上漂移比夏季弱, 且随着太阳活动增强, 差异更加明显, 这再次体现了极区热源驱动大气循环的影响.      

2009年7月22日日全食电离层效应观测

首次利用廊坊中频雷达和武汉、嘉兴、廊坊等三站GPS对2009年7月22日日全食电离层效应进行了观测．日食期间,中频雷达D层78km高度上电子密度减小了约67％,电子密度为200cm^-3的高度上升了近10km,GPS／TEC减小了1TECU左右,其变化的最大相位与日食最大相位几乎同步;日食后,观测到周期为2个多小时的电离层扰动现象．     

磁暴期间中低纬电离层暴形态和物理机制分析

本文利用东亚地区１２个低纬电离层台站的测高仪观测数据，对１９７８年８月２７日发生的一次曲型磁暴期间电离层峰值高度和密度的变化进行了分析。采用滑动平均区分开电离层中不同时间尺度的扰动，分析了影响中低纬度电离层暴的几种扰动形态特征，并对其物理机制进行了讨论。结果表明：伴随磁暴急始的磁层压缩，电离层中表现出峰值密度增加和峰值高度下降；磁暴主相期间热层大气暴环流及其所引起的中性大气成分变化控制着电离层的大     

电离层Es对HF和VHF传播信号的影响

Es层是存在于电离层中的电子密度非常高的偶发E层（Sporadic E）,其电子密度可达常规E层的100倍.电离层Es能够反射原本穿透F层的VHF低频段（30~150MHz）无线电波,而且对HF高频段（10~30MHz）无线电波传播具有显著影响.运用垂测和斜测观测数据,研究HF频段Es层电波传播特征,得到了不同类型及不同高度Es层的衰减系数.根据f₀E_s的日变化规律,可得HF频段衰减系数的日变化规律,进而分析并得到Es层对短波传播的影响.不存在电离层Es时,通常无法通过电离层实现VHF超远距离通信.为了对VHF链路通过电离层Es的传播衰减进行定量分析,根据EBU多条链路的观测结果,拟合并建立了电离层Es衰减模型.将该模型、ITU模型和观测数据进行对比,发现本文建立的模型准确度更高.利用建立的模型,对电离层Es不同临频f₀E_s条件下接收信号场强和电压随传播距离的变化进行了计算,结果可为VHF链路设计及建立提供参考.      

武汉中层、低热层大气角谱中频雷达观测

武汉中频雷达是利用中层、低热层中电子密度不均匀体的散射来测量大气的水平风场和电子密度剖面。雷达在计算风场的过程中可得到一些该层大气中电子密度不均匀体的寿命和空间尺度等参量值。结合这些参量和大气风场值可计算得到大气的角谱。其计算方法包括全相关分析技术的谱宽法和空间相关法。前者计算的值被认为是大气角谱的上限值。应用这两种方法,利用2001年2月9日武汉中频雷达的观测数据,对武昌上空中层、低热层大气的角谱进行了计算。结果得到大气角谱随高度增加略微增加,如在68km为6°,90km达最大为11°,其平均值为9.2°。利用2001年2月4—10日86km高度上的数据,得到一个7天平均的日变化曲线,发现大气角谱值在本地夜晚最小。     

Study of ionospheric D region changes during solar flares using MF radar measurements

During solar flares, the X-ray radiation suddenly increases, resulting in an increase in the electron density of the atmospheric D region and a strong absorption of short-wave radio waves. Based on Langfang medium frequency (MF) radar, this paper analyzed the variation characteristics of D region in the lower ionosphere from 62 km to 82 km. The analysis focused on multiple C-level and M-level solar flare events before and after the large-scale flare event at 11:53 (UT) on September 6, 2017. The results show that it is difficult to detect the electron density over 70 km in Langfang during solar flares, but the electron density value can be obtained as low as 62 km, and the stronger the flare intensity, the lower the detectable electron density height. Besides, the equal electron density height, the received power of X and O waves will also be significantly reduced during the flares, and the reduction of equal electron density height has a weak linear relationship with flare intensity.     

低纬地区夜间电离层底部电子浓度与磁扰的关系

本文利用100kHz的低频无线电波资料,计算分析了1986—1987年期间,几种不同磁扰情况下,低纬地区夜间电离层中100km以下区域积分电子浓度及其变化的起因.结果表明:该区域电子浓度的变化与地磁扰动关系密切.在磁静日期间,其值较小,且随磁扰而变化,但比磁扰滞后1到2天.在磁暴后,其值较大,会出现几次剧烈起伏.该区域积分电子浓度的起伏可大于一个量级.沉降电子产生的动致辐射可能是引起该区域电子浓度变化的主要原因之      

Lower ionosphere response to external forcing: A brief review

There are two ways of external forcing of the lower ionosphere, the region below an altitude of about 100 km: (1) From above, which is directly or indirectly of solar origin. (2) From below, which is directly or indirectly of atmospheric origin. The external forcing of solar origin consists of two general factors – solar ionizing radiation variability and space weather. The solar ionization variability consist mainly from the 11-year solar cycle, the 27-day solar rotation and solar flares, strong flares being very important phenomenon in the daytime lower ionosphere due to the enormous increase of the solar X-ray flux resulting in temporal terminating of MF and partly LF and HF radio wave propagation due to heavy absorption of radio waves. Monitoring of the sudden ionospheric disturbances (SIDs – effects of solar flares in the lower ionosphere) served in the past as an important tool of monitoring the solar activity and its impacts on the ionosphere. Space weather effects on the lower ionosphere consist of many different but often inter-related phenomena, which govern the lower ionosphere variability at high latitudes, particularly at night. The most important space weather phenomenon for the lower ionosphere is strong geomagnetic storms, which affect substantially both the high- and mid-latitude lower ionosphere. As for forcing from below, it is caused mainly by waves in the neutral atmosphere, i.e. planetary, tidal, gravity and infrasonic waves. The most important and most studied waves are planetary and gravity waves. Another channel of the troposphere coupling to the lower ionosphere is through lightning-related processes leading to sprites, blue jets etc. and their ionospheric counterparts. These phenomena occur on very short time scales. The external forcing of the lower ionosphere has observationally been studied using predominantly ground-based methods exploiting in various ways the radio wave propagation, and by sporadic rocket soundings. All the above phenomena are briefly mentioned and some of them are treated in more detail.     

The quiet nighttime low-latitude ionosphere as observed by TIMED/GUVI

In this paper, we examine the nighttime ionosphere climatology structure in the low latitude region and discrepancies between Global Ultraviolet Imager (GUVI) observations and the IRI model predictions using (1) the magnetic zonal mean of electron number density as a function of altitude and magnetic latitude, (2) vertical electron density profiles at various levels of F10.7 index, (3) nighttime descent and magnitude decrease of the ionosphere, (4) point-to-point comparisons of F-peak height (hmF2) and density (NmF2), and (5) the magnetic longitudinal variations of hmF2 and NmF2. The data collected from the Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED) mission since its launch in December 2001 have provided great opportunities for many scientific investigations of the ionosphere. In this analysis, we investigate the climatology of the nighttime low-latitude ionosphere under low geomagnetic activity (kp ? 4) using the electron density profiles inferred from the airglow measurements obtained by the GUVI aboard the TIMED spacecraft and compared with the results obtained from IRI (International Reference Ionosphere) model-2001. The observed climatology is an essential tool for further understanding the electrodynamics in the low-latitude region and improving the model’s prediction capability. The time range of the GUVI data used in this study is from 2002 (day 053) to 2006 (day 304), and the IRI model predictions were produced at every GUVI location. The ionosphere observed is generally of greater density than what IRI predicts throughout the night for all four seasons for low and moderate solar activity while the model over-predicts the electron density near the F-region peak at high solar activity before midnight. Observations show that the height of the F-region peak has a steep descent from dusk to midnight and near midnight the height of layer is insensitive to solar conditions, significantly different than what is predicted by IRI. Longitudinal features shown in GUVI data are present in the low-latitude ionosphere after sunset and continue through to midnight after which the low-latitude ionosphere is largely zonally symmetric.     

Comparative study of high-latitude,mid-latitude and low-latitude ionosphere on basis of local empirical models

The analysis of the regular features of the high-, mid- and low-latitude ionosphere characteristics has been carried out using local empirical models. The local empirical models were derived from the manual scaled ionogram data recorded by DPS-4 Digisondes located at Norilsk (69 N, 88E), Irkutsk (52 N, 104E) and Hainan (19 N, 109E) for a 6-year period from December, 2002 to December, 2008. The technique used to build the local empirical model is described. Primary focuses are diurnal, seasonal and solar cycle variations of the peak electron density and the peak height under low solar activity and their changes with increasing solar activity. The main objective of the paper is to reveal both common and specific features of high-, mid- and low-latitude ionosphere. Based on earlier comparisons with the International Reference Ionosphere model, we analyze how the common and specific features are reproduced by this model.     

昆明上空电离层D区电子密度季节变化的首次观测分析

通过分析2008年8月至2009年7月昆明站(25.6°N, 103.8°E) 中频(MF)雷达观测数据, 研究了太阳活动低年电离层D区电子密度的季节变化特性,发现D区电子密度主要呈现半年变化特征, 即在春秋季电子密度较大, 而在夏冬季则较小, 这与国际参考电离层(IRI)预测的年变化趋势不一致, 但与昆明站电离层测高仪的最低回波频率f_min的观测结果相符. 同时比较了D区电子密度半年变化与纬向风半年变化的关系, 发现二者之间保持了非常一致的变化趋势并对这种一致性的内在原因进行了分析.      

Sensing the Earth’s low ionosphere during solar flares using VLF signals and goes solar X-ray data

An analysis of D-region electron density height profile variations, induced by four isolated solar X-ray flares during period from September 2005 to December 2006, based on the amplitude and the phase delay perturbation of 22.1 kHz signal trace from Skelton (54.72 N, 2.88 W) to Belgrade (44.85 N, 20.38 E), coded GQD, was carried out. Solar flare data were taken from NOAA GOES12 satellite one-minute listings. For VLF data acquisition and recordings at the Institute of Physics, Belgrade, Serbia, the AbsPAL system was used. Starting from LWPCv21 code (Ferguson, 1998), the variations of the Earth-ionosphere waveguide characteristic parameters, sharpness and reflection height, were estimated during the flare conditions. It was found that solar flare events affected the VLF wave propagation in the Earth-ionosphere waveguide by changing the lower ionosphere electron density height profile, in a different way, for different solar flare events.