利用掩星技术研究南极地区顶部电离层特性

掩星观测能够提供地面到低轨卫星轨道高度处的整个电离层电子密度剖面,对于顶部电离层的研究有重要的作用。本文利用COSMIC(constellation observing system for meteorology ionosphere and climate)掩星数据反演了电子密度剖面,提取了F₂层峰值高度(hmF2)、F₂层峰值密度(NmF2)、垂直标尺高(vertical scale height,VSH)等电离层参数,研究了南极地区的F₂层在太阳活动周期内的变化、年际变化、周日变化等,并且重点分析了南极地区的顶部电离层的垂直结构特征,尤其是威德尔海异常在垂直方向上的变化。结果表明,整个南极的hmF2每日均值在250~300 km左右,NmF2每日均值在1~8×1011 el/m3之间,VSH每日均值在100~250 km,威德尔海异常主要表现在顶部电子密度的增强和底部电子密度的减少。     

中国区域掩星观测与IRI-2016电离层峰值参数的比较

对2008—2014年由气象、电离层和气候卫星联合观测系统(Constellation Observing System for Meteorology Ionosphere and Climate, COSMIC)掩星观测与最新版国际电离层参考模型IRI-2016输出得到的电离层峰值参数(峰值密度NmF2与峰值高度hmF2)在中国区域进行了比较。IRI-2016模型输出值与COSMIC掩星反演值的相关性在太阳活动高年(2011—2014年)整体上高于太阳活动低年(2008—2010年)。在低年春秋季的当地时间(local time,LT)12:00—14:00,IRI-2016相对于COSMIC掩星在30°N～55°N区域内对NmF2和hmF2分别存在低估和高估现象,在15°N～30°N区域内则恰恰相反。对于NmF2,采用IRI_CCIR和IRI_URSI两种选项的模型输出值在中午时分均存在高估,在低年高估更为显著。对于hmF2,采用IRI_CCIR和IRI_AMTB两种选项的模型输出值在低年各季节均存在高估,且IRI_AMTB选项高估更显著,冬季最突出。结果表明,在中国区域由IRI-2016模型计算NmF2和hmF2时,分别推荐使用IRI_CCIR和IRI_Shubin选项。     

GPS电离层掩星数据分析汶川大地震电离层响应

利用COSMIC和CHAMP掩星的电离层掩星资料研究汶川大地震引起的电离层响应。选取了2008年4月30日到5月18日共19天的北纬20°到40°,东经80°到120°的共309个掩星数据进行分析。采用人工神经网络插值技术,分析震中附近上空电离层F2区的临界频率的日变化。结果表明：震前6天电离层F2区的临界频率和电子密度的峰值高度开始明显下降,在震前的三天内其值达到最小。震后他它们的值开始逐步回升。     

利用FY3C与FY3D GPS掩星资料研究中低纬地区偶发E层形态的比较

偶发E层(sporadic E, Es)是主要发生在90~120 km高度的电子密度显著增强的电离层薄层,Es层的存在会导致掩星观测中全球导航卫星系统信号强度和相位的强烈波动。利用2019-01—2021-12风云三号C（Fengyun-3C,FY3C）和风云三号D(Fengyun-3D,FY3D)卫星GPS（global positioning system）掩星观测的50 Hz信噪比数据提取Es层信息,进而对两颗卫星数据分别反演得到的60°S~60°N中低纬地区Es层发生率的时空分布及季节变化进行比较。结果发现,虽然两颗卫星掩星资料得到的Es层发生率分布形态基本一致,均反映了Es层的发生率与地磁场和中性大气背景风场的相关性,但在大部分季节和地区,由FY3D得到的Es层发生率低于由FY3C得到的结果,北半球夏季中纬地区尤为明显,而FY3C反演结果与基于电离层与气候星座观测系统掩星数据的反演结果更为接近。导致差异的可能原因包括两颗卫星信噪比廓线的上边界高度分布和地方时覆盖上的差异、两颗卫星掩星接收机噪声水平的差异等。上述结果表明,后续融合两颗卫星的掩星数据进行Es层相关研究时,可能需要顾及两颗卫星接收机的不同噪声水平,在Es层发生的判定策略上进行针对性调整。     

基于COSMIC技术的震前电离层异常研究

介绍COSMIC技术反演电离层电子密度的研究现状、技术和方法,验证其可靠性;并利用COSMIC掩星数据计算分析2011-03-11东日本大地震震前5d及地震当天震区上空电离层电子密度分布,探测到显著的震前电离层异常现象,证明COSMIC技术研究地震电离层异常的可行性。     

奥斯特/GPS掩星与地面雷达联合观测电离层电子密度的初步结果

1999年2月丹麦发射了一颗奥斯特小卫星,它装载了两台GPS接收机,其中一台用于电离层和中性大气掩星测量,另一台用于自主定位。本文给出了1999年10月奥斯特/GPS掩星与日本MU雷达和数字测高仪的联合观测结果。奥斯特掩星反演得到的电子密度剖面与地面雷达观测的进行了比较,结果表明:天基和地基雷达观测的电子密度是一致的。文章还给出利用GPS的单频信号反演电离层电子密度剖面技术和3维射线追踪技术模拟掩星结果。     

奥斯特／GPS掩星与地面雷达联合观测电码离层电子密度的初步结果

1999年2月丹麦发射了一颗奥斯特小卫星,它装载了两台GPS接收机,其中一台用于电离层和中性大气掩星测量,另一台用于自主定位,本文给出了1999年10月奥斯特/GPS掩星与日本MU雷达和数字测高仪的联合观测结果。奥斯特掩星反演得到的电子密度剖面与地面雷达观测的进行了比较。结果表明：天基和地基雷达观测的电子密度是一致的。文章还给出利用GPS的单频信号反演电离层电子密度剖面技术和3维射线追踪技术模拟掩星结果。     

地基GPS与掩星联合的电离层层析成像方法研究

由于有限视角、稀疏布站等原因,地基G PS电离层层析成像存在垂直分辨率不高的问题,联合掩星进行电离层层析成像是一个重要的解决途径。一方面地基G PS观测能保证电离层层析成像的时间连续性和区域覆盖性;另一方面由于掩星信号能够提供地基G PS没有的水平射线信息,提升了电离层层析成像的垂直分辨率水平。利用地基GPS站与COSMIC卫星的掩星观测进行的仿真结果表明：与仅利用地基G PS相比,联合掩星进行电离层层析成像,不仅在电离层电子密度反演精度方面有明显提升,而且在电离层F2层峰值高度（hm F2）和电离层总电子含量（T EC ）的精度方面同样有明显提高。     

利用IGRA2探空数据和COSMIC掩星资料对FY-3C掩星中性大气产品进行质量分析

根据IGRA2无线电探空数据和气象、电离层和气候卫星联合观测系统(Constellation Observing System for Meteorology Ionosphere and Climate, COSMIC)掩星资料,对2015—2017年包含折射率、温度和比湿廓线在内的FY-3C掩星中性大气产品进行质量分析。结果表明:FY-3C掩星折射率廓线25 km以下整体无系统偏差,相对偏差标准差≤5%;温度廓线存在系统负偏差,标准差≥1.5 K;比湿廓线越趋地表质量越低,5 km下标准差≤1.2 g/kg。总体而言,FY-3C掩星数据质量较好,在2015—2017年间逐年提高,但在近地面和25km以上有待进一步提高。对2017年产品的进一步分析表明:就季节而言,折射率廓线和温度廓线质量均在冬季最差,比湿廓线质量在夏季最差;就纬度带而言,折射率廓线在低纬度带质量最差,温度廓线质量的分纬度带比较结果在不同高度范围存在差异,比湿廓线在低纬度带质量明显低于中高纬度带;此外,FY-3C掩星大气产品数据质量随昼夜和海陆变化也存在一定差异。     

一个基于GPS的三维电离层成像工具:过程和评估

介绍了一个检验基于地面和隐藏在IRI/GCPM组合模型中的GPSTEC数据的倍增代数重建技术(MART)吸收过程,评估了原始记录产生的电子密度场。通过GPS数据的同化,获得了高北纬地区的电子密度估值,用IRI/GCPM大约提高27.25%,完成了欧洲的7个电子电离层测高仪的比较,NmF2和hmF2规定的平均偏差分别仅为8.5×1010m-3和-6.1km。     

Analysis of ionospheric electron parameters versus geomagnetic index Dst from RO data of FORMOSAT-3/COSMIC

The FORMOSAT-3/COSMIC mission has provided ample ionospheric electron density profiles retrieved from the global positioning system radio occultation technique. Currently, there can be more than 2,000 electron density profiles acquired per day covering the global ionosphere from altitude 90 to 800 km. Utilizing the advantage of such a complete coverage, we statistically analyze how the ionospheric electron parameters NmF2, hmF2, and TEC respond to the geomagnetic index Dst for different magnetic latitudes and magnetic local time (MLT) and on quiet and storm times. A data set of 24 months is used for this study, in which most of the results focus on the low-latitude dayside regions. The results indicate that, in general, NmF2, hmF2, and TEC decrease as Dst increases at all seasons. Only during the sudden commencement phase (SSC) of storm events, NmF2 and TEC appear to increase as Dst increases.     

Seismo-ionospheric precursor of the 2008 Mw7.9 Wenchuan earthquake observed by FORMOSAT-3/COSMIC

The seismo-ionospheric precursor prior to the Mw7.9 earthquake near Wenchuan, China, on 12 May 2008 was observed by the FORMOSAT-3/COSMIC satellite constellation. By binning radio occultation observations, the three-dimensional ionospheric structure can be obtained to monitor the ionospheric electron density variation prior to the earthquake. It has been determined that near the epicenter the F2-peak height, hmF2, descends about 25 km and the F2-peak electron density, NmF2, decreases about 2 × 10⁵ el/cm³ around noon within 5 days prior to the earthquake. The integrated electron content decreases more than 2 TECU between 250 and 300 km altitude.     

Electron density profiles in the equatorial ionosphere observed by the FORMOSAT-3/COSMIC and a digisonde at Jicamarca

We examine for the first time the ionospheric electron density profiles concurrently observed by the GPS occultation experiment (GOX) onboard the FORMOSAT-3/COSMIC (F3/C) and the ground-based digisonde portable sounder DPS-4 at Jicamarca (12°S, 283°W, 1°N geomagnetic) in 2007. Our results show that the F3/C generally underestimates the F2-peak electron density NmF2 and the F2-peak height hmF2. On the other hand, when the equatorial ionization anomaly (EIA) pronouncedly appears during daytime, the total electron content (TEC) derived from the radio occultation of the GPS signal recorded by the F3/C GOX is significantly enhanced. This results in the NmF2 at Jicamarca being overestimated by the Abel inversion on the enhanced TEC during the afternoon period.     

Performance of the improved Abel transform to estimate electron density profiles from GPS occultation data

The Abel inversion is a straightforward tool to retrieve high-resolution vertical profiles of electron density from GPS radio occultations gathered by low earth orbiters (LEO). Nevertheless, the classical approach of this technique is limited by the assumption that the electron density in the vicinity of the occultation depends only on height (i.e., spherical symmetry), which is not realistic particularly in low-latitude regions or during ionospheric storms. Moreover, with the advent of recent satellite missions with orbits placed around 400 km (such as CHAMP satellite), an additional issue has to be dealt with: the treatment of the electron content above the satellite orbits. This paper extends the performance study of a method, proposed by the authors in previous works, which tackles both problems using an assumption of electron-density separability between the vertical total electron content and a shape function. This allows introducing horizontal information into the classic Abel inversion. Moreover, using both positive and negative elevation data makes it feasible to take into account the electron content above the LEO as well. Different data sets involving different periods of the solar cycle, periods of the day and satellites are studied in this work, confirming the benefits of this improved Abel transform approach.     

Is the long-term variation of the estimated GPS differential code biases associated with ionospheric variability?

The global positioning system (GPS) differential code biases (DCB) provided by the International GNSS Service (IGS) show solar-cycle-like variation during 2002–2013. This study is to examine whether this variation of the GPS DCBs is associated with ionospheric variability. The GPS observations from low earth orbit (LEO) satellites including CHAMP, GRACE and Jason-1 are used to address this issue. The GPS DCBs estimated from the LEO-based observations at different orbit altitudes show a similar tendency as the IGS DCBs. However, this solar-cycle-like dependency is eliminated when the DCBs of 13 continuously operating GPS satellites are constrained to zero-mean. Our results thus revealed that ionospheric variation is not responsible for the long-term variation of the GPS DCBs. Instead, it is attributed to the GPS satellite replacement with different satellite types and the zero-mean condition imposed on all satellite DCBs.     

Estimation and analysis of GPS satellite DCB based on LEO observations

The Global Positioning System (GPS) satellite differential code bias (DCB) should be precisely calibrated when obtaining ionospheric slant total electron content (TEC). So far, it is ground-based GPS observations that have been used to estimate GPS satellite DCB. With the increased Low Earth Orbit (LEO) missions in the near future, the real-time satellite DCB estimation is a crucial factor in real-time LEO GPS data applications. One alternative way is estimating GPS DCB based on the LEO observations themselves, instead of using ground observations. We propose an approach to estimate the satellite DCB based on Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) and Challenging Minisatellite Payload (CHAMP) GPS observations during the years 2002–2012. The results have been validated through comparisons with those issued by Center for Orbit Determination in Europe (CODE). The evaluations indicate that: The approach can estimate satellite DCB in a reasonable way; the DCB estimated based on CHAMP observations is much better than those on COSMIC observations; the accuracy and precision of DCB show a possible dependency on the ionospheric ionization level. This method is significance for the real-time processing of LEO-based GNSS TEC data from the perspective of real-time applications.     

现代低轨卫星对地球重力场探测的实践和进展

综述了现代低轨卫星对地球重力场测量的特点和近况，介绍了已经和即将发射的重力卫星CHAMP、GRACE、GOCE和新型测高卫星，讨论了作为现代重力卫星首次实践－－CHAMP卫星的进展和目前尚待解决的问题。     

GPS observations of the ionospheric F2-layer behavior during the 20th November 2003 geomagnetic storm over South Korea

The ionospheric F2-layer peak density (NmF2) and its height (hmF2) are of great influence on the shape of the ionospheric electron density profile Ne (h) and may be indicative of other physical processes within the ionosphere, especially those due to geomagnetic storms. Such parameters are often estimated using models such as the semiempirical international reference ionosphere (IRI) models or are measured using moderately priced to expensive instrumentation, such as ionosondes or incoherent scatter radars. Global positioning system (GPS) observations have become a powerful tool for mapping high-resolution ionospheric structures, which can be used to study the ionospheric response to geomagnetic storms. In this paper, we describe how 3-D ionospheric electron density profiles were produced from data of the dense permanent Korean GPS network using the tomography reconstruction technique. These profiles are verified by independent ionosonde data. The responses of GPS-derived parameters at the ionospheric F2-layer to the 20th November 2003 geomagnetic storm over South Korea are investigated. A fairly large increase in the electron density at the F2-layer peak (the NmF2) (positive storm) has been observed during this storm, which is accompanied by a significant uplift in the height of the F2 layer peak (the hmF2). This is confirmed by independent ionosonde observations. We suggest that the F2-layer peak height uplift and NmF2 increase are mainly associated with a strong eastward electric field, and are not associated with the increase of the O/N₂ ratio obtained from the GUVI instruments aboard the TIMED satellite. It is also inferred that the increase in NmF2 is not caused by the changes in neutral composition, but is related to other nonchemical effects, such as dynamical changes of vertical ion motions induced by winds and E × B drifts, tides and waves in the mesosphere/lower thermosphere region, which can be dynamically coupled upward to generate ionospheric perturbations and oscillations.     

FORMOSAT-3/COSMIC observations of the ionospheric auroral oval development

The ionospheric radiance and electron density observed by the tiny ionospheric photometer (TIP) and GPS occultation experiment (GOX) payloads on FORMOSAT-3/COSMIC satellites are applied to determine the boundaries of the auroral oval and its width in the winter nighttime ionosphere for both hemispheres. The TIP collects ionospheric emission at 135.6 nm due to electron impact excitation, while the GOX offers ionospheric electron density profiles with radio occultation (RO) technique. Comparison between them shows similar patterns of the plasma structure in the polar caps. The mean width of the auroral bands ranges between about 2 and 11° latitude in the winter nighttime and it varies with longitudes. The comparison by month suggests that the mean radius of the auroral ovals varies with the intensity of the auroral radiance.