Validation of international reference ionosphere models using in situ measurements from GRACE K-band ranging system and CHAMP planar Langmuir probe

The in situ measurements of electron contents from GRACE K-band (dual-frequency) ranging system and CHAMP planar Langmuir probe were used to validate the international reference ionosphere (IRI) models. The comparison using measurements from year 2003 to 2007 shows a general agreement between data and the model outputs. The improvement in the newer IRI model (IRI-2007) is evident with the measurements from the GRACE satellites orbiting at the higher altitude. We present the comparison between the models and data comprehensively for various cases in solar activity, local time, season, and latitude. The IRI models do not well predict the electron density in the years 2006 and later, when the solar activity is extremely low. The IRI models generally overestimate the electron density during local winter while they underestimate during local summer. In the equatorial region, the large difference at local sunrise lasts for all years and all seasons. The IRI models do not perform well in predicting the anomaly in the polar region such as the Weddell Sea Anomaly. These discrepancies are likely due to smoothed (12-month averaged) solar activity indices used in the IRI models and due to insufficient spherical harmonic representation not able to capture small spatial scales. In near future, further improvement on the IRI models is expected by assimilating those in situ satellite data by implementing higher resolution (spatial and temporal) parameterizations.     

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.     

一种适合MSNA区域的单站TEC经验模型:以ohi3站为例

中纬度夏季夜间异常(midlatitude summer nighttime anomaly,MSNA)区域内的总电子含量(total electron content,TEC)的日变特性存在季节性差异,能否有效描述MSNA的特征是检验电离层TEC经验模型精度的关键指标之一。针对MSNA现象,提出了一种适合MSNA区域的单站电离层TEC经验模型,命名为SSM-T2(single station model type2)模型;以位于MSNA区域内的南极半岛上的奥伊金斯站(ohi3)为例,验证了该模型的有效性。SSM-T2模型由3部分组成,分别是TEC日变化分量、季节变化分量和随太阳活动分量,模型中的系数由非线性最小二乘拟合得到。在南极半岛的ohi3站上,模型拟合测试结果表明,SSM-T2-ohi3模型与建模数据GPS-TEC拟合得很好,较好地描述了MSNA现象。通过模型对比分析发现,在外推时间点上,SSM-T2-ohi3与CODE GIMs、SSM-month模型符合得很好,能有效描述MSNA的特征,具有较好的预测能力,且总体上优于IRI2016模型。     

Temporal and spatial variability of the bias between TOPEX- and GPS-derived total electron content

Total electron content (TEC) predictions made with the GPS-based la plata ionospheric model (LPIM) and the International Reference Ionosphere (IRI95) model were compared to estimates from the dual-frequency altimeter onboard the TOPEX/Poseidon (T/P) satellite. LPIM and IRI95 were evaluated for the location and time of available T/P data, from January 1997 to December 1998. To investigate temporal and spatial variations of the TEC bias between T/P and each model, the region covered by T/P observations was divided into ten latitude bands. For both models and for all latitudes, the bias was mainly positive (i.e. T/P values were larger); the LPIM bias was lower and less variable than the IRI95 bias. To perform a detailed analysis of temporal and spatial variability of the T/P-LPIM TEC bias, the Earth’s surface was divided into spherical triangles with 9°-sides, and a temporally varying regression model was fitted to every triangle. The highest TEC bias was found over the equatorial anomalies, which is attributed to errors in LPIM. A significant TEC bias was found at 40°N latitude, which is attributed to errors in the T/P Sea State Bias (SSB) correction. To separate systematic errors in the T/P TEC from those caused by LPIM, altimeter range biases estimated by other authors were analysed in connection with the TEC bias. This suggested that LPIM underestimates the TEC, particularly during the Southern Hemisphere summer, while T/P C-band SSB calibration is worse during the Southern Hemisphere winter.     

利用区域地基GPS的电离层层析成像研究

电离层是近地空间的重要组成部分,如何对电离层的异常扰动进行合理监测与预报一直是空间物理领域的研究课题。本文将计算机层析成像技术引入到电离层扰动监测中,利用大量的区域地基GPS观测数据,借助代数层析迭代算法反演得到三维电离层电子密度;并将层析结果与国际电离层参考模型IRI2007进行对比分析,结果表明:地基GPS层析所得的电离层电子密度与IRI2007基本一致,但层析结果精度略高于IRI2007模型。     

基于CORS数据反演江苏上空电离层电子密度

本文利用IRI2007模型和江苏CORS网数据,结合附加约束的同时迭代重构算法,反演了地磁平静日江苏上空电子密度的结构分布。结果表明,整个研究区域电子密度在不同纬度和不同高度上存在明显的日变特征,电子密度最大值出现在北京时13～15时,随着时间推移,峰值幅度衰减;白天出现明显的E层,夜晚消失;反演结果说明了电离层三维层析技术为监测电离层时空结构提供了一种强有力的实验支持。     

Determination of the ionospheric foF2 using a stand-alone GPS receiver

The critical frequency of ionospheric F2 layer (foF2) is a measure of the highest frequency of radio signal that may be reflected back by the F2 layer, and it is associated with ionospheric peak electron density in the F2 layer. Accurate long-term foF2 variations are usually derived from ionosonde observations. In this paper, we propose a new method to observe foF2 using a stand-alone global positioning system (GPS) receiver. The proposed method relies on the mathematical equation that relates foF2 to GPS observations. The equation is then implemented in the Kalman filter algorithm to estimate foF2 at every epoch of the observation (30-s rate). Unlike existing methods, the proposed method does not require any additional information from ionosonde observations and does not require any network of GPS receivers. It only requires as inputs the ionospheric scale height and the modeled plasmaspheric electron content, which practically can be derived from any existing ionospheric/plasmaspheric model. We applied the proposed method to estimate long-term variations of foF2 at three GPS stations located at the northern hemisphere (NICO, Cyprus), the southern hemisphere (STR1, Australia) and the south pole (SYOG, Antarctic). To assess the performance of the proposed method, we then compared the results against those derived by ionosonde observations and the International Reference Ionosphere (IRI) 2012 model. We found that, during the period of high solar activity (2011–2012), the values of absolute mean bias between foF2 derived by the proposed method and ionosonde observations are in the range of 0.2–0.5 MHz, while those during the period of low solar activity (2009–2010) are in the range of 0.05–0.15 MHz. Furthermore, the root-mean-square-error (RMSE) values during high and low solar activities are in the range of 0.8–0.9 MHz and of 0.6–0.7 MHz, respectively. We also noticed that the values of absolute mean bias and RMSE between foF2 derived by the proposed method and the IRI-2012 model are slightly larger than those between the proposed method and ionosonde observations. These results demonstrate that the proposed method can estimate foF2 with a comparable accuracy. Since the proposed method can estimate foF2 at every epoch of the observation, it therefore has promising applications for investigating various scales (from small to large) of foF2 irregularities.     

Ionospheric tomography using GNSS: multiplicative algebraic reconstruction technique applied to the area of Brazil

Experimental analysis was performed using multiplicative algebraic reconstruction technique (MART) to map the ionosphere over Brazil. Code and phase observations from the global navigation satellite system (GNSS) together with the international reference ionosphere (IRI) enabled the estimation of ionospheric profiles and total electron content (TEC) over the entire region. Twenty-four days of data collected from existing ground-based GNSS receivers during the recent solar maximum period were used to analyze the performance of the MART algorithm. The results were compared with four ionosondes. It was demonstrated that MART estimated the electron density peak with the same degree of accuracy as the IRI model in regions with appropriate geometrical coverage by GNSS receivers for tomographic reconstruction. In addition, the slant TEC, as estimated with MART, presented lower root-mean-square error than the TEC calculated by ionospheric maps available from the International GNSS Service (IGS). Furthermore, the daily variations of the ionosphere were better represented with the algebraic techniques, compared to the IRI model and IGS maps, enabling a correlation of the elevation of the ionosphere at higher altitudes with the equatorial ionization anomaly intensification. The tomographic representations also enabled the detection of high vertical gradients at the same instants in which ionospheric irregularities were evident.     

Ionospheric F-layer global scintillation index variation using COSMIC during the period of 2007–2013

The three-dimensional global morphology and seasonal characteristics of the ionospheric scintillation index of the F-layer between 150 and 550 km altitudes are analyzed using the GPS radio occultation measurements from the Constellation Observing System for Meteorology, Ionosphere and Climate during the 7-year period of low and high sunspot activity from 2007 to 2013. The results show that the prominent scintillation intensity, which is confined within ±30° geomagnetic latitude, starts at post-sunset, reaches a maximum at around pre-midnight, and often persists until postmidnight. Moderate scintillation activity can be observed in the high-latitude region almost at any time, whereas weak scintillation prevails in the midlatitude region. The noticeable scintillation peak near midnight occurs at an altitude of approximately 250 km in most cases. However, the peak of the scintillation activity during the solar maximum extends to higher altitudes than observed during the solar minimum. Additionally, the local variation in time and altitude of the scintillation intensity is closely correlated with ionospheric HmF2. Statistical analysis indicates that an increase in solar activity or geomagnetic activity enhances the occurrence rate of scintillation and results in intense scintillation. The current research is beneficial for directly studying global ionospheric irregularities at GHz frequency based on high-rate L1 data and constructing a global scintillation model.     

A preliminary evaluation of the performance of multiple ionospheric models in low- and mid-latitude regions of China in 2010–2011

Ionospheric delay is a dominant error source in Global Navigation Satellite System (GNSS). Single-frequency GNSS applications require ionospheric correction of signal delay caused by the charged particles in the earth’s ionosphere. The Chinese Beidou system is developing its own ionospheric model for single-frequency users. The number of single-frequency GNSS users and applications is expected to grow fast in the next years in China. Thus, developing an appropriate ionospheric model is crucially important for the Chinese Beidou system and worldwide single-frequency Beidou users. We study the performance of five globally accessible ionospheric models Global Ionospheric Map (GIM), International Reference Ionosphere (IRI), Parameterized Ionospheric Model (PIM), Klobuchar and NeQuick in low- and mid-latitude regions of China under mid-solar activity condition. Generally, all ionospheric models can reproduce the trend of diurnal ionosphere variations. It is found that all the models have better performances in mid-latitude than in low-latitude regions. When all the models are compared to the observed total electron content (TEC) data derived from GIM model, the IRI model (2012 version) has the best agreement with GIM model and the NeQuick has the poorest agreement. The RMS errors of the IRI model using the GIM TEC as reference truth are about 3.0–10.0 TECU in low-latitude regions and 3.0–8.0 TECU in mid-latitude regions, as observed during a period of 1 year with medium level of solar activity. When all the ionospheric models are ingested into single-frequency precise point positioning (PPP) to correct the ionospheric delays in GPS observations, the PIM model performs the best in both low and mid-latitudes in China. In mid-latitude, the daily single-frequency PPP accuracy using PIM model is ~10 cm in horizontal and ~20 cm in up direction. At low-latitude regions, the PPP error using PIM model is 10–20 cm in north, 30–40 cm in east and ~60 cm in up component. The single-frequency PPP solutions indicate that NeQuick model has the lowest accuracy among all the models in both low- and mid-latitude regions of China. This study suggests that the PIM model may be considered for single-frequency GNSS users in China to achieve a good positioning accuracy in both low- and mid-latitude regions.     

Global morphology of ionospheric sporadic E layer from the FormoSat-3/COSMIC GPS radio occultation experiment

Ionospheric sporadic-E (Es) activity and global morphology were studied using the 50 Hz signal-to-noise ratio amplitude and excess phase measurements from the FormoSat-3/Constellation Observing System for Meteorology, Ionosphere and Climate (FS3/COSMIC) GPS radio occultation (RO) observations. The results are presented for data collected during the last sunspot cycle from mid-2006 to the end of 2017. The FS3/COSMIC generally performed more than 1000 complete E-region GPS RO observations per day, which were used to retrieve normalized L1-band amplitude standard deviation (SDL1) and relative electron density (N_e) profiles successfully. More or less 31% of those observations were identified as Es events based on SDL1 and peak SDL1 altitude criteria. We found that the peak Es-event i values are approximately proportional to the logarithms of the corresponding peak N_e differences. Five major geographical zones were identified, in which the seasonal and diurnal Es occurrence patterns are markedly different. These five zones include the geomagnetic equatorial zone (??5°?<?magnetic latitude (ML)?<?5°), two extended geomagnetic mid-latitude zones (15°?<?ML?<?55°, and ??55°?<?ML < ??15°), and two auroral zones (70°?<?ML, and ML < ??70°). The Es climatology, namely its variations with each identified zone, altitude, season, and local time has been documented.     

多GNSS监测下中国区域电离层格网模型可用性分析

多系统全球卫星导航系统(GNSS)的出现为天基增强系统(SBAS)电离层格网模型的性能提升提供了可能,但多系统GNSS测量对电离层格网模型性能提升是有条件的. 为此,利用中国区域GPS观测模拟分析了多系统GNSS测量对中国区域电离层格网模型可用性的影响. 结果表明:多系统GNSS测量可有效提高电离层格网模型的覆盖范围. 中国南方地区存在低纬赤道电离异常(EIA)现象,严重影响SBAS电离层格网模型实现性能,单纯增加GNSS测量不能有效应对低纬电离异常现象影响. 中国北方地区电离层延迟变化平缓,在多系统GNSS测量情况下可以考虑减少地面监测站数量,仍能保持系统原有性能.      

The international reference ionosphere today and in the future

The international reference ionosphere (IRI) is the internationally recognized and recommended standard for the specification of plasma parameters in Earth’s ionosphere. It describes monthly averages of electron density, electron temperature, ion temperature, ion composition, and several additional parameters in the altitude range from 60 to 1,500 km. A joint working group of the Committee on Space Research (COSPAR) and the International Union of Radio Science (URSI) is in charge of developing and improving the IRI model. As requested by COSPAR and URSI, IRI is an empirical model being based on most of the available and reliable data sources for the ionospheric plasma. The paper describes the latest version of the model and reviews efforts towards future improvements, including the development of new global models for the F2 peak density and height, and a new approach to describe the electron density in the topside and plasmasphere. Our emphasis will be on the electron density because it is the IRI parameter most relevant to geodetic techniques and studies. Annual IRI meetings are the main venue for the discussion of IRI activities, future improvements, and additions to the model. A new special IRI task force activity is focusing on the development of a real-time IRI (RT-IRI) by combining data assimilation techniques with the IRI model. A first RT-IRI task force meeting was held in 2009 in Colorado Springs. We will review the outcome of this meeting and the plans for the future. The IRI homepage is at .     

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。     

Combination of different space-geodetic observations for regional ionosphere modeling

Most of the space-geodetic observation techniques can be used for modeling the distribution of free electrons in the Earth’s ionosphere. By combining different techniques one can take advantage of their different spatial and temporal distributions as well as their different observation characteristics and sensitivities concerning ionospheric parameter estimation. The present publication introduces a procedure for multi-dimensional ionospheric modeling. The model consists of a given reference part and an unknown correction part expanded in terms of B-spline functions. This approach is used to compute regional models of Vertical Total Electron Content (VTEC) based on the International Reference Ionosphere (IRI 2007) and GPS observations from terrestrial Global Navigation Satellite System (GNSS) reference stations, radio occultation data from Low Earth Orbiters (LEOs), dual-frequency radar altimetry measurements, and data obtained by Very Long Baseline Interferometry (VLBI). The approach overcomes deficiencies in the climatological IRI model and reaches the same level of accuracy than GNSS-based VTEC maps from IGS. In areas without GNSS observations (e.g., over the oceans) radio occultations and altimetry provide valuable measurements and further improve the VTEC maps. Moreover, the approach supplies information on the offsets between different observation techniques as well as on their different sensitivity for ionosphere modeling. Altogether, the present procedure helps to derive improved ionospheric corrections (e.g., for one-frequency radar altimeters) and at the same time it improves our knowledge on the Earth’s ionosphere.     

Study on Regional Variations of Aerosol Loading Using Long Term Satellite Data Over Indian Region

Satellite-based measurements of aerosols are one of the most effective ways to understand the role of aerosols in climate in terms of spatial and temporal variability. In the present study, we attempted to analyse spatial and temporal variations of satellite derived aerosol optical depth (AOD) over Indian region using moderate resolution imaging spectrometer over a period of 2001–2011. Due to its vast spatial extent, Indian region and adjacent oceanic regions are divided into different zones for analysis. The land mass is sub divided into five different zones such as Indo Gangetic Plain (IGP), Indian mainland, North Eastern India (NE), South India-1 (SI-1), South India-2 (SI-2). Oceanic areas are divided into Arabian Sea and Bay of Bengal. Arabian Sea is further divided as three zones viz. Northern AS (NAS), Central AS (CAS) and Eastern AS (EAS) zones. Bay of Bengal is divided as North BoB (NBoB), West BoB (WBoB), Central BoB (CBoB), and East BoB (EBoB). The study revealed that among all the land regions, IGP showed the highest peak AOD value (0.52 ± 0.17) while SI-2 showed the lower values of AOD in all the months compared to all India average. The maximum AOD is observed during premonsoon season for all regions. During the winter, average AOD levels were substantially lower than the summer averages. Peak of aerosol loading (0.35 ± 0.159) is observed in March over NE region, whereas in all other regions, peak is observed during May. Frequency distribution of long term AOD (<0.2, 0.3–0.5, >0.5) shows a shift of frequency distribution of AOD from <0.3 to 0.3–0.5 during the study period in all regions except IGP. In IGP shift of frequency of AOD values occurs from 0.3–0.5 to >0.5. Oceanic areas also shows seasonal variation of AOD. Over Arabian Sea, high AOD values with greater variations were observed in summer monsoon season while in Bay of Bengal it is observed during winter monsoon. This is due to the high wind speed prevailing in Arabian Sea during monsoon season which results in production of more sea salt aerosol. Highest AOD values are observed over NAS during monsoon season and over NBOB during winter season. Lowest AOD values with its lower variations observed in both the central region of Arabian Sea and Bay of Bengal.     

Analysis of snow cover and climatic variability in Bhaga basin located in western Himalaya

Bhaga Basin has complex mountainous terrain; little study has been done on the spatial and temporal characteristics of snow cover in the region. The Moderate Resolution Imaging Spectroradiometer (MODIS) 8-day snow cover products between 2001 and 2012 for winter period (November–April) have been used to study the variation in snow cover area (SCA). The statistical analysis based on non-parametric Mann Kendall and Sen’s slope methods have been used for detecting and estimating trends for climatic variables (temperature and snowfall) and SCA for winter period. Results of statistical analysis indicate rise in minimum temperature (0.02 °C year^?1) and fall in maximum temperature (0.17 °C year^?1). It also shows decrease in mean seasonal snowfall (0.07 cm year^?1). The seasonal SCA was found to decrease at the rate of 0.002% year^?1. This study indicates that the climate change is probably one of the major causes for depleting SCA.     

Global morphology of ionospheric F-layer scintillations using FS3/COSMIC GPS radio occultation data

We report on the FormoSat-3/Constellation Observing System for Meteorology, Ionosphere and Climate (FS3/COSMIC) limb-viewing observations of GPS L-band scintillations since mid-2006 and propose to study global F-layer irregularity morphology. The FS3/COSMIC has generally performed more than 1000 ionospheric radio occultation (RO) observations per day. We reprocess 1-Hz amplitude data and obtain complete limb-viewing profiles of the undersampling (sampling frequency lower than Fresnel frequency) S4 scintillation index from about 80% of the RO observations. There are a few percent of FS3/COSMIC RO observations having greater than 0.09 undersampling S4max values on average. However, seven identified areas, Central Pacific Area (?20° to 20° dip latitude, 160°E–130°W), South American Area (?20° to 20° dip latitude, 100°W–30°W), African Area (?20° to 20° dip latitude, 30°W–50°E), European Area (30°–55°N, 0°–55°E), Japan Sea Area (35°–55°N, 120°–150°E), Arctic Area (>65° dip latitude), and Antarctic Area (<?65° dip latitude), have been designated to have a much higher percentage of strong limb-viewing L-band scintillations. During the years in most of the last sunspot cycle from mid-2006 to the end 2014, the scintillation climatology, namely, its variations with each identified area, season, local time, magnetic activity, and solar activity, have been documented.     

The geopotential to (14, 14) from a combination of satellite and gravimetric data

A set of2261 5°×5° mean anomalies were used alone and with satellite determined harmonic coefficients of the Smithsonian' Institution to determine the geopotential expansion to various degrees. The basic adjustment was carried out by comparing a terrestrial anomaly to an anomaly determined from an assumed set of coefficients. The (14, 14) solution was found to agree within ±3 m of a detailed geoid in the United States computed using1°×1° anomalies for an inner area and satellite determined anomalies in an outer area. Additional comparisons were made to the input anomaly field to consider the accuracy of various harmonic coefficient solutions. A by-product of this investigation was a new γ_E=978.0463 gals in the Potsdam system or978.0326 gals in an absolute system if −13.7 mgals is taken as the Potsdam correction. Combining this value of γ_E withf=1/298.25, KM=3.9860122·10 ²² cm ³ /sec ² , the consistent equatorial radius was found to be6378143 m.     

Investigating the vegetation and agricultural responses to El Nino/Southern Oscillation using AVHRR data

This study explores the possible linkages of El Nino/Southern Oscillation (ENSO) with vegetation and rainfall patterns, vegetation activity and food grain yields, in arid and semi-arid regions of western India. A sequence of 20-year (1981–2000) monthly maximum Normalized Difference Vegetation Index (NDVI) data from the Advanced Very High Resolution Radiometer (AVHRR) and monthly rainfall from 160 stations were examined to study the seasonal patterns and their relation to ENSO activity. In addition, a direct (ENSO-crop yield) linkage and an intermediate (ENSO-NDVI) linkage of agricultural responses to ENSO were also investigated. The results indicate below-normal seasonal NDVI and rainfall associated with El Nino (warm) events, except during 1997, while positive anomalies occur during La Nina (cold) events. Sea surface temperature (SST) anomalies from NINO 3 region (5°N–5°S; 150°W–90°W), as an indicator of ENSO were significantly correlated with NDVI anomalies, rainfall anomalies and yield anomalies but the Southern Oscillation Index (SOI) was significantly related to NDVI anomalies only. NDVI anomaly patterns correspond to rainfall variability including that associated with ENSO activity. The observed strong intermediate linkage between yield anomalies and NDVI anomaly signal (r = 0.609) indicates that NDVI is an ideal index for understanding and analysing agricultural response to ENSO climate teleconnections.