Ionospheric scintillations at Guilin detected by GPS ground-based and radio occultation observations

The occurrence of ionospheric scintillations with S4 ? 0.2 was studied using GPS measurements at Guilin, China (25.29°N, 110.33°E; geomagnetic: 15.04°N, 181.98°E), a station located near the northern crest of the equatorial anomaly. The results are presented for data collected from January 2009 to March 2010. The results show that nighttime amplitude scintillations only took place in February and March of the considered years, while daytime amplitude scintillations occurred in August and December of 2009. Nighttime amplitude scintillations, observed in the south of Guilin, always occurred with phase scintillations, TEC (Total Electron Content) depletions, and ROT (Rate Of change of TEC) fluctuations. However, TEC depletions and ROT fluctuations were weak during daytime amplitude scintillations, and daytime amplitude scintillations always took place simultaneously for most of the GPS satellites which appeared over Guilin in different azimuth directions. Ground-based GPS scintillation/TEC observations recorded at Guilin and signal-to-noise-ratio (SNR) measurements obtained from GPS-COSMIC radio occultation indicate that nighttime and daytime scintillations are very likely caused by ionospheric F region irregularities and sporadic E, respectively. Moreover, strong daytime amplitude scintillations may be associated with the plasma density enhancements in ionospheric E region caused by the Perseid and Geminid meteor shower activities.     

Using GPS-SCINDA observations to study the correlation between scintillation,total electron content enhancement and depletions over the Kenyan region

This paper presents the first results of total electron content (TEC) depletions and enhancement associated with ionospheric irregularities in the low latitude region over Kenya. At the low latitude ionosphere the diurnal behavior of scintillation is driven by the formation of large scale equatorial depletions which are formed by post-sunset plasma instabilities via the Rayleigh–Taylor instability near the magnetic equator. Data from the GPS scintillation receiver (GPS-SCINDA) located at the University of Nairobi (36.8°E, 1.27°S) for March 2011 was used in this study. The TEC depletions have been detected from satellite passes along the line of sight of the signal and the detected depletions have good correspondence with the occurrence of scintillation patches. TEC enhancement has been observed and is not correlated with increases in S₄ index and consecutive enhancements and depletions in TEC have also been observed which results into scintillation patches related to TEC depletions. The TEC depletions have been interpreted as plasma irregularities and inhomogeneities in the F region caused by plasma instabilities, while TEC enhancement have been interpreted as the manifestation of plasma density enhancements mainly associated with the equatorial ionization anomaly crest over this region. Occurrence of scintillation does happen at and around the ionization anomaly crest over Kenyan region. The presence of high ambient electron densities and large electron density gradients associated with small scale irregularities in the ionization anomaly regions have been linked to the occurrence of scintillation.     

Investigation of ionospheric irregularities and scintillation using TEC at high latitude

The occurrence of ionospheric irregularities at high latitudes, with dimensions of several kms down to decameter scale size shows strong correlation with geomagnetic disturbance, season and solar activity. Transionospheric radio waves propagating through these irregularities experience rapid random fluctuations in phase and/or amplitude of the signal at the receiver, termed scintillation, which can degrade GNSS services. Thus, investigation and prediction of this scintillation effect is very important. To investigate such scintillation effects, a GISTM (GPS Ionospheric Scintillation and TEC Monitoring) NovAtel dual frequency (L1/L2) GPS receiver has been installed at Trondheim, Norway (63.41°$63.41°$ N, 10.4°$10.4°$ E), capable of collecting scintillation indices at a 1 min rate as well as the raw data (phase and intensity) of the satellite signals at a 50 Hz sampling rate and TEC (Total Electron Content) at a 1 Hz rate. Many researchers have reported that both phase and amplitude scintillation is closely associated with TEC fluctuations or associated with a significant developing enhancement or depletion in the TEC. In this study, a novel analogous phase index is developed which provides samples at a 1 min rate. Generally the scintillation indices can help in estimating the irregularity scintillation effect at a one minute rate, but such procedures are time consuming if DFTs of the phase and/or amplitude at a 50 Hz data are required. In this study, instead, this analogous phase index is estimated from 1 Hz rate TEC values obtained from the raw signals and is then compared for weak, moderate and strong scintillation at Trondheim for one year of data collected from the installed GPS receiver. The spectral index of the irregularities (that is the inverse power law of their spatial spectrum) is determined from the resultant phase scintillation psd. The correlations of the scintillation indices and spectral indices with the analogous phase index have been investigated under different geomagnetic conditions (represented by the Kp index) and an approximate linear correlation of phase scintillation with the analogous phase index was found. Then a principal advantage of this index is that it achieves this correlation without requiring a high sampling data rate and the need for DFTs. Thus, the index seems a good candidate for developing a simple means of ionospheric scintillation prediction which could also be utilized in the development of alerts using regional mappings.     

Spread-F occurrence during geomagnetic storms near the southern crest of the EIA in Argentina

This work presents, for the first time, the analysis of the occurrence of ionospheric irregularities during geomagnetic storms at Tucumán, Argentina, a low latitude station in the Southern American longitudinal sector (26.9°S, 294.6°E; magnetic latitude 15.5°S) near the southern crest of the equatorial ionization anomaly (EIA). Three geomagnetic storms occurred on May 27, 2017 (a month of low occurrence rates of spread-F), October 12, 2016 (a month of transition from low to high occurrence rates of spread-F) and November 7, 2017 (a month of high occurrence rates of spread-F) are analyzed using Global Positioning System (GPS) receivers and ionosondes. The rate of change of total electron content (TEC) Index (ROTI), GPS Ionospheric L-band scintillation, the virtual height of the F-layer bottom side (h'F) and the critical frequency of the F2 layer (foF2) are considered. Furthermore, each ionogram is manually examined for the presence of spread-F signatures.The results show that, for the three events studied, geomagnetic activity creates favorable conditions for the initiation of ionospheric irregularities, manifested by ionogram spread-F and TEC fluctuation. Post-midnight irregularities may have occurred due to the presence of eastward disturbance dynamo electric fields (DDEF). For the May storm, an eastward over-shielding prompt penetration electric field, (PPEF) is also acting. A possibility is that the PPEF is added to the DDEF and produces the uplifting of the F region that helps trigger the irregularities. Finally, during October and November, strong GPS L band scintillation is observed associated with strong range spread-F (SSF), that is, irregularities extending from the bottom-side to the topside of the F region.     

基于多种观测手段的东亚低纬电离层不规则体事件分析

利用海南台站（19.5°N,109.1°E,dip:13.6°N）和磁赤道区的多种地基和天基观测数据,对2011年11月20日观测到的电离层不规则体事件进行了分析.海南台站VHF雷达、电离层闪烁和数字测高仪的综合观测结果表明,当天日落附近发生了强的电离层不规则体事件,主要表现为雷达羽和强闪烁的形态.结合磁赤道区GPS和C/NOFS卫星观测结果进行分析可知,海南台站日落附近出现的雷达羽和强闪烁与南海磁赤道区产生的主等离子体泡存在明显联系.      

A study of L band scintillations during the initial phase of rising solar activity at an Indian low latitude station

The ionospheric scintillation and TEC (Total Electron Content) variations are studied using GPS (Global Positioning System) measurements at an Indian low latitude station Surat (21.16°N, 72.78°E; Geomagnetic: 12.90°N, 147.35°E), situated near the northern crest of the equatorial anomaly region. The results are presented for data collected during the initial phase of current rising solar activity (low to moderate solar activity) period between January 2009 and December 2011. The results show that within a total number of 656 night-time scintillation events, 340 events are observed with TEC depletions, Rate of change of TEC (ROT) fluctuations and enhancement of Rate of change of TEC Index (ROTI). A comparison of night-time scintillation events from the considered period reveal strong correlation amongst the duration of scintillation activity in S₄ index_, TEC depletion, ROT fluctuations and ROTI enhancement in the year 2011, followed by the year 2010 and least in 2009. The statistical analyses of scintillation activity with enhancement of ROTI also show that about 70–96% scintillation activity took place in equinox and winter months. Moreover, from a nocturnal variation in occurrence of scintillation with (S₄ ? 0.2) and enhancement of ROTI with (ROTI ? 0.5), a general trend of higher occurrence in pre-midnight hours of equinox and winter seasons is observed in both indices during the year 2011 and 2010, while no significant trend is observed in the year 2009. The results suggest the presence of F-region ionospheric irregularities with scale sizes of few kilometers and few hundred meters over Surat and are found to be influenced by solar and magnetic activity.     

GPS phase difference variation statistics: A comparison between phase scintillation index and proxy indices

Scintillated GPS phase observations are traditionally characterized by the phase scintillation index, derived from specialized GPS receivers usually tracking at 50 Hz. Geodetic quality GPS receivers, on the other hand, are normally tracking at frequencies up to 1 Hz. However, availability of continuously operating geodetic receivers both in time and geographical location are superior to scintillation receiver’s coverage in many parts of the world. This motivates scintillation studies using regional and global geodetic GPS networks. Previous studies have shown the usefulness of GPS estimated total electron content variations for detecting ionospheric irregularities. In this paper, collocated geodetic and scintillation receivers are employed to compare proxy indices derived from geodetic receivers with the phase scintillation index during quiet and moderately disturbed ionospheric conditions. Sensitivity of the phase scintillation indices at high latitude stations to geomagnetic activity is discussed. Global mapping of ionospheric disturbances using proxy indices from real-time 1 Hz GPS stations are also presented.     

Response of the ionospheric F-region in the Brazilian sector during the super geomagnetic storm in April 2000 observed by GPS

The response of the ionospheric F-region in the equatorial and low latitude regions in the Brazilian sector during the super geomagnetic storm on 06–07 April 2000 has been studied in the present investigation. The geomagnetic storm reached a minimum D_st of −288 nT at 0100 UT on 07 April. In this paper, we present vertical total electron content (VTEC) and phase fluctuations (in TECU/min) from GPS observations obtained at Imperatriz (5.5°S, 47.5°W; IMPZ), Brasília (15.9°S, 47.9°W; BRAZ), Presidente Prudente (22.12°S, 51.4°W; UEPP), and Porto Alegre (30.1°S, 51.1°W; POAL) during the period 05–08 April. Also, several GPS-based TEC maps are presented from the global GPS network, showing widespread and drastic TEC changes during the different phases of the geomagnetic storm. In addition, ion density measurements on-board the satellite Defense Meteorological Satellite Program (DMSP) F15 orbiting at an altitude of 840 km and the first Republic of China satellite (ROCSAT-1) orbiting at an altitude of 600 km are presented. The observations indicate that one of the orbits of the DMSP satellite is fairly close to the 4 GPS stations and both the DMSP F15 ion-density plots and the phase fluctuations from GPS observations show no ionospheric irregularities in the Brazilian sector before 2358 UT on the night of 06–07 April 2000. During the fast decrease of D_st on 06 April, there is a prompt penetration of electric field of magnetospheric origin resulting in decrease of VTEC at IMPZ, an equatorial station and large increase in VTEC at POAL, a low latitude station. This resulted in strong phase fluctuations on the night of 06–07 April, up to POAL. During the daytime on 07 April during the recovery phase, the VTEC observations show positive ionospheric storm at all the GPS stations, from IMPZ to POAL, and the effect increasing from IMPZ to POAL. This is possibly linked to the equatorward directed meridional wind. During the daytime on 08 April (the recovery phase continues), the VTEC observations show very small negative ionospheric storm at IMPZ but the positive ionospheric storm effect is observed from BRAZ to POAL possibly linked to enhancement of the equatorial ionospheric anomaly.     

Relative amplitude of the total electron content variations depending on geomagnetic activity

We developed a method of estimation of a relative amplitude dI/I of the total electron content (TEC) variations in the ionosphere as deduced from the data of the global GPS receivers network. To obtain statistically significant results we picked out three latitudinal belts provided in the Internet by the maximum number of GPS sites. They are high-latitudinal belt (50–80°N, 200–300°E; 59 sites), mid latitude belt (20–50°N, 200–300°E; 817 sites), and equatorial belt (±20°N, 0–360°E; 76 sites). The results of the analysis of the diurnal and latitudinal dependencies of dI/I and dI/I distribution probability for 52 days with different levels of geomagnetic activity are presented. It was found that on average the relative amplitude of the TEC variations varies within the range 0–10% proportionally to the value of the K_p geomagnetic index. In quiet conditions the relative amplitude dI/I of the TEC variations at night significantly exceeds the daytime relative amplitude. At high levels of magnetic field disturbances, the geomagnetic control of the amplitude of TEC variations at high and middle latitudes is much more significant than the regular diurnal variations. At the equatorial belt, on average, the amplitude of TEC variations in quiet and disturbed periods almost does not differ. The obtained results may be useful for development of the theory of ionospheric irregularities.     

Change in refractivity of the atmosphere and large variation in TEC associated with some earthquakes,observed from GPS receiver

The present study reports the analysis of GPS based TEC for our station Surat (21.16°N, 72.78°E) located at the northern crest of equatorial anomaly region in India at times close to some earthquake events (M ? 5) during the year 2009 in India and its neighbouring regions. The TEC data used in the study are obtained from GPS Ionospheric Scintillation and TEC Monitoring (GISTM) system. The TEC data has been analysed corresponding to 11 earthquakes in low solar activity period and quiet geomagnetic condition. We found that, out of 11 cases of earthquakes (M > 5) there were seven cases in which enhancement in TEC occurred on earthquake day and in other four cases there was depletion in TEC on earthquake day. The variation in refractivity prior to earthquake was significant for the cases in which the epicentre lied within a distance of 600 km from the receiving station. By looking into the features on temporal enhancement and depletion of TEC a prediction was made 3–2 days prior to an earthquake (on 28 October 2009 in Bhuj – India). The paper includes a brief discussion on the method of potentially identifying an impending earthquake from ionospheric data.     

A study of GPS ionospheric scintillations observed at Shenzhen

Ionospheric scintillation variations are studied using GPS measurements at the low latitude station of Shenzhen (22.59°N, 113.97°E), situated under the northern crest of the equatorial anomaly region, from the Chinese Meridian Project. The results are presented for data collected during the current phase of rising solar activity (low to high solar activity) from December 2010 to April 2014. The results show that GPS scintillation events were largely a nighttime phenomenon during the whole observation period. Scintillation events mainly occurred along the inner edge of the northern crest of the equatorial anomaly in China. The occurrence of scintillations in different sectors of the sky was also investigated, and the results revealed that it is more likely for the scintillations to be observed in the west sector of the sky above Shenzhen. During the present period of study, a total number of 512 total electron content (TEC) depletions and 460 lock loss events were observed. In addition, both of these events are likely to increase during periods of high solar activity, especially because the strong scintillations are often simultaneously accompanied by TEC depletions and lock losses by GPS receivers.     

北驼峰区电离层GPS卫星闪烁事件时空特征及对通信的影响

基于子午工程北大深圳站（22.59°N,113.97°E）电离层GPS双频接收机在2011年1月1日至2017年12月31日连续7年的长时间序列闪烁和TEC观测数据,分析不同太阳活条件下华南赤道异常北驼峰区观测到的GPS卫星L波段电离层闪烁事件时空分布特征及其对通信的影响.结果表明:GPS闪烁事件几乎都发生在夜间,且主要发生在春秋分月份;在不同太阳活动条件下,夜间GPS闪烁事件都主要发生在北驼峰区域靠近磁赤道的一侧,且GPS闪烁事件存在明显的东-西侧天区不对称性,即在台站西侧天区发生的闪烁事件明显偏多;在不同太阳活动条件下,弱闪烁事件伴随的TEC耗尽和卫星失锁事件比例相对较低,强闪烁事件则大部分都伴随着TEC耗尽和卫星失锁事件的发生.      

Night-time total electron content enhancements at equatorial anomaly region in China

This paper presents small scale (duration ?1 h, ΔTEC ? 1TECU) night-time total electron content (TEC) enhancements observed at the equatorial anomaly region in China, for the first time. The data is from a GPS receiver chain established in 2005 by Institute of Center for Space Science and Applied Research, Chinese Academy of Sciences and a GPS receiver of International GPS Service (IGS), located between Fuzhou (26.1°N, 119.3°E) and Nanning (22.8°N, 108.3°E). Two other GPS observations of IGS taken at higher latitude were also used to investigate the localization of such phenomenon. The characteristics of the night-time TEC enhancement are examined with two case studies. The TEC increases about 1–3TECU, intermittently. While the night-time TEC enhancement mainly occurs at the equatorial anomaly region, it can be observed at middle latitude as well. The spatial size of the enhancement region is less than 5° in longitude. The primary statistical study shows that the TEC enhancement is more often observed in spring and autumn, but rarely in summer. It has no dependence on geomagnetic activity. The enhancement can occur both before and after midnight.     

Low-latitude ionospheric scintillations and total electron content obtained with the CITRIS instrument on STPSat1 using radio transmissions from DORIS ground beacons

The primary objective of the Scintillation and Tomography Receiver in Space (CITRIS) is to detect ionospheric irregularities from space at low latitude. For this purpose, the satellite receiver uses the UHF and S-Band transmissions of the ground network of Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) beacons. CITRIS, developed at the Naval Research Laboratory, differs from the normal DORIS receiver by being able to capture and store the complex amplitude of the 401.25 and 2036.25 MHz transmissions at 200 Hz sample rate. Ground processing of the CITRIS data yields total electron content (TEC) and both phase and amplitude scintillations. With CITRIS flying on the US Space Test Program (STP) satellite STPSat1, 2 years of data were collected and processed to determine the fluctuations in ionospheric TEC and radio scintillations associated with equatorial irregularities. CITRIS flights over DORIS transmitters yield direct measurements of the horizontal plasma density fluctuations associated with equatorial plasma bubbles. Future flights of CITRIS can provide valuable complements to other satellite instruments such as GPS occultation receivers used to estimate vertical electron density profiles in the ionosphere.     

Low latitude ionospheric scintillation and zonal irregularity drifts observed with GPS-SCINDA system and closely spaced VHF receivers in Kenya

In this study we have used VHF and GPS-SCINDA receivers located at Nairobi (36.8°E, 1.3°S, dip −24.1°) in Kenya, to investigate the ionospheric scintillation and zonal drift irregularities of a few hundred meter-scale irregularities associated with equatorial plasma density bubbles for the period 2011. From simultaneous observations of amplitude scintillation at VHF and L-band frequencies, it is evident that the scintillation activity is higher during the post sunset hours of the equinoctial months than at the solstice. While it is noted that there is practically no signatures of the L-band scintillation in solstice months (June, July, December, January) and after midnight, VHF scintillation does occur in the solstice months and show post midnight activity through all the seasons. VHF scintillation is characterized by long duration of activity and slow fading that lasts till early morning hours (05:00 LT). Equinoctial asymmetry in scintillation occurs with higher occurrence in March–April than in September–October. The occurrence of post midnight VHF scintillation in this region is unusual and suggests some mechanisms for the formation of scintillation structure that might not be clearly understood. Zonal drift velocities of irregularities were measured using cross-correlation analysis with time series of the VHF scintillation structure from two closely spaced antennas. Statistical analyses of the distribution of zonal drift velocities after sunset hours indicate that the range of the velocities is 30–160 m/s. This is the first analysis of the zonal plasma drift velocity over this region. Based on these results we suggest that the east–west component of the plasma drift velocity may be related to the evolution of plasma bubble irregularities caused by the prereversal enhancement of the eastward electric fields. The equinoctial asymmetry of the drift velocities and scintillation could be attributed to the asymmetry of neutral winds in the thermosphere that drives the eastward electric fields.     

Variation of ionospheric total electron content in Indian low latitude region of the equatorial anomaly during May 2007–April 2008

The ionospheric total electron content (TEC), derived by analyzing dual frequency signals from the Global Positioning System (GPS) recorded near the Indian equatorial anomaly region, Varanasi (geomagnetic latitude 14°, 55′N, geomagnetic longitude 154°E) is studied. Specifically, we studied monthly, seasonal and annual variations as well as solar and geomagnetic effects on the equatorial ionospheric anomaly (EIA) during the solar minimum period from May 2007 to April 2008. It is found that the daily maximum TEC near equatorial anomaly crest yield their maximum values during the equinox months and their minimum values during the summer. Using monthly averaged peak magnitude of TEC, a clear semiannual variation is seen with two maxima occurring in both spring and autumn. Statistical studies indicate that the variation of EIA crest in TEC is poorly correlated with Dst-index (r = −0.03) but correlated well with Kp-index (r = 0.82). The EIA crest in TEC is found to be more developed around 12:30 LT.     

Coordinated in situ measurements of plasma irregularities and ground based scintillation observations at the crest of equatorial anomaly

First comparison of in situ density fluctuations measured by the DEMETER satellite with ground based GPS receiver measurements at the equatorial anomaly station Bhopal (geographic coordinates (23.2°N, 77.6°E); geomagnetic coordinates (14.29°N, 151.12°E)) for the low solar activity year 2005, are presented in this paper. Calculation of the diurnal maximum of the strength of the equatorial electrojet, which can serve as precursor to ionospheric scintillations in the anomaly region is also done. The Langmuir Probe experiment and Plasma Analyzer onboard DEMETER measure the electron and ion densities respectively. Irregularities in electron density distribution cause scintillations on transionospheric links and there exists a close relationship between an irregularity and scintillation. In 40% of the cases, DEMETER detects the irregularity structures (dNe/Ne ? 5% and dNi/Ni (O⁺) ? 5%) and GPS L band scintillations (S4 ? 0.2) are also observed around the same time, for the low solar activity period. It is found that maximum irregularity intensity is obtained in the geomagnetic latitude range of 10–20° for both electron density and ion density. As the GPS signals pass through this irregularity structure, scintillations are recorded by the GPS receiver installed at the equatorial anomaly station, Bhopal it is interesting to note that in situ density fluctuations observed on magnetic flux tubes that pass over Bhopal can be used as indicator of ionospheric scintillations at that site. Many cases of density fluctuations and associated scintillations have been observed during the descending low solar activity period. The percentage occurrence of density irregularities and scintillations shows good correspondence with diurnal maximum of the strength of electrojet, however this varies with different seasons with maximum correspondence in summer (up to 66%) followed by equinox (up to 50%) and winter (up to 46%). Also, there is a threshold value of EEJ strength to produce density irregularities ((dNe/Ne)_max ? 5%) and for moderate to strong scintillations (S4 ? 0.3) to occur. For winter this value is found to be ∼40 nT whereas for equinox and summer it is around 50 nT.     

电离层不规则结构对GPS性能的影响

电离层不规则结构的存在可引起无线电信号的幅度和相位发生随机起伏, 这 种电离层闪烁现象会影响全球定位系统(Global Positioning System, GPS)的 性能, 降低定位精度, 严重时导致信号失锁. 电离层不规则结构对GPS性能的 影响涉及电离层物理、接收机设计和表征卫星几何分布的精度衰减因子(Dilution of Positioning, DOP)等多方面因素. 本文通过对表征电离层不规则结构参数 的分析, 根据GPS接收机跟踪环路和闪烁信号模型, 综合研究了电离层闪烁对 GPS接收机载波跟踪环和码跟踪环跟踪误差的影响; 结合实际观测, 评述 了电离层不规则结构对单频和双频GPS接收机定位性能的影响, 在此基础上 提出了有待深入研究的问题及具体建议.      

用GPS 观测研究电离层TEC 水平梯度

双频GPS 用户能自动修正电离层总电子含量(TEC) 引起的延时误差, 但是对于电离层中的不规则体造成的信号闪烁而引起的误差则不能消除. 即使是差分GPS 系统, 电离层误差仍然是其主要的误差源, 其中电离层TEC 梯度将会影响到系统的定位精度和性能. 本文用GPS 方法研究了电离层TEC 的水平梯度问题, 用处于赤道异常区NTUS 台站的GPS 观测数据作了具体计算. 结果表明, 在日落以后到子夜前后电离层垂直TEC 出现了大的涨落, 电离层中的不规则体导致L 波段信号强的闪烁, 同时还伴随着大而快速变化的电离层~TEC 水平梯度. 对比发现, ROTI指数、电离层TEC 水平梯度和电离层垂直TEC 三者之间有很好的对应关系, 它们的变化特征均由电离层中的不规则体引起. 我们认为研究电离层闪烁, 特别是在缺乏S4指数时, 电离层TEC 梯度也可以作为一个重要的可选参数.      

Comparison of observed TEC values with IRI-2007 TEC and IRI-2007 TEC with optional foF2 measurements predictions at an equatorial region,Chumphon, Thailand

In this research, as part of working towards improving the IRI over equatorial region, the total electron content (TEC) derived from GPS measurements and IRI-2007 TEC predictions at Chumphon station (10.72°N, 99.37°E), Thailand, during 2004–2006 is analyzed. The seasonal variation of the IRI-2007 TEC predictions is compared with the TEC from the IRI-2007 TEC model with the option of the actual F2 plasma frequency (foF2) measurements as well as the TEC from the GPS and International GNSS service (IGS). The Chumphon station is located at the equatorial region and the low latitude of 3.22°N. For a declining phase of the solar cycle (2004–2006), the study shows that the IRI-2007 TEC underestimates the IRI-2007 TEC with the foF2 observation at the nighttime by about 5 TECU. The maximum differences are about 15 TECU during daytime and 5 TECU during nighttime. The overestimation is more evident at daytime than at nighttime. When compared in terms of the root-mean square error (RMSE), we find that the highest RMSE between GPS TEC and IRI 2007 TEC is 14.840 TECU at 1230 LT in 2004 and the lowest average between them is 1.318 TECU at 0630 LT in 2006. The noon bite-out phenomena are clearly seen in the IRI-2007 TEC with and without optional foF2 measurements, but not on the GPS TEC and IGS TEC. The IRI TEC with optional foF2 measurements gives the lowest RMSE values between IRI TEC predicted and TEC measurement. However, the TEC measurements (GPS TEC and IGS TEC) are more correct to use at Chumphon station.