2004年11月强磁暴期间武汉电离层TEC的响应和振幅闪烁特征的GPS观测

本文利用设在武汉(11436°E,3053°N,磁纬194°)的GPS电离层TEC和电波闪烁监测仪的测量数据,分析了2004年11月强磁暴期间TEC的响应以及电波闪烁和TEC起伏的特征.结果表明,在这次强磁暴期间,武汉及其邻近地区电离层TEC的响应以正暴相为主,正暴相分别出现在两次主相期间,最大正偏离达到50 TECU.这次磁暴另一个重要影响是主相期间L波段振幅闪烁的活动性及其强度显著增强.S4指数最大接近10.伴随增强的闪烁活动,多次观测到深度耗尽的等离子体泡与TEC起伏,TEC变化率的标准差ROTI指数也显著增强.分析揭示, ROTI指数与S4指数呈正相关,相关系数达到097.线性回归计算得到,ROTI和S4的比率为964.     

A comparison of TEC fluctuations and scintillations at Ascension Island

With increasing reliance on space-based platforms for global navigation and communication, concerns about the impact of ionospheric scintillation on these systems have become a high priority. Recently, the Air Force Research Laboratory (AFRL) performed amplitude scintillation measurements of L1 (1.575 MHz) signals from GPS satellites at Ascension Island (14.45° W, 7.95° S; magnetic latitude 16° S) during February–April, 1998, to compare amplitude scintillations with fluctuations of the total electron content (TEC). Ascension Island is located in the South Atlantic under the southern crest of the equatorial anomaly of F2 ionization where scintillations will be much enhanced during the upcoming solar maximum period. Ascension Island is included in the global network of the International GPS Service (IGS) and the GPS receivers in this network report the carrier to noise (C/N) ratio, the dual frequency carrier phase and pseudorange data at 30-s intervals. Such data with a sampling interval of 30 s were analyzed to determine TEC, the rate of change of TEC (ROT) and also ROTI, defined as the standard deviation of ROT. The spatial scale of ROTI, sampled at 30 s interval, will correspond to 6 km when the vector sum of the ionospheric projection of the satellite velocity and the irregularity drift orthogonal to the propagation path is of the order of 100 m/s. On the other hand, the scale-length of the amplitude scintillation index corresponds to the Fresnel dimension which is about 400 m for the GPS L1 frequency and an ionospheric height of 400 km. It is shown that, in view of the co-existence of large and small scale irregularities in equatorial irregularity structures, during the early evening hours, and small magnitude of irregularity drifts, ROTI measurements can be used to predict the presence of scintillation causing irregularities. The quantitative relationship between ROTI and S4, however, varies considerably due to variations of the ionospheric projection of the satellite velocity and the ionospheric irregularity drift. During the post-midnight period, due to the decay of small scale irregularities leading to a steepening of irregularity power spectrum, ROTI, on occasions, may not be associated with detectable levels of scintillation. In view of the power law type of irregularity power spectrum, ROTI will, in general, be larger than S4 and the ratio, ROTI/S4, in the present dataset is found to vary between 2 and 10. At high latitudes, where the ionospheric motion, driven by large electric fields of magnetospheric origin, is much enhanced during magnetically active periods, ROTI/S4 may be considerably larger than that in the equatorial region.     

Effects of geomagnetic storm on GPS ionospheric scintillations at Sanya

The effects of geomagnetic storm on GPS ionospheric scintillations are studied here using GPS scintillation data recorded at Sanya (18.3°N, 109.5°E; geomagnetic: 7.6°N, 180.8°E), the southmost station in the Chinese longitude region. GPS scintillation/TEC and DMSP data are utilized to show the development of irregularities during the period year 2005 (solar minimum). Statistical analysis of K planetary index (Kp) and amplitude scintillation index (S4) indicates that most storms of the year did not trigger the scintillation occurrence at Sanya. However, cases of scintillation occurring during moderate and strong storm (Dst<−100) periods show clearly that the development of irregularities producing scintillations can be triggered by geomagnetic storms during the low scintillation occurrence season. The effects (trigger or not trigger/inhibit) depend on the maximum dDst/dt determined local time sector, and can be explained by the response of the equatorial vertical drift velocities to magnetospheric and ionospheric disturbance electric fields. For station Sanya, the maximum dDst/dt determined local time is near the noon (or post-midnight) sector for most storms of the year 2005, which inhibited (or did not trigger) the post-sunset (or post-midnight) scintillation occurrence and then led to the phenomena that the statistical results presented.     

The response of TEC at quasi-conjugate point GPS stations during solar flares

Global Positioning System (GPS) derived total electron content (TEC) measurements were analyzed to investigate the ionospheric response during the X-class solar flare event that occurred on 5-6 December 2006 at geomagnetic conjugate stations: Syowa, Antarctica (SYOG) (GC: 69.00°S, 39.58°E; CGM: 66.08°S, 71.65°E) and árholt, Iceland (ARHO) (GC: 66.19°N, 342.89°E; CGM: 66.37°N, 71.48°E). Bernese GPS software was used to derive the TEC maps for both stations. The focus of this study is to determine the symmetry or asymmetry of TEC values which is an important parameter in the ionosphere at conjugate stations during these solar flare events. The results showed that during the first flares on 5 December, effects were more pronounced at SYOG than at ARHO. However, on 6 December, the TEC at ARHO showed a sudden spike during the flare with a different TEC variation at SYOG.     

我国低纬电离层闪烁和周跳的统计特征比较

本文利用位于我国中南部电离层闪烁监测台网2012年至2015年的观测数据,比较分析了GPS（Global Positioning System）信号闪烁与周跳的统计特征以及太阳活动和地磁扰动对闪烁与周跳的影响.结果表明,闪烁活动与周跳出现随地方时、月份、太阳活动和地磁扰动变化的统计特征类似,且周跳出现的可能性随S4指数增高显著增大,说明闪烁与周跳存在密切的关联,是引起周跳的一种重要因素.一天之中,闪烁和周跳主要出现在日落后至黎明前,午夜前出现最频繁,白天仅偶尔出现.在赤道异常峰及其邻近区域,一年之中,闪烁和周跳主要出现在春秋季,春季闪烁活动和周跳出现明显比秋季频繁,呈现春秋不对称性,冬夏季节闪烁和周跳都很少出现.闪烁活动与周跳出现的逐年变化显著依赖太阳活动水平,随太阳活动水平升高而增强,而地磁扰动与闪烁活动与周跳出现呈负相关,地磁扰动对闪烁活动与周跳出现整体上起抑制作用.平均而言,越靠近磁赤道的台站闪烁活动越频繁,随纬度升高,闪烁活动频次逐渐降低,且闪烁活动的开始时间随纬度升高而滞后,暗示引起GPS信号闪烁的电离层不规则结构主要起源于磁赤道区.此外,分析还发现,闪烁活动与周跳出现的空域有相当好的一致性,主要分布在观测点上空仰角55°以下、方位角150°~240°的空域内.     

Ionospheric disturbances in the east-Asian region during the geomagnetic period in November 2004

Results of the studies of ionospheric parameter variations during the intense geomagnetic storm on November 7–11, 2004, in the 20°–80° N, 60°–180° E sector are presented. The data of ionospheric stations and the results of total electron content (TEC) measurements at the network of the GPS ground-based receivers and of the GPS receiver onboard the CHAMP satellite were used. Periods of total absorption and blanketing sporadic E layers were observed at high latitudes, whereas durable negative disturbances typical of geomagnetic storms of high intensity were detected at midlatitudes. In the afternoon hours of local time on November 8, 2004, a large-scale ionospheric disturbance of a frontal type was detected on the basis of foF2 and TEC measurements. The disturbance propagated southwestward at a mean velocity of about 200 m/s. The comparison of the relative amplitude of this large-scale disturbance according to the total electron content (～70%) and foF2 (～80%) measurements made it possible to assume a large vertical scale of the disturbance.     

Studies of ionospheric variations during geomagnetic activities at the low-latitude station, Ile-Ife, Nigeria

The dual frequency SCINDA NovAtel GSV 4004B GPS receiver installed at the Ile-Ife (low-latitude station) has been in operation since December 2009. Data records for the year 2010 were processed to obtain Total Electron Content (TEC) and S ₄ index. These were interpreted to analyze the ionospheric condition during low geomagnetic activity period (when Dst is from ?40 to 0 nT) and during geomagnetic storm events (with Dst about ?100 nT). Seasonal variations of the TEC and S ₄ index were also investigated. The occurrence of scintillations is closely linked to the peak value of TEC during the daytime; this is very evident during the equinox months when TEC ≥ 30 TECu. When the maximum TEC value is below 30 TECu, as shown by most of the days in the summer months, the scintillation phenomenon does not occur. During geomagnetic storms, the daytime segment of the TEC plot experiences fluctuations (even bifurcations) in values with the peak TEC value of about 40 TECu. From the interpreted data, the occurrence of geomagnetic storm does not necessarily suggest an increase in the level of scintillations at a low-latitude region. Also, there is a remarkable difference between the IRI 2007 model and the observed TEC values, as the daytime TEC peak differs in magnitude and time of occurrence from the observed TEC.     

电离层不规则结构漂移的GPS测量及其初步结果

本文阐述了利用GPS接收机台阵测量到的闪烁和TEC变化率ROT快速起伏图样估计F层不规则结构漂移的原理和方法,并利用实测数据估计了静日和暴时电离层不规则结构的水平漂移速度.短间距台网和超短间距台链观测实例的计算结果表明,暴时武汉地区引起TEC快速起伏的电离层不规则结构沿纬圈向西漂移,21∶30至03∶00 LT,西向漂移速度在约40 m/s至130 m/s的范围内变化;在桂林地区,磁静日午夜前后引起L波段电波闪烁的电离层不规则结构沿纬圈向东漂移,漂移速度从约70 m/s下降到约55 m/s,磁扰日午夜前不规则结构向西漂移,速度从约150 m/s下降到约50 m/s,午夜后转为向东漂移,速度从约25 m/s上升到约65 m/s.文中还提出了由单站多卫星观测估计F层不规则结构漂移的设想.实例分析与计算结果表明,利用单站多卫星观测估计电离层不规则结构漂移是一种合理可行的方法.     

利用GPS对磁暴期间极区TEC变化与极区碎片（Polar Patches）的研究

利用2004年11月6～10日磁暴发生期间南极区域内的中国中山站GPS常年跟踪站（ZHON）和国际GPS服务站（CAS1, MCM4, SYOG, MAW1）的GPS观测数据,计算了可观测卫星传播路径上的TEC和ROT值,进而依据TEC的波动频率和幅度推估出极区碎片的个数,分析了极区磁暴期间电离层响应及其极区碎片特性. 最终所得TEC和ROT结果与极区地磁场D_st和K_p指数信息相吻合,如实地反映了磁暴事件和极区碎片的出现. 本文所做工作在国内尚未开展,因此所用方法和结论为将来这一方向的研究提供了一定的参考.     

Plasmaspheric electron content (PEC) over low latitude regions around the magnetic equator in the Indian sector during different geophysical conditions

The variation of plasmaspheric electron content (PEC) is an important parameter for studying the effects of space weather events in the low latitude ionosphere. In the present study, the vertical TEC (VTEC) measurements obtained from co-located dual-frequency Global Positioning System (GPS) and Coherent Radio Beacon Experiment (CRABEX) systems have been used. The daytime PEC variations under different geophysical conditions have been estimated (around the magnetic equator) over the Indian sector, for the first time. The first observations of the nighttime PEC variations over the Indian sector are also estimated from the simultaneous measurements of Faraday rotation, differential Doppler and modulation phase delay made using the CRABEX system on-board the Indian geostationary satellite GSAT2. The study shows that the PEC varies over a range of 10–22% (of the total electron content (TEC)) during daytime of magnetically quiet period. There is an increase in PEC with latitude during magnetically quiet period. During a magnetically disturbed period of 9 November 2004, the PEC increased to ∼30% of the TEC over the magnetic equatorial location of Trivandrum (8.5°N, 76.9°E, dip 0.5°N), while at Bangalore (13°N, 78°E, dip 10°N) it showed a large depletion. The implications of the new observations are discussed.     

暴时低纬电离层不规则体响应特征的多手段观测

2010年10月11日发生了一次中等强度的磁暴.本文利用三亚(18.4°N,109.6°E)数字测高仪、VHF雷达和GPS TEC/闪烁监测仪数据以及120°E子午线附近我国漠河(53.5°N,122.4°E)、北京(40.3°N,116.2°E)和武汉(30.6°N,114.4°E)的数字测高仪和GPS TEC/闪烁监测仪数据,分析了磁暴期间我国中低纬地区电离层不规则体的响应特征.结果表明:这次磁暴触发了10月11日午夜前后两个时段低纬(三亚)电离层不规则体事件,而在较高的纬度地区(武汉及以北),并没有观测到电离层不规则体与闪烁.在午夜前,电离层不规则体的发生受磁暴主相期间快速穿透电场激发;在午夜后,电离层不规则体受磁暴恢复相的扰动发电机电场触发,该时段伴随行星际磁场北向翻转的过屏蔽穿透电场也可能是扰动源之一.此外,磁暴期间不同尺度的电离层不规则体会伴随发生.     

Onset location of scintillation-producing spread-F plume over Sanya

Equatorial spread-F (ESF) backscatter plumes are often observed in radar range-time-intensity (RTI) maps at low latitude. Except case studies, few statistical investigations on the onset locations of scintillation-producing ESF plumes at given sites have been conducted. In this study, a statistical analysis is carried out on onset locations of ESF backscatter plumes observed at a low-latitude location Sanya (18.4°N, 109.6°E; dip lat 12.8°N) during equinoctial months of 2013. By employing a tracing method to locate backscatter plumes, we estimate the onset longitudes of periodic plumes obtained from the Sanya VHF radar five-beam steering measurements. The results show that the inter-plume distances (in longitude) are mostly confined within 200–600 km, and the ESF plumes producing ionospheric scintillations over Sanya are almost exclusively generated at the longitudes of 94°–110°E. The results indicate the necessity to monitor ESF plume initial generation in the longitude region of 94°–110°E to better understand the day-to-day variability in the occurrence of ionospheric scintillations over Sanya.     

Climatology of total electron content near the dip equator under geomagnetic quiet-conditions

This study analyzes the TEC data during 1998–2007, observed by the AREQ (16.5°S, 71.5°W) GPS station to investigate the equatorial ionospheric variations under geomagnetic quiet-conditions. The diurnal TEC values generally have a maximum value between 1330 and 1500 LT and a minimum around 0500 LT. For the seasonal variation, the semi-annual variation apparently exists in the daytime TEC with two peaks in equinoctial months. In contrast, this semi-annual variation is not found in the nighttime. Furthermore, the results of the annual variation show that the correlation between the daytime TEC value and the solar activity factor is highly positive.     

Characterization of low latitude GPS-TEC during very low solar activity phase

We present the mean diurnal, seasonal and annual variations in TEC during the lowest solar activity phase from low latitude Indian zone recorded at Udaipur (Geog. Lat. 24.6°N, Geog. Long.73.7°E, Geomag. Lat. 15.6°N) using a GPS receiver. Seasonal variations in daytime TEC show a semiannual periodicity, with a minimum in winter. Results of seasonal variations have been compared with that of the IRI-2007 model. Model calculations reveal significant seasonal as well as longitudinal differences in TEC. Seasonal variations in the nighttime TEC reveal an annual periodicity. Near the crest of the EIA, TEC shows a very good correlation with the solar flux. The results also point to weakening of the anomaly crest as well as its spatial and temporal contraction with declining solar activity.     

The structure of the mid- and high-latitude ionosphere during the November 2004 storm event obtained from GPS observations

GPS data from the International GNSS Service (IGS) network were used to study the development of the severe geomagnetic storm of November 7–12, 2004, in the total electron content (TEC) on a global scale. The TEC maps were produced for analyzing the storm. For producing the maps over European and North American sectors, GPS measurements from more than 100 stations were used. The dense network of GPS stations provided TEC measurements with a high temporal and spatial resolution. To present the temporal and spatial variation of TEC during the storm, differential TEC maps relative to a quiet day (November 6, 2004) were created. The features of geomagnetic storm attributed to the complex development of ionospheric storm depend on latitude, longitude and local time. The positive, as well as negative effects were detected in TEC variations as a consequence of the evolution of the geomagnetic storm. The maximal effect was registered in the subauroral/auroral ionosphere during substorm activity in the evening and night period. The latitudinal profiles obtained from TEC maps for Europe gave rise to the storm-time dynamic of the ionospheric trough, which was detected on November 7 and 9 at latitudes below 50°N. In the report, features of the response of TEC to the storm for European and North American sectors are analyzed.     

低纬电离层TEC起伏与相位闪烁的统计特征

穿过电离层不规则体传播后的无线电波,其振幅和相位出现快速随机起伏,即电离层闪烁.为了量化电离层不规则体和相位闪烁的强度,本文提出用TEC起伏δTEC作为特征参量,并用δTEC的标准差构建一种新指数σtec.文中证明指数σtec与相位闪烁指数完全等效.在电离层强闪烁期间,经常出现信号短暂失锁和周跳,导致TEC值突跳和不连续.为此,本文设计了一种周跳检测与校正的批处理算法,用于消除TEC值突跳.在此基础上,利用位于我国中南部电离层闪烁监测台网2012—2015年的观测数据,考察了GPS信号相位闪烁和不规则体的统计特征.结果表明,我国低纬电离层不规则体和相位闪烁与振幅闪烁随地方时和月份变化的特征类似,一天之中主要出现在日落后至黎明前,一年之中,春季不规则体出现最频繁、秋季次之,呈现春秋不对称性,冬夏季出现很少.此外,我们还比较分析了指数S4与σtec的联系,两者之间显著正相关表明,小于第一菲涅尔带尺度的小尺度不规则体和大于第一菲涅尔带尺度的大尺度不规则体一般同时存在.     

Variability of total electron content near the crest of the equatorial anomaly during moderate geomagnetic storms

The ionospheric responses to a large number (116) of moderate (?50≥D_st>?100 nT) geomagnetic storms distributed over the period (1980–1990) are investigated using total electron content (TEC) data recorded at Calcutta (88.38°E, 22.58°N geographic, dip: 32°N). TEC perturbations exhibit a prominent dependence on the local times of main phase occurrence (MPO). The storms with MPO during daytime hours are more effective in producing larger deviations and smaller time delays for maximum positive deviations compared to those with nighttime MPO. Though the perturbations in the equinoctial and winter solstitial months more or less follow the reported climatology, remarkable deviations are detected for the summer solstitial storms. Depending on the local times of MPO, the sunrise enhancement in TEC is greatly perturbed. The TEC variability patterns are interpreted in terms of the storm time modifications of equatorial electric field, wind system and neutral composition.     

Ionospheric Scintillation and Dynamics of Fresnel-Scale Irregularities in the Inner Region of the Equatorial Ionization Anomaly

This paper reports differences in the occurrence statistics of global positioning system (GPS) L-band scintillations at observational sites located in the inner regions of the northern and southern crests of the equatorial ionization anomaly. Ground-based GPS data acquired at the closed magnetically aligned stations of Manaus (3.1°S; 59.9°W; dip lat. 6.2°N) and Cuiabá (15.5°S; 56.1°W; dip. lat. 6.2°S), Brazil, from December 2001 to February 2007 are used in the analysis. The drift dynamics of Fresnel-scale ionospheric irregularities at the southern station of Cuiabá are also investigated. Only geomagnetically quiet days with the sum of daily Kp < 24 were used in the analysis statistics and in the irregularity drift studies. The results reveal a clear dependence of the scintillation occurrence with the solar activity, but there exists an asymmetry in the percentage of scintillation occurrence between the two stations throughout the period analyzed. The nocturnal occurrence of the scintillations over Cuiabá is predominantly larger than over Manaus, but this scenario seems to change with the decline in the solar activity (mainly during local post-midnight hours). A broad minimum and maximum in the scintillation occurrence appears to occur over both the stations, respectively, during the June solstice (winter) and December solstice (summer) months. The dynamics of the Fresnel-scale irregularities, as investigated from the estimations of the mean zonal drift velocities, reveals that the amplitude of the eastward drifts tends to reduce with the decline in the solar activity. The magnitude of the zonal drift velocities during the December solstice months is larger than during the equinoxes, with the differences being more pronounced at solar maximum years. Other relevant aspects of the observations, with complementary data from a low-latitude ionospheric model, are highlighted and discussed.     

GPS L1-Frequency Observations of Equatorial Scintillations and Irregularity Zonal Velocities

In this work, the climatology of ionospheric scintillations at global positioning system (GPS) L-band frequency and the zonal drift velocities of scintillation-producing irregularities were depicted for the equatorial observatory of São Luis (2.33°S; 44.21°W; dip latitude 1.3°S), Brazil. This is the first time that the hourly, monthly, and seasonal variations of scintillations and irregularity zonal drifts at São Luis were characterized during periods of different solar activity levels (from December 1998 to February 2007). The percentage occurrence of scintillations at different sectors of the sky was also investigated, and the results revealed that the scintillations are more probable to be observed in the west sector of the sky above São Luis, whereas the north–south asymmetries are possibly related to asymmetries in the plasma density distribution at off-equatorial latitudes. The scintillations on GPS signals occurred more frequently around solar maximum years, but it is also clear from the results of a strong variability in the scintillation activity in the years with moderate solar flux during the descending phase of the solar cycle. The equatorial scintillations occur predominantly during pre-midnight hours with a broad maximum near the December solstice months. In general, weak level of scintillations (S ₄ index between 0.2 and 0.4) dominated at all seasons; however, during the winter months around solar maximum years (although the scintillation occurrence is extremely low), stronger levels of scintillations (S ₄ > 0.6) may occur at comparable rate with the weak scintillations. The irregularity zonal velocities, as estimated from the GPS spaced-receiver technique, presented a different scenario for the two seasons analyzed; during the equinoxes, the magnitude of the zonal velocities appeared not to change with the solar activity, whereas during the December solstice months, the larger magnitudes were observed around solar maximum years. Other relevant aspects of the observations are highlighted and discussed.     

Variations in the total electron content during the powerful typhoon of August 5–11, 2006, near the southeastern coast of China

The variations in the total electron content (TEC), obtained from the data of 11 ground-based GPS stations in the region (5°S–80°N; 110–160°E) in the period August 2–15, 2006, have been analyzed in order to search for possible ionospheric manifestations of the SAOMAI powerful typhoon (August 5–11, 2006) near the south-eastern coast of China. The global TEC maps (GIM) have also been used. In the region of the typhoon action during the magnetic storm of August 7, 2006, an intensification of the TEC variations in the evening local time within the 32–128 min periods range was detected. However, this effect was most probably caused by the dynamics of the irregular structure of the equatorial anomaly and by the disturbed geomagnetic situation (Kp ～ 3–6, Dst varied from ?74 to ?153 nT). The analysis of the diurnal variations in the absolute values of TEC and TEC variations with periods of 2–25 min did not reveal a substantial increase in the intensity and changes in the spectrum of the TEC variations in the period of typhoon action as compared to the adjacent days. Thus, we failed to detect ionospheric disturbances unambiguously related to the SAOMAI typhoon.