120°E赤道电离异常区电子浓度总含量分析与预测

基于国际全球定位服务中心(International GPS Service,IGS)提供的120°E 上空1999-2009年IONEX格式电离层电子浓度总含量(TEC)资料,分析赤道电离异常驼峰区TEC峰值Inc和Isc的年变化和季节变化以及与太阳、地磁活动的相关性.在11年时间尺度上,Inc和Isc与太阳辐射P指数的日均值有较好的相关性(r=0.90和r=0.84),而与地磁活动指数Dst、Kp和Ap日均值的相关性均不好.驼峰区TEC峰值Inc和Isc都是在北半球春、秋季出现极大值,而且冬季值大于夏季值,即Inc呈现"半年异常"和"冬季异常"现象,我们认为Inc和Isc相似的半年变化特征是与赤道上空电离层电急流相关的东向电场半年变化导致的.利用支持向量回归方法构建了EIA指数的预报模型,预报试验结果表明,该预报模型能较准确地描述Inc和Isc的变化,对南北驼峰TEC峰值预报的平均相对误差分别为22.96%和10.2%.基于支持向量机回归的预测方法为赤道电离异常特征指数预报的实现提供一条有效方法途径和好的应用前景.     

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.     

Harmonic analysis and an empirical model for TEC over Palehua

An empirical model of total electron content (TEC) for a low-latitude station, Palehua, has been developed using harmonic analysis of TEC data measured at this station during the period 1980–1990; the TEC data were obtained from Faraday rotation measurements of linearly polarised signals transmitted by geostationary satellites. The analysis reveals that monthly mean values of the daily mean and the first four harmonics vary in phase with solar activity and exhibit annual, semi-annual variations and equinoctial asymmetries. A set of 81 coefficients of zero and the first four orders were determined which were found to be sufficient to model the TEC. The model strongly depends on the sunspot number. The harmonic components derived from the 81 coefficients are scaled by this property. The modelled monthly mean TEC values agree quantitatively with the measured data, the maximum deviation being limited to ±15%. The model reasonably reproduces the features observed in the diurnal, seasonal and solar cycle variations of the measured data. The annual variation of observed TEC exhibits opposite equinoctial asymmetries at solar minimum and solar maximum. Also, the mean and first four harmonics show a saturation/decreasing effect when the sunspot number exceeds about 170. The observed features are discussed qualitatively.     

Comparison between IRI predictions and digital ionosonde measurements of hmF2 at New Delhi during low and moderate solar activity

This paper deals with the diurnal and seasonal variations of height of the peak electron density of the F2-layer (hmF2) derived from digital ionosonde measurements at a low–middle-latitude station, New Delhi (28.6°N, 77.2°E, dip 42.4°N). Diurnal and seasonal variations of hmF2 are examined and comparisons of the observations are made with the predictions of the International Reference Ionosphere (IRI-2001) model. Our study shows that during both the moderate and low solar activity periods, the diurnal pattern of median hmF2 reveals a more or less similar trend during all the seasons with pre-sunrise and daytime peaks during winter and equinox except during summer, where the pre-sunrise peak is absent. Comparison of observed median hmF2 values with the IRI during moderate and low solar activity periods, in general, reveals an IRI overestimation in hmF2 during all the seasons for local times from about 06 LT till midnight hours except during summer for low solar activity, while outside this time period, the observed hmF2 values are close to the IRI predictions. The hmF2 representation in the IRI model does not reproduce pre-sunrise peaks occurring at about 05 LT during winter and equinox as seen in the observations during both the solar activity periods. The noontime observed median hmF2 values increase by about 10–25% from low (2004–2005) to high solar activity (2001–2002) during winter and equinox, while the IRI in the same time period and seasons shows an increase of about 10–20%. During summer, however, the observed noontime median hmF2 values show a little increase with the solar activity, as compared to the IRI with an increase of about 12%.     

Temporal and spatial variations in periglacial soil movements on alpine crest slopes

This paper describes up to ten years of continuous monitoring of frost heave, creep and associated parameters on high mountain crest slopes in the Japanese and Swiss Alps, aiming to evaluate spatial and interannual variations in the rates and controls of soil movement. Shallow frost creep re?ecting diurnal frost heave activity dominates the crest slopes that lack a vegetation mat and have a thin debris mantle with good drainage. Seasonal frost heave activity can induce slightly deeper movement where ?ne soil exists below the depth reached by diurnal freeze–thaw penetration, although the shallow bedrock impedes movements below 20 cm depth. As a result, downslope velocity pro?les display strong concavity with surface velocities of 2–50 cm a^?1. The frost creep rates vary spatially, depending on the soil texture, slope gradient, frequency of temperature cycling across 0 °C and moisture availability during freeze–thaw periods. Soil movements recur in every freeze–thaw period, although with some interannual variations affected by the length of seasonal snow cover and the occurrence of precipitation during freeze–thaw periods. The Swiss Alps encounter more signi?cant interannual variations than the Japanese Alps, re?ecting the large variability of the annual snow regime. Copyright © 2005 John Wiley & Sons, Ltd.     

A study on chaotic behaviour of equatorial/low latitude ionosphere over Indian subcontinent,using GPS-TEC time series

The deterministic chaotic behaviour of ionosphere, over Indian subcontinent falling under equatorial/low latitude region, ?0.3 to 22.19°N (geomagnetic), was studied using GPS-TEC time series. The values of Lyapunov exponent are low at Thiruvananthapuram and Agatti (?0.30 and 2.38°N, geomagnetic, respectively), and thereafter increase through Bangalore and Hyderabad (4.14 and 8.54°N, geomagnetic, respectively), and attain maximum at Mumbai (10.09°N, geomagnetic), which is near/at the edge of an anomaly crest. The values of correlation dimension computed for TEC time series are in the range 3.1–3.6, which indicate that equatorial/low latitude ionosphere can be described with four variables. Entropy values estimated for TEC time series show no appreciable latitudinal variabilites. The values of non-linear prediction error exhibit a trough, around the latitude sector, 4.14–16.15°N (Geomagnetic). Based on the values of the above quantifiers, the features of chaotic behaviour of equatorial/low latitude ionosphere are briefly discussed.     

Climatological monthly characteristics of middle atmosphere gravity waves (10 min-10 h) during 1979–1993 at Saskatoon

Saskatoon (52° N, 107°W) medium frequency (MF) radar data from 1979 to 1993 have been analyzed to investigate the climatology of irregular wind components in the height region 60–100 km. This component is usually treated in terms of internal gravity waves (IGW). Three different band-pass filters have been used to separate the intensities of IGWs having periods 0.2-2.5; 1.5-6 and 2–10 h, respectively. Height, seasonal and inter-annual variations of IGW intensities, anisotropy and predominant directions of propagation are investigated. Mean over 14 years’ seasonal variation of the intensity of long-period IGWs shows a dominant annual component with winter maximum and summer minimum. Seasonal variations of the intensity of short-period waves have a strong semi-annual component as well, which forms a secondary maximum in summer. Predominant azimuths of long-period IGWs are generally zonal, though they vary with season. For short-period IGWs, the predominant azimuth is closer to the meridional direction. Anisotropy of IGW intensity is larger in summer, winter and at lower altitudes. The IGW intensity shows apparent correlation with both solar and geomagnetic activity. In most cases, this correlation appears to be negative. The variations versus solar activity is larger for longer-period IGW. Possible reasons and consequences of the observed climatological variations of IGW intensity are discussed.     

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.     

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.     

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.     

Solar and latitude dependence of TEC and SLAB thickness

Several years of the vertical total electron content (TEC) and of the equivalent slab thickness data measured at different European locations are analysed by using the linear regression technique to demonstrate the annual response of these two ionospheric parameters to changes in solar activity and to their dependence on latitude. It is shown that TEC is highly correlated with the latitude and solar activity, whereas the slab thickness is independent of both solar activity and latitude. Their intercepts relative to solar activity show clearly a seasonal dependence.     

Comparison of ionospheric equivalent slab thickness with bottomside digisonde profile over Wuhan

A comparison of the diurnal and seasonal variations in the ionospheric equivalent slab thickness (τ) and bottomside slab thickness (B0) is presented based on the observation during high solar activities at a mid-latitude station—Wuhan (114.4°E, 30.6°N). The investigated data include foF2, hmF2, B0, B1, and TEC, and are derived from the measured ionogram and GPS receiver over Wuhan from April 1999 to March 2000. The results show that τ and B0 are highly/weakly correlated during the day/night, respectively. Furthermore, a comprehensive discussion of the relation between τ, B0, and hmF2 for geomagnetic storm events is provided in this paper.     

The variation of ionospheric slab thickness over equatorial ionization area crest region

The GPS-derived total electron content (TEC) and NmF2 are measured at the Chung-Li ionosonde station (24.9°N, 121°E) in order to study the variations in slab thickness (τ) of the ionosphere at low-latitudes ionosphere during 1996–1999, corresponding to half of the 23rd solar cycle. This study presents the diurnal, seasonal, and solar flux variations in τ for different solar phases. The seasonal variations show that the average daily value is greater during summer and the reverse is true during equinox in the equatorial ionization anomaly (EIA) region. Moreover, the τ values are greater during the daytime (0800–1600 LT) and nighttime (2000–0400 LT) for summer and winter, respectively. The diurnal variation shows two abnormal peaks that appear during the pre-sunrise and post-sunset hours. The peak values decrease as the sunspot number increases particularly for the pre-sunrise peak. Furthermore, the variation in the F-peak height (hpF2) indicates that a thermospheric wind toward the equator leads to an increase in hpF2 and an enhancement in τ during the pre-sunrise period. Furthermore, the study shows the variations of τ values for different geophysical conditions such as the geomagnetic storm and earthquake. A comprehensive discussion about the relation between τ and the geophysical events is provided in the paper.     

Global electron content during solar cycle 23

Global electron content (GEC) as a new ionospheric parameter was first proposed by Afraimovich et al. [2006]. GEC is equal to the total number of electrons in the near-Earth space. GEC better than local parameters reflects the global response to a change in solar activity. It has been indicated that, during solar cycle 23, the GEC dynamics followed similar variations in the solar UV irradiance and F _10.7 index, including the 11-year cycle and 27-day variations. The dynamics of the regional electron content (REC) has been considered for three belts: the equatorial belt and two midlatitude belts in the Northern and Southern hemispheres (±30° and 30°–65° geomagnetic latitudes, respectively). In contrast to GEC, the annual REC component is clearly defined for the northern and southern midlatitude belts; the REC amplitude is comparable with the amplitude of the seasonal variations in the Northern Hemisphere and exceeds this amplitude in the Southern Hemisphere by a factor of ～1.7. The dayside to nightside REC ratio, R(t), at the equator is a factor of 1.5 as low as such a GEC ratio, which indicates that the degree of nighttime ionization is higher, especially during the solar activity maximum. The pronounced annual cycle with the maximal R(t) value near 8.0 for the winter Southern Hemisphere and summer Northern Hemisphere is typical of midlatitudes.     

Anomalous Variations of Ionosphere Associated with the Strong Earthquake at Pakistan-Iran Border at a Low Latitude Station Agra,India

In this paper, we analyze the TEC data for April 2013 observed at Agra station, India (geogr. lat. 27.2° N, long. 78° E) to examine the effect of earthquake of magnitude M = 7.8 which occurred on 16 April 2013 at Pakistan–Iran border region. We process the TEC data using the s statistical criterion to find out anomalous variation in TEC data. We also study the VLF propagation signal from NPM, Hawaii (21.42° N, 158° W), which is monitored at the same station (Agra station) in the light of this earthquake as well as solar flares. The nighttime fluctuation method is used to analyze the VLF data for the period of ±5 days from the day of earthquake (11–21 April 2013). The anomalous enhancements and depletions are found in TEC data on 1–9 days before the occurrence of event.     

THE METHOD OF CONSTRUCTING IONOSPHERIC TEC BACKGROUND FIELD BASED ON SVR MODEL

There are many factors related to the variations of TEC, and the changes of TEC caused by earthquake only occupy a small portion. Therefore, it is vital how to exclude the ionospheric interference of non-seismic factors accurately in the process of seismic ionospheric anomaly extraction. This study constructed a TEC non-seismic dynamic background field considering the influence of solar and geomagnetic activities. Firstly, the TEC components of half-year cycle and annual cycle are extracted by wavelet decomposition. Then, it establishes a regression model between TEC in which periodic factors are removed and solar activity index, geomagnetic activity index with SVR method(support vector regression)in non-seismic period. Finally, based on the constructed model, the solar activity index and geomagnetic activity index is used to reconstruct aperiodic components of TEC in earthquake's period. From the reconstructed aperiodic components of TEC plus the half-year periodic components and annual periodic components of TEC in the same period, the non-seismic dynamic background field is obtained. Comparing the residuals relative to original TEC values in non-seismic dynamic background field and traditional sliding window background, there are apparent monthly periodic change and semi-annual periodic change in the residuals of sliding window background, which can have obvious impacts on the subsequent seismic ionospheric anomaly detection. In order to test the validity of seismic TEC anomaly detection based on the background field construction method, this paper investigated the long time series TEC anomalies near Wenchuan city(30°N, 100°E)from March 1 to September 26 in 2008. It is found that under the condition of non-seismic disturbance such as solar activity and geomagnetic activity, TEC abnormal disturbance is rarely detected by non-seismic dynamic background field method, when compared with the traditional sliding time-window method. And before the earthquake, more TEC anomalies were detected based on the proposed method, also, they were more intense than those extracted by sliding window method. Therefore, the TEC background field construction method based on SVR(support vector regression)has superiorities in both system errors elimination, which are caused by solar, geomagnetism, the non-seismic ionospheric disturbance events and periodic fluctuations of TEC, and in reducing the false alarm rate of seismic TEC anomaly. Moreover, it can also improve the seismic TEC anomaly detection ability. In addition, this paper analyzed the time-spatial distribution of TEC anomaly before three earthquakes on May 12, August 21 and August 30, 2008. They were mainly negative abnormal perturbations and often distributed on the equatorial side of epicenter.     

Long term ionospheric electron content variations over Delhi

Ionospheric electron content (IEC) observed at Delhi (geographic co-ordinates: 28.63°N, 77.22°E; geomagnetic co-ordinates: 19.08°N, 148.91E; dip Latitude 24.8°N), India, for the period 1975/80 and 1986/89 belonging to an ascending phase of solar activity during first halves of solar cycles 21 and 22 respectively have been used to study the diurnal, seasonal, solar and magnetic activity variations. The diurnal variation of seasonal mean of IEC on quiet days shows a secondary peak comparable to the daytime peak in equinox and winter in high solar activity. IEC_max (daytime maximum value of IEC, one per day) shows winter anomaly only during high solar activity at Delhi. Further, IEC_max shows positive correlation with F_10.7 up to about 200 flux units at equinox and 240 units both in winter and summer; for greater F_10.7 values, IEC_max is substantially constant in all the seasons. IECmax and magnetic activity (A_p) are found to be positively correlated in summer in high solar activity. Winter IEC_max shows positive correlation with A_p in low solar activity and negative correlation in high solar activity in both the solar cycles. In equinox IEC_max is independent of A_p in both solar cycles in low solar activity. A study of day-to-day variations in IEC_max shows single day and alternate day abnormalities, semi-annual and annual variations controlled by the equatorial electrojet strength, and 27-day periodicity attributable to the solar rotation.     

利用GPS观测数据和IRI模型对比分析太阳活动低年广州地区电离层TEC变化

电离层TEC是描述电离层特性的一个重要参量,利用GPS观测数据（包括广州站接收的GPS-TEC数据和国际GNSS提供的IGS-TEC数据）与IRI-2007模型计算的TEC预测值对太阳活动低年2008年的广州地区TEC周日和季节变化以及年变化特征等进行了多方面的对比分析。结果表明：TEC观测值白天较高且变化迅速,夜间较低且变化缓慢,同时表现出明显的季节依赖性和半年变化特性,全年在春秋分季节出现两次峰值,IRI-TEC预测值能较好地反映GPS观测值,但局部上也存在着一些偏差,并对其中的物理机制和产生差异的原因给出了合理的分析和解释。     

基于IGS台网观测的2001—2008年赤道电离异常的强度和半球不对称特征

赤道电离异常（Equatorial Ionization Anomaly,EIA）是低纬电离层中的一个重要现象.本文基于IGS台网提供的2001—2008年期间的电离层总电子含量（Total Electron Content,TEC）数据,分析了120°E区的EIA强度和磁南北半球不对称性在磁平静时期的变化特征,包括对地方时、季节和太阳活动的依赖.本文结果表明,（1） EIA强度表现出显著的随地方时和季节的变化特征.EIA强度在0200LT和2000LT附近分别出现一个极值,且2000LT附近的EIA强度更大;EIA强度通常在春/秋季较大,在夏/冬季较小,且冬季要大于夏季.（2） EIA南北半球不对称也表现出随地方时和季节变化特征.EIA半球不对称在0200LT和2000-2200LT附近分别出现一个极值;EIA半球不对称的季节变化特征还依赖于太阳活动,太阳活动高年期间,EIA半球不对称通常在春/秋季更显著;太阳活动低年期间,EIA半球不对称通常在冬季更显著.（3） EIA强度和半球不对称性的逐日变化和月变化表现出对太阳活动存在一定的依赖,但依赖性并不显著.2000LT （0200LT）附近的EIA强度的月变化与太阳活动整体呈正（负）相关,而2200LT （0200LT）附近的EIA半球不对称的月变化与太阳活动整体呈负（负）相关.（4）影响EIA强度变化的主要因素可归于纬圈电场和中性风场;影响EIA半球不对称变化的主要因素可归因为子午中性风场.     

A study of GPS ionospheric scintillations observed at Guilin

The occurrence of strong ionospheric scintillations with S4≥0.2 was studied using global positioning system (GPS) measurements at Guilin (25.29°N, 110.33°E; geomagnetic: 15.04°N, 181.98°E), a station located near the northern crest of equatorial anomaly in China. The results are presented for data collected from January 2007 to December 2008. The results show that amplitude scintillations occurred only during the first five months of the considered years. Nighttime amplitude scintillations, observed mainly in the south of Guilin, always occurred with phase scintillations, total electron content (TEC) depletions, and Rate Of change of TEC (ROT) fluctuations. However, TEC depletions and ROT fluctuations were weak during daytime amplitude scintillations, and daytime amplitude scintillations usually occurred in most of the azimuth directions. GPS scintillation/TEC observations recorded at Guilin and signal-to-noise-ratio 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.