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1.
The paper is based on the ionospheric variations in terms of vertical total electron content (VTEC) for the low solar activity period from May 2007 to April 2009 based on the analysis of dual frequency signals from the Global Positioning System (GPS) satellites recorded at ground stations Varanasi (Geographic latitude 25°16′ N, Longitude 82°59′ E), situated near the equatorial ionization anomaly crest and other two International GNSS Service (IGS) stations Hyderabad (Geographic latitude 17°20′ N, longitude 78°30′ E) and Bangalore (Geographic latitude 12°58′ N, longitude 77°33′ E) in India. We describe the diurnal and seasonal variations of total electron content (TEC), and the effects of a space weather related event i.e. a geomagnetic storm on TEC. The mean diurnal variation during different seasons is brought out. It is found that TEC at all the three stations is maximum during equinoctial months (March, April, September and October), and minimum during the winter months (November, December, January and February), while obtaining intermediate values during summer months (May, June, July and August). TEC shows a semi-annual variation. TEC variation during geomagnetic quiet as well as disturbed days of each month and hence for each season from May 2007 to April 2008 at Varanasi is examined and is found to be more during disturbed period compared to that in the quiet period. Monthly, seasonal and annual variability of GPS-TEC has been compared with those derived from International Reference Ionosphere (IRI)-2007 with three different options of topside electron density, NeQuick, IRI01-corr and IRI 2001. A good agreement is found between the GPS-TEC and IRI model TEC at all the three stations.  相似文献   

2.
The present study examines the ionospheric Total Electron Content (TEC) variations in the lower mid-latitude Turkish region from the Turkish permanent GNSS network (TPGN) and International GNSS Services (IGS) observations during the years 2009 to 2017. The corresponding vertical TEC (VTEC) predicted by Kriging and NeQuick-2 models are evaluated to realize their efficacy over the country. We studied the diurnal, seasonal and spatial pattern of VTEC variation and tried to estimate by a new mathematical model using the long term of 9 years VTEC data. The diurnal variation of VTEC demonstrates a normal trend with its gradual enhancement from dawn to attain a peak around 09:00–14.00 UT and reaching the minimum level after 22.00 UT. The seasonal behavior of VTEC indicates a strong semi-annual variation of VTEC with maxima in September equinox followed by March equinox and minima in June solstice followed by December solstice. Also, the spatial variation in VTEC depicts a meaningful longitudinal/latitudinal pattern altering with seasons. It decreases longitudinally from the west to the east during March equinox and June solstice increases with latitude. The comparative analysis among the GNSS-VTEC, Kriging, NeQuick and the proposed mathematical model are evaluated with the help one way ANOVA test. The analysis shows that the null hypothesis of the models during storm and quiet days are accepted and suggesting that all models are statistically significantly equivalent from each other. We believe the outcomes from this study would complement towards a relatively better understanding of the lower mid-latitude VTEC variation over the Turkish region and analogous latitudes over the globe.  相似文献   

3.
The Total Electron Content (TEC) prediction performances of the empirical IRI model and IRI-PLAS model were investigated by comparing the GPS-based TEC values provided by the IONOLAB group. TEC values were obtained on equinox (March 21 and September 23) and solstice (June 21, and December 21) days in low (2009), medium (2012) and high (2015) solar activity periods at Istanbul, Turkey. The prediction performances of the models were statistically analyzed based on the differences between the GPS-TEC and the empirical models, considering the maximum and minimum deviations, the correlation analysis and the root mean square error (RMSE). As a result of the investigation, it is seen that the empirical models have similar predictive performances when the plasmaspheric effects are neglected, and the IRI-PLAS estimations are generally a little closer to the observed GPS-TEC values than all options of IRI-2016 model. Also, it can be said that “IRI2001”, one of the IRI-2016’s “topside” options, can make better predictions than other options and “IG” solar proxy option of IRI-PLAS model is a more appropriate option than the others in TEC calculations over Istanbul, Turkey.  相似文献   

4.
A time-dependent one-dimensional model of Saturn's ionosphere has been developed as an intermediate step towards a fully coupled Saturn Thermosphere-Ionosphere Model (STIM). A global circulation model (GCM) of the thermosphere provides the latitude and local time dependent neutral atmosphere, from which a globally varying ionosphere is calculated. Four ion species are used (H+, H+2, H+3, and He+) with current cross-sections and reaction rates, and the SOLAR2000 model for the Sun's irradiance. Occultation data from the Voyager photopolarimeter system (PPS) are adapted to model the radial profile of the ultraviolet (UV) optical depth of the rings. Diurnal electron density peak values and heights are generated for all latitudes and two seasons under solar minimum and solar maximum conditions, both with and without shadowing from the rings. Saturn's lower ionosphere is shown to be in photochemical equilibrium, whereas diffusive processes are important in the topside. In agreement with previous 1-D models, the ionosphere is dominated by H+ and H+3, with a peak electron density of ∼104 electrons cm−3. At low- and mid-latitudes, H+ is the dominant ion, and the electron density exhibits a diurnal maximum during the mid-afternoon. At higher latitudes and shadowed latitudes (smaller ionizing fluxes), the diurnal maximum retreats towards noon, and the ratio of [H+]/[H+3] decreases, with H+3 becoming the dominant ion at altitudes near the peak (∼1200-1600 km) for noon-time hours. Shadowing from the rings leads to attenuation of solar flux, the magnitude and latitudinal structure of which is seasonal. During solstice, the season for the Cassini spacecraft's encounter with Saturn, attenuation has a maximum of two orders of magnitude, causing a reduction in modeled peak electron densities and total electron column contents by as much as a factor of three. Calculations are performed that explore the parameter space for charge-exchange reactions of H+ with vibrationally excited H2, and for different influxes of H2O, resulting in a maximum diurnal variation in electron density much weaker than the diurnal variations inferred from Voyager's Saturn Electrostatic Discharge (SED) measurements. Peak values of height-integrated Pedersen conductivities at high latitudes during solar maximum are modeled to be ∼42 mho in the summer hemisphere during solstice and ∼18 mho during equinox, indicating that even without ionization produced by auroral processes, magnetosphere-ionosphere coupling can be highly variable.  相似文献   

5.
On 11 April 2012, a strong earthquake of magnitude Ms8.6 occurred near the west coast of Northern Sumatra, Indonesia. In this paper, we investigated the morphological characteristics of anomalous variations in Global Positioning System Total Electron Content (GPS TEC) prior to the earthquake by the method of the statistical analysis. It was found the TEC anomaly was firstly decreased, then, it became more enhanced, finally, it decreased, the peak of anomaly enhancement arose from 13:00–17:00 LT on April 5 lasted for ～4 hours and the anomalous ionospheric regions extended to ～40° in longitude and ～20° in latitude, its location did not coincide with the vertical projection of the epicenter, but lies at the north and south of the geomagnetic equator, meanwhile, corresponding ionospheric anomalies are also observed in the magneto conjugate region. Potential causes of these results are discussed,eliminating the ionospheric anomalies that may be caused by solar activities and magnetic storms, it can be concluded that the observed obvious anomalous variation in GPS TEC on April 5 were possibly related to the earthquake.  相似文献   

6.
Autospectra in the 2–13 month range, computed from mean monthly horizontal intensity on quiet days at Trivandrum, situated close to the dip equator, suggest an exceedingly large semi-annual modulation of the field confined to an interval of about 5 hr centred at 1000 LT. The amplitude of the semi-annual oscillation at this station, derived from power density, is greater than 19 γ at 1000 LT. Between 1900 and 0500 LT, spectral lines, corresponding to a period of six months, are not observed above the continuum. Spectral densities from observations at two other electrojet stations in India, Kodaikanal and Annamalainagar, and at Alibag, outside the electrojet, establish the existence of an appreciable enhancement of the semi-annual oscillation of the field in the equatorial electrojet belt. Similar computations of spectra using observations on all days, however, suggest a secondary component in the evening sector. This component is not enhanced in the equatorial electrojet belt. It is concluded that while in low latitudes the daytime component is largely associated with the modulation of Sq currents, in the electrojet belt it appears to be due entirely to a semi-annual modulation of the equatorial electrojet. It is also concluded that the secondary component, observed in the evening sector in low latitude and equatorial stations, is associated purely with the modulation of the ring current by disturbance. The two components of the semi-annual variation observed at the Indian stations have also been noticed at several stations between geomagnetic latitudes N54.6° and S41.8°. It is also observed that the association of the semi-annual component with geomagnetic latitude is confined to the evening-night component.  相似文献   

7.
The position of the mid-latitude trough in the topside ionosphere is determined from electron density data of ESRO 4 during quiet solar conditions. An analysis of about 300 trough observations in the northern and southern hemispheres showed that the trough was generally seen only during the night from 19 to 05 hr LT. During this period, the invariant latitude of the minimum of the trough was found to decrease both with increasing geomagnetic disturbance represented by Kp and with increasing LT after sunset.  相似文献   

8.
Measurements of electron content (NT) near the crest of the equatorial ionosphere anomaly in South America have been made and analysed to investigate NT variations with solar hour, solar rotation and geomagnetic storms. The annual mean of diurnal ratio, defined as the ratio of the maximum to the minimum electron content of the day is found to be 5.0. Anomalous increases in night time electron content are observed with maxima around 2100 LMT and 2300 LMT during summer and equinoctial months. These increases are found to be linked with vertical motion of the F-layer. Spatial resonance in equatorial F-layer plasma appears to be the possible cause of these increases.  相似文献   

9.
The present paper analyzes the dual frequency signals from GPS satellites recorded at Varanasi (Geographic latitude 25°, 16′ N, longitude 82°, 59′ E) near the equatorial ionization anomaly (EIA) crest in India, to study the effect of geomagnetic storm on the variation of TEC, during the low solar active period of May 2007 to April 2008. Three most intense—but still moderate class—storms having a rapid decrease of Dst-index observed during the GPS recorded data have been analyzed, which occurred on 20 November 2007, 9 March 2008 and 11 October 2008 were selected and storm induced features in the vertical TEC (VTEC) have been studied considering the mean VTEC value of quiet days as reference level. The possible reasons for storm time effects on VTEC have been discussed in terms of local time dependence, storm wind effect as well as dawn-dusk component of interplanetary electric field (IEF) Ey intensity dependence.  相似文献   

10.
The comparison of the IRI model with TEC estimated using the continuous GPS observations of European part of IGS network is considered. The variability of a measured TEC was analyzed for the low solar activity in the period 1996–1997. We used IRI 90 version with upped height limit 2000 km and IRI 95 up to 1000 km. It is shown that the difference between model and measured TEC depends on latitude and season.Generally, IRI 95 underestimate and IRI 90 overestimate the TEC relative to GPS measurements. The relative difference may reach over 50%.  相似文献   

11.
The seasonal variation of Titan's atmospheric structure with emphasis on the stratosphere is simulated by a three-dimensional general circulation model. The model includes the transport of haze particles by the circulation. The likely pattern of meridional circulation is reconstructed by a comparison of simulated and observed haze and temperature distribution. The GCM produces a weak zonal circulation with a small latitudinal temperature gradient, in conflict with observation. The direct reason is found to be the excessive meridional circulation. Under uniformly distributed opacity sources, the model predicts a pair of symmetric Hadley cells near the equinox and a single global cell with the rising branch in the summer hemisphere below about z = 230 km and a thermally indirect cell above the direct cell near the solstice. The interhemispheric circulation transports haze particles from the summer to the winter hemisphere, causing a maximum haze opacity contrast near the solstice and a smaller contrast near the equinox, contrary to observation. On the other, if the GCM is run under modified cooling rate in order to account for the enhancement in nitrites and some hydrocarbons in the northern hemisphere near the vernal equinox, the meridional cell at the equinox becomes a single cell with rising motions in the autumn hemisphere. A more realistic haze opacity distribution can be reproduced at the equinox. However, a pure transport effect (without particle growth by microphysics, etc.) would not be able to cause the observed discontinuity of the global haze opacity distribution at any location. The stratospheric temperature asymmetry can be explained by a combination of asymmetric radiative heating rates and adiabatic heating due to vertical motion within the thermally indirect cell. A seasonal variation of haze particle number density is unlikely to be responsible for this asymmetry. It is likely that a thermally indirect cell covers the upper portion of the main haze layer. An artificial damping of the meridional circulation enables the formation of high-latitude jets in the upper stratosphere and weaker equatorial superrotation. The latitudinal temperature distribution in the stratosphere is better reproduced.  相似文献   

12.
The effects of F-region neutral winds on the distribution of He+ in the equatorial ionosphere have been examined using a theoretical model and an observational data set. It is shown by the model that components of neutral wind in the magnetic meridian up to only 50 m s? can produce He+ gradients in the northern and southern sectors of a flux tube that differ by more than 80%. This is associated with interhemisphere transport velocities of He+ as large as 15 m s?1 at 800 km. A substantial latitude gradient in the He+ distribution across the dip equator also results from the redistribution of He+ The changes in the He+ concentration at the dip equator and the latitude distribution of He+ in response to different neutral wind components is determined from the model and used to construct longitude distributions of He+ to compare with observations made at equinox. Good agreement between the calculations and observations is obtained both at the geographic and geomagnetic equators using the relationship between neutral winds, interhemispheric transport velocity and He+ concentration derived from the model. If these relationships can be extrapolated to accommodate the different conditions expected during solstice, we can also discuss the He+ distributions expected during this season.  相似文献   

13.
A three-dimensional, semi-empirical dynamic model of the neutral thermosphere is used to examine the effect of the displaced geomagnetic and geographic poles on the daily variation of neutral gas motion. The global-scale pressure distribution to drive the neutral gas motion is derived from the empirical model of Jacchia (1965). The ionization distribution is obtained from the Pennsylvania State M.K 1 model ionosphere using the first few longitudinal Fourier coefficients of the ionization distribution. The calculations were made at various latitudes at equinox and solstice and for various values of solar activity. The results show that the calculated neutral winds for the case where the geomagnetic and geographic poles are coincident differ at most only a few per cent from the winds calculated assuming the poles displaced. With the poles coincident, longitude and local time are interchangeable, and one dimension in any dynamic model of the thermosphere may be eliminated.  相似文献   

14.
Singular Value Decomposition (SVD) model is implemented to recognize the Total Electron Content (TEC) time series of daily, temporal as well as seasonal characteristics throughout the 24th solar cycle period of the year 2015 in the study. The Vertical (vTEC) analysis has been carried out with Global Positioning System (GPS) data sets collected from five stations from India namely GNT, Guntur (16.44° N, 80.62° E), and IISC, Bangalore (12.97° N, 77.59° E), LCK2, Lucknow (26.76° N, 80.88° E), one station from Thailand namely AITB, Bangkok (14.07° N, 100.61° E), and one station from South Andaman Island namely PBR, Port Blair (11.43° N, 92.43° E), located in low latitude region. The first five singular value modes constitute about 98% of the total variance, which are linearly transformed from the observed TEC data sets. So it is viable to decrease the number of modeling parameters. The Fourier Series Analysis (FSA) is carried out to characterize the solar-cycle, annual and semi-annual dependences through modulating the first three singular values by the solar (F10.7) and geomagnetic (Ap) indices. The positive correlation coefficient (0.75) of daily averaged GPS–TEC with daily averaged F10.7 strongly supports the temporal variations of the ionospheric features depends on the solar activity. Further, the significance and reliability of the SVD model is evaluated by comparing it with GPS–TEC data and the standard global model (Standard Plasma-Spherical Ionospheric Model, SPIM and International Reference Ionosphere, IRI 2016).  相似文献   

15.
A general circulation model is used to evaluate changes to the circulation and dust transport in the martian atmosphere for a range of past orbital conditions. A dust transport scheme, including parameterized dust lifting, is incorporated within the model to enable passive or radiatively active dust transport. The focus is on changes which relate to surface features, as these may potentially be verified by observations. Obliquity variations have the largest impact, as they affect the latitudinal distribution of solar heating. At low obliquities permanent CO2 ice caps form at both poles, lowering mean surface pressures. At higher obliquities, solar insolation peaks at higher summer latitudes near solstice, producing a stronger, broader meridional circulation and a larger seasonal CO2 ice cap in winter. Near-surface winds associated with the main meridional circulation intensify and extend polewards, with changes in cap edge position also affecting the flow. Hence the model predicts significant changes in surface wind directions as well as magnitudes. Dust lifting by wind stress increases with obliquity as the meridional circulation and associated near-surface winds strengthen. If active dust transport is used, then lifting rates increase further in response to the larger atmospheric dust opacities (hence circulation) produced. Dust lifting by dust devils increases more gradually with obliquity, having a weaker link to the meridional circulation. The primary effect of varying eccentricity is to change the impact of varying the areocentric longitude of perihelion, l, which determines when the solar forcing is strongest. The atmospheric circulation is stronger when l aligns with solstice rather than equinox, and there is also a bias from the martian topography, resulting in the strongest circulations when perihelion is at northern winter solstice. Net dust accumulation depends on both lifting and deposition. Dust which has been well mixed within the atmosphere is deposited preferentially over high topography. For wind stress lifting, the combination produces peak net removal within western boundary currents and southern midlatitude bands, and net accumulation concentrated in Arabia and Tharsis. In active dust transport experiments, dust is also scoured from northern midlatitudes during winter, further confining peak accumulation to equatorial regions. As obliquity increases, polar accumulation rates increase for wind stress lifting and are largest for high eccentricities when perihelion occurs during northern winter. For dust devil lifting, polar accumulation rates increase (though less rapidly) with obliquity above o=25°, but increase with decreasing obliquity below this, thus polar dust accumulation at low obliquities may be increasingly due to dust lifted by dust devils. For all cases discussed, the pole receiving most dust shifts from north to south as obliquity is increased.  相似文献   

16.
With the increased number of low Earth orbit (LEO) satellites equipped with Global Positioning System (GPS) receiver, the LEO based GPS slant total electron content (STEC) data play a more important role in ionospheric research due to better global coverage. The accuracy of LEO TEC is hardly evaluated by comparison with the independent TEC measurement simultaneously. We propose an approach based on the simulated data to verify the accuracy of TEC determination. The simulated data (i.e., the pseudorange and carrier phase observations) was generated based on the consideration of the effect of the ionosphere, the so-called differential code bias (DCB) and observational noise. The errors of carrier phase to code leveling process and DCB estimation are analyzed quantitatively. Also, the effect of observational noise, solar activity and LEO orbit altitude on the accuracy of TEC determination will be discussed in detail. The accuracy of TEC determination is relative to solar activity and LEO orbit altitude, the higher LEO orbit and lower F10.7 index, the higher accuracy of TEC determination. It is found by the first time that, with the amplification of the pseudorange noise, the accuracy of leveling process and TEC determination declines almost linearly. With the LEO missions in the near future, it is hoped that the GPS satellite DCBs estimated based on LEO observations would be better than those based on ground-based observations.  相似文献   

17.
Ground-based spectrophotometric measurements of night airglow OH (8-3) band absolute intensities in the polar cap region (78.4°N) during winter solstice are reported. A mean value of 425 ± 40 R is found for the absolute intensity of the OH (8-3) band. Maximum and minimum daily mean values were 770 and 320 R respectively with hourly mean values ranging from 180 to 1020 R. Neither a winter solstice minimum or maximum in the intensity is obvious from the data. No consistent correlation was found between the absolute intensity and geomagnetic and solar activity. A mean transport of O and O3 into the polar cap region corresponding to a meridional wind speed of at least 20 m s?1 at 90 km height seems necessary to maintain the observed intensity. A dominant semidiurnal tide component is found in the intensity data, both on a 20-day and a 3-day time scale.  相似文献   

18.
It is the purpose of this paper to study whether the non-ducted propagation in the inner plasmasphere in the presence of the equatorial anomaly might be relevant to daytime whistlers observed on the ground at low latitudes. Realistic models of the equatorial anomaly simulating the satellite observations have been incorporated in the ray tracing computations. It is found that there are two different non-ducted modes able to penetrate through the ionosphere onto the ground; (1) whispering gallery mode around the anomaly field line which is trapped just by the outer boundary of the anomaly, and (2) pro-longitudinal (PL) mode at a latitude around 30° which is supported by the horizontal gradient in the tail of the anomaly. These modes may provide a new interpretation for some whistlers observed on the ground. The properties of these modes are examined in detail and then compared with those of ducted propagation. This study may be useful for distinguishing the propagation mode in future ground-based experiments.  相似文献   

19.
Continuous measurements of the ionospheric total electron content (TEC) provide a parameter well suited for the study of F-region disturbance effects. In this investigation, five cases of large and rapid drops in TEC observed near the sunset period are interpreted as being due to the contraction of the plasmasphere to L values less than 3. A model calculation is performed for the specific case of 1 November 1968 using simultaneous Alouette I data to define the position and magnitude of the ionospheric trough. The results indicate that the motion of a deep trough across the ray path from a geostationary satellite to an observing station can cause drastic changes in the measured amounts of Faraday rotation and therefore in the derived values of TEC. All of the TEC data sources available for the five events are then examined in an attempt to describe more completely the latitude dependence of the effects. It is suggested that during severe geomagnetic storms, the large and rapid decays in TEC during the 18–21 LT period to values significantly below normal can be used as a criterion to determine the approximate latitudinal extent of the contracted plasmasphere.  相似文献   

20.
利用GPS双频观测量可获取电离层总电子含量(Total Electron Content,TEC),以监测区域上空电离层的分布和变化特征,从而可以发现不同尺度的电离层异常。该文采用2004年上海地区GPS综合应用网(SCGAN)以及中国地壳运动监测网络(CMONC)的部分台站的观测,计算得到1年时间序列的TEC数据,来研究长三角地区上空的电离层TEC的变化与活动。应用这些数据,综合利用高斯权函数和滑动平均等几种数据处理方法,重点分析和讨论了长三角地区上空电离层的周日变化、周年变化和季节性变化特性,揭示了电离层冬季异常等现象。同时,通过对1年时间序列TEC进行谱分析,得到了其相应的变化周期。  相似文献   

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