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1.
Observations made by the Hinotori satellite have been analysed to determine the yearly variations of the electron density and electron temperature in the low-latitude topside ionosphere. The observations reveal the existence of an equinoctial asymmetry in the topside electron density at low latitudes, i.e. the density is higher at one equinox than at the other. The asymmetry is hemisphere-dependent with the higher electron density occurring at the March equinox in the Northern Hemisphere and at the September equinox in the Southern Hemisphere. The asymmetry becomes stronger with increasing latitude in both hemispheres. The behaviour of the asymmetry has no significant longitudinal and magnetic activity variations. A mechanism for the equinoctial asymmetry has been investigated using CTIP (coupled thermosphere ionosphere plasmasphere model). The model results reproduce the observed equinoctial asymmetry and suggest that the asymmetry is caused by the north-south imbalance of the thermosphere and ionosphere at the equinoxes due to the slow response of the thermosphere arising from the effects of the global thermospheric circulation. The observations also show that the relationship between the electron density and electron temperature is different for daytime and nighttime. During daytime the yearly variation of the electron temperature has negative correlation with the electron density, except at magnetic latitudes lower than 10°. At night, the correlation is positive. 相似文献
2.
G. F. Deminova 《Geomagnetism and Aeronomy》2010,50(2):181-186
Using the data of the topside ionosphere sounding from the Intercosmos-19 satellite, longitudinal variations in foF2 at low latitudes at the daytime hours are considered. It is obtained that these variations in particular days in the majority
of cases have a regular wave-like character with periods of about 75°–100° in longitude and amplitudes on the average of 2–4
MHz. In other words, along the valley and crests of the equatorial anomaly, a structure with four maximums and four minimums
which have a tendency to be located near certain longitudes (the same in all seasons) is observed. The variations in foF2 along the crests of the equatorial anomaly are usually in anti-phase to variations along its valley. Comparing the characteristics
of this wavelike structure at the daytime and nighttime hours, we obtained that the average positions of its extremes at the
nighttime hours are shifted eastwards by 10°–50° relative to the daytime extremes. As a cause of formation of such a structure,
high harmonics of atmospheric tides are assumed which, uplifting from below to heights of the E region, via the electric currents in this region influence the longitudinal structure of the electrodynamic plasma drift
over the equator and by that impact the structure of the entire daytime low-latitude ionosphere. 相似文献
3.
《Journal of Atmospheric and Solar》2003,65(1):47-57
Ionospheric data observed in 30 stations located in 3 longitude sectors (East Asia/Australia Sector, Europe/Africa Sector and America/East Pacific Ocean Sector) during 1974–1986 are used to analyse the characteristics of semiannual variation in the peak electron density of F2 layer (NmF2). The results indicate that the semiannual variation of NmF2 mainly presents in daytime. In nighttime, except in the region of geomagnetic equator between the two crests of ionospheric equatorial anomaly, NmF2 has no obvious semiannual variation. In the high latitude region, only in solar maxima years and in daytime, there are obvious semiannual variations of NmF2. The amplitude distribution of the semiannual variation of daytime NmF2 with latitude has a “double-humped structure”, which is very similar to the ionospheric equatorial anomaly. There is asymmetry between the Southern and the Northern Hemispheres of the profile of the amplitude of semiannual variation of NmF2 and longitudinal difference. A new possible mechanism of semiannual variation of NmF2 is put forward in this paper. The semiannual variation of the diurnal tide in the lower thermosphere induces the semiannual variation of the amplitude of the equatorial electrojet. This causes the semiannual variation of the amplitude of ionospheric equatorial anomaly through fountain effect. This process induces the semiannual variation of the low latitude NmF2. 相似文献
4.
5.
利用太阳活动高年（2000年）IGS提供的全球TEC数据,采用傅里叶展开的方法,分析了白天电离层TEC周年和半年变化的全球特征.结果显示：电离层TEC周年变化幅度在南北半球中高纬度地区较大、赤道和低纬地区很小.半年变化的幅度在“远极地区”（远离地球南北地磁极点的东北亚和南美地区） 比“近极地区”（靠近地球南北地磁极点的北美和澳大利亚）大得多.进一步的统计显示,全球大部分地区TEC在春秋月份出现最大值,北半球近极地区最大值在冬季出现.南半球的南美和澳大利亚部分地区,最大值出现在夏季.同样,采用傅里叶方法分析了中性大气模式MSIS90计算的全球大气原子分子浓度比值（［O/N2］）的数据,发现在南北半球中高纬度地区,中性成分［O/N2］周年变化幅度较大且有明显的冬季异常现象,依据Rishbeth等提出的理论,我们认为大气成分［O/N2］可能对TEC周年变化的产生有重要作用,并且也是TEC在近极地区出现冬季异常现象的主要原因.TEC半年变化的全球分布特征形成的原因较复杂,我们初步分析可能是由于中性成分［O/N2］、太阳天顶角控制的电离层光化学产生率变化共同作用而产生的. 相似文献
6.
Zhi Wang 《地震科学（英文版）》2010,23(6):555-565
Three-dimensional P-and S-wave velocities(vP,vS),Poisson's ratio(σ),crack-density(ε) and bulk-sound velocity(vφ) structures along the slab upper boundary beneath the fore-arc regions were determined using a large number of high-quality P-wave and S-wave arrival times from both onshore and offshore earthquakes in Japan.The velocity and Poisson's ratio images provide a compelling evidence for a highly hydrated and serpentinized fore-arc mantle and fluid-bearing anomalous low velocity and high Poisson's ratio associated with slab dehydration under the fore-arc areas.Most great thrust earthquakes(M7.5) occurred at or close to the high-velocity areas along the slab interface under the fore-arc areas,suggesting strong interplate coupling(asperities) with slab subduction.On the other hand,prominent low-velocity anomalies were revealed along the slab upper boundary,which may reflect weak coupled or decoupled patches(aseismicity) of the plates due to serpentinization of the fore-arc mantle wedge.The crack-density and bulk-sound velocity images,calculated from the corresponding velocity models,indicate that the interplate coupling in northeastern Japan is different from that under central and southwestern Japan owing to differences between the tectonic backgrounds of the subduction system,such as the geological age,thermal regime and dipping angle of the oceanic plates.A comparison between fluid-related anomalies of Japan,Cascadia,Chile,and Costa Rica subduction zones suggests that seismic mantle may be common in fore-arc settings and these reflect similar 3-D seismic structures relatively to fluid liberating processes.We consider that the fluid-bearing anomalies along the interface of the subducting slab,attributing to processes such as slab dehydration and serpentinization of the fore-arc mantle,are mainly contributed to the interplate coupling and the repeated generation of the great thrust earthquakes under the fore-arc regions in Japan. 相似文献
7.
A.T. Karpachev G.F. Deminova N. Beloff T.D. Carozzi P.F. Denisenko M. Lester T. Karhunen 《Journal of Atmospheric and Solar》2011,73(5-6):567-577
We used bottomside ground observations and topside sounding data from the Intercosmos-19 satellite to study a Travelling Ionospheric Disturbance (TID) that occurred in response to Large-Scale Internal Gravity Wave (LSIGW) propagation during a substorm on November 30, 1979. We built a global scheme for the wavelike ionospheric variations during this medium substorm (AEmax ～800 nT). The area where the TID was observed looks like a wedge since it covers the nighttime hours at subauroral latitudes but contracts to a ～02 h local sector at low latitudes. The ionospheric response is strongly asymmetric because the wedge area and the TID amplitude are larger in the winter hemisphere than in the summer hemisphere. Clear evidence was obtained indicating that the more powerful TID from the Northern (winter) hemisphere propagated across the equator into the low latitude Southern (summer) hemisphere. Intercosmos-19 observations show that the disturbance covers the entire thickness of the topside ionosphere, from hmF2 up to at least the 1000 km satellite altitude at post-midnight local times. F-layer lifting reached ～200 km, Ne increases in the topside ionosphere by up to a factor of ～1.9 and variations in NmF2 of both signs were observed. Assumptions are made concerning the reason for the IGW effect at high altitudes in the topside ionosphere. The relationship between TID parameters and source characteristics determined from a global network of magnetometers are studied. The role of the dayside cusp in the generation of the TID in the daytime ionosphere is discussed. The magnetospheric electric field effects are distinguished from IGW effects. 相似文献
8.
Bruce R. Bowman W. Kent Tobiska Michael J. Kendra 《Journal of Atmospheric and Solar》2008,70(11-12):1482-1496
The goal of this study was to characterize the thermospheric semiannual density response to solar heating during the last 35 years. Historical radar observational data have been processed with special orbit perturbations on 28 satellites with perigee heights ranging from 200 to 1100 km. Approximately 225,000 very accurate average daily density values at perigee have been obtained for all satellites using orbit energy dissipation rates. The semiannual variation has been found to be extremely variable from year to year. The magnitude of the maximum yearly difference, from the July minimum to the October maximum, is used to characterize the yearly semiannual variability. It has been found that this maximum difference can vary by as much as 100% from one year to the next. A high correlation has been found between this maximum difference and solar EUV data. The semiannual variation for each year has been characterized based on analyses of annual and semiannual cycles, using Fourier analysis, and equations have been developed to characterize this yearly variability. The use of new solar indices in the EUV and FUV wavelengths is shown to very accurately describe the semiannual July minimum phase shifting and the variations in the observed yearly semiannual amplitude. 相似文献
9.
Heejun Kim Kyoungwook Min Jaeheung Park Jaejin Lee Ensang Lee Hyosub Kil Vitaly P. Kim Sunmie Park 《Journal of Atmospheric and Solar》2006,68(18):2107-2118
In this paper, we report the results of our comparison study between satellite measurements and the International Reference Ionosphere (IRI) model on the seasonal and longitudinal changes of the low-latitude nighttime topside ionosphere during the period of solar maximum from June 2000 to July 2001. Satellite measurements were made by KOMPSAT-1 and DMSP F15 at 685 km altitude and 840 km altitude, respectively. The results show that the IRI2001 model gives reasonable density estimations for the summer hemisphere and the March equinox at both altitudes. However, the observed wintertime densities are smaller than the predictions of the IRI2001 model, especially at a higher (840 km) altitude, manifesting strong hemispheric asymmetries. The observed electron temperatures generally reside between the two estimations of IRI2001, one based on the Aeros–ISIS data and the other based on Intercosmos, and the latter estimation better represents the observations. With more or less monotonic increase with latitude, the temperature profiles of the IRI2001 model do not predict the enhancement seen around 15° magnetic latitude of the winter hemisphere. Longitudinal variation, probably caused by the zonal winds, is seen in all seasons at both altitudes, while the IRI2001 model does not show a large variation. The observed density and temperature show significant changes according to the F10.7 values in the whole low-latitude region from 40°S to 40°N geomagnetic latitude. The effect is manifested as increases in the density and temperature, but not in the hemispheric asymmetry or in the longitudinal variation. 相似文献
10.
本文利用DMSP卫星测量数据和傅里叶分解和重构方法,研究了地磁平静期顶部电离层总离子密度(Ni)经度结构的多重波数特征及波数4的年变化、逐年变化、地方时差异和随倾角的变化.傅里叶分解和重构的结果表明,顶部电离层平均Ni的经度结构中同时含有以波数1至波数4为主的多重分量,不同波数分量的幅度和相位各不相同.对波数4分量的分析表明,波数4的幅度在春秋季最强,北半球夏季高于冬季;随太阳活动水平增强,波数4分量的幅度增高,至太阳活动高年幅度达到最高,此后随太阳活动水平降低而减小,与F10.7呈正相关;春秋季和北半球夏季波数4分量在傍晚最强,晚上和上午次之,黎明最弱,从09LT到21LT,波数4的相位依次滞后,暗示向东移动.分析还发现,日落期间波数4幅度依赖倾角,春秋季随倾角的变化呈双峰结构,两个极大出现在倾角±18°附近,暗示赤道等离子体喷泉效应对顶部电离层经度结构的控制作用. 相似文献
11.
Alfven波在低纬地区电离层的传播有其特殊性,一方面,低纬地区同样存在Alfven速度梯度的巨大变化,导致电离层Alfven谐振器(Ionospheric Alfven resonator, IAR)的形成;另一方面,由于在低纬地区磁倾角很小,所以剪切Alfven波在传播的过程中纬度方向跨度很大,不同纬度电离层参数将共同对其产生影响;并且,由于电离层水平分层,故磁力线与电离层不正交.本文选取双流体力学模型,在忽略场向电场的条件下,利用非正交坐标系,结合IRI07模型与MSISE00模型模拟低纬地区Alfven波的传播,得到其反射及耦合特性.结果表明,低纬地区同样存在电离层Alfven谐振现象,由耦合产生的压缩模有向磁赤道方向传播的趋势,夜间电离层状态相对于白天更适合IAR的形成,谐振频率沿磁力线L值增大单调递增. 相似文献
12.
The main factors controlling NmF2 longitudinal variations at mid- and subauroral latitudes have been studied. The data of the Intercosmos-19 topside sounding,
obtained at high solar activity for summer nighttime conditions, have been used in the analysis. The contributions of the
solar ionization, neutral wind, and temperature and composition of the thermosphere to NmF2 longitudinal variations have been estimated based on ionospheric models. It has been indicated that NmF2 variations in the unsunlit midlatitude ionosphere mainly depends on the residual electron density and its decay under the
action of recombination. At subauroral latitudes under summer nighttime conditions, the ionosphere is partially sunlit, and
ionization by solar radiation mainly contributes to NmF2 longitudinal variations, whereas the effect of the neutral wind is slightly less significant. These results also indicate
how the contribution of different factors to NmF2 longitudinal variations changes at different latitudes. 相似文献
13.
本文用日本电离层探测卫星ISS-b的资料,假设F2层峰顶以上电离层中各类离子随高度呈扩散平衡分布,得到了太阳活动高年（1978年8月-1979年8月）确定顶外电离层电子密度剖面形状的一个重要参数,即O+-H+离子过渡高度hT的一些变化特征.指出hT的日变化特性主要受电离层中O+离子的产生与复合作用的控制.太阳活动高年在不同纬度和所有经度区域,平均说来日间hT为大约1500-2500km变化,而夜间位于800-1400km,冬夜甚至接近中性氢、氧原子的化学平衡高度.过渡高度也表现出明显的纬度关系,在接近±20°的磁赤道地区,hT基本上不随纬度变化;但在大于±30°磁纬区域hT随纬度很快增加.义中还就hT随时间和地磁（或地理）纬度变化的机制作了简要讨论. 相似文献
14.
利用一个电离层理论模式,模拟了太阳活动低年、地磁宁静情况下,中低纬和赤道地区电离层F2区峰值电子浓度(NmF2)的半年变化规律,重点讨论了电离层电场对NmF2半年变化的影响.模拟结果表明,当输入的电场没有周年和半年变化时,磁赤道地区电离层NmF2本身就具有一定的半年变化特征,而在稍高的纬度上,NmF2半年变化的强度较弱.当输入的电场具有一定的半年变化时,电离层NmF2的半年变化强度有明显的改变,且这种改变随地方时和地磁纬度不同有明显的差别.在地磁赤道附近的电离层赤道槽地区,从上午到午夜的时间内,具有半年变化的电场对电离层NmF2半年变化的强度是减弱的作用,在其他的时间内,电场对电离层NmF2半年变化强度是加强的作用.而在稍高纬度的电离层驼峰地区,情况明显不同.从上午一直到翌日日出前,具有半年变化的电场对电离层NmF2半年变化的幅度都是加强的作用.在其他的时间内,电场对电离层NmF2半年变化的幅度是减弱的作用.同时,研究表明电离层电场对NmF2半年变化的作用和“赤道喷泉”现象强烈相关. 相似文献
15.
S. S. Starjinsky 《Geomagnetism and Aeronomy》2008,48(2):265-276
Results of studying the lunar daily geomagnetic variations in the spectral and time regions at the network of observatories are presented. The seasonal variations in the amplitudes of the fundamental harmonic constituents of three lunar variation components have been revealed. The seasonal time variations have been analyzed using the digital bandpass filtering and harmonic synthesis based on the data of the Kakioka and Memambetsu geomagnetic observatories. The 11-year solar cycle and annual and semiannual periods have been distinguished in the seasonal variation spectrum. Studying the spectral singularities of the lunar daily variation at these observatories and the sea level variations in daytime and nighttime hours has made it possible to identify the contribution of the oceanic dynamo to the lunar variation vertical component. 相似文献
16.
The occurrence of Traveling Ionospheric Disturbances (TIDs) at a midlatitude location (London, Canada, 43°N, 81°W) has been examined using data recorded by standard 5 min ionograms during the year 2000. It is found that the dominant source of TIDs during daytime appears to be the sunrise terminator but during nighttime the sunset terminator and magnetic disturbances both contribute to the TIDs. The daytime TIDs show a weak semiannual variation with maxima in solstices. The nighttime TIDs show insignificant annual variation. 相似文献
17.
The variations in the electron number density of the ionospheric F2 layer maximum (NmF2) under the action of the zonal plasma drift in the geomagnetic west-geomagnetic east direction perpendicularly to the electric (E) and geomagnetic (B) fields during a geomagnetically quiet period on December 7, 1989, at high solar activity have been studied based on a three-dimensional nonstationary theoretical model of electron number densities and temperatures in the ionospheric F region. Calculated and measured NmF2 values for 12 low-latitude ionospheric sounding stations have been compared. When the zonal E × B plasma drift is ignored, the NmF2 values become smaller by up to a factor of 3 under nighttime conditions in the low-latitude ionosphere. The average effect of the zonal E × B plasma drift on NmF2 in the low-latitude ionosphere is larger during winter nights than under summer nighttime conditions. 相似文献
18.
Annual and seasonal variations in the low-latitude topside ionosphere are investigated using observations made by the Hinotori satellite and the Sheffield University Plasmasphere Ionosphere Model (SUPIM). The observed electron densities at 600 km altitude show a strong annual anomaly at all longitudes. The average electron densities of conjugate latitudes within the latitude range ±25° are higher at the December solstice than at the June solstice by about 100% during daytime and 30% during night-time. Model calculations show that the annual variations in the neutral gas densities play important roles. The model values obtained from calculations with inputs for the neutral densities obtained from MSIS86 reproduce the general behaviour of the observed annual anomaly. However, the differences in the modelled electron densities at the two solstices are only about 30% of that seen in the observed values. The model calculations suggest that while the differences between the solstice values of neutral wind, resulting from the coupling of the neutral gas and plasma, may also make a significant contribution to the daytime annual anomaly, the E × B drift velocity may slightly weaken the annual anomaly during daytime and strengthen the anomaly during the post-sunset period. It is suggested that energy sources, other than those arising from the 6% difference in the solar EUV fluxes at the two solstices due to the change in the Sun-Earth distance, may contribute to the annual anomaly. Observations show strong seasonal variations at the solstices, with the electron density at 600 km altitude being higher in the summer hemisphere than in the winter hemisphere, contrary to the behaviour in NmF2. Model calculations confirm that the seasonal behaviour results from effects caused by transequatorial component of the neutral wind in the direction summer hemisphere to winter hemisphere. 相似文献
19.
The F-region peak electron densities NmF2 measured during daytime quiet geomagnetic conditions at low solar activity on January 22, 2008, April 8, 1997, July 12, 1986, and October 26, 1995, are compared. Ionospheric parameters are measured by the ionosonde and incoherent scatter radar at Millstone Hill and calculated with the use of a 1D nonstationary ionosphere–plasmasphere model of number densities and temperatures of electrons and ions at middle geomagnetic latitudes. The formation of the semiannual anomaly of the midlatitudinal NmF2 under daytime quiet geomagnetic conditions at low solar activity is studied. The study shows that the semiannual NmF2 anomaly occurs due to the total impact of three main causes: seasonal variations in the velocity of plasma drift along the geomagnetic field due to the corresponding variations in the components of the neutral wind velocity; seasonal variations in the composition and temperature of the neutral atmosphere; and the dependence of the solar zenith angle on a number of the day in the year at the same solar local time. 相似文献
20.
Parts of the consecutive apparent resistivity monitoring stations of China have recorded clear diurnal variations. The relative amplitudes of diurnal variations at these stations range from 1.3‰ to 5.8‰. The daily accuracies of apparent resistivity observation are better than 1‰, because the background electromagnetic noise is rather low at these stations. Therefore, the diurnal variations of apparent resistivity recorded at these stations are real phenomena. The diurnal variation shapes can be divided into two opposite types according to their characteristics. One type is that the apparent resistivity data decreases during the daytime but increases during the nighttime(Type 1). The other type is the apparent resistivity data increases during the daytime but decreases during the nighttime(Type 2). There is a correspondence between the diurnal and annual variation patterns of apparent resistivity. For the monitoring direction with diurnal variation of Type 1, the apparent resistivity decreases in summer and increases in winter. However, for the monitoring direction with diurnal variation of Type 2, the apparent resistivity increases in summer and decreases in winter.
We take an analysis on the mechanism of apparent resistivity diurnal variation, combining the influence factors of water-bearing medium's resistivity, the electric structure of stations, and the apparent resistivity sensitivity coefficient(SC)theory. Intuitively, diurnal variation of apparent resistivity is caused by diurnal variation of medium resistivity in the measured area. The diurnal variation of medium resistivity will inevitably be caused by the factors with diurnal variation. Among the possible factors, there is diural variation in earth tide and temperature.
Our analysis displays that apparent resistivity diurnal variation is not caused by the usually-believed earth tide, but by the ground temperature difference between daytime and nighttime. The earth tide strain is too small to cause remarkable effects on the apparent resistivity data. On the other hand, the daily tide strain has two peak-valley variations, and its phase and amplitude has a period of approximate 28 days. However, the apparent resistivity data do not show these corresponding features to earth tide. Furthermore, the detection range of current apparent resistivity stations is within a depth of several hundred meters. Within this depth range, the medium deformation caused by solid tide can be regarded as uniform change. Therefore, all monitoring directions and all stations will have the same pattern of diurnal variation.
In general, the temperature increases in the daytime but decreases in the nighttime. For most water-bearing rock and soil medium, its resistivity decreases as temperature increases and increases as temperature decreases. Diurnal temperature difference affects about 0.4m of soil depth. Therefore, resistivity of this surface thin soil layer decreases in the daytime while increases in the nighttime. Under layered medium model, SC of each layer represents its contribution to the apparent resistivity. For the stations with positive SC of surface layer, apparent resistivity decreases in the daytime but increases in the nighttime. While for the stations with negative SC of surface layer, apparent resistivity diurnal variations display the opposite shape. 相似文献