Analysis Of The Feature Of Ionospheric Tec Variations And Its Influencing Factors Along Meridian Chain And Latitudinal Zone

Ionospheric total electron content (TEC for short), one of the key parameters for the ionosphere, is the major cause resulted in the additional time delay in the propagation of GNSS signals. The TEC variation feature plays an important role in ionosphere-magnetosphere-thermosphere coupling since the spatial and temporal variations of TEC can effectively reflect the coupling process. Therefore the research on the spatial and temporal variation of TEC and its influencing factors along the meridian chain and latitudinal zone is of great scientific and application value.Owing to the liftoff of the global positioning system (GPS for short) and the observation network spreading all over the world, TEC monitoring and research based on GPS dual-frequency beacon measurement is greatly enhanced and many new achievements have been gained. In this paper, based on the global vertical TEC data from Madrigal database along35°N latitudinal zone and75°W and135°W meridian chains for about a solar cycle from2001to2011, the characteristics of the spatial and temporal variation along35°N latitudinal zone and75°W and135°W meridian chains and the impact of thermospheric wind and declination to the longitudinal difference for midlatitude ionosphere are studied in detail, as well as the statistical analysis on the characteristics of TEC storm-variation and the relationship between the TEC storm-variation and magnetic disturbance.The main studies performed and results obtained in this paper can be concluded as follows:1. The characteristics of spatial and temporal variation of TEC along35°N latitudinal zone and75°W and135°W meridian chains from2001to2011are analyzed, the results show:(1) The monthly averaged diurnal TEC variations are affected by solar activity, the maxima and noon-averaged values are affected by solar activity very, much, followed by the day-averaged value, the minima and night-averaged values are affected by solar activity a little. During the high solar activity, the local time of the maximum and minimum values of monthly averaged diurnal TEC is fairly concentrated, but it's quite dispersed in other years. (2) There are amplitude-comparable components variation of TEC, such as1/3-day variation, semidiurnal variation, diurnal variation and2-day variation. The amplitude of diurnal variation is the greatest, followed by the amplitude of semidiurnal variation, the amplitudes of2-day variation and1/3-day variation are the smallest. Distributions of the amplitude of diurnal variation and2-day variation along latitude are nearly symmetrical about the geomagnetic equator, and their relative minima are on geomagnetic equator. Distributions of the amplitude of semidiurnal variation and diurnal variation are nearly symmetrical about the geographic equator, and their relative maxima are on geomagnetic equator while their relative minima are on geographic equator. Hemispheric asymmetry exists in these periodic components, wherein the diurnal variation is the most obvious.(3) The anomaly variation depended on season exists in75°W mid-latitude (±45°in magnetic latitude) ionospheric magnetic conjugate. During the11years, in spring, autumn and winter of the Northern Hemisphere, the maxima of TEC diurnal variation occur around14:00LT, but they occur later around18:00LT in summer. In spring, autumn and winter of the Southern Hemisphere, the maxima of TEC diurnal variation occur around12:00LT, while the minima of TEC diurnal variation occur around04:00to06:00LT, but the maxima of TEC diurnal variation occur in the dawn and dusk to midnight each for once in summer while the minima occur around noon to dusk. This diurnal morphological abnormalities of the mid-latitude of the Southern Hemisphere, which is near the Weddell Sea, this anomaly phenomenon might be an act of ionospheric Weddell Sea anomaly.2. Based on the vertical TEC data for North America, South America and Oceania from Madrigal database of about a solar activity cycle from2001to2011, the difference for the mid-latitude ionosphere in both sides of the longitudes with zero declination and dynamical coupling of ionosphere and thermosphere under the influence of magnetic field are analyzed, the results show:(1) For nearly all seasons from2001to2010and in both sides of the longitudes with zero declination, there exist systematic differences for the mid-latitude ionospheric TEC in the regions mentioned above and the features of these differences markedly depend upon the local time but less depend upon seasons and the level of solar activity.(2) Theory analysis of dynamical coupling of ionosphere and thermosphere under the influence of magnetic field shows that the longitude variations of both declination and zonal thermospheric winds are important factors to cause differences of the mid-latitude ionospheric TEC in both sides of the longitudes with zero declination.(3) during strong magnetic storms and under stronger influence of magnetosphere-ionosphere electrodynamical coupling, the longitude differences of mid-latitude ionospheric TEC and their diurnal variations for the east and west sides of the longitude with zero declination greatly differ from the results given above. With regard to the influence of magnetosphere-ionosphere electrodynamical coupling on longitude differences of the mid-latitude ionospheric TEC.3. The characteristics of ionospheric TEC storm-variation along35°N latitudinal zone and75°N and135°E meridian chains during6super storms from2001to2005, and the evolutionary process of the ionospheric TEC storm-variation for the Northern America during one super storm are investigated. The statistical analysis of the parameter of characteristics of TEC storm-variation and the relationship between TEC storm-variation and magnetic disturbance is performed. The results show:(1) During the6super storms, the TEC storm-variations are mainly positive; strong TEC storm-variations occur mainly during day-time with the local time from dawn to dusk; along the meridian chain, hemisphere asymmetry of the TEC storm-variation appears; along the latitude zone, extending up to several thousand kilometers, the TEC storm-variations occur almost in the same universe time. This hemisphere asymmetry of the TEC storm-variations along the meridian chain is supposed to be the result of the effect on the uplift of the ionosphere caused by the eastward electric field and meridian wind with the same direction of both north and south hemisphere in day-time.(2) Analysis shows that during the super storm of November20,2003, TEC storm-variation of the Northern America appeared firstly in the mid-latitude of the east part of the Northern America, then expanded to the west and north, the main TEC storm-variation lasted for about8hours; the low-latitude TEC storm-variation appeared about5hours later, appeared first in the middle east part of the Northern America, then expanded to the west and north, lasted for about20hours. In the same time, there was an obvious ionospheric quiet zone between the higher and lower ionospheric storm regions. The evolutionary process shows, the evolutionary of the TEC storm-variation between the high-and low-latitude of the Northern America might be developed separately, the former supposed to be the direct response of the storm time energy input of the high-latitude, while the latter supposed to be the indirect response of the coupling of high-and low-latitude ionosphere-thermosphere system.(3) The statistical analyses of the amplitude and duration of the TEC storm-variations show that overall the positive TEC storm-variations appear more often than the negative ones with the ratio around3.2:1; the amplitudes of both the positive and negative TEC storm-variations are mainly between double and triple the diurnal variation standard deviation, the amplitude of the positive storm-variation achieves over fourfold the diurnal variation standard deviation for more than10times, while the amplitude of the negative storm-variation never achieves; the duration of the positive storm-variation is mainly between1to4hours while that of the negative storm-variation is mainly between half to1hours, that is to say the duration of the positive storm-variation is longer than that of the negative storm-variation.(4) The statistical analysis shows that a strong relationship between the TEC storm-variation and magnetic disturbance, under the strong disturbance condition with the Kp index over6, TEC storm-variation appears100%. Nevertheless, the statistical analysis also shows that some TEC storm-variations appear after a fairly magnetic quite time, which supposes the factors other than the magnetic disturbance lead to the TEC storm-variation.The characteristics and innovations in this paper can be concluded as:1. The monthly averaged TEC diurnal variation anomaly depending on season exists in75°W mid-latitude magnetic conjugate ionosphere (±45°in geomagnetic latitude).2. For nearly all seasons from2001to2011and in both sides of the longitudes with zero declination, there exist systematic differences for the mid-latitude ionospheric TEC in the regions mentioned above and the features of these differences markedly depend upon the local time but less depend upon seasons and the level of solar activity.3. The longitude variations of both declination and zonal thermospheric winds are one of important factors to cause differences of the mid-latitude ionospheric TEC in both sides of the longitudes with zero declination.4. The ionospheric TEC storm-variations are mainly positive; the positive storm-variations last longer than the negative storm-variations on average; TEC storm-variations are strongly linked to the magnetic disturbance, but there are still some TEC storm-variation having nothing to do with the magnetic disturbance.