Variations in ionospheric parameters during solar flares

Results of studying the ionospheric response to solar flares, obtained based on the incoherent scatter radar observations of the GPS signals and as a result of the model simulations, are presented. The method, based on the effect of partial “shadowing” of the atmosphere by the globe, has been used to analyze the GPS data. This method made it possible to estimate the value of a change in the electron content in the upper ionosphere during the solar flare of July 14, 2000. It has been shown that a flare can cause a decrease in the electron content at heights of the upper ionosphere (h > 300 km) according to the GPS data. Similar effects in the formation of a negative disturbance in the ionospheric F region were also observed during the solar flares of May 21 and 23, 1967, at the Arecibo incoherent scatter radar. The mechanism by which negative disturbances are formed in the upper ionosphere during solar flares has been studied based on the theoretical model of the ionosphere-plasmasphere coupling. It has been shown that an intense ejection of O⁺ ions into the above located plasmasphere under the action of a sharp increase in the ion production rate and the thermal expansion of the ionospheric plasma cause the formation of a negative disturbance in the electron concentration in the upper ionosphere.     

Disturbances in the topside ionosphere during solar flares

The results of studying the ionospheric response to solar flares, obtained from the data of the GPS signal observations and incoherent scatter radars and as a result of the model calculations, are presented. It is shown that, according to the GPS data, a flare can cause a decrease in the electron content at altitudes of the topside ionosphere (h > 300 km). Similar effects of formation of a negative disturbance in the ionospheric F region were also observed during the solar flares of May 21 and 23, 1967, with the Arecibo incoherent scatter radar. The mechanism by which negative disturbances appear in the topside ionosphere during solar flares has been studied in this work based on the theoretical model of the ionosphere-plasmasphere coupling. It has been indicated that the formation of the electron density negative disturbance in the topside ionosphere is caused by an intense removal of O⁺ ions into the overlying plasmasphere under the action of an abrupt increase in the ion production rate and thermal expansion of the ionospheric plasma.     

On ionospheric disturbances caused by the solar activity of November 1960

Summary On November 1960 a special solar activity occured, producing characteristic disturbances in the ionosphere. Athens center followed this activity by vertical sounding and by Riometer recordings. They are evidences for proton events producing deep absorption in the ionosphere and aurora extending till 38° latitude. Some observations concerning July 1961 solar activity are reported. World wide decrease of the maximum electron density reported during the November period was observed also in Athens. Ionospheric disturbances following vertical sounding observations are of «negative» type.     

Configuration of the upper boundary of the ionosphere

Variations of the upper boundary of the ionosphere (UBI) are investigated based on three sources of information: (i) ionosonde-derived parameters: critical frequency foF2, propagation factor M3000F2, and sub-peak thickness of the bottomside electron density profile; (ii) total electron content (TEC) observations from signals of the Global Positioning System (GPS) satellites; (iii) model electron densities of the International Reference Ionosphere (IRI^*) extended towards the plasmasphere. The ionospheric slab thickness is calculated as ratio of TEC to the F2 layer peak electron density, NmF2, representing a measure of thickness of electron density profile in the bottomside and topside ionosphere eliminating the plasmaspheric slab thickness of GPS-TEC with the IRI^* code. The ratio of slab thickness to the real thickness in the topside ionosphere is deduced making use of a similar ratio in the bottomside ionosphere with a weight Rw. Model weight Rw is represented as a superposition of the base-functions of local time, geomagnetic latitude, solar and magnetic activity. The time-space variations of domain of convergence of the ionosphere and plasmasphere differ from an average value of UBI at ∼1000 km over the earth. Analysis for quiet monthly average conditions and during the storms (September 2002, October–November 2003, November 2004) has shown shrinking UBI altitude at daytime to 400 km. The upper ionosphere height is increased by night with an ‘ionospheric tail’ which expands from 1000 km to more than 2000 km over the earth under quiet and disturbed space weather. These effects are interposed on a trend of increasing UBI height with solar activity when both the critical frequency foF2 and the peak height hmF2 are growing during the solar cycle.     

A neural network approach for regional vertical total electron content modelling

A Neural Network model has been developed for estimating the total electron content (TEC) of the ionosphere. TEC is proportional to the delay suffered by electromagnetic signals crossing the ionosphere and is among the errors that impact GNSS (Global Navigation Satellite Systems) observations. Ionospheric delay is particularly a problem for single frequency receivers, which cannot eliminate the (first-order) ionospheric delay by combining observations at two frequencies. Single frequency users rely on applying corrections based on prediction models or on regional models formed based on actual data collected by a network of receivers. A regional model based on a neural network has been designed and tested using data sets collected by the Brazilian GPS Network (RMBC) covering periods of low and high solar activity. Analysis of the results indicates that the model is capable of recovering, on average, 85% of TEC values.     

Investigation of Ionospheric Variations During Magnetic Storm Over Turkey

Geomagnetism and Aeronomy - Variations in solar, geomagnetic, and seismic activity cause disturbance in the ionosphere that can be monitored effectively using GPS observations. Total electron...     

Behavior of the polar lower ionosphere during the solar flares in January 2005 according to the data of the partial reflections method

The results of the observations, using partial reflections, of the lower ionosphere over Tumannyi station in the Murmansk region (69.0° N, 35.7° E) during strong solar flares on January 15–20, 2005, are presented. The structure of the D region of the polar ionosphere and the effects of X-ray flares and fluxes of high energy protons on this region are considered. The anomalous values of electron density in the lower part of the D region, unusually low values of the height of the lower ionospheric boundary, complete or partial absorption of short and medium radiowaves, and bursts of the meter cosmic radio emission were detected during solar disturbances.     

基于北斗、GLONASS和GPS系统的中低纬电离层特性联合探测

基于GNSS(Global Navigation Satellite Systems)的发展,我们利用具有北斗、GLONASS和GPS三系统信号接收功能的接收机观测的数据,结合电离层总电子含量(Total Electron Content, TEC)的反演算法,提取出GNSS三系统观测的电离层TEC;同时,将GNSS三系统获取的TEC应用到电离层TEC地图、行进式扰动、不规则体结构和电离层的太阳耀斑响应等方面的研究中,这也是首次使用三种GNSS系统数据对电离层进行联合探测研究.研究结果表明,增加了北斗系统的GNSS三系统在研究中国地区电离层TEC地图、周日变化、逐日变化,行进式扰动以及电离层的实时监测等方面较单系统的GPS具有明显的优势.     

Joint observation results of Na layer and ionosphere in Wuhan during the Total Solar Eclipse

During the total solar eclipse on July 22, 2009 in Wuhan, the joint observation test of Na layer and ionosphere was conducted by using the daytime observation atmospheric lidar and the GPS ionosphere detector. The results show that the full width at half maximum (FWHM) of Na layer density slightly narrowed during the total solar eclipse and broadened after the eclipse, while the height of Na peak slightly decreased in the eclipse and increased after the eclipse. These implying that Na layer changes reflect the rapid process of sunrise and sunset. The ionosphere total electron content (TEC) and the sky background light noise also presented an obvious fluctuation characteristic with the changes of solar irradiation during the process of total solar eclipse. The difference lies in that the changes of FWHM of Na layer atoms are much slower than that of ionosphere, the reason for this might be that the Na layer, after being disturbed by the total solar eclipse, will generate a series of complicated photochemical reactions and momentum transport processes, and then recombine the Na atoms. The Na atoms to be detected by the lidar need a lag process, which rightly conforms to the theoretical simulated results.     

Dependence of the ionospheric response on the solar flare parameters based on the theoretical modeling and GPS data

The measurements of an increase in the total electron content (TEC) of the ionosphere during solar flares, obtained based on the GPS data, indicated that up to 30% of TEC increments corresponded to the ionospheric regions above 300 km altitude in some cases, and TEC increased mainly below altitudes of 300 km in other cases. The theoretical model of the ionosphere and plasmasphere was used to study the obtained effects. The altitude-time variations in the charged particle density in the ionospheric region from 100 to 1000 km were used depending on the solar flare spectrum. An analysis of the modeling results indicated that an intensification of the flare UV emission in the 55–65 and 85–95 nm spectral ranges results in a pronounced increase in the electron density in the topside ionosphere (above 300 km). The experimental dependences of the ionospheric TEC response amplitude on the localization and peak power of flares on the Sun in the X-ray range, obtained based on the GPS data, are also presented in the work.     

Analysis of observations backing up the existence of VLF and ionospheric TEC anomalies before the Mw6.1 earthquake in Greece,January 26, 2014

The present work integrates ground-based ionosphere measurements using very-low-frequency radio transmissions with satellite measurements of the total electron content to draw common conclusions about the possible impact that the Mw6.1 earthquake that took place in Greece on January 26, 2014, had on the ionosphere.Very-low-frequency radio signals reveal the existence of an ∼4-day anomaly in the wavelet spectra of the signals received inside the earthquake preparation zone and a significant increase in the normalized variance of the signals prior to the earthquake (approximately 1 day before).Through total electron content analysis, it was possible to identify a clear anomaly from 15:00 until 20:00 UT on the day before the earthquake that appears again on the day of the earthquake between 07:00 UT and 08:00 UT. The anomalous values reach TEC1Sigma ∼4.36 and 3.11, respectively. Their spatial and temporal distributions give grounds to assume a possible link with the earthquake preparation. The geomagnetic, solar and weather conditions during the considered period are presented and taken into account.This work is an initial and original step towards a multi-parameter approach to the problem of the possible earthquake-related effects on the ionosphere joining observations made from both ground stations and satellites. A well-founded knowledge of these phenomena is clearly necessary before dealing with their application to earthquake prediction purposes.     

日食电离层效应

本文分析了1987年9月23日日环食期间,我国14个电离层站和1988年3月18日日全食期间两个站的垂测仪和偏振仪记录,并综合50年代以来历次日食电离层效应的观测结果,证实:1.E层和F₁层光食效应明显,F₂层动力学效应显著;2.f₀F₂存在日食日值大于、小于或等于控制日值三种典型情况;3.TEC食变曲线有凹陷和不凹陷两种典型情况,甚至出现日食日值大于控制日值的异常现象. 本文对F₂层和外电离层的动力学特征作了定性讨论,认为:空间等离子体温度急剧下降和沿场扩散是F₂层和外电离层日食效应的最主要因素;而磁赤道上空等离子体的沿场扩散、“喷泉”效应,热层风和全（环）食带方位是影响位于磁赤道异常区各电离层站日食电离层效应的主要因素.     

Ionospheric response to the partial solar eclipse of March 29, 2006, according to the observations at Nizhni Novgorod and Murmansk

The results of observations of the solar eclipse ionospheric effects on March 29, 2006, are presented. The observations were conducted using the partial reflection method near Nizhni Novgorod and the vertical sounding method at the automatic ionospheric station near Murmansk. It has been obtained that the electron density at altitudes of 77 and 91 km decreases by a factor of more than 4; in this case the response of the ionosphere at an altitude of 91 km lags behind the eclipse maximum phase on the Earth by approximately 20 min. It has been established that the eclipse in the E and F1 regions of the polar ionosphere causes a change in the electron density by 15–20%. The delay time of this effect varies from 12 to 24 min depending on the altitude. It has been registered that the reflection virtual altitude at altitudes of the ionospheric F region increases in Murmansk and Nizhni Novgorod.     

Time delays in the correlation between solar activity and the F2 region plasma frequency

The upper ionosphere electron density characterized by the critical frequency foF2 is correlated with solar activity when using monthly medians or averages from longer intervals. When shorter intervals are studied, time delays of different lengths in solar activity effects in the ionosphere are observed. The correlation between the foF2 values and the solar radiation intensity, given by the F10.7 index, is studied using the 1967–2003 data of mid-latitude ionosonde stations spaced at distances greater than 100° in geographical longitude. At which longitude the reaction of foF2 to the changes in solar activity appears sooner depends on the position of the interval studied in the 22-year solar cycle.     

Specification and forecasting of scintillations in communication/navigation links: current status and future plans

The ionosphere often becomes turbulent and develops electron density irregularities. These irregularities scatter radio waves to cause amplitude and phase scintillation and affect satellite communication and GPS navigation systems. The effects are most intense in the equatorial region, moderate at high latitudes and minimum at middle latitudes. The thermosphere and the ionosphere seem to internally control the generation of irregularities in the equatorial region and its forcing by solar transients is an additional modulating factor. On the other hand, the irregularity generation mechanisms in the high-latitude ionosphere seem to be driven by magnetospheric processes and, therefore, high-latitude scintillations can be tracked by following the trail of energy from the sun in the form of solar flares and coronal mass ejections. The development of a global specification and forecast system for scintillation is needed in view of our increased reliance on space-based communication and navigation systems, which are vulnerable to ionospheric scintillation. Such scintillation specification systems are being developed for the equatorial region. An equatorial satellite equipped with an appropriate suite of sensors, capable of detecting ionospheric irregularities and tracking the drivers that control the formation of ionospheric irregularities, has also been planned for the purpose of specifying and forecasting equatorial scintillations. In the polar region, scintillation specification and forecast systems are yet to emerge although modeling and observations of polar cap plasma structures, their convection and associated irregularities have advanced greatly in recent years. Global scintillation observations made during the S-RAMP Space Weather Month in September 1999 are currently being analyzed to study the effects of magnetic storms on communication and navigation systems.     

Regional ionosphere modeling in support of IRI and wavelet using GPS observations

Dual-frequency global navigation satellite systems (GNSS) observations provide most of the input data for development of global ionosphere map (GIM) of vertical total electron content (VTEC). The international GNSS service (IGS) develops different ionosphere products. The IGS tracking network stations are not homogeneously distributed around the world. The large gaps of this network in Middle East, e.g., Iran plateau, reduce the accuracy of the IGS GIMs over this region. Empirical ionosphere models, such as international reference ionosphere (IRI), also provide coarse forecasts of the VTEC values. This paper presents a new regional VTEC model based on the IRI 2007 and global positioning system (GPS) observations from Iranian Permanent GPS Network. The model consists of a given reference part from IRI model and an unknown correction term. Compactly supported base functions are more appropriate than spherical harmonics in regional ionosphere modeling. Therefore, an unknown correction term was expanded in terms of B-spline functions. The obtained results are validated through comparison with the observed VTEC derived from GPS observations.     

Reconstructing Regional Ionospheric Electron Density: A Combined Spherical Slepian Function and Empirical Orthogonal Function Approach

The computerized ionospheric tomography is a method for imaging the Earth’s ionosphere using a sounding technique and computing the slant total electron content (STEC) values from data of the global positioning system (GPS). The most common approach for ionospheric tomography is the voxel-based model, in which (1) the ionosphere is divided into voxels, (2) the STEC is then measured along (many) satellite signal paths, and finally (3) an inversion procedure is applied to reconstruct the electron density distribution of the ionosphere. In this study, a computationally efficient approach is introduced, which improves the inversion procedure of step 3. Our proposed method combines the empirical orthogonal function and the spherical Slepian base functions to describe the vertical and horizontal distribution of electron density, respectively. Thus, it can be applied on regional and global case studies. Numerical application is demonstrated using the ground-based GPS data over South America. Our results are validated against ionospheric tomography obtained from the constellation observing system for meteorology, ionosphere, and climate (COSMIC) observations and the global ionosphere map estimated by international centers, as well as by comparison with STEC derived from independent GPS stations. Using the proposed approach, we find that while using 30 GPS measurements in South America, one can achieve comparable accuracy with those from COSMIC data within the reported accuracy (1 × 10¹¹ el/cm³) of the product. Comparisons with real observations of two GPS stations indicate an absolute difference is less than 2 TECU (where 1 total electron content unit, TECU, is 10¹⁶ electrons/m²).     

电离层预报模型研究

当利用无线电电磁波进行远程通信、卫星导航时,传递信号要受到电离层的影响,因此,对电离层中电子含量的研究显得特别重要.虽然国际上有几种电离层的电子含量预报模型,但其预报只能精确到电子含量的50%～60%.本文提出了一种新的电离层电子含量预报方法:即用球谐函数对IGS(国际GPS服务)所给出的离地面450 km高的球面上的每一网点的电离层电子含量进行拟合,对不同的时间所得到的拟合系数所形成的时间序列用时间序列分析理论中的ARMA(p,q)模型进行预报,从而实现全球的电离层电子含量预报.利用本方法对2004年和2005年IGS所给电离层电子含量资料在地理框架中做了分析预报,5天内电子含量预报相对精度在90%左右.