电离层电场半年变化的模拟研究

利用一个中低纬电离层电场理论模式，模拟太阳活动低年、地磁活动平静情况下，中低纬地区电离层电场全年的变化情况．结果显示，单独计算南、北半球(去耦合)得到电离层电场具有明显的周年变化特征，且两个半球电场的相位相差半年左右．而同时计算南、北半球(计及耦合)时，电场则是以半年变化为主，且这种半年变化的幅度和相位随地方时和地磁纬度有变化．提出一个南、北半球耦合电路的简单物理模型给予解释．电路模型初步计算发现，即使两个半球电离层电场分别具有周年变化，只要它们变化的幅度相当，相位相差半年左右，由于跨越南北半球磁力线的耦合效果，耦合的电离层电场会产生明显的半年变化分量．由于缺少连续的电离层电场观测资料，将模拟结果与Richmond基于非相干散射雷达数据建立的经验模式(ISR Model)相比较，结果符合较好．     

GNSS single frequency ionospheric range delay corrections: NeQuick data ingestion technique

A study of the performance of the NeQuick model and the Klobuchar model for GNSS single frequency range delay correction on a global scale was done using data for moderate solar activity. In this study NeQuick was used in the way intended for Galileo. This study is to assess the performance of the two models at each ionospheric geographic region during moderate solar activity as previously published studies were concentrated only on high solar activity. The results obtained showed that NeQuick outperformed Klobuchar for the whole year at the three geographical regions of the ionosphere. In terms of monthly root mean square of mismodeling, NeQuick outperformed Klobuchar by 15 TECU or more at low-latitudes, 5 TEC or more at mid-latitudes, and 1 TECU or more at high-latitudes.     

Variations of total electron content over high latitude region during the ascending phase of 24th solar cycle

The solar cycle variation and seasonal changes significantly affects the ionization process of earth’s ionosphere and required to be monitored in real time basis for regional level refinement of existing models. In view of this, the present study has been carried out by using the ionospheric Total Electron Content (TEC) data observed with the help of Global Ionospheric Scintillation and TEC monitoring (GISTM) system installed at Indian Antarctic Research Station, “Maitri” [70°46′00″S 11°43′56″E] during the ascending phase of 24th solar cycle. The daily values of solar extreme ultraviolet (EUV) flux (0.1–50?nm wavelength), 10.7?cm radio flux F_10.7 and Sunspot number (SSN) has been taken as a proxy to represent the solar cycle variation to correlate with TEC. The linear regression results revels better correlation of TEC with EUV flux rather than F_10.7 and SSN. Also, the EUV and TEC show better agreement during summer as compared to winter and equinox period. Correlation between TEC and EUV appears significantly noticeable during ten internationally defined quiet days of each month (stable background geophysical condition) as compared to the overall days (2010–2014). Further, saturation effect has been observed on TEC values during the solar maxima year 2014. The saturation effects are more prominent during the night hours of winter and equinox season due to transportation losses manifested by the equator-ward direction of meridional wind.     

Day-to-day variability of equatorial anomaly in GPS-TEC during low solar activity period

The ionospheric total electron content (TEC) in the northern hemispheric equatorial ionization anomaly (EIA) crest region is investigated by using dual-frequency signals of the Global Positioning System (GPS) acquired from Rajkot (Geog. Lat. 22.29°N, Geog. Long. 70.74°E; Geom. Lat. 14.21°N, Geom. Long. 144.90°E), India. The day-to-day variability of EIA characteristics is examined during low solar activity period (F10.7∼83 sfu). It is found that the daily maximum TEC at EIA crest exhibits a day-to-day and strong semi-annual variability. The seasonal anomaly and equinoctial asymmetry in TEC at EIA is found non-existent and weaker, respectively. We found a moderate and positive correlation of daily magnitude of crest, Ic with daily F10.7 and EUV fluxes with a correlation coefficient of 0.43 and 0.33, respectively indicating an existence of a short-term relation between TEC at EIA and the solar radiation even during low solar activity period. The correlation of daily Ic with Dst index is also moderate (r = −0.35), whereas no correlation is found with the daily Kp index (r = 0.14) respectively. We found that the magnitude of EIA crest is moderately correlated with solar flux in all seasons except winter where it is weakly related (0.27). The magnitude of EIA crest is also found highly related with EEJ strength in spring (r = 0.69) and summer (r = 0.65) than autumn (0.5) and winter (r = 0.47), though EEJ is stronger in autumn than spring.     

Dynamics of global electron content in 1998–2005 derived from global GPS data and IRI modeling

We analyzed the dynamics of global electron content (GEC) for the period 1998–2005 and compared the estimated GEC with variations of the 10.7-cm solar radio emission and with and with GEC values obtained with IRI-2001. We found a strong resemblance between the curves’ shapes for the experimental and modeled GEC: strong semiannual variations are discernible in these series and both curves tend to increase the absolute GEC value during the period of maximum of solar activity. However, there are some significant distinctions, such as absence of 27-day fluctuations in the series of GEC computed by the IRI-2001. On the contrary, observational GEC reflects well dynamics of solar activity: 27-day variations of GEC are very similar to the ones of the index F10.7, but GEC undergoes a lagging of about of 30–60 h as compared to value of the F10.7 index. The relative amplitude of 27-day variations decreases from 8% at the rising and falling solar activity to 2% at the period of its maximum.     

EUV-TEC proxy to describe ionospheric variability using satellite-borne solar EUV measurements: First results

Primary photoionisation of major ionospheric constituents is calculated from satellite-borne solar EUV measurements. Number densities of the background atmosphere are taken from the NRLMSISE-00 climatology. From the calculated ionisation rates, a proxy termed EUV-TEC, which is based on the global total ionisation is calculated, and describes the ionospheric response to solar EUV and its variability. The proxy is compared against the global mean ionospheric total electron content (TEC) derived from GPS data. Results show that the EUV-TEC proxy provides a better overall representation of global TEC than conventional solar indices like F10.7 do. The EUV-TEC proxy may be used for scientific research, and to describe the ionospheric effects on radio communication and navigation systems.     

Variation of ionospheric total electron content in Taiwan region of the equatorial anomaly from 1994 to 2003

The ionospheric total electron content (TEC) in the northern hemispheric equatorial ionospheric anomaly (EIA) region is studied by analyzing dual-frequency signals of the Global Position System (GPS) acquired from a chain of nine observational sites clustered around Taiwan (21.9–26.2°N, 118.4–112.6°E). In this study, we present results from a statistical study of seasonal and geomagnetic effects on the EIA during solar cycle 23: 1994–2003. It is found that TEC at equatorial anomaly crests yield their maximum values during the vernal and autumnal months and their minimum values during the summer (except 1998). Using monthly averaged I_c (magnitude of TEC at the northern anomaly crest), semi-annual variations is seen clearly with two maxima occurring in both spring and autumn. In addition, I_c is found to be greater in winter than in summer. Statistically monthly values of I_c were poorly correlated with the monthly Dst index (r = −0.22) but were well correlated with the solar emission F_10.7 index (r = 0.87) for the entire database for the period during 1994–2003. In contrast, monthly values of I_c were correlated better with Dst (r ? 0.72) than with F_10.7 (r ? 0.56) in every year during the low solar activity period (1994–1997). It suggests that the effect of solar activity on I_c is a longer term (years), whereas the effect of geomagnetic activity on I_c is a shorter term (months).     

The solar cycle variation of plasma parameters in equatorial and mid latitudinal areas during 2005–2010

Based on the ISL data detected by DEMETER satellite, the solar cycle variation in electron density (Ne) and electron temperature (Te) were studied separately in local daytime 10:30 and nighttime 22:30 during 2005–2010 in the 23rd/24th solar cycles. The semi-annual, annual periods and decreasing trend with the descending solar activity were clearly revealed in Ne. At middle and high latitudes, there exhibited phase shift and even reversed annual variation over Southern and Northern hemisphere, and the annual variation amplitudes were asymmetrical at both hemispheres in local daytime. In local nighttime, the annual variations of Ne at south and north hemispheres were symmetrical at same latitudes, but the annual variation amplitudes at different latitudes differed largely, showing obviously zonal features. As for Te, the phase shift in annual variations was not as apparent as Ne with the increase of latitudes at Southern and Northern hemisphere in local daytime. While in local nighttime the reversed annual variations of Te were shown at low latitudinal areas, not at high latitudes as those in Ne. The correlation study on Ne and Te illustrated that, in local daytime, Ne and Te showed strong negative correlation at equator and low latitudes, but during the solar minimum years the correlation between Ne and Te changed to be positive at 25–30° latitudes in March 2009. The correlation coefficient R between Ne and Te also showed semi-annual periodical variations during 2005–2010. While in local nighttime, Ne and Te exhibited relatively weak positive correlation with R being about 0.6 at low latitudes, however no correlation beyond latitudes of 25° was obtained.     

广州地磁Z分量日变幅的谱特征

利用1972—1993年广州地磁资料，分析了Z分量日变幅的年平均、年变化和半年变化等低频成分的逐年变化，以及小于60天的短周期变化特征．同时对1972—1993年的F10.7日均值进行了谱分析．结果指出，广州地磁Z分量日变幅的年平均与太阳活动指数F10．7的年平均存在良好的线性相关；具有幅度大约5nT夏季极大的年变化，与太阳活动没有明显相关，是一种季节效应；存在春秋分极大的半年变化，幅度与太阳活动有关，高年的幅度明显大于低年；具有明显的与太阳自转相关的27天左右的变化和明显的与行星波有关的接近16日、10日、5日、2日等短周期变化．广州地磁Z分量日变幅的这些谱特征，有助于更深入地了解中低层大气对电离层影响的物理机制．     

Symmetry and asymmetry of ionospheric weather at magnetic conjugate points for two midlatitude observatories

Variations of the ionospheric weather W-index for two midlatitude observatories, namely, Grahamstown and Hermanus, and their conjugate counterpart locations in Africa are studied for a period from October 2010 to December 2011. The observatories are located in the longitude sector, which has consistent magnetic equator and geographic equator so that geomagnetic latitudes of the line of force are very close to the corresponding geographic latitudes providing opportunity to ignore the impact of the difference of the gravitational field and the geomagnetic field at the conjugate points on the ionosphere structure and dynamics. The ionosondes of Grahamstown and Hermanus provide data of the critical frequency (foF2), and Global Ionospheric Maps (GIM) provide the total electron content (TECgps) along the magnetic field line up to the conjugate point in the opposite hemisphere. The global model of the ionosphere, International Reference Ionosphere, extended to the plasmasphere altitude of 20,200 km (IRI-Plas) is used to deliver the F2 layer peak parameters from TECgps at the magnetic conjugate area. The evidence is obtained that the electron gas heated by day and cooled by night at the summer hemisphere as compared with the opposite features in the conjugate winter hemisphere testifies on a reversal of plasma fluxes along the magnetic field line by the solar terminator. The ionospheric weather W-index is derived from NmF2 (related with foF2) and TECgps data. It is found that symmetry of W-index behavior in the magnetic conjugate hemispheres is dominant for the equinoxes when plasma movement along the magnetic line of force is imposed on symmetrical background electron density and electron content. Asymmetry of the ionospheric storm effects is observed for solstices when the plasma diffuse down more slowly into the colder winter hemisphere than into the warmer summer hemisphere inducing either plasma increase (positive phase) or decrease (negative phase of W-index) in the ionospheric and plasmaspheric plasma density.     

Ionospheric anomalies observed over South Korea preceding the Great Tohoku earthquake of 2011

We investigated the ionospheric anomalies observed before the Tohoku earthquake, which occurred near the northeast coast of Honshu, Japan on 11 March, 2011. Based on data from a ground-based Global Positioning System (GPS) network on the Korean Peninsula, ionospheric anomalies were detected in the total electron content (TEC) during the daytime a few days before earthquake. Ionospheric TEC anomalies appeared on 5, 8 and 11 March. In particular, the ionospheric disturbances on 8 March evidenced a remarkable increase in TEC. The GPS TEC variation associated with the Tohoku earthquake was an increase of approximately 20 total electron content units (TECU), observed simultaneously in local and global TEC measurements. To investigate these pre-earthquake ionospheric anomalies, space weather conditions such as the solar activity index (F10.7) and geomagnetic activity indices (the Kp and Dst indices) were examined. We also created two-dimensional TEC maps to visual the spatial variations in the ionospheric anomalies preceding the earthquake.     

2008—2009年电离层对重现型地磁活动的响应

利用2008—2009年的GPS TEC数据,分析了电离层对冕洞引起的重现型地磁活动的响应. 结果表明,在太阳活动低年,电离层TEC表现出与地磁 ap指数（采用全球3h等效幅度指数ap来表征）和太阳风速度相似的9天和13.5天短周期变化,表明TEC的这种短周期特性主要与重现型地磁活动相关. 地磁纬度和地方时分析表明,夜间高纬地区正负相扰动明显,中低纬地区则以正相扰动为主,较大的TEC变幅主要发生在南北半球高纬地区,夜间南半球高纬地区TEC变化相对ap指数变化有相位延迟. 白天中低纬地区正负相扰动明显,TEC短周期变化与ap指数变化相位基本一致. 2008年TEC的9天和13.5天周期变化幅度大于2009年.      

Electron densities and temperatures in the Venus ionosphere: Effects of solar EUV,solar wind pressure and magnetic field

The Venus ionosphere is influenced by variations in both solar EUV flux and solar wind conditions. On the dayside the location of the topside of the ionosphere, the ionopause, is controlled by solar wind dynamic pressure. Within the dayside ionosphere, however, electron density is affected mainly by solar EUV variations, and is relatively unaffected by solar wind variations and associated magnetic fields induced within the ionosphere. The existence of a substantial nightside ionosphere of Venus is thought to be due to the rapid nightward transport of dayside ionospheric plasma across the terminator. Typical solar wind conditions do not strongly affect this transport and consequently have little direct influence on nightside ionospheric conditions, except on occasions of extremely high solar wind dynamic pressure. However, both nightside electron density and temperature are affected by the presence of magnetic field, as in the case of ionospheric holes.     

武汉地区电离层电子浓度总含量的统计经验模式研究

由武汉电离层观象台一个太阳黑子周期（1980-1990年）的实测电离层电子浓度总含量（TEC）资料，统计分析得出了武汉地区的一个TEC经验模式，模式很好地再现了武汉地区的TEC观测值，其预测误差在太阳活动高年稍太，低年较小；在春秋两季稍大，冬夏两季较小；在当地时间白天和傍晚稍大，夜间和早晨较小。此外，与国际参考电离层模式IRI的计算结果比较，本模式预测的TEC值更接近于实际观测结果，同时，本文也初步探讨了TEC的半年变化特征和冬季异常现象。     

Hemispheric,seasonal and latitudinal dependence of storm-time ionosphere during low solar activity period

Analysis of the seasonal, hemispheric and latitudinal variation of the ionospheric F2 peak during periods of disturbed geomagnetic conditions in 2011, a year of low solar activity, had been studied using hourly data obtained from low- and mid-latitude ionosonde stations. Our results showed an enhancement in F2-layer maximum electron density (NmF2) at daytime over low latitudes. For the mid-latitude stations, NmF2 depletion pre-dominates the daytime and overturned at nighttime. In general, the variation in terms of magnitude is higher in the low-latitude than at mid-latitude. The nighttime decrease in NmF2 is accompanied by a corresponding F2 peak height (hmF2) increase and overturned at daytime. The hmF2 response during the equinoctial months is lower than the solstices. NmF2 shows distinct seasonal, hemispheric and latitudinal dependence in its response. Appearance of a significant ionospheric effect in southern hemisphere is higher than in the northern hemisphere, and is more pronounced in the equinoxes at low latitudes. At mid-latitudes, the ionospheric effect is insignificant at both hemispheres. A negative ionospheric response dominates the whole seasons at the mid-latitude except for March equinox. The reverse is the case for the hmF2 observation. The amplitudes of both the NmF2 and hmF2 increase with increasing latitude and maximize in the southern hemisphere in terms of longitude.     

A nonlinear background removal method for seismo-ionospheric anomaly analysis under a complex solar activity scenario: A case study of the M9.0 Tohoku earthquake

A precise determination of ionospheric total electron content (TEC) anomaly variations that are likely associated with large earthquakes as observed by global positioning system (GPS) requires the elimination of the ionospheric effect from irregular solar electromagnetic radiation. In particular, revealing the seismo-ionospheric anomalies when earthquakes occurred during periods of high solar activity is of utmost importance. To overcome this constraint, a multiresolution time series processing technique based on wavelet transform applicable to global ionosphere map (GIM) TEC data was used to remove the nonlinear effect from solar radiation for the earthquake that struck Tohoku, Japan, on 11 March, 2011. As a result, it was found that the extracted TEC have a good correlation with the measured solar extreme ultraviolet flux in 26–34 nm (EUV_26–34) and the 10.7 cm solar radio flux (F_10.7). After removing the influence of solar radiation origin in GIM TEC, the analysis results show that the TEC around the forthcoming epicenter and its conjugate were significantly enhanced in the afternoon period of 8 March 2011, 3 days before the earthquake. The spatial distributions of the TEC anomalous and extreme enhancements indicate that the earthquake preparation process had brought with a TEC anomaly area of size approximately 1650 and 5700 km in the latitudinal and longitudinal directions, respectively.     

Prompt effects of solar wind variations on the inner magnetosphere and midlatitude ionosphere

It is well known that the solar wind can significantly affect high-latitude ionospheric dynamics. However, the effects of the solar wind on the middle- and low-latitude ionosphere are much less studied. In this paper, we report observations that large perturbations in the middle- and low-latitude ionosphere are well correlated with solar wind variations. In one event, a significant (20–30%) decrease of the midlatitude ionospheric electron density over a large latitudinal range was related to a sudden drop in the solar wind pressure and a northward turning of the interplanetary magnetic field, and the density decrease became larger at lower latitudes. In another event, periodic perturbations in the dayside equatorial ionospheric E × B drift and electrojet were closely associated with variations in the interplanetary electric field. Since the solar wind is always changing with time, it can be a very important and common source of ionospheric perturbations at middle- and low-latitudes. The relationship between solar wind variations and significant ionospheric perturbations has important applications in space weather.     

Study the behavior of the ionospheric frequencies at Karachi during three consecutive solar minima of cycle 21, 22 & 23 and their comparison with IRI-2016

The behavior of critical frequencies of ionospheric E and F2 layers (foE & foF2) along with minimum ionospheric frequency (fmin) is studied for solar minima of cycle 21 (1986), 22 (1996) and 23 (2008) over Karachi (24.95$°$N, 67.13$°$E), Pakistan. The station is located at the crest of equatorial ionization anomaly region. Beside seasonal differences, pronounced change in the values of frequencies is noted from one solar minimum to another solar minimum. A strong and direct correlation of foF2 with Smoothed Sunspot Number (SSN) and F10.7?cm solar flux is observed. In the minimum of cycle 23, reduction in foF2 is noted due to reduction of solar EUV as compared to other minima. Also disappearance of semi-annual variations in foF2 is noted in cycle 23 minimum. Unexpectedly higher values of foE and fmin are observed in minimum of cycle 23 as compared to other minima. It is difficult to explain this unusual behavior of fmin and foE along with disappearance of semi-annual variation in foF2. It is possible that during very low solar activity, thermospheric conditions are changed which in turn altered the ionosphere. Further investigation of atmosphere-ionosphere coupling is required to understand this complex behavior. On comparison of observed values with IRI-2016, higher deviations are observed in foE before noon hours while in case of foF2, large deviations are noted during daytime. The absence of foF2 semi-annual variation in cycle 23 is not reproduced by IRI-2016. It is suggested that IRI-2016 need some modification for extremely low solar activity condition.     

The impact of large solar events on the total electron content of the ionosphere at mid latitudes

Ionospheric disturbances associated with solar activity may occur via two basic mechanisms. The first is related to the direct impact on the ionosphere of EUV photons from a flare, and the second by prompt electric field penetration into the magnetosphere during geomagnetic storms. In this paper we examine the possibility that these two mechanisms may have an impact at mid latitudes by calculating the total electron content (TEC) from GPS stations in Mexico during several large X-ray flares. We have found that indeed large, complex flares, which are well located, may affect the mid latitude ionosphere. In fact, in the solar events of July 14, 2000 and April 2001 storms, ionospheric disturbances were observed to increase up to 138 and 150 TECu, respectively, due to the influence of EUV photons. Also, during the solar events of July 2000, April 2001, Halloween 2003, January 2005 and December 2006, there are large ionospheric disturbances (up to 393 TECu in the Halloween Storms), due to prompt penetration electric field, associated with CME producing geomagnetic storm.     

太阳极紫外辐射对热层大气密度的影响

对2001-2021年SOHO卫星的极紫外辐射测量数据,以及CHAMP,GRACE-A和SWARM-C卫星资料推导出的高分辨率大气密度数据进行统计分析,发现大气密度与极紫外测量值的相关系数大于密度与F10.7指数的相关系数,证实极紫外辐射在不同地方时的影响程度存在显著差异,从而驱动大气密度的周日变化。利用三颗卫星的高度差异揭示极紫外辐射对大气密度的加热效应在350～500 km范围随着高度增加而减弱。统计得到极紫外辐射影响在地方时和纬度上的空间差异：对夏季半球的影响大于冬季半球;在白天,对中纬度地区的影响高于赤道和高纬度地区;在夜间,密度对辐射的斜率在夏季半球高纬度地区存在峰值,在冬季半球中纬度存在谷值,模型DTM2000和NRLMSISE00未能准确刻画。为了改进经验模型,提出基于球谐函数的拟合方法,优于主流模型周日效应采用的表达式,对周日效应建模和修正提供有益借鉴。利用昼夜间能量传输和热层大气经向环流机制探讨了统计结果的物理机制。