全球电离层对2000年4月6-7日磁暴事件的响应

利用分布于全球的电离层台站的测高仪观测数据,对扰动期间,foF2值与其宁静期间参考值进行比较,研究了2000年4月6—7日磁暴期间全球不同区域电离层的响应形态,并通过对比磁扰期间NmF2的变化与由MSISR90经验模式估算的中性大气浓度比(no／nN2)的变化,探讨了本次事件期间的电离层暴扰动机制．结果表明,在磁暴主相和恢复相早期,出现了全球性的电离层F2层负相暴效应．最大负相暴效应出现在磁暴恢复相早期,即电离层暴恢复相开始时间滞后于磁暴恢复相开始时间．在磁暴恢复相后期,一些台站出现正相扰动．研究结果表明,本次事件期间的电离层暴主要是由磁暴活动而诱发的热层暴环流引起的．     

磁暴期间中国中低纬电离层不规则体与扰动分析

2017年9月8日发生了一次强磁暴，Kp指数最大值达到8.利用区域电离层格网模型（Regional Ionosphere Map，RIM）和区域ROTI（Rate of TEC Index）地图，分析了磁暴期间中国及其周边地区电离层TEC扰动特征和低纬地区电离层不规则体的产生与发展情况，同时利用不同纬度IGS（International GNSS Service）测站BJFS（39.6°N，115.9°E），JFNG（30.5°N，114.5°E）和HKWS（22.4°N，114.3°E）的GPS双频观测值，获取各测站的ROTI和DROT（Standard Deviation of Differential ROT）指数变化趋势.结果表明:此次磁暴发生期间电离层扰动先以正相扰动为主，主要发生在中低纬区域，dTEC（differential TEC）最大值达到14.9TECU，随后电离层正相扰动逐渐衰减，在低纬区域发生电离层负相扰动，dTEC最小值达到-7.2TECU；在12:30UT-13:30UT时段，中国南部低纬地区发生明显的电离层不规则体事件；相比BJFS和JFNG两个测站，位于低纬的HKWS测站的ROTI和DROT指数变化更为剧烈，这表明电离层不规则体结构存在纬度差异.      

地磁扰动期间日本Kokubunji站电离层的扰动特征分析

利用日本Kokubunji站(139.5°E,35.5°N)1959年1月到2004年12月共46年的F2层临界频率foF2参数,统计分析了Kokubunji站电离层F2层峰值电子浓度NmF2随地磁活动、太阳活动、季节和地方时变化的形态特征.结果表明,总体来看,磁暴期间Kokubunji站电离层响应以正暴为主,其中在太阳高年夏季为负暴,冬季为正暴,春秋季以负暴为主但幅度较小;在太阳低年夏季以正暴为主,冬季为正暴,春秋季以正暴为主.NmF2扰动与ap指数在夏季太阳高年负相关,在冬季无论太阳高年低年均为正相关,春秋季中4月和9月在太阳高年类似夏季,3月和10月在太阳低年类似冬季.电离层最大负相扰动对最大地磁活动的延迟时间约为12～15 h;正相扰动的延迟时间则分别为3 h和10 h.地磁活跃期间地方时黄昏后到午夜前倾向于正相扰动,清晨倾向于负相扰动.      

1998年5月磁暴事件期间全球电离层响应形态与机制的研究

利用全球40余个电离层台站的f0F2观测数据，采取对经度进行分区处理的方法，通过计算各台站f0F2参数对其月中的偏离百分比，对1998年5月大磁爆期间的电离层扰动形态进行了分析，并对可能的扰动机制进行了探讨，结果表明本次磁暴事件中，在磁暴主要活动相期间的电离层扰动与暴环流理论所描述的电脑层扰动特征相符，但在恢复相后期欧洲扇区台站出现的正相扰动似不能用暴环流理论来解释，它可能对应期间的行星行条件（太阳风与行星际磁场）的变化有关。     

2004年11月一次磁暴期间全球电离层TEC扰动分析

利用全球分布的GPS原始观测数据提取的电离层总电子含量(TEC)分析了2004年11月6日至12日期间全球电离层暴的形态特点与发展过程.结果表明,11月8日磁暴主相期间电离层暴以大范围的强烈正暴为主,在11月10日的恢复相,Dst又一次降到最低值前后期间,电离层再次受到很强的扰动,大范围的正暴和负暴交替出现.这次磁暴期间夏季半球的负暴更加强烈,反映出负暴偏向于在夏季半球发生的季节变化特点.另外,磁暴期间,夜晚TEC值普遍比磁暴前的平静期要低,具体是什么机制导致还需要进一步收集数据和分析.      

磁暴期间中低纬电离层暴形态和物理机制分析

本文利用东亚地区１２个低纬电离层台站的测高仪观测数据，对１９７８年８月２７日发生的一次曲型磁暴期间电离层峰值高度和密度的变化进行了分析。采用滑动平均区分开电离层中不同时间尺度的扰动，分析了影响中低纬度电离层暴的几种扰动形态特征，并对其物理机制进行了讨论。结果表明：伴随磁暴急始的磁层压缩，电离层中表现出峰值密度增加和峰值高度下降；磁暴主相期间热层大气暴环流及其所引起的中性大气成分变化控制着电离层的大     

一次中等磁暴期间全球电离层TEC及ROTI指数变化分析

地磁暴发生时,电离层会有偏离平均水平的强烈扰动.基于全球电离层TEC及其时间变化率ROTI(Rate of TEC Index)数据,对2014年8月一次中等强度磁暴期间的全球电离层影响进行了分析,探讨了磁暴所引发电离层暴的可能机制.研究发现,本次磁暴伴随有明显的电离层暴效应.磁暴期间:南半球电离层以正相暴为主,北半球电离层暴则整体表现为短暂正相暴后长时间强的负相暴;电离层在北半球的下降比南半球强,并且这种下降持续了约一周时间;低纬区域电离层变化幅度明显小于中纬区域,高纬区域则主要表现为负暴效应;赤道北驼峰出现了明显的南移现象,直至磁赤道两侧双驼峰结构消失.对磁暴期间三个不同扇区的电离层ROTI变化的分析表明:欧洲-非洲扇区磁暴前有电离层闪烁发生,磁暴发生后消失,而东亚-澳大利亚及美洲扇区则无此现象出现.研究结果表明,此次磁暴期间的电离层变化存在明显的时间和空间差异.     

一次中等磁暴期间全球电离层TEC及ROTI指数变化分析

地磁暴发生时,电离层会有偏离平均水平的强烈扰动.基于全球电离层TEC及其时间变化率ROTI(Rate of TEC Index)数据,对2014年8月一次中等强度磁暴期间的全球电离层影响进行了分析,探讨了磁暴所引发电离层暴的可能机制.研究发现,本次磁暴伴随有明显的电离层暴效应.磁暴期间:南半球电离层以正相暴为主,北半球电离层暴则整体表现为短暂正相暴后长时间强的负相暴;电离层在北半球的下降比南半球强,并且这种下降持续了约一周时间;低纬区域电离层变化幅度明显小于中纬区域,高纬区域则主要表现为负暴效应;赤道北驼峰出现了明显的南移现象,直至磁赤道两侧双驼峰结构消失.对磁暴期间三个不同扇区的电离层ROTI变化的分析表明:欧洲-非洲扇区磁暴前有电离层闪烁发生,磁暴发生后消失,而东亚-澳大利亚及美洲扇区则无此现象出现.研究结果表明,此次磁暴期间的电离层变化存在明显的时间和空间差异.     

中纬电离层暴负相开始时间与磁暴主相开始时间的对应关系

本文假设在磁暴主相期间,由于极光椭圆带处的空气被加热上升,从而使高纬高空出现富含分子的气体。这些气体由于扩散及与中性风的相互作用会向中低纬移动,其所到之处电子消失系数增加,从而导致负相电离层暴的发生。计算给出了全球中纬电离层暴负相的开始时间与磁暴主相开始时间之间的关系,并讨论了负相电离层暴发生的"时间禁区"问题。结果与有关统计结果符合得很好。      

中纬电离层爆负相开始时间与磁暴主相开始时间的对应关系

本文假设以磁暴主相期间，由于极光椭圆带处的空气被加热上升，从而使高纬高空出现富含分子的气体，这些气体由于扩散及与中性风的相互作用会向低纬移动，其所到之处电子消失系数增加，从而导致负相电离层暴的发生，计算给出了全球中纬电离层暴负相的开始时间与磁暴主相开始时间之间的关系，并讨论了负相电离层暴发生的“时间禁区”问题，结果与有关统计结果符合得很好。     

Modelling of Geomagnetic Storm Effects in the Ionosphere of East Asia

This paper presents simulated results of the ionospheric behavior during few geomagnetic storms,which were occurred in the different seasons. The numerical model for ionosphere-plasmasphere coupling was used to interpret the observed variation of ionosphere structure. Reasons why the positive storms are dominant in the winter whereas the negative ones are dominant in the summer season present the special interest for the mid-latitude ionosphere. A theoretical analysis of the processes controlling the ionospheric response to the geomagnetic storms has showed a good agreement between the simulated results and measurements, as well as the crucial role of the neutral composition variations to fit the calculated and the observed ionospheric parameters.      

Statistical study of the time delay of ionospheric TEC storms to geomagnetic storms in Taoyuan,Taiwan

We have studied the time delay of ionospheric storms to geomagnetic storms at a low latitude station Taoyuan (25.02°N, 121.21°E), Taiwan using the Dst and TEC data during 126 geomagnetic storms from the year 2002 to 2014. In addition to the known local time dependence of the time delay, the statistics show that the time delay has significant seasonal characteristics, which can be explained within the framework of the seasonal characteristics of the ionospheric TEC. The data also show that there is no correlation between the time delay and the intensity of magnetic storms. As for the solar activity dependence of the time delay, the results show that there is no relationship between the time delay of positive storms and the solar activity, whereas the time delay of negative storms has weakly negative dependence on the solar activity, with correlation coefficient −0.41. Especially, there are two kinds of extreme events: pre-storm response events and long-time delay events. All of the pre-storm response events occurred during 15–20 LT, manifesting the Equator Ionospheric Anomaly (EIA) feature at Taoyuan. Moreover, the common features of the pre-storm response events suggest the storm sudden commencement (SSC) and weak geomagnetic disturbance before the main phase onset (MPO) of magnetic storms are two main possible causes of the pre-storm response events. By analyzing the geomagnetic indices during the events with long-time delay, we infer that this kind of events may not be caused by magnetic storms, and they might belong to ionospheric Q-disturbances.     

Time delay of ionospheric TEC storms to geomagnetic storms and pre-storm disturbance events in East Asia

Using the TEC data at Beijing (39.61°N, 115.89°E)/Yakutsk (62.03°N, 129.68°E) stations of East Asia regions and relevant geomagnetic data from 2010 to 2017, we have studied the time delay of ionospheric storms to geomagnetic storms and compare it with our previous results of Taoyuan (25.02°N, 121.21°E) station (Zhang et al., 2020). The data shows a well-known local time dependence of the time delay, and seasonal dependences are different at these stations. In addition, there is no correlation between the time delay and the magnetic storm intensity /solar activity, except the time delay of negative storms has weakly negative dependence on the solar activity. Comparing with the results of Taoyuan station which is located at EIA region in East Asia, we find that the time delay increases nonlinearly as the latitude decreases due to different ionospheric backgrounds at these places. Moreover, the pre-storm disturbance events are found to have similar statistical characteristics as the pre-storm enhancement in Europe middle latitudes (Bure?ová and La?tovi?ka, 2007). By subtracting the common features of the pre-storm disturbance events, we preliminarily infer that auroral activity might be main driver of the pre-storm disturbance events.     

Moderate geomagnetic storms and their ionospheric effects at middle and low latitudes

This paper reports the global response of the mid high and low latitude ionosphere in four longitudinal sectors to two moderate geomagnetic storms that occurred during 2007 (the more intense storms occurred that year). The results obtained during these storms show that the ionospheric effects in general are not moderate in magnitude, showing an important degree of complexity as during intense storms. The outstanding features produced during the storms are significant positive storm effects at mid-high latitudes during the main phase/first part of the recovery, positive effects after the onset of the storm followed by negatives effects at middle latitudes and delayed positive effects during the night-time hours in the trough of the equatorial anomaly (“dusk” effect). Possible physical mechanisms for controlling the morphology of the ionosphere during these events are considered.     

Responses of the African equatorial ionization anomaly (EIA) to some selected intense geomagnetic storms during the maximum phase of solar cycle 24

This study investigates the morphology of the GPS TEC responses in the African Equatorial Ionization Anomaly (EIA) region to intense geomagnetic storms during the ascending and maximum phases of solar cycle 24 (2012–2014). Specifically, eight intense geomagnetic storms with Dst ≤ ?100 nT were considered in this investigation using TEC data obtained from 13 GNSS receivers in the East African region within 36–42°E geographic longitude; 29°N–10°S geographic latitude; ± 20°N magnetic latitude. The storm-time behavior of TEC shows clear positive and negative phases relative to the non-storm (median) behavior, with amplitudes being dependent on the time of sudden commencement of the storm and location. When a storm starts in the morning period, total electron content increases for all stations while a decrease in total electron content is manifested for a storm that had its sudden commencement in the afternoon period. The TEC and the EIA crest during the main phase of the storm is significantly impacted by the geomagnetic storm, which experiences an increase in the intensity of TEC while the location and spread of the crest usually manifest a poleward expansion.     

Effects of geomagnetic storms on the bottomside ionospheric F region

The geomagnetic storm is a complex process of solar wind/magnetospheric origin. The variability of the ionospheric parameters increases substantially during geomagnetic storms initiated by solar disturbances. Various features of geomagnetic storm act at various altitudes in the ionosphere and neutral atmosphere. The paper deals with variability of the electron density of the ionospheric bottomside F region at every 10 km of altitude during intense geomagnetic storms with attention paid mainly to the distribution of the F1 region daytime ionisation. We have analysed all available electron density profiles from some European middle latitude stations (Chilton, Pruhonice, Ebro, Arenosillo, Athens) for 36 events that occurred in different seasons and under different levels of solar activity (1995–2003). Selected events consist of both depletion and increase of the F2 region electron density. For European higher middle and middle latitude the F1 region response to geomagnetic storm was found to be negative (decrease of electron density) independent on the storm effect on the F2 region. For lower middle latitude the F1 response is weaker and less regular. Results of the analysis also show that the maximum of the storm effect may sometimes occur below the height of the maximum of electron density (NmF2).     

Seasonal variations of storm-time TEC at European middle latitudes

Global Navigation Satellite System (GNSS) measurements of the Total Electron Content (TEC) from local (Dourbes, 50.1°N, 04.6°E) and European IGS (International GNSS Service) stations were used to obtain the TEC changes during the geomagnetic storms of the latest solar activity cycle. A common epoch analysis, with respect to geomagnetic storm intensity, season, and latitude, was performed on data representing nearly 300 storm events. In general, the storm-time behaviour of TEC shows clear positive and negative phases, relative to the non-storm (median) behaviour, with amplitudes that tend to increase during more intense storms. The most pronounced positive phase is observed during winter, while the strongest and yet shortest negative phase is detected during equinox. Average storm-time patterns in the TEC behaviour are deduced for potential use in ionosphere prediction services.     

Empirical model of the TEC response to the geomagnetic activity over the North American region

The paper presents an empirical model of the total electron content (TEC) response to the geomagnetic activity described by the K_p-index. The model is built on the basis of TEC measurements covering the region of North America (50°W–150°W, 10°N–60°N) for the period of time between October 2004 and December 2009. By using a 2D (latitude-time) cross-correlation analysis it is found that the ionospheric response to the geomagnetic activity over the considered geographic region and at low solar activity revealed both positive and negative phases of response. The both phases of the ionospheric response have different duration and time delay with respect to the geomagnetic storm. It was found that these two parameters of the ionospheric response depend on the season and geographical latitude. The presence of two phases, positive and negative, of the ionospheric response imposed the implementation of two different time delay constants in order to properly describe the two different delayed reactions. The seasonal dependence of the TEC response to geomagnetic storms is characterized by predominantly positive response in winter with a short (usually ∼5–6 h) time delay as well as mainly negative response in summer with a long (larger than 15 h) time delay. While the TEC response in March and October is more close to the winter one the response in April and September is similar to the summer one.     

Midlatitude ionospheric F2-layer response to eruptive solar events-caused geomagnetic disturbances over Hungary during the maximum of the solar cycle 24: A case study

In our study we analyze and compare the response and behavior of the ionospheric F2 and of the sporadic E-layer during three strong (i.e., Dst?<??100nT) individual geomagnetic storms from years 2012, 2013 and 2015, winter time period. The data was provided by the state-of the art digital ionosonde of the Széchenyi István Geophysical Observatory located at midlatitude, Nagycenk, Hungary (IAGA code: NCK, geomagnetic latitude: 46.17° geomagnetic longitude: 98.85°). The local time of the sudden commencement (SC) was used to characterize the type of the ionospheric storm (after Mendillo and Narvaez, 2010). This way two regular positive phase (RPP) ionospheric storms and one no-positive phase (NPP) storm have been analyzed. In all three cases a significant increase in electron density of the foF2 layer can be observed at dawn/early morning (around 6:00 UT, 07:00 LT). Also we can observe the fade-out of the ionospheric layers at night during the geomagnetically disturbed time periods. Our results suggest that the fade-out effect is not connected to the occurrence of the sporadic E-layers.     

Response of the ionosphere to super geomagnetic storms: Observations and modeling

The relative importance of the main drivers of positive ionospheric storms at low-mid latitudes is studied using observations and modeling for the first time. In response to a rare super double geomagnetic storm during 07–11 November 2004, the low-mid latitude (17°–48°N geomag. lat.) ionosphere produced positive ionospheric storms in peak electron density (NmF2) in Japan longitudes (≈125°–145°E) on the day of main phase (MP1) onset (06:30 LT) and negative ionospheric storms in American longitudes (≈65°–120°W) on the following day of MP1 onset (13:00–16:00 LT). The relative effects of the main drivers of the positive ionospheric storms (penetrating daytime eastward electric field, and direct and indirect effects of equatorward neutral wind) are studied using the Sheffield University Plasmasphere Ionosphere Model (SUPIM). The model results show that the penetrating daytime (morning–noon) eastward electric field shifts the equatorial ionisation anomaly crests in NmF2 and TEC (total electron content) to higher than normal latitudes and reduces their values at latitudes at and within the anomaly crests while the direct effects of the equatorward wind (that reduce poleward plasma flow and raise the ionosphere to high altitudes of reduced chemical loss) combined with daytime production of ionisation increase NmF2 and TEC at latitudes poleward of the equatorial region; the later effects can be major causes of positive ionospheric storms at mid latitudes. The downwelling (indirect) effect of the wind increases NmF2 and TEC at low latitudes while its upwelling (indirect) effect reduces NmF2 and TEC at mid latitudes. The net effect of all main drivers is positive ionospheric storms at low-mid latitudes in Japan longitude, which qualitatively agrees with the observations.