Some Investigations on the Ionosphere during 2012 – 2014 in China

In this national biannual report, we will outline some recent progresses in ionospheric studies conducted by Chinese scientists since 2012. The mentioned aspects include: (1) the solar control of the ionosphere; (2) couplings between the ionosphere, lower atmosphere and plasmasphere; (3) ionospheric climatology and disturbances; (4) ionospheric irregularities and scintillation; (5) models, data assimilation and simulations; (6) unusual phenomena of the ionosphere; (7) possible seismic signatures presented in ionospheric observations, and (8) some methodology progresses. These progresses will enhance our ability to observe the ionosphere, provide more reasonable understanding about the states of the ionosphere and underlying fundamental processes, and stimulate ionospheric modeling, forecasting and related applications.

Le et al. [1] quantitatively investigated Solar Zenith Angle (SZA) dependence of the Total Electron Content (TEC) enhancements (ΔTEC) during flares by using data from GOES X-ray and EUV and GPS TEC during about 100 X-class flares in 1999-2006.The response of the ionosphere to a solar limb flare is found to be weaker than to a central one.Such a feature is called the CMD effect, which is found to be less distinct with decreasing X-ray class of the flare.The TEC response to flares is closely related to the intensity of EUV flux enhancement, but not highly related to that of X-ray flux.The flare effect is more significant in equinoxes than in solstices, under the influence of the seasonal variation of the background neutral density.
It has potential application to estimate the intensity of a solar flare using ionospheric observations.Xiong et al. [2] proposed an ionospheric solar flare indicator in terms of global network TEC observations during 1439 M-class and 126 X-class flares in solar cycle 23  (1996-2008).The indicator enables us to detect the extreme X-class flares, and also flares one order of magnitude or even more weaker (such as C-class flares).The validation shows that the indicator can detect 80% M-class and 92% X-class flares during 1996-2008, but insensitive to some limb flares.The outstanding advantage of the indicator, high sensitivity and temporal resolution, indicates, as Figure 1 illustrated, that it can be utilized to monitor detailed information about solar flares in space weather applications.
The radio wave emission shortly after a solar flare, Solar Radio Burst (SRB), originates from the solar atmosphere.During a SRB event, some specific frequency radio waves could interfere with the signals of technical systems, such as Global Navigation Satellite System (GNSS).Yue et al. [3] made case and statistical analyses of the SRB effect on the Radio Occultation (RO) technique during April 2006 to September 2012.RO signals of multiply Low Earth Orbit (LEO) based high-resolution GNSS were used to study the effect of SRB statistically.They showed more details in higher resolution response of RO signals in response to SRB event.The SRB could affect GNSS RO significantly by decreasing data volume and quality.
Ma et al. [4] investigated the effect of ~27 day solar rotation on the ionospheric F 2 region peak electron densities (N m F 2 ) observed in the East Asian-Australian sector from 1969 to 1986.They found that the ~27 day variations in solar radiation and geomagnetic activity, Fig. 1 Temporal evolution of the flare indicator and X-ray and EUV fluxes on 24 April 2001 [2]  caused by solar rotation, have large impact on the ionosphere, mainly driving the ionospheric ~27 day variations.At low magnetic latitudes, the contribution of solar EUV radiation to the ~27 day variation was greater than that of geomagnetic activity, but the results are opposite at higher latitudes, especially at midnight.
Chen et al. [5] investigated the correlations between SOHO/Solar EUV Monitor 26-34 nm EUV and the F 10.7 and Mg II proxies at timescales of solar cycle (long-term) and solar rotation (short-term).The long-term components of EUV and proxies are well correlated with each other, and the general relation between them can be captured by the 81-day averaged EUV and proxies.Short-term EUV-proxy correlation is poorer and variable during the solar cycle.The slopes of short-term EUV against proxies vary from solar rotation to solar rotation, and they are generally lower than those of long-term.The variability of solar EUV and proxies has significant signatures in the ionosphere.Proxies are improved to effectively capture the variations in EUV, although they cannot follow the short-term variability completely.
The solar activity has an extremely extended low in 2007-2009, with a record of spotless days since the discovery of the ionosphere.It is critical what this deep solar minimum had brought to the Earth's space environment.Liu et al. [6] and Yang et al. [7] analyzed the DPS ionograms measured at Jicamarca (12.0° S, 283.2°E) to explore what happened in the equatorial ionosphere during the deep minimum.Compared to 1996-1997, the seasonal median values of the critical frequency of F 2 -layer (f 0 F 2 ) remarkably reduced during the deep solar minimum.It is the first time to find that decreasing trend prevails in the F 2 -layer peak height (h m F 2 ) and Chapman scale height (H m ).Furthermore, the electron density profiles retrieved from ionogram demonstrate that the topside ionosphere in 2008-2009 is contracted strongly and more perceptible in the afternoon hours.The decrease in N e is strongest on September equinox and weakest on June solstice.In contrast, the bottomside profile thickness (B 0 ) in 2008-2009 shows higher values than that in 1996-1997 in some daytime intervals.

Couplings between the Ionosphere, Lower Atmosphere and Plasmasphere
Wan et al. [8] numerically investigated the couplings between the non-migrating tide (diurnal eastward wavenumber-3 DE3 tide) in the mesosphere and low thermosphere (MLT) and the response of the upper thermosphere and ionosphere.The tides were included as lower boundary sources to Global Coupled Ionosphere-Thermosphere-Electrodynamics Model developed by the Institute of Geology and Geophysics, Chinese Academy of Sciences (GCITEM-IGGCAS) [9] .The longitudinal wavenumber-4 (WN4) structure in the upper thermosphere and ionosphere was reasonably reproduced by GCITEM-IGGCAS.The simulations with some devised control conditions were used to discuss the relative contributions of the upward tidal wave propagation and the electro-dynamical coupling.
Quantitative analysis shows that the electro-dynamical coupling mechanism is responsible for almost all the ionospheric and thermospheric WN4 waves, while the direct wave propagation mechanism is important only for the thermospheric WN4 waves under low and moderate solar activity conditions.Xiong and Lühr [10] investigated the tidal signatures in the magnitude and the inter-hemisphere asymmetry of the Equatorial Ionization Anomaly (EIA) based on nine years of CHAMP and GRACE observations.Both the EIA magnitude and Crest-to-Trough Ratio (CTR) parameters show WN4/WN3 patterns are dependent on season.The contribution of tidal components to EIA in different months was identified.
Sudden Stratospheric Warming (SSW) is another manifestation of the coupling between the lower atmospheres and the ionosphere.The SSW in the winter of 2008/2009 is the strongest event recorded.Xiong et al. [11] found the semidiurnal variation of TEC enhancement with phase shifting forward in Beijing (40.30°N, 116.19°E) during 22 to 27 January 2009.Wind observations from the Beijing all-sky meteor radar illustrated that the semidiurnal solar tide in the mesosphere starts to increase before the SSW and maintains oscillation with period 16-20 days during this period.The phase comparison showed that lunar tides in TEC and zonal wind reach their maxima almost simultaneously, but lag 2-4 h behind the meridional wind.They proposed that the semidiurnal variation of TEC is attributed to the coupling between the mesosphere and ionosphere through both solar and semidiurnal lunar tides, and the enhancement in semidiurnal lunar tide is responsible for the TEC peak's forward shift.
For the 2005-2006 SSW event, Mo et al. [12] revealed a quasi-16-day periodicity in the meridional movement of Equatorial Ionization Anomaly (EIA) crest, which is derived from GPS observations at low latitude over China.Such large-scale periodic movement of EIA crest is likely related to the globally enhanced stratospheric planetary waves.The Equatorial Electrojet (EEJ) and h m F 2 at two Chinese ionosonde stations also vary in phase with the EIA movement.
Comparing the tide structures and the corresponding signatures in the ionosphere should shed some light on the coupling between the ionosphere-thermosphere and the lower atmosphere.Luan et al. [13] derived the latitudinal, seasonal and solar cycle variations of the signature of terdiurnal westward migrating wave number 3 (TW3) tide in JPL TEC during 1999-2011.The TW3 absolute amplitude peaks in the magnetic equatorial region, and the TW3 relative amplitude has a main peak in the magnetic equatorial region and a secondary one at magnetic middle latitudes.Moreover, the relative amplitude of zonal mean TEC in equatorial region shows little solar activity dependence.
Gong et al. [14] presented an extensive analysis of atmospheric tides in the low-latitude thermosphere and their responses to a major SSW event (18-23 January 2010).Their analysis was based on the Arecibo dual-beam ISR observations, which overlapped with the SSW event.In order to investigate the thermospheric tidal response to the SSW, nine consecutive days of data were separated into two periods: the non-SSW period and the SSW period.The power spectral density analysis shows several strong tidal components in both periods but with large period-to-period variability.This large variability is most likely due to the effects of the SSW.
Tang et al. [15] studied the ionospheric quasi-biennial oscillations (QBO) through analyzing the TEC observations during the period of 1999-2011.QBO signal only appears in TEC during solar maximum, existing at latitudes from 50°S to 50°N with 22-34 months period.In the equatorial region, the QBO exhibits a significant feature of EIA, where the transition of phases occurs 2-6 months later than at high latitude.Stratospheric QBO possibly influences the QBO phenomenon in the ionosphere.
The atmosphere-ionosphere coupling may influence the ionospheric absorption.Hao and Zhang [16] checked the manifestations of atmospheric influences on ionospheric absorption in the East Asia sector.They detected a dominant 27-day periodic variation in the absorption level recorded at Beijing with a sweep frequency technique for 4 years.Unusual enhancements of the absorption level in winters (winter anomaly) vary with periods mainly in the range of 8-12 days.Comparing to the solar 27-day rotation component, the oscillations with shorter periods are related with planetary wave activities in the lower atmosphere.
Chong et al. [17] investigated the patterns of the plasmaspheric electron content (PEC) derived from the observations of the incoherent scatter radar (ISR) and GPS at Millstone Hill (42.6°N, 288.5°E).Both PEC and its relative contribution to GPS-TEC exhibit obviously diurnal variations, with higher values and lower relative contribution during daytime than nighttime.The PEC value is higher for the High Solar Activity (HSA) and lower for the Low Solar Activity (LSA).The relative PEC contribution at night can be as high as 60% for HSA and 70% for LSA.
Wang and Lühr [18] performed a statistical study to investigate the seasonal effect of Subauroral Polarization Stream (SAPS) on the ion upflow in the northern hemisphere duskside ionosphere by using two years' DMSP plasma observations.Obvious upflows appear in the topside ionosphere around the SAPS region, indicating an important relationship between SAPS and the local plasma upward motion.Both SAPS and ion upward velocities show similar seasonal variations, largest in winter and smallest in summer, irrespective of geomagnetic activity.The SAPS-related frictional heating at mid-latitudes play an important role in the local formation of the strong upward flow, which might provide a direct ionospheric ion source for the ring current and plasmasphere in the duskside sector.
Dispersive Alfven Waves (DAWs) play a significant role in auroral generation of the magnetosphere-ionosphere coupling system.Zhao and Lu [19] showed that dispersive standing Alfven waves can generate the field-aligned currents which transport energy into the auroral ionosphere, where it is dissipated by Joule heating and energy lost due to electron precipitation.The Joule dissipation can heat the ionospheric electrons and influence the ionospheric Pedersen conductivity.The conducting ionosphere can also strongly affect the magnetospheric currents.The ponderomotive force can cause the plasma to move along the field line, and generate ionospheric density cavity.The nonlinear structuring can lead to a dispersive scale to accelerate auroral particle, and the Alfven waves can be trapped within the density cavity.

Ionospheric Climatology
In the middle latitude regions over America, the ionosphere over the east and west coast exists significant differences.The configuration of the geomagnetic field is proposed to be the main controller of the neutral field effect.Zhao et al. [20] showed that strong east-west differences (R ew ) also exist in the ionosphere over the Far East zone, where a clear diurnal variation is quite similar to that in US.New features over Far East zone include: (1) The noontime negative R ew is most pronounced in April-June, while in US during February-March; (2) The positive R ew at night is much less evident than in US, especially no winter enhancement; (3) The magnitude of negative Rew tends to enhance toward solar maximum and the trend reverses in US.
Xu et al. [21] compared the east-west differences of the mid-latitude TEC in North American, South American and Oceania regions.They used ground-based networks of TEC data covering about a solar cycle from 2001 to 2010.Their results show that, for nearly all seasons from 2001 to 2010, systemic differences exist in mid-latitude TEC in both sides of the three zero declination longitudes.The east-west differences vary markedly with local time but depend weakly on season and level of solar activity.Theoretical analysis confirms that the effects of the longitudinal variations of both declination and zonal wind are well consistent with the observed differences of TEC in both sides of the zero declination longitudes.
Xu et al. [22] collected the N m F 2 data from 33 stations in three longitude sectors from 1969 to 1986 to study the seasonal variation of N m F 2 .There is a ter-annual variation in daytime N m F 2 , especially at middle latitude, but no obvious ter-annual oscillation signature in the nighttime.The ter-annual amplitude is also correlated with the product of the amplitudes of annual and semiannual oscillations, which suggests that the ter-annual oscillation might be related to the nonlinear interaction between the annual and semiannual oscillations.These three oscillations vary with solar activity, with larger amplitudes during solar maximum.
The possible differences of the ionosphere in two equinoctial seasons are named the equinoctial asymmetry.Chen et al. [23] collected the data of several solar cycles of N m F 2 and one solar cycle of JPL TEC maps to investigate the equinoctial asymmetries in ionospheric electron density and its solar activity dependency.They found different latitudinal patterns of solar cycle dependency of equinoctial asymmetry in noontime N m F 2 and TEC.With increasing solar activity, the equinoctial asymmetry TEC becomes stronger at all latitudes, while that of N m F 2 increases at middle latitudes, decreases or changes little at low latitudes.The solar sensitivity of N m F 2 and TEC also shows equinoctial asymmetries.
Xiong et al. [24] studied the morphology of the Equatorial Ionization Anomaly (EIA) at CHAMP (~400 km) and GRACE (~480 km) altitudes, focusing on the magnitude and inter-hemispheric asymmetry of EIA.About nine years (2001-2009) of observations from CHAMP and GRACE were analyzed.The electron density and the magnetic latitudes of the EIA crests both peak around 14:00 LT, while the EIA CTR peaks much later.The magnetic latitude of the EIA can reach 13° at CHAMP altitude around December solstice, while the crests move much closer to the dip equator at GRACE altitude during low solar activity years.An inter-hemispheric asymmetry can be seen clearly in solstice EIA.At CHAMP altitude the EIA has higher electron density in the winter hemisphere during morning to noontime hours and subsequently in the summer hemisphere.At GRACE altitude, the electron density of the EIA crest is always stronger in the summer hemisphere at the whole daytime.The SAMI2 simulations can reproduce such features at the two altitudes.
The north-south asymmetry of the EIA can be explored using the TEC observations from the IGS GPS network.Huang et al. [25] examined the hemispheric asymmetry in the EIA crest regions in the 110°E longitude sector under geomagnetic quiet conditions during the period from 2000 to 2011.The strength, latitude and occurrence time of the EIA crests are found to be asymmetric about the magnetic equator.The solar activity levels have a significant effect on EIA crest and the north-south asymmetry.
Wu et al. [26] explored the variations of TEC in 2004 over China using the TEC observed from China Crustal Movement Observation Network.The diurnal pattern has a peak in post-noon hours, and the seasonal variation peaks around equinox months.The TEC has higher values with decreasing latitude, and obvious moderate longitudinal differences in both sides of the zero magnetic declination longitude.The day-to-day variation of TEC is less significant than that of N m F 2 .
Xiong et al. [27] compared the predictions of the International Reference Ionosphere (IRI-2007) model with the CHAMP and GRACE in-situ measurements for the years from 2005 to 2010 in the subauroral regions.N e in the trough region peaks in local summer and attains valley in local winter.Around 100°W and 60°E, two larger electron density sectors feature can be seen in both hemispheres, which attributed to the electron extending from middle latitude to trough region.The IRI generally overestimates N e in the trough region.In detail, the model predicts quite well in the southern hemisphere and needs significant improvement in northern hemisphere.

Ionospheric Disturbances
Severe space weather events may drive strong disturbances in the ionosphere, which seriously influence accurate navigation, telecommunication, and other applications.With the development of ground and space observation techniques, various efforts have been made to understand the long-term statistical variations and sudden disturbances of the ionosphere.Xiao et al. [28] outlined some progresses of researches on ionospheric disturbances achieved in recent years.
Large-Scale Traveling Ionospheric Disturbances (LSTIDs) are the ionospheric manifestations of Atmospheric Gravity Waves (AGWs), which were launched from intense high latitude sources, such as Joule heating and particle precipitation.Song et al. [29] studied the global propagation features of the LSTIDs during the 7-10 November 2004 magnetic storm, and found that the horizontal phase velocities of LSTIDs were different among different regions, and the attenuations of the daytime LSTID amplitude were larger than the nighttime ones.Song et al. [30] analyzed the LSTIDs in China during the 28 May 2011 geomagnetic storm.Two LSTIDs were detected: one is propagating northeastward in southwestern China, and the other is propagating southwestward in northeastern China.The observations of Song [29−30] reflect the variation of the propagation properties of LSTIDs with respect to latitude and local time.During the medium storm on 28 May 2011, a substorm onset and initiated a slow-speed LSTID over North America just after midnight [31] .Several hours after that, Ding et al. [31] observed two LSITDs in East Asia using data from Chinese GPS network, combined with observations from an ionosonde chain.Although the nighttime LSTID over China travelled farther south than the earlier dusk event, both disappeared in South China.This was due to an increase of the attenuation and uplift of the ionosphere at low latitudes; both resulted in reduced amplitude of TEC perturbations.
Ding et al. [32] compared two poleward-propagating LSTID cases during a moderate storm.The observations from the GPS network and ionosondes in China and Southeast Asia display a northeastward-propagating LSTID in the 30 May morning and the other during the nighttime of 1 June.Both LSTIDs occurred during the storm's recovery phase and experienced severe dissipation.Although the relative amplitude of the nighttime LSTID was ~60% larger than that of the morning event initially, strong ion-drag dissipation induced by a strong nighttime enhancement in background TEC led to dissipation quicker than the morning event.Therefore, poleward propagating LSTIDs were likely excited by some local sources, possibly excited through the dissipation of some primary medium-scale disturbances from the lower atmosphere.
Ding et al. [33] conducted a comparative study of the climatology of LSTIDs over North America and China in 2011-2012 based on the GPS observations from two dense regional coverage networks.They identified a total of 390 LSTIDs in China and 363 events in North America.These LSTIDs were categorized into three types, according to the propagating directions.The southward LSTIDs over North America show similar diurnal and seasonal variations to those of geomagnetic disturbances, but over China such type LSTIDs do not show the similar variations.Northward LSTIDs occur much less frequently and are mainly observed in China.Westward LSTIDs are seen in both regions during local sunrise and may be excited by the moving solar terminator.The propagation direction of westward events changed from northwestward during winter solstice to southwestward in summer solstice, which is consistent with the seasonal orientation of the solar terminator.
Song et al. [34] further statistically studied the LSTIDs triggered by the solar terminator over China.The occurrence rate of LSTIDs has maximum in winter and minimum in summer, and the propagation direction shows a highly seasonal dependence.
Cai et al. [35] presented further observational evidence for the transpolar propagation of LSTIDs from their nightside source region to the dayside reported earlier.Slant TEC (STEC) from GPS receiver chains longitudinally aligned in North American and European sectors was analyzed to demonstrate presences of LSTIDs in both nightside and dayside mid-latitude.The derived periods of the ionospheric disturbances are in good agreement with that of the transpolar AGW recorded by EISCAT/ESR radars.The observed daytime and nighttime mid-latitude LSTIDs are likely to origin from auroral latitude.After launched from the nightside, the waves propagate simultaneously equatorward and poleward, which are recorded by North American GPS receiver chain and by EISCAT/ESR radars and GPS receiver chains, respectively.
The zonal electric field strongly modulates the low-latitude ionosphere, especially under disturbance conditions.Sun et al. [36] compared the disturbance vertical drift in ionospheric F layer during the initial and main phases of 50 intense storms in terms of historical ionosonde data observed at Haikou and Chongqing.The disturbance drift is taken from an empirical model.The drifts at the base and peak height are comparable and both increase in magnitude under stronger disturbances.Moreover, drifts over Haikou are larger than over Chongqing.
A positive ionospheric storm occurred during the main phase of the 20 November 2003 superstorm.Zhao et al. [37] investigated this event using observations from ground-based GPS TEC and the meridian chain of ionosondes distributed along the Latin America longitude of ~280° E. At mid-and low latitudes the maximum enhancement is 3.2-7.7 times in the topside ionospheric electron content compared to the bottomside.Moreover, h m F 2 at middle to low latitudes exceeds 400 km and increases by 100 km compared with that on the quiet day over the South American area, which might be resulted from a continuous eastward penetration electric field and storm-generated equatorward winds.
Wu et al. [38] studied the high energy particle precipitation effects in Arctic ionosphere during the main phase of the 14-15 December 2006 magnetic storm.Observations are from GPS TEC, EISCAT radar, RO from both CHAMP and COSMIC, and the ionospheric absorption of cosmic radio noise measured by the Imaging Riometer for Ionospheric Studies at Kilpisjärvi (69.05°N, 20.79°E).During the main phase, significant increases in electron density were found in the Arctic ionosphere, in Scandinavian, Northwest part of Russia and Svalbard (SNRS) regions.Increase in electron density is primarily at an altitude of about 110 km.The increases were accompanied by the ionospheric absorption enhancement at altitude of about 90 km.The observations provide direct evidence that the storm-time enhancement in E-layer electron density (e.g., the sporadic E) can contribute dominantly to the observed TEC increase.
Luan et al. [39] performed a superposed epoch analysis to investigate the relative impact of the solar wind/ Interplanetary Magnetic Field (IMF) on geomagnetic activity, auroral hemispheric power, and auroral morphology during Corotating Interaction Regions (CIRs) events between 2002 and 2007.They compared the CIRs effects under different solar wind/IMF conditions.Interesting auroral morphology is found around CIR onset time.Their work suggests during CIR events, southward B z plays the most critical role in determining geomagnetic and auroral activity, whereas solar wind speed is the next most important contributor.The solar wind dynamic pressure is the least important factor, as compared with B z and solar wind speed.
The observations of CHAMP thermospheric mass density (normalized to 400 km) during 2001-2008 and TIMED/GUVI O/N 2 from 2002 to 2008 are used by Liu et al. [40] to investigate the solar cycle and seasonal dependencies of the thermospheric response to CIRs.The relative changes of the thermosphere vary with solar cycle, decreasing pronouncedly in neutral density and increasing in high latitude O/N 2 at higher solar levels.The relative deviations present a seasonal asymmetry.On the dayside, the peak increases of neutral density at high latitudes in average are higher in the summer hemisphere than in the winter hemisphere.In addition, neutral density changes are more remarkable at nighttime than at daytime.
To estimate the high-speed stream influence on the ionosphere during the recent deep solar minimum year 2008, Liu et al. [41] analyzed the equatorial f 0 F 2 , h m F 2 and TEC in the American sector.Their results reveal a prominent 9-day oscillation in dip equatorial h m F 2 and f 0 F 2 , whose amplitudes are not always positively correlated with those of TEC.The band-passed-filtered TEC displays latitudinal patterns, including tilt latitudinal configuration, opposite correlation between the crests and trough, and south-north asymmetry.
Knowledge of the solar sector polarity effects on the ionosphere may provide some clues in understanding the day-to-day variability and "hysteresis" effect of the ionosphere.Liu et al. [42] made a comprehensive investigation on the ionospheric response to changes in solar sector polarity in terms of solar cycle, season and local time dependencies.The solar sector polarity effect is more appreciable in equinoctial months than in solstitial months, which is mainly caused by larger southward B z components in equinox.
Several solar events occurred on November 6-10, 2004, including solar flares and Coronal Mass Ejections (CMEs), which triggered two large geomagnetic storms and continuous energy proton events.Zhang et al. [43] detected that, in the East-Asian region, a large positive ionospheric storm happened on November 8 and strong spread-F at mid-latitude appeared on November 10.They took the Rate of TEC (ROT) derived from dualfrequency GPS measurement to study the characteristics of the spatial distribution of the ionosphere fluctuations and their temporal evolution.Strong fluctuating activity in the ionosphere occurred in the mid-latitude regions between longitudes of 100°E and 180°E in both hemispheres.
A long-lasting southward turning of Interplanetary geomagnetic Field (IMF) for 30 h below -10 nT triggered a major geomagnetic storm on July 15-16, 2012.Wang et al. [44] observed prominent large-scale ionospheric disturbances in North China during this storm event.They used the newly built China Seismic-ionospheric Ground-based Monitoring Network (CSGMN), including GPS network and oblique and vertical sounding systems, to investigate the ionospheric responses at different storm phases.A moderate and a strong positive storm were observed around the noon and the sunset sector on 15 July, respectively.Then, an intense negative storm effect occurred on the entire day of July 16.
Xiao et al. [45] reconstructed the low-latitude electron density distributions during the November 2004 superstorm from GPS observations of ground-based IGS network and onboard CHAMP and GRACE satellites using time-dependent 3D tomography method.The reconstructed electron densities are validated with CHAMP and GRACE in situ measurements.The long-lived positive storm phase during the first main phase of the storm (November 8) is mainly attributed to enhancement of electron density in the upper F region.The top-hat-like double layers occurred in the equatorial ionization anomaly region during the main phase of the storm.At the time of the minimum of Dst The column-like enhanced electron density structures are found in the longitudinal sector about 157°E, which extend from the topside ionosphere toward plasmasphere, reaching at least about 2000 km as high.Their footprints stand on the two peaks of the EIA.
Xiong et al. [46] investigated the relationship between the polar cap potential and its temporal variation rate and the disturbed equatorial ion velocity during 2001 to 2003.The equatorial vertical ion drift is measured by DMSP, and polar cap potential from AMIE output.Their analysis indicates that the electric field penetrates more easily under rapidly varying solar wind input.The optimal delay time of electric field penetration from the high-latitude magnetosphere to equatorial ionosphere has local time dependence, being longer at nightside.The penetration efficiency is lower at day than at night.

Ionospheric Irregularities and Scintillation
A VHF coherent scatter radar with frequency of 47.5 MHz and peak power of 24 kW has been operated since February 2009 at Sanya (18°N, 109°E, dip 13ºN), a southern city of Hainan Is., China.The characteristics of Field-Aligned Irregularities (FAIs) in the low latitude ionosphere have been extensively studied using the observations of the Sanya VHF coherent radar.Several interesting aspects of the FAIs observed include the E-region continuous [47−48] and quasi-periodic FAI echoes [49] , the post-midnight F region FAIs [50] , and the Range Spread Trail Echoes (RSTEs) produced by meteoroid ablation [51] .Li et al. [52] analyzed the data from the continuous observations of F region irregularities by the Sanya VHF radar, an ionosonde and a GPS scintillation/TEC receiver during the equinoctial and June solstitial months of 2009-2010.The F region 3 m FAIs appeared frequently at post-sunset hours in equinox, but initiated mostly at midnight/post-midnight in June solstice.Comparison of FAIs observed from GPS scintillations, TEC fluctuations and spread-F shows that the equinoctial FAIs coincided well with GPS scintillations and TEC fast depletions, which are associated with the development of Equatorial Plasma Bubbles (EPBs).For the June solstitial FAIs, satellite in-situ measurements show that some post-midnight FAI structures over Sanya may originate from equatorial region and then extend to Sanya latitude along magnetic field lines, and the others have local origins.Both types of post-midnight FAIs in June solstice are associated with the occurrence of spread-F, not accompanied by GPS scintillations and TEC depletions.
In attempts to understand the day-to-day variability in Equatorial Spread-F (ESF), studies in recent years have revealed the role of seeding perturbation as a dominant source of such variability.Using the Sanya VHF radar five-beam scanning measurements in east-west direction, Li et al. [53] investigated the correspondence between the Large-Scale Wave Structures (LSWS) and the development of ESF over Sanya.The LSWS and ESF have nearly a one-to-one relationship when the F layer undergoes an abrupt Post-Sunset Rise (PSSR).However, under weak or even moderate PSSR conditions, more factors other than the LSWS could play crucial roles favoring the growth of ESF instabilities responsible for ionospheric scintillations.Further, Li et al. [54] conducted simultaneous ESF backscatter plume observations using the Equatorial Atmosphere Radar and the Sanya VHF radar to investigate the temporal and spatial evolutions of plumes and their smaller scale longitudinal differences in Southeast Asia.The simultaneous radar beam steering measurements revealed that the LSWS could trigger the sunset equatorial plasma plumes occurring quasi-periodically at a wide longitudinal span of about 25°.On the other hand, the observations indicated that the small-scale waves, unlike the LSWS which occurred at a larger longitudinal span of more than 1000 km, might occur only at a narrower longitudinal range.It is quite possible that the appearance of small-scale wave structure in the bottomside F region may be required for further evolution of bottomside spread F or plumes under weak PSSR.
Luo et al. [55] investigated the three-dimensional linear growth rate of Rayleigh-Taylor (R-T) instability.The peak growth rate of the R-T instability significantly relies on local time, season, solar activity and longitude, displaying similar characteristics with that of the irregularities and scintillations in equatorial and low-latitude ionosphere.Therefore, the peak growth rate can be used to indicate the occurrence of the ionospheric irregularity and scintillation.
Luo et al. [56] studied how the hemispheric asymmetry of the background ionosphere and the longitudinal variations of neutral wind and declination influence the development and evolution of generalized R-T instability.They adopted the method of flux-tube integration in each geomagnetic hemisphere.The flux-tube integrated linear R-T instability growth rate shows significant hemispheric asymmetry, and the neutral wind with hemispheric asymmetry may result in an asymmetric distribution of ionospheric irregularities.Longitudinal variations of the neutral wind and the declination could cause the longitudinal variation of ionospheric irregularities.
Five years of CHAMP Fluxgate Magnetometer (FGM) data were analyzed by Xiong et al. [57] to investigate the characteristics of Equatorial Plasma Bubbles (EPBs).Band-passes with four different cut-off periods were used to filter the FGM data to retrieve the EPBs with selected spatial scale sizes in the meridional plane ranging from 76-608 km.By comparing the properties of EPB occurrence for different scale sizes with the global distribution of plasma vertical drift from ROCSAT-1, they found that EPBs reaching higher altitudes are more structured than those sampled near the topside of the depleted fluxtube.Small-scale EPB structures are observed by CHAMP, primarily in the Brazilian sector around November, with high post-sunset vertical plasma drift.
A puzzling phenomenon in the daytime equatorial ionosphere is the so-called 150 km coherent backscatter echoes induced by the irregularities in the height range of 140-180 km.Using the Sanya VHF radar measurements, Li et al. [58] presented first results of daytime 150 km echo from a magnetic latitude close to the northern anomaly crest region.It is found that the rare observation of daytime 150 km echoes with the Sanya VHF radar was preceded by the occurrence of an unusual intermediate layer, which is identified as abnormal traces at the upper E region in corresponding ionograms.They suggested that the critical conditions favoring the irregularity growth were linked with the upper E region abnormal density layer traces that were possibly associated with gravity wave activity.
A layered model was taken by Shi [59] to analyze the influence of electrical conductivity and magnetic dip angle on the Ionospheric Alfven resonant (IAR) feedback instability.Numerical calculations show that the dip angle effectively modifies the resonant frequencies and growth rate of the IAR, which consequently influence the IAR feedback instability.Considering Hall conductance can increase the growth rate, especially for large dip angle.

Models, Data Assimilation and Simulations
Ren et al. [9] further developed ionospheric theoretical model, and built a new three-dimension mid-and low-latitude theoretical ionospheric model, named Three-Dimension Theoretical Ionospheric Model of the Earth in the Institute of Geology and Geophysics, Chinese Academy of Sciences (TIME3D-IGGCAS).The new model covers the domain of mid-and low-latitude ionosphere and whole plasmasphere, self-consistently solves the equations of mass continuity, motion and energy of electron and ions to provide the main ionospheric and plasmaspheric parameters (number density, velocity, and temperature) in a realistic configuration of the geomagnetic field.
Ren et al. [60] simulated the temporal-spatial distribution of the summer nighttime anomaly at mid-latitude (MSNA) and the influences of thermospheric meridional and zonal winds on the formation of MSNA using the TIME3D-IGGCAS.In their simulations, MSNA mainly appears in three regions, East Asia, North Atlantic-Europe and the South Pacific region, in local summer, also frequently in equinox.MSNA in the third region is obviously stronger than in the other regions.The formation of MSNA is mainly related with the meridional wind, and the zonal wind also plays an important role in the South Pacific region.
The vertical drift plays a key role in forming of the equinoctial asymmetry of the low latitude ionosphere.Ren et al. [61] simulated how the equatorial vertical drifts are influenced by lower thermospheric tidal winds using the TIDM-IGGCAS-II model.The tidal winds below 105 km are taken from TIMED/TIDI observations.The simulations show larger daytime vertical drift in March Equinox than in September Equinox in most longitudinal sectors.Asymmetry in daytime vertical drift is stronger in the eastern hemisphere than in the western hemisphere, which is driven by DW2 tides and geomagnetic fields.Moreover, the DE3 tides drive the equinoctial asymmetry in the wave number 4 structures in vertical drift.
Yue et al. [62] constructed a global ionospheric data assimilation model based on empirical models and the Kalman filter.They performed a series of simulations based on observing systems.A sparse matrix method is used to relief the huge computation and storage problems.The slant TEC from various ground-based, low-Earth orbit satellites, and cross-link between COSMIC-2 low and high inclination satellites are also simulated.The joint system shows huge impact in specifying the ionosphere.Especially, the COSMIC-2 mission can potentially complement and optimize the global ionospheric specification, contributing significantly to accurate ionospheric nowcasting.
Based on a two-layer Empirical Orthogonal Function (EOF) technique, Zhang et al. [63] built an updated global model of h m F 2 by combining data from COSMIC/ FORMOSAT-3 RO measurements and from global ionosonde stations (including 10 Chinese stations).Their model reproduces reasonably well, compared to the measurements of COSMIC RO and digisondes, and it also has a better performance than the well-known International Reference Ionosphere (IRI) model.
Yu et al. [64] proposed a technique based on Canonical Correlation Analysis (CCA) to estimate N m F 2 at a single station or globally from TEC.At a single station, the obtained CCA modes consist of the patterns and corresponding amplitudes, which reflect the short-term and long-term variations of N m F 2 .This method can reasonably estimate the missing values of N m F 2 at a single station from continuous measurements of the TEC.Global maps of N m F 2 (N m F 2 -GIMs) can be constructed with the global ionospheric maps for TEC (TEC-GIMs), successfully reproducing their temporal and spatial structures.Furthermore, the correlation coefficient (root mean square error) of the CCA N m F 2 versus the observed N m F 2 is relatively higher (lower) than that of IRI-07 N m F 2 over global stations (Figure 2).Wan et al. [65] adopted Empirical Orthogonal Functions (EOF) to model the global TEC through separating the temporal and spatial variation of TEC into some eigen modes and their amplitudes.The eigen modes give the mapping patterns (latitude and longitude) of TEC, and the corresponding amplitudes depict the variations at different time scales (the solar cycle, annual and semiannual, and the diurnal universal time variations).It is found that the first two modes capture most of the ionospheric climate properties.The EOF analysis has outstanding advantages in ionospheric climatology modeling.A similar model was also built [66] .
Liu et al. [67] constructed a middle-latitude single station model based on a long-term dataset of hourly values of f 0 F 2 recorded at Wakkanai (45.4°N, 141.7°E).The model consists of two modules, geomagnetic quiet and active modules.The geomagnetic quiet module incorporates local time, seasonal, and solar variability of climatological f 0 F 2 and its upper and lower quartiles.It is the first attempt to predict the upper and lower quartiles of f 0 F 2 to account for the notable day-to-day variability in ionospheric f 0 F 2 .In the geomagnetically disturbed module, the storm-induced deviations are described by diurnal and semidiurnal waves modulated by a magnetic activity index, reflecting the delayed responses of f 0 F 2 to geomagnetic activity forcing.The introduced Kalman filter algorithm optimizes the coefficients of the model in real time.Their model demonstrated that introducing the Kalman filter algorithm is promising for improving the accuracy of short term ionospheric predication.
Sun et al. [68] established an empirical model for real-time ionospheric correction, using the nonlinear relations between ionospheric f 0 F 2 data from two low-latitude ionosondes and equatorial disturbance field data from an empirical model during fifty storms.They evaluated the performance of the model during twelve intense storms compared with another empirical correction model, STORM.The comparison of the performance of the models indicates that including the disturbance electric fields is important for improving the storm-time ionospheric corrections at low latitude.
Zhao et al. [69] constructed a model for 1-hour ahead prediction of f 0 F 2 with the AdaBoost-BP algorithm.In their model, twenty-two years' f 0 F 2 data from nine ionosonde stations in the East-Asian sector are used.The introducing of the AdaBoost method improves the modeling/prediction of ionospheric parameters, compared to BP Neural Network, Support Vector Regression and the IRI model.
Wang et al. [70] explored the diurnal and seasonal variations of ionospheric profile parameter B 2bot at Hainan station under low solar activity conditions.They used a local time function in different seasons to correct the B 2bot formula of NeQuick-2 to improve its performance.
Based on the EISCAT Svalbard Radar (ESR) observations, He et al. [71] analyzed the diurnal variations of N m F 2 in a solar minimum year 2007.The soft precipitation electron had an evident effect on the N m F 2 over ESR.The number of electrons in lower energy band becomes less under more active geomagnetic disturbances, resulting in a lower N m F 2 .A comparison is conducted between the IRI model and observations, which illustrates that the IRI model has a better N m F 2 prediction when the photoionization is dominant and becomes worse when the electron precipitation is dominant.
Shi et al. [72] constructed a one-dimensional kinetic model to simulate the electron acceleration by inertial Alfven waves.The cold and hot electrons are treated separately.The including of cold electrons brings intense variation of Alfven speed.The model results show that the exponential decrease of the plasma density leads to the sharp gradient in both Alfven velocity and electron inertial length.When Alfven waves encounter such sharp gradient at lower altitudes, the electrons are accelerated by the waves and become super-Alfvenic, and the width of burst structures becomes much wider than the electron inertial length.Consequently, the background electrons carry the opposite field-aligned current due to plasma oscillation.The electron carried current exceeding the wavefront is balanced by the reverse current carried by background electrons, which reasonably explains observations of the electron bursts accompanied by little net field-aligned current.Furthermore, there is another type of Alfven wave reflection due to mirror force and wave-particle interaction.

Unusual Phenomena of the Ionosphere
The F 3 layer is a distorted ionospheric height structure in ionograms.Zhu et al. [73] analyzed the features of F 3 layer at low latitude under low and moderate solar activity conditions.The F 3 layer signature was more distinct in spring to summer and less distinct during LSA periods.The ionograms were recorded at a low latitude ionosonde in 2010-2012.They found that the average duration time of the F 3 layer becomes longer and the differences between h′F 2 and h′F 2 show a semiannual variation more significantly with increasing solar activity level.Zhu et al. [74] outlined morphological features of the F 3 layer traces in magnetic equatorial (Kwajalein) and low-latitude (Sanya) regions under geomagnetically quiet conditions.They found two formation patterns in F 3 layer.When the F 3 layer appears, the F 3 peak density may increase, with upward moving of the peak of F 3 layer.The F 3 peak density for the second type changes little, even decrease.This work fixed the previous view that the occurrence of the F 3 layer is always accompanied by the density-enhanced region.
The F region electron density occasionally rises pronouncedly during the nighttime, which are termed ionospheric nighttime enhancements.The post-midnight enhancement is an unresolved issue in ionospheric physics.Liu et al. [75] analyzed the manually scaled ionograms, which were measured by a Lowell DPS-4D ionosonde operated at Sanya, to explore post-midnight enhancement events occurred in 2012, a year of moderate solar activities.Outstanding features in these cases (see Figure 3) over Sanya include, post-midnight enhancements have a high occurrence, lower h m F 2 and sharper profiles accompanying the rise in f 0 F 2 , and earlier rise in electron density at higher altitudes.Moreover, downward plasma drift is detected under the enhancement event, revealing the essential role of the westward electric field in forming the post-midnight enhancements in electron density of ionospheric F-layer at such low latitudes.In contrast, former investigations thought the equatorward winds driving plasma upward, which forms the nighttime enhancement.
The 15 January 2010 solar eclipse swept over Asia around sunset.Two ionosondes at Wuhan and Beijing and an oblique incidence ionosonde network in North China were operated to measure the ionospheric response to the solar eclipse by Chen et al. [76] .Strong premidnight and postmidnight enhancements appeared in the ionosphere.The two enhancements were resulted from the plasma flux downward from the top ionosphere, possibly due to the steep decrease of the ionospheric electron density and plasma temperature during the solar eclipse.The different magnitudes of greatest eclipse in the umbra and outside may explain the different occurrence times of the plasma flux.
The Weddell Sea Anomaly (WSA) appeared to be an extreme manifestation of the longitudinal variations in the Southern Hemisphere.Luan and Dou [77] analyzed the longitudinal variations of the nighttime N m F 2 at southern midlatitudes using the COSMIC observations between 2006 and 2008.Significant longitudinal difference relative to the minimum density prevails in all seasons, although the WSA is only evident in summer under this solar minimum condition.In summer, the maximum longitudinal differences occur around midnight rather than in the evening.The maximum longitudinal differences are associated with the strongest wind-induced vertical plasma drifts in the Western Hemisphere. .
Nighttime ion temperature in the F region is generally thought to be close to neutral temperature.The similarities and differences of the nighttime ion and neutral temperatures in the F region over Millstone Hill were examined by Ruan et al. [78] using the incoherent scatter radar and Fabry-Pérot interferometer observations during the periods of 1-11 August 2011 and 12-23 January 2012.Their results showed that in winter, both neutral and ion temperatures exhibited the postmidnight enhancements, which occurred during 03:00 LT-04:00 LT and had amplitudes of about 30-90 K.The extension of midnight temperature maximum over Millstone Hill is the preferred mechanism for the observed enhancements of neutral temperature in winter and summer.the 2-D profile of electron density from CT reconstruction based on the LEO-GPS data.Some reconstructed plasmasphere electron densities are generally unevenly distributed with latitude.Some interested phenomena have been detected in low and equatorial regions, including local enhanced density clusters in the plasmasphere, and column-like enhanced structures usually extending from the topside ionosphere up to the plasmasphere almost perpendicular to the magnetic field lines.
The ionospheric Computerized Tomography (CT) is an under-determined and ill-posed inverse problem.The constraint of limited-angle geometry and sparse receivers severely limits the precision and reliability of the CT imaging, especially in the vertical direction.Zhao et al. [87] put forward a method for combining vertical and oblique sounding data with the TEC retrieved from a tri-band beacon of a planned Chinese satellite for seismological studies.The initial bottomside profile takes the vertical/oblique observation, and the topside uses the Chapman layer.The attempts show that precision of the reconstructed electron density is significantly improved when the combined data are used, compared to that only from the three-frequency technique TEC.
Zhang et al. [88] studied the features of the temporal variation of Differential Code Biases (DCB).Besides the satellite DCB data from the Center for Orbit Determination in Europe (CODE) from 1999 to 2011, they also estimated DCB.They classified three types of variations in DCB at different time scales, day-to-day, annual and monotonously descending tendency from 1999 to 2010.The day-to-day and annual variations of the estimated GPS DCB are related to the ionospheric variability.The variation of DCBs on solar cycle time scale mixes the real hardware DCBs and pseudo-DCBs induced by ionospheric variation.
Detecting Doppler frequency shifts from ionospheric high frequency echoes is an efficient way to study ionospheric disturbances.Chen et al. [89]  Ionograms are always contaminated by noises.Chen et al. [90] obtained high-quality ionograms with a curvelet transform denoising algorithm, which is based on image processing and can well preserve information about the layer traces.They proposed an adaptive threshold based on Bayes theory to improve the performance of the method.The curvelet transform method is combined with the traditional method to deal with a variety of noise for practical ionogram denoising.It can be used for ionogram automatic scaling.
Automatic scaling of ionograms is fundamental for obtaining ionospheric information.Chen et al. [91] proposed a new algorithm to scale F layer traces and recognize ordinary ray and extraordinary ray traces separately from ionograms in an automatic way.The method is developed from mathematical morphology, graph theory, and the echo characteristics of the ionosphere.
The vertical sounding has the advantage of easy providing many key ionospheric parameters, and the oblique ionosonde has the ability to detect wider regions.Chen et al. [92] conducted an experimental comparison between oblique and vertical detections in the EIA region of south China on 25 and 26 August 2010.The oblique detecting path was from Wuhan to Shenzhen and the VI ionosonde was set at the midpoint of the oblique path.The oblique ionogram reversion showed that both techniques provide comparable critical frequency, minimum virtual height and electron density profile.
To improve the spatial resolution of ionospheric tomography, Li et al. [93] combined simultaneous observations from space-based GPS onboard LEO satellites and from ground-based GPS to retrieve the image of Ionospheric Electron Density (IED).High precision dual-frequency GPS data were taken from the Crustal Movement Observation Network of China and several International GPS Service stations, LEO GPS data were from the COSMIC.Some retrieved IED profiles were compared with the COSMIC RO data, which were processed by COSMIC Data Analysis and Archival Center.Validations showed that N m F 2 agrees with the ionosonde data.They also discussed the imaging effectiveness and potential benefits of including the COSMIC data into the IED inversion.
Cheng et al. [94] reported an EISCAT ionospheric heating experiment carried out on 13 September 2010 at Tromsø, Norway.Long-lasting enhanced ion-and plasma-lines were observed by the VHF incoherent scatter radar.The VHF data show that the enhanced ion-lines were descending in altitude with time during the heater-on period.The enhanced lines may be caused by Parametric Decay Instability.They gave a possible mechanism for the altitude evolution.
Zhang et al. [95] proposed a method for extracting

Fig. 3
Fig.3 Mass plot of f 0 F 2 , h m F 2 , and H m (Chapman scale height) of F-layer observed at Sanya in May 2012.The red line gives the values on May 28th and the black curves plot the values on individual 31 days in that month.In each panel the grey shaded area indicates the interval of local nighttime and the short black bar marks local midnight.The blue arrow shows the onset time of the enhancement in f 0 F 2 on May 28th and the horizontal line gives the duration of the f 0 F 2 enhancement at the development phase.∆f 0 F 2 , ∆h m F 2 , and ∆H m are the change of f0 F 2 , h m F 2 ,and H m at the enhancement peak from the onset time, respectively[75] developed a new technique to detect ionospheric disturbances through combining the coded pulses and echo phase measurement analysis.They developed this on the Canadian Advanced Digital Ionosonde (CADI) platform.The mode can acquire accurate Doppler ionogram (Dopplionogram) and obtain the velocity of ionospheric disturbances in real time, which opens a new and effective way to enhance the ability of the available ionospheric instruments to monitor ionospheric information.