Recent Advances in Research of the Chinese Meridian Project*

WANG Chi CHEN Zhiqing XU Jiyao

Recent Advances in Research of the Chinese Meridian Project*

WANG Chi CHEN Zhiqing XU Jiyao

(100190)

The Chinese Meridian Project is a ground-based space environment monitoring facility in China. The first phase of the project has been put into formal operation since 2012 after 4-year’s construction. It consists of 15 observatories located roughly along 120°E longitude and 30°N latitude, with each observatory equipped with multiple instruments to monitor space environment. Based on the huge observational data accumulated, significant scientific achievements have been made with more than 300 peer-reviewed journal papers published. In this report, scientific results from the past two years have been reviewed with topics covering fields of geomagnetic, atmosphere, ionosphere, and their responses to solar activities. The excellent achievements from the Phase I of Chinese Meridian Project lay a good foundation for Phase II, which has already been approved with the official kick-off of construction in November 2019. It will conceive an unprecedented contribution to global space weather community from China.

ChineseMeridian Project, Space weather, Ionosphere, Upper atmosphere

1 Brief Introduction of the Project

The Chinese Meridian Project, with a full name of Space Environment Ground Based Comprehensive Monitoring Network (SEGCMN), is a Major Science and Technology Infrastructure funded by the Chinese government. The project was scheduled to be constructed in two steps: the first step (Phase I) has led to tens of instruments deployed across China’s territory and the Antarctic region, which has been put into operation since 2012. The second step (Phase II) of construction kicked off in November 2019, and is expected to complete in 2023. This report will briefly summarize the recent research progresses based on observations made by instruments that were constructed in Phase I of the Chinese Meridian Project.

Phase I of the project, also named East-sphere Space Environment Ground-based Comprehensive Monitoring Chain, consists of 15 ground-based observatories located roughly along 120°E longitude and 30°N latitude[1]. The longitudinal chain of observatories starts from Mohe, the northernmost city of China, and runs southward roughly through Beijing, Wuhan, Guangzhou, and the island of Hainan (with instruments at Haikou, Fuke, andSanya) and extends to China’s Zhongshan station in Antarctica[1]. Distances between neighbouring stations are roughly 4°~5° in latitude, or about 500 km near 120°E longitude, except the Zhongshan station in Antarctica. A chain of stations was also constructed roughly following 30°N, spanning from Lhasa to Shanghai. Each observatory is equipped with multiple instruments to comprehensively measure the key para­meters such as the baseline and time-varying geomagnetic field, as well as the middle and upper atmosphere and ionosphere from about 20 to 1000 km. Parameters of the solar wind are also tentatively measured.

Instruments of the Meridian Project (Phase I) mainly include magnetometers, ionosondes, an incoherent scatter radar, a high-frequency backscatter radar, mesosphere-stratosphere-troposphere radars, meteor radars, lidars (light detection and ranging), Fabry-Perot Interferometers (FPI), and aurora spe­c­trographs. Overall, the instruments can be grouped into four categories, named geomagnetic (geoelectric) field, radio wave, optical, and sounding rocket. Al­together, 87 instruments were built and installed at 15 stations.

The Phase I of the project was funded by China’s National Development and Reform Commission as part of a series of major scientific infrastructures. Construction of the project started in January 2008 and completed in December 2012. It is a joint effort of 12 institutions or universities in China, led by National Space Science Center (NSSC), the Chinese Academy of Sciences.

2 Scientific Achievements in the Past Two Years

The Meridian Project was put into formal operation in December 2012. Up to the end of 2019, about 15570 thousand data files (6.54 TB) have been accumulated. Using these data, scientists have made significant progress in space physics and space weather study, publishing more than 300 peer-revi­ewed papers. More than 10 national or provincial level awards were obtained.

Among these achievements, there are more than 80 journal papers published in recent two years (2018–2019), which will be briefly summarized as follows.

2.1 Polar Region Ionosphere

The polar region ionosphere is distinctive from that of the mid-to-low latitudes, features with plasma patches, strong radio wave scintillations, particle precipitation and upflows,. A bundle of instruments deployed by Chinese Meridian Project at Antarctic region, such as the SuperDARN radar[2]and scintillation monitors, were available for relevant researches.

Recently, a newly identified high-density irregularity named polar cap hot patch was found at high latitudes. Compared with classical polar cap patches, hot patches are associated with higher electron temperature and particle precipitation. By using five years of in situ plasma observations from DMSP satellites, with SuperDARN data in comparison, Ma.[3]investigated the characteristics of hot patches versus classical patches. For the first time, they show that the difference/ratio of ion/ele­c­tron temperature can be used to distinguish bet­ween classical and hot patches. The vertical ion flux is generally downward in the classical patches (i/e>0.8 orei+600 K). They also found that thei/eratio (ori -e difference) has a definite influence on behaviors of polar cap patches as, upon increasingi/e ratio, patches turn from hot to classical ones and vertical flux turns from upward to downward. The highest upflow occurrence was found in those hot patches which are near the polar cap boundary, and are associated with particle precipitation, strong convection speed, and localized field-aligned currents. This result shows that the polar cap hot patches may play a very important role in the solar wind-magne­tosphere-ionosphere coupling processes.

For a long time, it was widely accepted that in the polar region the amplitude scintillation should be much weaker than the phase scintillation, even ignorable. Recently, this popular opinion has been challenged more and more based on simulations and some tests, which usually focused on the way ado­pted to obtaining these scintillation indices especially over the polar region. However, a direct experimental evidence is still required to deep the new understanding. On the plasma flow around the noon sector of polar ionosphere, Wang.[4]presented the first experimental proof of a clear and strong dependence of the standard phase scintillation index by using the observation measurements from Global Positioning System and SuperDARN radars. They reported that the phase scintillation index presents a strong linear dependence on the plasma drift speed, but the amplitude scintillation index (4) does not. These different dependencies on the relative drift can be explained to be a consequence of changing Fresnel frequency, but also the fixed cutoff frequency (0.1 Hz) used to detrending the raw data to calculating the classical phase scintillation index. Moreover, other than the amplitude scintillation and the so-called phenomenon “phase without amplitude” scintillation in the high latitude region, a higher occurrence of phase scintillation can be probably explained by this dependence pattern. Furthermore, it can be concluded that the phase scintillation index in classical is much more sensitive to plasma drift. Therefore, if you are looking at the phase scintillation phenomenon especially over the polar region where the ionospheric plasma flow is much faster than those at equatorial and mid-latitude regions, one should be much careful to investigating the scintillation behaviors.

By analyzing five years’ (2010–2014) DMSP plasma data, with SuperDARN data in comparison, Ma.[5]investigated the plasma parameters of ion upflow above the north polar cap under different Solar Zenith Angles (SZA), solar activity (10.7) le­v­els, and convection speeds. In spatial distributions, high upflow occurrence rates are found in the dawn sector associated with regions of higher convection speed, while higher upflow fluxes are found in the dusk sector associated with higher density. By taking the solar illumination and activity into consideration, they find that the upflow occurrence shows a positive correlation with convection speed and solar activity, while it shows a negative correlation with SZA. Thus, when SZA>100º and the convection speed is low, the lowest upflow occurrence is observed. A clear seasonal dependence is found for upflow velocity and flux, which shown higher speed in the winter (high SZA) and higher flux in the summer (low SZA) during low convection conditions. It suggests that upflow density mainly contributes to upflow flux in summer, and upflow velocity mainly contributes to upflow flux in winter. For the first time, upflow velocity and density are found to be both higher in summer and high convection conditions, which implies the combined effect of evolution of flux tubes and joule/frictional heating variation done on the convection speeds. These results suggest that ion upflow in the polar cap is controlled by the combination of convection, solar activity, and solar illumination.

2.2 Study on Ionospheric Irregularities

Using Wuhan ionosonde of the Chinese Median Project, Wuhan Very High Frequency (VHF) cohe­rent scatter radar and Mengcheng airglow imager, Zhou.[6]investigated the nighttime disturbances in E and F regions at the midlatitude China region. They presented two case studies of simultaneous observations of diffuse sporadic E (Es) layers, quasiperiodic echoes of E region irregularities, spread F, and medium-scale traveling ionospheric disturbances. Results indicate that diffuse Es layers and E region irregularities can be associated with the F region medium-scale traveling ionospheric disturbances or spread F through polarization electric field mapping along the field lines. Further analysis on the dataset of Wuhan ionosonde and Wuhan VHF radar during 2015–2016 finds that, the probability of coincidence of diffuse Es layers, quasiperiodic echoes of E region irregularities, and spread F is high at local nighttime. The results provide the first observational evidence in midlatitude China region to support the concept that polarization electric field generated in the E region irregularities could map along the magnetic field lines and excite electrodynamic disturbances in F region, such as Perkins instability.

Brazil is in the South American Magnetic Ano­maly region. Research on the ionosphere, geomag­netic field, and other space weather physical pro­cesses in this region is a hot topic in international space weather communities. Moro.[7]did a co­m­parative study using ionospheric sounding data of Chinese Meridian Project Wuhan station (30.5ºN, 114.4ºE, dip is 46º), together with data from Santa Maria station (29.7ºS, 53.7ºW, dip is–38º) which is located in the South American Magnetic Anomaly area in Brazil. The differences of ionospheric para­meters (0,02,m2, and thickness parameter0) between the northern and southern hemispheres, the geomagnetic anomaly area, and the non-anomaly area are revealed. It is found that the ionosphere in the South American Magnetic Anomaly area exhibits a greater variation than in the northern hemisphere under the same conditions. This study is of great significance as to deepening understanding of the influences of the South American Magnetic Anomaly on space weather.

Based on ground-based instruments, including an all-sky airglow imager, a spectrometer, and a Fabry-Perot Interferometer (FPI) from the Chinese Meridian Project and a Digisonde in midlatitude of China, Sun.[8]report a special mid-latitudinal Medium-Scale Traveling Ionospheric Disturbance (MSTID) event, accompanied by a poleward surge of airglow depletion/enhancement and a bifurcation of depletion during the magnetically quiet period. It is found that interaction of the passing MSTID and Nighttime Plasma Density Enhancement (NPDE) resulted in the poleward surge and bifurcation of airglow depletion. For the first time, the article provided observational evidence that midlatitudinal bubble-like bifurcation structure can be directly generated by the propagating MSTID, that most likely caused by the Secondary Gradient Drift Instability (SGDI). Meanwhile, for the first time, the article suggested the Midnight Brightness Wave (MBW) as one of the possible sources of the NPDE at midlatitude regions.

In the study of Chen.[9], the oblique-inci­dence ionosonde network in North China and the Beijing Digisonde were used to investigate the night­time ionospheric disturbances, including the Premidnight Enhancements (PRMEs) and the Post­mi­dnight Enhancements (POMEs) in January and February of 2011. Most of the two kinds of electron density enhancements present opposite latitudinal dependence, namely, the PRMEs appear earlier in the north and then travel southward and the POMEs occur in the south and then move to north. Thus, the two enhancements should be induced by different mechanisms. Further analysis reveals that the PR­MEs are the peaks of the Large-Scale Travelling Ionospheric Disturbances (LSTIDs), which may be induced by the southward propagating gravity waves. Most of the POMEs are composed by an enhancement and a following depletion, which is considered to be induced by the downward×drift due to the westward electric field.

Equatorial Plasma Bubbles (EPBs) are frequ­ently occurred phenomena in the ionosphere at low latitude of China, which have a great impact on navigation and communication. Wu.[10]used observational data from all-sky imager of the Chinese Meridian Project and the C/NOFS satellite to study the edge plasma enhancements of EPBs. By researc­hing on four-year observations from 2012 to 2015, it was found that edge plasma enhancements are usually accompanied by EPBs. The observations of four years show that it is a high-incidence phenomenon, and the average incidence rate reaches about 82%. And the plasma enhancements only appeared at the east and west edges of EPBs, while none of them appeared at poleward edges. They show similar geo­graphical longitude and local time distribution rela­tive to pertaining EPBs. The results of observations show their occurrence rate made a peak between 20:00 LT to 22:00 LT. Zonal extents of those plasma enhancements present different scale characteristics at different altitudes. Occurrence characteristics of edge plasma enhancements of EPBs were also stu- died. Results indicate that the plasma related to EPBs is more likely to have been redistributed and, consequently formed depletions and enhancements. This study has provided a new perspective on the forma­tion of EPBs.

In general, ionospheric plasma blobs as the density enhancement in the Equatorial Ionization Anomaly (EIA) regions are caused by the polarization electric field, which was generated within Equatorial Plasma Bubbles (EPBs) and mapped to the EIA regions along the magnetic field lines. A major characteristic of this view is that plasma blobs occur at the poleward edges of EPBs. Wang.[11]reported three cases of concurrent plasma blobs and bubbles around 22:30 LT from low latitude stations Vanimo (geog. 2.7°S, 141.3°E; geom. 11.2°S, 146.2°W) and Hainan (geog. 19.5°N, 109.1°E; geom. 9.1°N, 179.1°W) in the same magnetic meridian in Asi­an-Oc­eanian sector during solar maximum, taking advantages of simultaneous observations by ROC­­­SAT-1 satellite and ground ionosonde/GPS scintil­­lations to examine the relative location of plasma bubbles and blobs. In these cases, the blobs were at the equatorward edges of equatorial plasma bubbles, contradicting to the traditional perception. All pla­­s­ma blobs were observed at about 600 km height near to the equator. ESF and amplitude scintillations near the same magnetic meridian line indicated the exis­tence of bubbles at higher latitude. Considering that both plasma bubbles and blobs are field-aligned elongated structures, in two of these cases, blobs were above bubbles, indicating an explanation that the eastward polarization electric field inside EPBs drives plasma upward and causes plasma blobs just above the upper boundary of the EPBs in the int­ermediate stage of the bubble development. In the third case, the blob might be slightly lower than EPBs, and the EPB and blob might be generated by eastward and westward electric fields in the parts of the perturbation produced by charging by gravitational electric current below the ionospheric peak.

2.3 Responses of Ionosphere/Thermosphere to Solar Activities and Geomagnetic Activities

Based on data collected by the meteor radars at the Davis Station (68.6°S, 77.9°E), Svalbard (78.3°N, 16°E), Tromsø (69.6°N, 19.2°E), Mohe (53.5°N, 122.3°E) and Beijing (40.3°N, 116.2°E), Yi.[12]presented a first inter-hemisphere observation of neutral mesospheric density response to geomagnetic storms. A superposed epoch analysis results show that strong geomagnetic storms can cause a greater than about 10% decrease at the polar region and about 5% decrease at higher mid-latitudes, which may indicate that geomagnetic storms could influ­ence the polar mesospheric dynamics.

Using multiple data sets including Beidou geo­­­stationary orbit satellites Total Electron Contents (TECs), ionosonde, and meteor radar from Chinese Meridian Project combined with model simulations, Lei.[13]investigated the ionospheric responses during the September 2017 geomagnetic storm in the Asian-Australian sector. It was found that long-dur­­a­tion daytime TEC enhancements that lasted from 7 to 12 September 2017 were observed by the Beidou geostationary orbit satellite constellation. This is a unique event as the prominent TEC enhancements persisted during the storm recovery phase when geomagnetic activity became quiet. The model res­­u­lts predicted that the TEC enhancements on 7–9 September were associated with the geomagnetic activity, but it showed significant electron density depletions on 10 and 11 September in contrast to the observed TEC enhancements. The results suggested that the observed long-duration TEC enhancements from 7 to 12 September are mainly associated with the interplay of ionospheric dynamics and electro­­dynamics. Nevertheless, the root causes for the obs­­er­ved TEC enhancements seen in the storm recov­ery phase are unknown and require further observations and model studies.

Using the nighttime thermospheric temperature (about 250 km) measured by an FPI deployed at Xinglong (40.2°N, 117.4°E) station between 2010 and 2018, Liu.[14]studied the responses of the mul­­tiday oscillations in thermospheric temperature to oscillations in10.7andindex. The results showed that the 27, 13.5, 9, and 7-day oscillations depen­­­­ded on solar phases. The 27-day oscillation was pre­dominant during solar maximum and highly corre­­­­lated with the10.7andindex. The 13.5, 9, and 7-day oscillations were important and highly corre­­­­lated withindex during solar ascending phase.

2.4 Responses of Ionosphere/thermosphere to Disturbances in Lower Atmosphere

Exploiting variation of the Equatorial Ioniza­tion Anomaly (EIA) crest derived from GPS obser­vations in China and Brazilian sector, Mo[15]investigated the longitudinal dependence of periodic meridional movement of EIA crest during Sudden Stratospheric Warming (SSW) events in 2003, 2006 and 2009. Results show that the locations of EIA crests in both China and Brazilian sectors exhibit obvious and constant 14~15 days periodic in-phase oscillation, which coincide with the half of the lunar revolution period (29.53 days) and the lunar phase. The temporal extent of wave power at 14~15 days is consistent with the temporal extent of stratospheric zonal wind, indicating that 14~15 days periodic meridional movement of EIA crest is due to enh­anced lunar tide modulated by zonal wind. In addi­tion, it is also found that the amplitude of 14~15 days periodic oscillation of EIA crest in China sector is larger than that in Brazilian sector, which may be caused by the longitudinal variation of tides and neutral wind pattern.

With ionospheric TEC, geomagnetic field data in East-Asia and American sectors, Liu.[16]studied the longitudinal differences of low-latitudinal ionospheric responses during the SSW winters from 2009–2018. The Time-shifted Semi-diurnal (TS) pa­tterns and the amplitude, phase angle, and relative strength of the lunar semi-diurnal tide (M2) harmonic in TEC are compared between the two sectors. The study came out with main results as: (i) TS patterns, M2 amplitudes, and M2 relative strengths tend to be more discernible or larger in the American sector than in the East Asian sector; (ii) TS patterns and M2 phase angles correlated well to the lunar phase, and the occurrence of TS patterns coincides well with the enhancement of M2 amplitudes and M2 relative strengths; (iii) such patterns can sometimes occur before the polar peak warming and experience several cycles during one event, but the most significant one tends to follow the peak warming. These longitudinal differences suggest that the influences of M2 on the low-latitudinal ionosphere tend to be more prominent in the American sector than in the East Asian sector during SSWs. It has probably resulted from a combined effect of the longitudinal variety in atmospheric and electrodynamic processes.

To study the response characteristics of ionosphere during a strong lower atmospheric process, Liu.[17]analyzed the ionospheric TEC data measured by Chinese Meridian Project during a Sudden Stratospheric Warming event in 2018. Research result shows that the deviation of TEC presents remarkable perturbation after the reversal of the zonal wind, especially around the temperature peak. Wavelet power spectra analysis indicates that the variation of TEC exhibits a strong semidiurnal and diurnal oscillation pattern during the SSW period. The cross-correlation analysis reveals that the equatorial electrojet played a key role in the anomaly perturbation and periodic oscillation of TEC, while the correlation is weak at middle latitude. These results definitely confirm the energy transmission and coupling between the lower atmosphere and ionosphere, and the dynamic process interplaying between high latitude and low latitude.

Using TEC associated with Beidou geostationary satellites from GNSS receivers of Chinese Meridian Project and IGS network, Huang.[18]statistically analyzed the daytime periodic wave-like structures for the first time in the low-latitude ionosphere over the Asian-Australian sector during 2016–2017. These structures have periods of about 18~28 min, which frequently occur during 11:00 LT–17:00 LT in the winter at latitudes ranging between 17°N and 25°N (10°–18°N magnetic latitude MLAT) in the Northern Hemisphere, where they have a maximum occurrence rate of 80% at about 21°N (14°N MLAT). In the Southern Hemisphere, daytime periodic wave-like structures are also obser­ved during 11:00 LT–15:00 LT in the winter within latitudes ranging between 6.0°S and 11.1°S (15.4°– 21.6°S MLAT), although the peak occurrence rate is only approximately 40%. Compared with stratospheric Gravity Waves (GWs), the seasonal and latitudinal variations of daytime periodic wave- like structures are generally consistent with those of stratospheric GWs. This gives a possible argument that daytime periodic wave-like structures in the low-latitude ionosphere could be generated in the low-latitude ionosphere and triggered by GWs from the lower atmosphere. This study provides a typical example that the Chinese Meridian Project combined with other observation networks have been used to explore the coupling of the different Earth’s atmosphere and provide a new understanding of the stru­cture of the low-latitude daytime ionosphere.

By the help of ionosonde observations from Chinese Meridian Project and GNSS radio occultation measurements from COSMIC, Yu.[19]analyzed the long-term climatology of the intensity of ionospheric sporadic E (Es) layers during 2006–2014. A high-spatial-resolution map of Es intensity shows a peak of strong occurrence and intensity of Es layers in the mid-latitudes of the summer hemisphere. Some interesting distinctions between the occurrence and intensity of sporadic E layers were found. At high latitudes, the occurrence rates of Es are generally low, but the intensity of Es is relatively high. This pattern is more evident over the magnetic poles,likely resulting from the vertical motions of polar-gap gravity waves in concentrating ionizations within Es layers. Using wind fields from Whole Atmosphere Community Climate Model (WACCM), the summer maxi­mum of Es layers can be explained by the vertical ion convergences of wind shear. However, the wind shear convergences at higher E-region altitudes cannot explain the observations. The results indicate that other dynamical and chemical processes of me­tallic ions, such as meteoric mass influx and geo-ma­­gnetic forces, should be considered in the temporal and spatial variation in the Es layers.

2.5 Study on Atmospheric Gravity Waves

Li.[20]reported a study of mesospheric Gravity Waves (GWs) for one year (August 2015 to July 2016) in the latitudinal band from 45°N to 75°N using an OH all sky airglow imager over Kazan (55.8°N, 49.2°E), Russia, for the first time. The observation, sponsored by Chinese Meridian Project, fills a huge airglow imaging gap in Europe and Russia region. Most of the waves propagate northeastward in all seasons, which was significantly different from airglow imager observations at other latitudes, such as Meridian Project Xinglong station that the pro­pagation directions were seasonal dependent. The European Centre for Medium Range Weather Fore­casts reanalysis data indicates that the convections near Caucasus Mountains region are the dominant source of the GWs in spring, summer, and autumn seasons. This study extends our knowledge that convection might also be an important source of GWs in the higher latitudes. Jet stream systems are considered to be the controllinggeneration mechanism of the GWs in winter. Another important finding is that theoccurrence frequency of ripple is much lower than that of other stations. The present study provides some constraints in GW parameteri­zation schemes in general circulation models in Eur­ope and Russian region.

Using data from the meteor radar chain along 120°E, Jia.[21]revealed multiyear high-frequency Gravity Wave (GW) momentum fluxes and variances in the mesosphere and lower thermosphere region at Northern Hemisphere mid-latitudes for the first time. The meteor radars are located at Mohe (53.5°N, 122.3°E), Beijing (40.3°N, 116.2°E), Mengcheng (33.3°N, 116.5°E), and Wuhan (30.5°N, 114.2°E) respectively, with two of them from Chinese Meridian Project. The directions of the monthly mean zonal momentum fluxes are mostly against the background mean zonal winds, which agree well with the selective filtering mechanism. The seasonal vari­ations of meridional momentum fluxes have similar trends over all four stations. The latitudinal diffe­r­ences in the seasonal variation of GW momentum fluxes are mainly due to the latitudinal differences of background winds and GW sources. Observations show unexpected eastward momentum flux in winter over Beijing, that is likely caused by the secondary GWs propagating eastward from the source region over the Tibetan Plateau (25°–40°N, 70°–100°E). The GW variances show a V-shaped structure indicating annual and semiannual variations over four stations in zonal component. A quasi-4-month oscil­lation was observed over Mohe, Mengcheng, and Wu­han in meridional component. The background winds play decisive roles in these GW variance structures.

Li.[22]presented a mesospheric bore event observed in the airglow layers of both OH and OI (557.7 nm) bands by two all-sky airglow imagers in Lhasa (29.66°N, 90.98°E) on the Tibetan Plateau and the Day Night Band (DNB) of the Visible Inf­rared Imaging Radiometer Suite (VIIRS) onboard the Suomi National Polar-orbiting Partnership (NPP) satellite on the night of 16–17 December 2014. They also made a comparative study of the bore event observed at Chinese Meridian Project Xinglong Sta­tion. Temperature and OH intensity measurements from the Sounding of the Atmosphere using Broa­dband Emission Radiometry (SABER) instrument onboard the TIMED satellite and wind observation by a meteor radar were used to study the environment of the bore propagation. SABER temperature shows a large mesospheric inversion layer. Through hydraulic jump theory and observations, it is found that the duct initially shrank followed by an expansion. The horizontal wavelengths and observed phase speeds of the bore packet increased with the expansion of the duct and decreased with the contraction of the duct. The bore may leak out of the duct with the variation of the depth of the duct. This study provides new insight into mesospheric bore evolution and how the ducted environment influences the propagation of bores.

2.6 Study on Atmospheric Tide and Planetary Waves

Most previous studies about tidal climatology assume that tidal variations are quite stable over a couple of weeks, and it’s safe to ignore its day-to-day variability. However, the day-to-day variability of tides is in itself an important scientific question that needs to investigate further. Zhou.[23]attempted to fix these questions and proposed a new approach to derive tidal climatology with its day-to-day variability taken into consideration. Combined with TIMED Doppler Interferometer (TIDI) wind measurements, 4-year continuous horizontal winds meas­ured by a meteor radar chain over Mohe, Beijing, Wuhan, and Sanya are used in the study. Taking ad­vantage of Empirical Tidal Mode (ETM), which is first derived from Global Scale Wave Model (GSWM) using Empirical Orthogonal Function (EOF) analysis, the daily variations of different mesosphere and lower thermosphere tides are then obtained. ETM displays latitudinal and vertical features of each tidal component in a realistic background atmosphere with dissipation effects. After fitting the observations by ETM day by day, the monthly mean of daily fitted results is used to describe tidal monthly features. Seasonal variations of three major tidal components were identified: DW1 in both zonal and meridional winds usually has two maxima around equinoxes, DE3 in zonal winds achieves its maximum in September, while that in meridional winds becomes strongest in February and November. SW2 in zonal winds reaches the largest amplitudes in May in the southern hemisphere, and meridional winds have minor peaks in February and November in the nort­hern hemisphere.

Using data by a meteor radar chain of the Chinese Meridian Project in the period from December 2008 to November 2017, Gong.[24]presented an extensive analysis of quasi 5-Day Waves (5DWs) in the Mesosphere and Lower Thermosphere (MLT) and their responses to a major Sudden Stra­tospheric Warming (SSW) event. The ter-annual oscillation of the 5DWs is found to be as important as the commonly recognized annual oscillation and semiannual oscillation at the three stations in both neutral wind components. The 5DWs are strong mainly during the August/September in the meri­dional component, while they are primarily enhanced in the period of January, April/May, and late sum­mer in the zonal component. An enhancement of 5D­Ws is observed during the 2013 SSW event in the MLT region at the three stations. The amplitudes during the SSW are more than 2 times larger than the January average. The strength of the amplification is most prominent at Mohe and reduces as latitude decreases. Results indicate that the amplification of the 5DWs is very likely associated with the 2013 SSW. This is the first time that an enhanced 5DW during a major SSW has been investigated.

By using observations from three meteor radars and a MST radar supported by the Chinese Meridian Project and reanalysis data, Yu.[25]studied the SFW in 2015 spring and PW activities from the troposphere to the MLT. The reanalysis data indicates that in the SFW, the polar mean temperature at 10 hPa level increases about 20 K, and the polar mean zonal wind decreases from about 30 m·s–1to –10 m·s–1. In this way, the polar circulation completes a seasonal transition. The radar observations show that the Q10DW and Q16DW occur around the SFW. In the troposphere and stratosphere, the wave activi­ties are intense before the SFW, while in the MLT, the waves are amplified following the SFW with an amplitude peak of about 10-day lag, and their latitudinal variation is roughly consistent with the Hough modes. This means that the Q10DW and Q16DW in the MLT are likely to be generated and strengthened in situ in the upper stratosphere and MLT.

The propagation and excitation of ISOs were extensively studied in the tropics. However, there are few reports on ISOs at higher latitudes. Using mea­surements of meteor radar and MST radar established by the Chinese Meridian Project and reanalysis data, Huang.[26]reported an ISO with about a 30-day period at mid and high latitudes. The oscillation propagates downward below 9 km and has an amplitude peak at about 9 km. At about 9~16 km, the oscillation gradually decays with height, and then strengthens again as it propagates into the stratosphere. In the mesosphere, the oscillation is robust at 78~86 km. Reanalysis data shows that in the troposphere, the oscillation propagates southward. In the lower stratosphere, the oscillation is gradually reflected back to propagate northward. Refractive index can explain these complex propagation characteristics very well. The phase progression indicates that the oscillation comes from the polar lower atmosphere. Hence, ISOs can not only originate from but also propagate in the atmosphere at mid and high latitudes.

Because the measurements of mesopause density remain scarce, the seasonal variations in the mesopause densities, especially with regard to its global structure, are still unclear. Yi.[27]reported a climatology of mesopause density estimated by using multiyear observations from a global distribution of meteor radars (including the meteor radars in the Chinese Meridian Project). They find that the seasonal variations of the southern polar mesopause densities are mainly dominated by an Annual Oscillation (AO). The mesopause densities of northern high latitudes show mainly an AO and a relatively weak SAO (semiannual oscillation). The mesopause densities over low latitudes show an Intra-Seasonal Oscillation (ISO) with a periodicity of 30~60 days. The latitudinal variation of mesopause densities ex­h­ibit a clear seasonal asymmetry, which may be related to latitudinal changes influenced by forcing of waves from the low atmosphere.

2.7 Characteristics of Metal Layers in Mesopause Region

Qiu.[28]used the combined observations from the sodium fluorescence lidar (Hefei, 31°N, 117°E) and ionosonde (Wuhan, 30°N, 114°E) of the Chinese Meridian Project to study the possibility of icy dust existing at the mesopause. The calculations for icy dust acting as a sodium reservoir are experimentally tested for the first time in details. The icy dust could form under extremely low temperature and appropriate humidity. Simultaneous temperature results from the nearby USTC T/W lidar show that the temperature minimum of 134.4 K was lower than the theoretical frost-point temperature, indicating the nucleation and growth for the water vapor. The formed icy-dust particles may have the capability to absorb enough sodium atoms in a given time period. Based on these results, an empirical model for the subtropical SSLs is proposed: first, icy dust is formed and then absorbed sodium species as a sodium store; after that, sodium atoms were released by the reser­voirspecial triggering (., by gravity wave braking). The overall finding of the study is such that three important conditions regarding zonal wind shear, temperature, and water vapor cycle must coincide for SSLs to form. Accordingly, if one or more of the three conditions vanish, then the sudden sodium layer disappears, which explains the occasional intermittency of the SSL phenomenon as observed. As it stated, this study discussed the mechanism of the formation of a sporadic sodium layer from a rather innovative viewpoint.

Xun.[29]reported the first concurrent observations of thermospheric Na layers from two nearby lidar stations, located at Yanqing (40.5°N, 116.0°E) and Pingquan (41.0°N, 118.7°E) respectively. From one year data set, they identified four thermospheric Na layer events, including an unprecedented one reaching to the height of 200 km with a maximum density of 35 cm–3and a very fast descending rate. Taking main Na layers into comparison, for three nights, thermospheric Na layers were observed only in one station (Yanqing). This suggests that thermospheric Na layers usually occur locally with a horizontal scale less than about 250 km.

Based on the sodium lidars, ionosondes, and meteor radars observations provided by Chinese meridian Project, the Solar-Terrestrial Environment Research Network and the China Research Institute of Radiowave Propagation, Ma.[30]analyzed the co-observed enhanced sodium layers and their association with the background wind, temperature as well as the ionospheric sporadic E layers from 8 years observational data. The co-observed enhanced sodium layers are the sporadic sodium layers or the thermospheric enhanced sodium layers, which expanded to a horizontal scale of approximate 350 km (., the distance between Hefei and Wuhan). In a statistical point of view, the co-observed enhanced sodium layers occurred more frequently during the summer time. Moreover, among the total 19 co- obs­erved enhanced sodium layers, about 85% have been observed simultaneously over Hefei and Wuhan wit­hout a time delay. Most of them were associated with the enhancement of critical frequency of sporadic E layer. Further analysis showed that the enhanced sod­ium layers observed over Hefei and Wuhan, which last more than 2 hours, were highly correlated. It indicates the possible role of the large-scale tide/gr­avity wave-induced wind shear in formation of the large horizontal enhanced sodium layers. Meanwhile, the apparent time delay between Hefei and Wuhan was observed for a few enhanced sodium layers, which suggests the horizontal transport induced by wind might exist.

2.8 Comparisons between Observations and Modeling

Jiang.[31]investigated the characteristics of nig­ht time thermospheric wind during geomagnetically quiet times by using the observations of three mid­latitude FPIs at Xinglong (geog. 40.2°N, 117.4°E; geom. 35°N), Kelan (geog. 38.7°N, 111.6°E; geom. 34°N) and Millstone Hill (geog. 42.6°N, 71.5°W; geom. 52°N) and the model calculations of TIEGCM and HWM14. The results show that good agreement between TIEGCM winds and FPI winds occurs in the months around winter, especially for the NS winds; discrepancies between TIEGCM and the measurements occur mainly in the months around summer, especially for zonal winds. The output of TIEGCM overestimates the NS wind speeds at the two Asian stations from February to September, while generally produces the best replication to measurements at Millstone Hill. HWM14 generally agrees with quiet-time mid-latitude neutral wind measurements, especially the meridional component does very well throughout most of a year; discrepancies between HWM14 model and observations occur mainly in zonal winds during the winter season. HWM14 gives a preferable reproduction for the FPI wind dataset of Millstone Hill which already was used to construct the model, so inclusion of data from these new stations in the HWM empirical database will more likely improve the ability of HWM model products to be closer to the real thermospheric wind.

Ma.[32]presented a study of the mean wind variations in the MLT region from 2009 to 2017, using observations made by the Chinese Meridian Project meteor radar chain. The wind structures over Mohe, Beijing, Wuhan, and Sanya are analyzed based on the long-term observations and are further compared with the Horizontal Wind Model-07 (HWM-07). The annual oscillation dominates at mid-latitudes while both the annual oscillation and semiannual oscillations are important at low latitudes. A reversal of the mean zonal winds is observed in mid-latitudes around the spring equinox, which is likely due to the rapid change of the gravity wave forcing. In addition, a three-cell southward wind pattern is observed over Wuhan and Sanya. Composite-year comparisons between the observations and the HWM-07 show large discrepancies during winter time. HWM-07 predictions agree with the meteor radar observations better in the zonal component than in the meridional component. The model predictions at low latitudes are not as accurate as that of the middle latitudes.

In the ionosphere and thermosphere community, it is Roble and Dickinson[33], who did the simulation for the first time to demonstrate the greenhouse gas effect on the ionosphere. After that, the community has done comprehensive studies on this topic involving both data analysis and modeling. However, there still exist differences between observations and simulations. Yue.[34]made use of Wuhan iono­sonde data during 1947–2017 interval, which was digitized and unified recently, to evaluate the NC­AR-TIEGCM simulation, which was driven by the realistic geomagnetic field and CO2level. They found that both data and model show the similar long term02(–0.0021 MHz·a–1) andm2(–0.106 km·a–1) tre­nd in terms of mean value and local time variation during the interval. Further control simulations indicate that CO2and geomagnetic field have a comparable effect onm2trend, while geomagnetic field effect dominates02trend over Wuhan. In addition, the trend due to geomagnetic field is more complex than CO2increase versus years. Their results also testified the high quality of Wuhan iono­sonde long-term data that produced by a series of different engineers during 70 years.

3 Conclusions and Outlook

In recent two years, using data from the Chinese Meridian Project, about 80 peer-reviewed journal papers were published embodying significant progresses in space weather and space physics research. The research topics covered a wide range, from ion­os­phere dynamics and regional characteristics to coupling between different spheres within the solar- terrestrial space. In this report, we categorized the scientific achievements into 8 subjects.

In addition to basic research, space science communities in China have made huge progresses to build a world leading ground-based space environment monitoring system, namely the Phase II of the Chinese Meridian Project. Meanwhile, the International Meridian Circle Program (IMCP) advocated by China is progressively stepping into realization stage. Altogether, these achievements will certainly contribute significantly to the global space science community.

Acknowledgements The article was written up based on materials provided by authors of the cited papers in the references. Much thanks for their generous help. We are also grateful to all members of the whole Chinese Meridian Project team, who have been making continuous efforts to perfect the opera­tion of the project.

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[4] Wang Y, Zhang Q H, Jayachandran P T,. Experimental evidence on the dependence of the standard GPS phase scintillation index on the ionospheric plasma drift around noon sector of the polar ionosphere [J].., 2018, 123:2370-2378

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[15] MO X H, ZHANG D H. Lunar tidal modulation of periodic meridional movement of equatorial ionization anomaly crest during sudden stratospheric warming [J]., 2018, 123:1488-1499

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[19] YU Bingkun, XUE Xianghui, KUO Chengling,. The intensification of metallic layered phenomena above thunderstorms through the modulation of atmospheric tides [J]., 2018. doi.org/10.5194/acp- 2018-1025

[20] LI Q, YUSUPOV K, AKCHURIN A,. First OH airglow observation of mesospheric gravity waves over European Russia region [J]., 2018, 123:2168-2180

[21] JIA M, XUE X, GU S,. Multiyear observations of gravity wave momentum fluxes in the midlatitude mesosphere and lower thermosphere region by meteor radar [J]., 2018, 123:5684-5703

[22] LI Q, XU J, YUE J,. Evolution of a mesospheric bore in a duct observed by ground-based double-layer imagers and satellite observations over the Tibetan Plateau region [J]., 2019, 124:1377-1388

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[24] GONG Y, LI C, MA Z,. Study of the quasi-5-day wave in the MLT region by a meteor radar chain [J]., 2018, 123:9474-9487

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P35

WANG Chi, CHEN Zhiqing, XU Jiyao. Recent Advances in Research of the Chinese Meridian Project., 2020, 40(5): 679-690. DOI:10.11728/cjss2020.05.679

* Supported by the Open Research Project of Large Research Infrastructures of Chinese Academy of Sciences, the Study on the Interaction between Low/Mid-latitude Atmosphere and Ionosphere Based on the Chinese Meridian Project, and the Chinese Meridian Project

March 16, 2020

E-mail: cw@spaceweather.ac.cn