Recent Advances in Chinese Meridian Project

To develop an understanding of near-Earth space’s response to solar activities and the coupling among different layers in geospace, China has initiated a ground base program to monitor China’s geospace environment called the Meridian Space Weather Monitoring Project (Chinese Meridian Project). The effort consists of a chain of 15 ground-based observatories located roughly along 120°E longitude and 30°N latitude. Each observatory is equipped with multiple instruments to measure 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 kilometers. This project started collecting data in 2012. We will give a brief introduction to the Chinese Meridian Project, and present recent scientific results mainly in ionospheric and atmospheric studies.


A Brief Introduction to Chinese Meridian Project
The Chinese Meridian Space Weather Monitoring Project (Meridian Project for short) is a ground-based geospace monitoring chains in China [1] .It consists of a chain of 15 ground-based observatories located roughly along 120°E longitude and 30°N latitude.Each observatory is equipped with multiple instruments to measure 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 kilometers.This project was funded by China's National Development and Reform Commission as part of a series of major scientific infrastructure construction projects involving15 stations, 38 observing sites and 94 instruments.After 58 months of construction, the world's most extensive and integrated ground-based system for geospace weather monitoring passed national acceptance check in Beijing on October 23, 2012, and officially started data collection and scientific research.
With the joint efforts of Meridian Project's innovative team since the official kick-off, the Meridian Project has achieved abundant accomplishments in the aspect of space weather exploration and research [2] .The Meridian Project's innovative team has been carrying out investigation on the characteristics of the weather in geospace above China, the disturbances of space weather along meridian circle, and the couplings of different regions in geospace, and made significant progress in the polar ionosphere detection, ionosphere disturbances, the coupling of the ionosphere and plasmasphere, ionospheric and geomagnetic disturbances due to a large earthquake, sporadic and thermospheric enhanced sodium layers etc.More than 20 papers have been published in famous domestic and foreign journals, e.g.JGR and ANGEO, and 2 patents have been authorized.A total of 155 papers have been published, including 73 SCI papers, 32 EI papers, 38 international conference papers and 54 papers published in domestic journals.

Research Progress and Achievements
The Meridian Project has provided the scientists with the first-hand monitoring data of the space environment over China.The scientists are carrying out studies on the evolution and characteristics of the space environment, space weather disturbance phenomena along meridian and the couplings of different geospace regions with those monitoring data.A series of significant research achievements have been made.

Simultaneous Observations of Plasmaspheric and Ionospheric Variations during Magnetic Storms in 2011: First Result from Chinese Meridian Project
The plasma transport between plasmasphere and ionosphere during magnetic storms is a long-standing problem and not fully understood yet.Simultaneous observations of the plasmasphere and ionosphere are vital to understand the coupling between the two regions.Using the measurements from Chinese Meridian Project, Wang et al. [3] investigated the plasmaspheric density at L ≈ 2 inferred from ground magnetometers and the ionospheric electron density inferred by digital ionosondes and GPS signals during magnetic storms in 2011.Five moderate magnetic storms with minimum Dst index between -47 nT and -103 nT during this period were investigated.
The observations showed that the plasmaspheric density drops significantly by more than half of the pre-storm value.The ionospheric F 2 layer electron density N m F 2 and the Total Electron Content (TEC) show about 20%-50% decreases, and the N m F 2 and TEC reductions took place before the plasmaspheric density reaches its minimum.These findings suggest that the plasmaspheric depletion is very likely due to the reduced plasma supply from the ionosphere for the five moderate magnetic storms in 2011.Therefore, the plasmasphere dynamics seem to be controlled by the ionosphere during magnetic storms.

Observations of Poleward-propagating Large-scale Traveling Ionospheric Disturbances in Southern China
Ding et al. [4] reported two cases of poleward propagating during their propagation from south to north.Although the initial relative amplitude of the nighttime LSTID was ~60% larger than that of the morning event, the nighttime event dissipated more quickly than the morning event because of a strong nighttime enhancement in background TEC during storm time, which led to strong ion-drag dissipation during the evening.The poleward-propagating LSTIDs exhibited a narrower latitudinal range, smaller amplitude, and slightly higher elevation compared with the equatorward moving LSTIDs observed in the same region.Given these features, the poleward-propagating LSTIDs were likely excited by some local source near southern China.Excitation of secondary LSTIDs during the dissipation of some primary medium-scale disturbances from the lower atmosphere is a possible mechanism.

Sporadic and Thermospheric Enhanced Sodium Layers Observed by a Lidar Chain over China
Dou et al. [5] reported the statistic features of Sporadic Sodium Layers (SSLs) and the Thermospheric enhanced Sodium Layers (TeSLs) observed by a lidar chain located at Beijing (40.The co-observations of SSLs at three lidar site pairs, i.e., Hefei-Beijing, Hefei-Wuhan, and Hefei-Haikou, indicated that a large-scale SSL extended horizontally for at least a few hundred kilometers and exhibited a tidal-induced modulation.Moreover, the SSLs were better correlated for the Hefei-Wuhan and Hefei-Haikou pairs than the Hefei-Beijing pair, which suggested a difference in the dynamical/chemical process in Mesosphere and Lower Thermosphere (MLT) between Beijing and the other sites.

Teleseismic Magnetic Effects of 2011 Tohoku Earthquake
Anomalous magnetic variations were observed by ground magnetometers in East Asia area after the 2011 Tohoku earthquake.Some earlier reports showed that the seismomagnetic variations have obvious amplitude around the epicenter.Hao et al. [6] emphasized that variations could still be notable at stations 2000-4000 km away from epicenter, which were defined as Teleseismic Magnetic Disturbances (TMDs).TMDs appeared about 8 min later after the arrival of seismic Rayleigh waves at teleseismic distances and propagated at a horizontal velocity of 3.9±0.1 km•s -1 .The wave-like TMDs lasted for no longer than 10 min and had a main period of 2.1-3.3 min.TMDs were not generated by direct effects of processes in focal area crust or tsunami waves, instead, their properties were consistent with the Rayleigh wave model of seismo-ionospheric disturbances.Hence, it can be concluded that the TMDs are magnetic manifestation of Seismotraveling Ionospheric Disturbances (STIDs) generated by the interaction between ionosphere and atmosphere through acoustic waves launched by traveling Rayleigh waves.These findings contribute to the knowledge of seismo-electromagnetic effects in the atmosphere-ionosphere system and further the understanding of couplings between various spheres of the Earth.

First Observations of Polar Mesosphere Summer Echoesby SuperDARN Zhongshan Radar
Liu et al. [7] reported the first observations of Polar Mesosphere Summer Echoes (PMSE) by SuperDARN Zhongshan radar in Antarctica and presented a statistical analysis of PMSE from 2010 to 2012.The seasonal variations of occurrence were consistent with those before, with an obvious enhancement at the beginning of summer and a maximum several days after

Statistical Characteristics of Atmospheric Gravity wave in the Mesopause Region Observed with a Sodium Lidar at Beijing, China
Nightly and seasonal variations of Atmospheric Gravity Wave (AGW) activity in the sodium layer were investigated by analyzing 253 nights of sodium lidar observations by Gong et al. [8] .These observations were made from April 2010 to September 2011 at Yanqing, Beijing.Vertical wavelengths, periods and phase velocities of 162 quasi-monochromatic AGWs were extracted from the lidar data.An average parameter relationship λ z = 0.212 T ob 0.544 was found.Nightly statistical results showed that the activity of AGW had maximum from 22 to 24 LST at night.Monthly statistical results revealed that AGW activity maximized in summer and minimizes in winter.Seasonal variation characteristic of AGW activity was interpreted based on a qualitative analysis of probable main sources in Northern China.Influences of seasonally varying temperature and background winds on AGW activity were discussed.

Investigation of a Mesospheric Bore Event over Northern China
Li et al. [9] observed a mesospheric bore event using an OH All-sky Airglow Imager (ASAI) of Meridian Project at Xinglong in northern China, on the night of 8-9 January 2011.And they investigated the characteristics and environment of the bore propagation and the possible relations with the Na density perturbations using Simultaneous observations by a Doppler meteor radar, a broadband sodium Lidar, and TIMED/SABER OH intensity and temperature measurements.The bore Simultaneous Na Lidar observations at Yanqing suggested that there was a downward displacement of Na density during the passage of mesospheric bore event.

FPI Observations of Nighttime Mesospheric and Thermospheric Winds in China and Their Comparisons with HWM07
Yuan et al. [10] analyzed the nighttime horizontal neutral There are large discrepancies between the observed and HWM07 winds at ~250 km.This discrepancy is largely due to the strong semiannual zonal wind in the model and the phase difference in the annual variation of the meridional wind.The FPI annual variation coincides with the results from Arecibo, which has similar geomagnetic latitude as Xinglong Station.In general, the consistency of FPI winds with model winds is better at ~87 km and ~98 km than that at ~250 km.

Summary
Generally speaking, the Chinese Meridian Project has been operated smoothly so far, and provided valuable space environment parameters along 120°E longitude and 30°N latitude in China.Significant scientific achievements have been made by Chinese space weather community, mainly in ionospheric and atmospheric studies.With the accumulation of the data, combined with space-borne observations, more scientific results and better understanding of the geospace are expected.

Fig. 1
Fig.1 From the top to bottom panels, shown are Dst index, fundamental mode frequencies of field line resonances at L ≈ 2 for the local time 15:00, equatorial plasma mass densities, the ionospheric F 2 layer electron density values N m F 2 , and the TEC values during the time period of the 26 September 2011 magnetic storm event.The solid and dashed lines in the last panel indicate the pre-storm and the minimum TEC daily peak values, respectively Large-Scale Traveling Ionospheric Disturbances (LSTIDs) in China during a medium-scale storm between 27 May and 1 June 2011.The observations were conducted by making use of the Global Positioning System network and ionosondes in China and Southeast Asia.One northeastward propagating LSTID occurred on the morning of 30 May, while the other was observed during the nighttime of 1 June.Both poleward-traveling LSTIDs occurred during the storm's recovery phase in southern China's low latitude region (geomagnetic latitude ~7.3°N -24°N) and experienced severe dissipation

Fig. 2
Fig.2 TEC variation as a function of horizontal distance and Universal Time (UT) during the periods when the LSTIDs occurred on the morning of 30 May (Panel a) and the evening of 1 June (Panel b)

Fig. 5 Fig. 6
Fig.5 The occurrence variation of (a)PMSE echoes, (b)ionospheric echoes at Gate 0-2, (c) ionospheric echoes at Gate5-9, (d) ionospheric echoes at Gate10-15 with Day of Year (DOY).The blue vertical dashed line indicates the DOY corresponding to summer solstice (December 22).The red curves are for the 9-day sliding average value of the data points

Fig. 7
Fig.7 Contour plot of (a) hourly zonal wind, (b) meridional wind, and (c) background wind in the wave propagation direction measured by the meteor radar on the night of 8-9 January 2011.The vertical solid lines indicate the time when the bore was observed.The vertical dashed line indicates the time when the leading edge of the bore passed through the zenith of the airglow observation.winds in the middle atmosphere (~87 km and ~98 km) and thermosphere (~250 km) derived from a Fabry-Perot Interferometer (FPI) of Meridian Project, which was installed at Xinglong Station in central China, covering the period from April to July in 2012.They studied the annual, semiannual and terannual variations of the midnight winds at ~87 km, ~98 km and ~250 km for the first time and compared them with Horizontal Wind Model 2007 (HWM07).Their results showed the following results.(1) At ~87 km, both the observed and model zonal winds have similar phases in the annual and semiannual variations.However, the HWM07 amplitudes are much larger.(2) At ~98 km, the model

Fig. 8
Fig.8 Midnight zonal (eastward positive) and meridional (northward positive) winds obtained from FPI (black dot) and HWM07 (green line) at the altitudes of ~ 87 km (lower row), ~ 98 km (middle row), and ~ 250 km (upper row), respectively.The monthly averaged winds and their standard deviations are shown as red triangles and error bars