Equatorial plasma bubbles (EPBs) can cause rapid fluctuations in amplitude and phase of radio sig... more Equatorial plasma bubbles (EPBs) can cause rapid fluctuations in amplitude and phase of radio signals traversing the ionosphere and in turn produce serious ionospheric scintillations and disrupt satellite-based communication links. Whereas numerous studies on the generation and evolution of EPBs have been performed, the prediction of EPB and ionospheric scintillation occurrences still remains unresolved. The generalized Rayleigh–Taylor (R–T) instability has been widely accepted as the physical mechanism responsible for the generation of EPBs. But how the factors, which seed the development of R–T instability and control the dynamics of EPBs and resultant ionospheric scintillations, change on a short-term basis are not clear. In the East and Southeast Asia, there exist significant differences in the generation rates of EPBs at closely located stations, for example, Kototabang (0.2°S, 100.3°E) and Sanya (18.3°N, 109.6°E), indicating that the decorrelation distance of EPB generation is small (hundreds of kilometers) in longitude. In contrast, after the initial generation of EPBs at one longitude, they can drift zonally more than 2000 km and extend from the magnetic equator to middle latitudes of 40° or higher under some conditions. These features make it difficult to identify the possible seeding sources for the EPBs and to accurately predict their occurrence, especially when the onset locations of EPBs are far outside the observation sector. This paper presents a review on the current knowledge of EPBs and ionospheric scintillations in the East and Southeast Asia, including their generation mechanism and occurrence morphology, and discusses some unresolved issues related to their short-term forecasting, including (1) what factors control the generation of EPBs, its day-to-day variability and storm-time behavior, (2) what factors control the evolution and lifetime of EPBs, and (3) how to accurately determine ionospheric scintillation from EPB measurements. Special focus is given to the whole process of the EPB generation, development and disruption. The current observing capabilities, future new facilities and campaign observations in the East and Southeast Asia in helping to better understand the short-term variability of EPBs and ionospheric scintillations are outlined.
Equatorial plasma bubbles (EPBs) can cause rapid fluctuations in amplitude and phase of radio sig... more Equatorial plasma bubbles (EPBs) can cause rapid fluctuations in amplitude and phase of radio signals traversing the ionosphere and in turn produce serious ionospheric scintillations and disrupt satellite-based communication links. Whereas numerous studies on the generation and evolution of EPBs have been performed, the prediction of EPB and ionospheric scintillation occurrences still remains unresolved. The generalized Rayleigh-Taylor (R-T) instability has been widely accepted as the physical mechanism responsible for the generation of EPBs. But how the factors, which seed the development of R-T instability and control the dynamics of EPBs and resultant ionospheric scintillations, change on a short-term basis are not clear. In the East and Southeast Asia, there exist significant differences in the generation rates of EPBs at closely located stations, for example, Kototabang (0.2°S, 100.3°E) and Sanya (18.3°N, 109.6°E), indicating that the decorrelation distance of EPB generation is small (hundreds of kilometers) in longitude. In contrast, after the initial generation of EPBs at one longitude, they can drift zonally more than 2000 km and extend from the magnetic equator to middle latitudes of 40° or higher under some conditions. These features make it difficult to identify the possible seeding sources for the EPBs and to accurately predict their occurrence, especially when the onset locations of EPBs are far outside the observation sector. This paper presents a review on the current knowledge of EPBs and ionospheric scintillations in the East and Southeast Asia, including their generation mechanism and occurrence morphology, and discusses some unresolved issues related to their short-term forecasting, including (1) what factors control the generation of EPBs, its day-today variability and storm-time behavior, (2) what factors control the evolution and lifetime of EPBs, and (3) how to accurately determine ionospheric scintillation from EPB measurements. Special focus is given to the whole process of the EPB generation, development and disruption. The current observing capabilities, future new facilities and campaign observations in the East and Southeast Asia in helping to better understand the short-term variability of EPBs and ionospheric scintillations are outlined.
The ionospheric sporadic E (E s) layer is a thin metallic layer produced by wind shears in the he... more The ionospheric sporadic E (E s) layer is a thin metallic layer produced by wind shears in the height of 90-130 km (Whitehead, 1989). The density of E s is much higher than the background E region and can be even higher than the F layer of the ionosphere, thus density irregularities of various scales can be embedded in the E s structures and cause scintillation or even loss of lock of radio signals within various frequency bands (e.g., Chatterjee et al., 2013; Seif et al., 2015). It is demonstrated that the occurrence of E s is the highest during local summer (e.g., Chu et al., 2014). Physical mechanisms, such as the meteoric influxes, the electric
Understanding the day-today variability of equatorial plasma bubble (EPB) irregularities has been... more Understanding the day-today variability of equatorial plasma bubble (EPB) irregularities has been a major challenge for many decades that makes the deterministic prediction of EPBs remain elusive. In the present study, we report a case of intense and periodic EPBs observed during 8 and 9 April 2013 by the 47-MHz Equatorial Atmosphere Radar at Kototabang, Indonesia. The periodic EPBs separated by about 200-250 km were initiated before sunset. The presunset onset and development of these periodic EPBs were discussed in light of the gravity waves (GWs) excited in connection with the deep convection due to the tropical cyclone (TC) Victoria. The outgoing long-wave radiation measurement by very high-resolution radiometer (VHRR) onboard Indian meteorological satellite Kalpana-1 shows the occurrence of deep convective activity during these days. The presence of upward propagating GWs from the deep convective region associated with TC Victoria was confirmed using the GPS radio occultation (GPS-RO) observations in the troposphere and stratosphere. The GW signatures at ionospheric altitudes were also observed from the Ionosonde observations over magnetic equator, and medium-scale (~300 km) GWs were observed from the GPS-TEC (Total Electron Content) data near to the magnetic equator and cyclone center. From the GW parameters observed from GPS-TEC and GPS-RO, we surmise that the secondary GWs generated by the dissipation of primary GWs associated with TC Victoria could have served as a seeding source on the generation of periodic EPBs during these two consecutive days.
Ionospheric F-region irregularity backscatter plumes are commonly regarded as a nighttime phenome... more Ionospheric F-region irregularity backscatter plumes are commonly regarded as a nighttime phenomenon at equatorial and low latitudes. Using the Sanya (18.4°N, 109.6°E, dip lat. 12.8°N) VHF radar, F-region backscatter echoes were observed at daytime during 0700-1800 LT, with an unexpected high occurrence in June solstice of solar minimum. Radar interferometry and ICON satellite in situ results show that the daytime F-region echoes were from plume structures consisting of field-aligned irregularities. The daytime echoing structures appeared mostly above 350 km altitude, extending up to 650 km or more with apparent westward drifts at times. We surmise that the daytime F-region echoes were due to equatorial plasma bubble irregularities generated on the previous night around 100-125°E, where the irregularities could survive unexpectedly long time, beyond sunrise as vertically elongated fossil plumes. Under the ionospheric background dynamics, the fossil plumes could be elevated to high altitudes and drift zonally over Sanya. Plain Language Summary Equatorial plasma plumes, which are known as vertically elongated irregularity structures over the magnetic equator, have been widely accepted as being generated at nighttime. The plumes usually disappear before sunrise. At daytime, there are very few reported cases of F-region backscatter echoes. It is still not clear what caused the daytime echoes. During November 2016 to August 2020, the Sanya VHF radar was operated for observing F-region echoes at daytime. The observations show that the daytime F-region echoing structures could appear at any time during 0700-1800 LT, with a maximum occurrence around 0900 LT. Radar interferometry and ICON satellite in situ results reveal that these daytime echoes were from field-aligned irregularities, which are shown as plume structures in the topside ionosphere. It is suggested that the plume structures could be remnants of equatorial plasma bubble (EPB) irregularities generated on the previous night around 100-125°E. They rise to high altitudes and drift zonally together with background plasma, causing the daytime F-region backscattering structure over Sanya. Our results indicate that the EPBs could maintain their vertically elongated structures and meter-scale irregularities at F-region topside for much longer time than previously thought and have important implications for understanding their dynamics.
The occurrences of equatorial plasma bubble (EPB) irregularities over Asian and American sectors ... more The occurrences of equatorial plasma bubble (EPB) irregularities over Asian and American sectors often show a different behavior due to significantly different geometries of the geomagnetic field. By using the GPS total electron content observations over equatorial and low latitudes sectors of Asian (about 120°E) and American (about 60°W) during 1997-2018, we present a comparative study of long-term occurrences of EPB kilometer-scale irregularities over the two longitude sectors. The results show that post-sunset EPB irregularities over the Asian sector were positively correlated with solar activity. However, in the American sector, such a positive dependence was not apparent in 2000-2002, which could be partly induced by limited data available during the high occurrence season of solar maximum when the GPS receivers frequently lost lock. The geomagnetic disturbance tends to inhibit the generation of EPB irregularities over both sectors during post-sunset but enhance their generation during post-midnight. The latitudinal distribution of EPB irregularities exhibited a doublepeak structure centered within ± 10° (dip lat.), with the latitudinal limit below ± 25° at solar maximum in both sectors and below ± 20° and ± 10° at solar minimum in American and Asian sectors, respectively. Specifically, in the American sector, the occurrence rates were about two times higher than those in the Asian sector, and the occurrence rates in the northern hemisphere were apparently lower than those in the southern hemisphere. The enhanced ionization by particle precipitation over the South Atlantic Magnetic Anomaly region could cause the north-south asymmetry of occurrence rates of EPB irregularities. The results help in designing experiments to understand better the generation of EPB irregularities under the future international meridian circle project. Keywords Equatorial plasma bubble irregularity • Long-term occurrence characteristics • South American and East Asian low latitudes * Guozhu Li
A Meteor and ionospheric Irregularity Observation System (MIOS)-optical subsystem, which currentl... more A Meteor and ionospheric Irregularity Observation System (MIOS)-optical subsystem, which currently consists of 26 video cameras (4 spectrographs) at two stations Sanya (18.3°N, 109.6°E) and Ledong (18.4°N, 109°E) separated by about 70 km was developed. One of the major goals of MIOS is to study how the entry of meteoroids into the Earth's atmosphere changes the ionosphere through combining measurements of optical meteor with radar specular and non-specular meteor echoes and ionospheric irregularity echoes. This paper outlines the MIOS optical subsystem design for optical meteor detecting and methods for inferring physical and chemical properties. The spectrum and common volume optical trail observations from the two stations allow identifying emissions from meteor and atmospheric species, and calculating the meteor velocity, trajectory and orbital parameters. Preliminary results of meteors detected by the MIOS optical subsystem during July-August 2019 are presented and discussed.
The ionospheric sporadic E (E s) layers over middle and low latitudes are generally believed to b... more The ionospheric sporadic E (E s) layers over middle and low latitudes are generally believed to be produced by neutral wind shear (Whitehead, 1989). Based on Global Navigation Satellite System (GNSS) radio occultation technique, previous statistical studies on the global distribution of E s revealed that its occurrence always peaks in local summer (Chu et al., 2014). The planetary and tidal waves, and electric fields were proposed to modulate the occurrences and dynamics of E s (e.g., Abdu et al., 2014; Arras et al., 2009; Patra et al., 2009). The theory of gravity waves (GWs) has long been proposed to be responsible for modulating E s generation and morphology (Woodman et al., 1991), and is supported by a series of observational results from very high frequency (VHF) radars and ionosondes (e.g., Li et al., 2014; Ogawa et al., 2002). However, the horizontal shape of E s structures has remained unclear for decades. Although observations from radars and airglow imagers revealed the quasiperiodic and wave-like feature of E s (e.g., Djuth et al., 1999; Hysell Abstract Sporadic E (E s) structures have been observed occasionally covering a large horizontal scale of more than 1,000 km over China. Their onset locations and propagation features, and related generation mechanisms still remain unknown. In this study, a statistical analysis of large-scale E s structures is performed based on the ionospheric total electron content obtained from ground-based receiver networks, in combination with data from multiple ionosondes in China. The large-scale strong E s structures mainly occur during summer months, with dominant horizontal azimuth in the east-west and northwestsoutheast directions and dimensions of 1,000-3,000 km along the elongation. They predominantly drift southwestward at the speed of 30-210 m/s. The main onset region for the large-scale E s structures over China is identified for the first time, which is around 20°-45°N and 100°-125°E. Based on the morphological features of large-scale E s structures, and the observation of concurrent cases of traveling ionospheric disturbances, we surmise that gravity waves could play an important role in the generation of large-scale E s structures. Plain Language Summary The ionospheric sporadic E (E s) layer, which is of high occurrence over mid-to-low latitude regions of China, was observed simultaneously or sequentially at locations separated by thousands of kilometers at times. Based on the traditional observational techniques, e.g., ionosonde, it is hard to determine whether the E s layers observed at different locations belong to the same structure with a large horizontal scale or belong to regional E s patches developed independently. In this regard, the ground-based ionospheric total electron content measurements provide a good opportunity for characterizing large-scale strong E s structures, which were observed to elongate up to more than 1,000 km. Whereas the large-scale E s structures have been reported occasionally, their general features and onset locations remain unclear. This constrains further understanding on the generation mechanism for largescale strong E s occurrence. In this study, a statistical analysis of occurrences and dynamics of large-scale strong E s structures over China is performed. Most of the large-scale strong E s structures were initially observed around 20°-45°N and 100°-125°E. It was suggested that gravity waves could play an important role in the initial generation of large-scale strong E s structures. SUN ET AL.
Using total electron content (TEC) data from a chain of Beidou geostationary satellite TEC receiv... more Using total electron content (TEC) data from a chain of Beidou geostationary satellite TEC receivers along ∼110°E during 2016-2019, the local time, seasonal, and latitudinal occurrence characteristics of periodic TEC perturbation associated with medium scale traveling ionospheric disturbances (MSTIDs) are investigated. The results show that the daytime periodic ionospheric disturbances appear prominently in winter with dominant periods ∼30-40 min. An interesting feature is that the latitudinal variations of periodic ionospheric disturbances show two occurrence peaks at higher and lower latitudes, with a minimum centering around 30°N-33°N, where almost no periodic ionospheric disturbance was detected under the threshold of absolute TEC perturbation greater than 0.3 TECU. Further, using two short-baseline GNSS receiver arrays at middle and low latitudes, the propagation characteristics of the periodic ionospheric disturbances were derived. The median values of propagation azimuth, phase velocity, and period of the ionospheric disturbances at middle (low) latitude are 153.1° (157.4°), 36.7 min (33 min), and 97.2 m/s (62.5 m/s), respectively, showing typical characteristics of MSTIDs. We surmise that the perturbation source and background electron density could play important roles in causing the latitudinal variation of daytime periodic ionospheric disturbances along 110°E. HU ET AL.
Ionospheric scintillation is one of the main error sources that can reduce the quality of communi... more Ionospheric scintillation is one of the main error sources that can reduce the quality of communication and navigation. Ionospheric scintillations mainly occur at equatorial and low latitudes, and polar regions (e.g.,
The bottom-type irregularity scattering layer (BSL) that can appear in the ionospheric F region b... more The bottom-type irregularity scattering layer (BSL) that can appear in the ionospheric F region bottomside has been observed generally after sunset, serving as a possible telltale of equatorial spread F (ESF). Using simultaneous multibeam radar measurements over two low-latitude stations, Sanya (18.3°N, 109.6°E; dip latitude 13°N) and Fuke (19.3°N, 109.1°E; dip latitude 14°N) in China, we report, for the first time, a thin BSL that initially occurred at presunset (~1720 LT), much earlier than the occurrence of BSL generated from the equatorial plasma shear vortex-driven instability. The presunset BSL was situated around 225 km altitude and continued to exist until the appearance of ESF plumes after sunset (~1930 LT). Interestingly, the Doppler velocities of the presunset BSL echoes measured by the radar and the F layer virtual heights obtained from the collocated Digisonde measurements over Sanya both show oscillations with a period of about 1 h, suggesting a close link between the occurrences of the BSL and of F region plasma density large-scale wave structure before sunset. These observations could imply an important role of gravity waves in the generation of the presunset F region bottom-type irregularities.
Meteoroids entering the Earth's atmosphere can create meteor trail irregularity seriously disturb... more Meteoroids entering the Earth's atmosphere can create meteor trail irregularity seriously disturbing the background ionosphere. Although numerous observations of meteor trail irregularities were performed with VHF/UHF coherent scatter radars in the past, no simultaneous radar and optical instruments were employed to investigate the characteristics of meteor trail irregularity and its corresponding meteoroid. By installing multiple video cameras near the Sanya VHF radar site, an observational campaign was conducted during the period from November 2016 to February 2017. A total of 242 optical meteors with simultaneous non-specular echoes backscattered from the plasma irregularities generated in the corresponding meteor trails were identified. A good agreement between the angular positions of non-specular echoes derived from the Sanya radar interferometer and those of optical meteors was found, validating that the radar system phase offsets have been properly calibrated. The results also verify the interferometry capability of Sanya radar for meteor trail irregularity observation. The non-specular echoes with simultaneous optical meteors were detected at magnetic aspect angles greater than ~78°. Based on the meteor visual magnitude estimated from the optical data, it was found that the radar non-specular echoes corresponding to brighter meteors survived for longer duration. This could provide observational evidence for the significance of meteoroid mass on the duration of meteor trail irregularity. On the other hand, the simultaneous radar and video common-volume observations showed that there were some cases with optical meteors but without radar non-specular echoes. One possibility could be that some of the optical meteors appeared at extremely low altitudes where meteor trail irregularities rarely occur.. (2018). First results of optical meteor and meteor trail irregularity from simultaneous Sanya radar and video observations. Earth Planet. Phys., 2, 15-21. http://doi.
Unexpected daytime F-region irregularities following the appearance of an ionospheric hole have b... more Unexpected daytime F-region irregularities following the appearance of an ionospheric hole have been observed over low latitude. The irregularities developed initially above the F-region peak height (~ 360 km) with a thickness of about 30 km and an east-west extension of more than 200 km around 1057 LT and then expanded upward to 500 km altitude behaving like the equatorial spread-F (ESF) irregularities of the nighttime ionosphere. These daytime F-region irregularities cannot be explained on basis of an earlier suggestion that the F-region irregularities observed during daytime are the continuation of the irregularities initially generated on the previous night. Based on the coincidence, both in space and time, with the appearance of an ionospheric hole, which was generated after the passage of a rocket, we conclude that the daytime F-region irregularities must have been artificially generated locally through a manifestation of plasma instability triggered by the rocket exhaust-induced ionospheric hole over low latitude.
Storm time development of equatorial plasma bubbles (EPBs) around the meridian 120°E/60°W during ... more Storm time development of equatorial plasma bubbles (EPBs) around the meridian 120°E/60°W during early September 2017, when the Bz component of interplanetary magnetic field (IMF) experienced two large southward excursions, producing a strong geomagnetic storm that included two main phase decreases, was investigated. The observations from networks of Global Navigation Satellite Systems total electron content receivers, very high frequency radars, and ionosondes operated around the meridian reveal that in the American and Asian sectors, intense EPB irregularities developed and extended to dip latitudes of ~30°N and 46°N, respectively, following rapid sunset F layer height rises during two episodes of strong southward IMF Bz excursions. The storm-enhanced EPB irregularities, however, were not observed following the sunset terminator in the Pacific sector, where the sunset rise of F layer was not detected. More interestingly, the EPBs in the Asian sector were observed to drift toward the west, with velocity increasing from ~30 m/s at low latitude to ~95 m/s at middle latitude. The poleward increasing westward drifts drove the formation of west-titled structure of irregularities. For the EPBs in the American sector, no apparent west-tilted structure was detected. The results indicate that the prompt penetration undershielding electric fields (PPEF) of eastward polarity resulting from the two IMF Bz southward excursions dominated the generation of postsunset EPBs in the American and Asian sectors, respectively. The westward drifts of PPEF-induced EPBs in the Asian sector could be attributed dominantly to disturbance westward wind, with a possible contribution to it arising from the PPEF. Plain Language Summary The development and evolution of equatorial plasma bubbles (EPBs) exhibit complex global behavior during geomagnetic storms. In recent years, an international space weather meridian circle program, which aims to provide a global picture of unfolding space weather events by using diverse instruments along the approximate meridian 120°E/60°W, that is, the Asian and American longitude sectors, was launched. Considering the sunset interval (~12 hr) between the two longitudes, it is expected that the development of postsunset EPBs, if enhanced in one region by short-lived prompt penetration electric fields (PPEF), would be inhibited in the other region under the delayed and long duration effect of disturbance dynamo electric fields. Here we report a unique case of significantly enhanced postsunset EPBs developments by PPEF in both the American and Asian sectors, but their total absence by disturbance dynamo electric fields in the Pacific sector during the September 2017 geomagnetic storm sequence. Moreover, the PPEF-induced EPBs along the meridian show different characteristics, with apparent west-tilted structure in the Asian sector but not in the American sector. This sort of study based on the international space weather meridian circle program observations will strengthen our understanding on the generation and evolution characteristics of EPBs during geomagnetic storms.
A number of ionosondes have been operated in China to detect ionospheric disturbances for many ye... more A number of ionosondes have been operated in China to detect ionospheric disturbances for many years. These ionosondes, however, are not very suitable for the short-period (<15 min) disturbances due to their poor time resolutions (>5 min). During recent years, the Institute of Geology and Geophysics, Chinese Academy of Sciences, together with the South Central University for Nationalities has been developing a portable digital ionosonde (PDI) equipped with the capability to detect and characterize small-scale/short-period ionospheric disturbances and to be quickly assembled and set up at temporary field stations for low-latitude campaign coordinated observations. The PDI uses a set of technologies (e.g., code multiplexing and antenna/transmitter matching) that allow it to obtain quality Doppler ionograms at a good time resolution with small transmitting antennas. A preliminary analysis of observations by the PDI at Sanya (18.3°N, 109.6°E) shows the presence of ionospheric disturbances with periods ranging between several and tens of minutes. Interestingly, the disturbances (with different periods) were found to simultaneously occur at different F region altitudes, for example, with periods of ~5 and 20 min below and above ~180 km, respectively. The absence of shorter-period disturbance at higher altitude is consistent with acoustic gravity waves through the region with intrinsic periods above the Brunt-Väisälä period. The short-period disturbances observed in F region bottomside are not evident in total electron content. The results demonstrate the capability of PDI to detect ionospheric disturbances with temporal scales down to a few minutes in routine ionogram mode. Future prospects of PDI are outlined.
An Ionospheric Observation Network for Irregularity and Scintillation in East/Southeast Asia was ... more An Ionospheric Observation Network for Irregularity and Scintillation in East/Southeast Asia was recently deployed. Using ionospheric total electron content (TEC) from the two crossed Beidou geostationary satellite receiver chains of the network along 110°E and 23°N and Doppler velocity measurements from the Sanya (18.3°N, 109.6°E) portable digital ionosonde, we report first observations of low latitude TEC oscillations synchronized over a wide longitude range in East/Southeast Asia, which occur at nighttime, after the main phase of the geomagnetic storm on 20 April 2018. A comparison among TEC and Doppler velocity and interplanetary magnetic field (IMF) Bz component shows that the periodic TEC enhancements correlate with F region downward plasma drifts and IMF Bz southward turnings. The results suggest that the quasiperiodic southward turnings of IMF Bz could produce multiple short-lived westward prompt penetration electric fields, which contribute to driving the nighttime low latitude TEC oscillations simultaneously over the wide longitude range. Plain Language Summary Regional-scale positive/negative ionospheric storm effects have been widely investigated using GPS total electron content (TEC) and ionosonde observations. Here we report significant nighttime TEC oscillations detected at latitudes lower than the equatorial ionization anomaly crest in the longitudes of 95-120°E by the Ionospheric Observation Network for Irregularity and Scintillation in East/Southeast Asia during geomagnetic storm. Periodic TEC enhancements are nearly synchronized over the wide longitude range, in close association with downward plasma drifts at low latitude. The good consistence between the changes of plasma vertical drift and of interplanetary magnetic field Bz polarity indicates that multiple short-lived westward prompt penetration electric fields could drive the periodic downward plasma drifts, which are very likely to cause low-pressure region of topside ionosphere at low latitude and subsequent flowing of plasma from higher latitudes (equatorial ionization anomaly crest) to the low-pressure region. The extra plasma sourced from higher latitudes, which is against the loss of increased recombination due to F region height decrease, could contribute to the TEC oscillations. Compared with the TEC measurements by GPS receiver network that are affected by satellite motion, the Ionospheric Observation Network for Irregularity and Scintillation in East/Southeast Asia, which measures TEC at fixed ionospheric pierce points along the same longitude/latitude by receiving Beidou geostationary satellite signals, provides a unique means for fine-scale observations of ionospheric perturbations not previously possible.
The small-scale wave-like structure (SSWS) of F region bottomside plasma density was proposed to ... more The small-scale wave-like structure (SSWS) of F region bottomside plasma density was proposed to be an important seeding for equatorial plasma bubble (EPB) generation, and employed in theoretical simulations of EPBs in recent years. The seeding role of SSWS, however , is waiting to be demonstrated by observation. Here we present two cases of SSWS and EPB observed by the Fuke all-sky airglow imager (19.3°N, 109.1°E; dip latitude 14.3°N). For each case, the results show that two large-scale wave-like structures (LSWSs) initially appeared around sunset in the longitude regions separated by 3-4°, but EPB irregularities were only generated in one of the LSWSs where SSWSs were seen riding on LSWS. For the other LSWS, no SSWS and EPB irregularities were seen. Considering that the two LSWSs were situated closely in longitude where the amplitude of pre-reversal enhancement of background eastward electric field should be similar, the observation that EPB was only generated in the longitude with simultaneous LSWS and SSWS could provide supporting evidence for SSWS seeding of EPB.
All-sky meteor radars are primarily used for meteor observations. This paper reports the first ob... more All-sky meteor radars are primarily used for meteor observations. This paper reports the first observations of ionospheric E-region field-aligned irregularities (FAIs) from a conventional all-sky meteor radar located at Wuhan (31°N, 114°E) for the period of March-June 2018. E-region FAI echoes evident in range-time intensity (RTI) maps show quasiperiodic striations with positive and negative slopes, which are consistent with multiple FAI structures moving across the wide beam of the meteor radar. A statistical analysis shows that out of a total of 111 d, there are 73 d with E-region FAI echoes detected by the meteor radar. The FAI events correspond well with the presence of sporadic-E layers which provide the necessary plasma density gradient for the development of gradient drift instability producing FAIs. The results demonstrate the capability of conventional meteor radars to make simultaneous routine observations of meteors and ionospheric E-region FAIs through incorporating RTI and spectral analysis into the online realtime data processing. Meteor radar observations could potentially address the limitations of iono-spheric radars, which cannot provide simultaneous measurements of neutral winds and irregularity structures, and thereby contribute to better understanding of the dynamical processes producing E-region irregularities. meteor radar, ionospheric irregularities, sporadic E layer Citation: Xie H Y, Li G Z, Ning B Q, et al. The possibility of using all-sky meteor radar to observe ionospheric E-region field-aligned irregularities. Sci China
The coupling of equatorial and low-latitude ionosphere has been an active area of research for ma... more The coupling of equatorial and low-latitude ionosphere has been an active area of research for many years. At low latitude, the occurrences of E and valley region irregularities during postsunset hours are obviously different from those around the magnetic equator. In this work, we statistically investigated the occurrences of postsunset ionospheric E, valley, and F region irregularities, the postsunset rise of F layer, and their correlations by using the Sanya (18.4°N, 109.6°E, dip latitude 12.8°N) very high frequency radar and ionosonde observations during equinoctial months of 2012-2016. A statistically significant correlation between the presence of valley region irregularities and that of equatorial spread F (ESF) was found over Sanya. For the days without ESF, valley region irregularity was rarely observed. The Doppler velocities of valley region irregularities were mainly upward which resemble those of simultaneous ESF irregularities. On the other hand, an occurrence peak of E region irregularities was detected around sunset. After sunset, E region irregularity echo was frequently disrupted. The disruption can appear on both ESF and non-ESF days, when the amplitudes of F layer postsunset rise were significantly different. The effects of polarization electric field associated with ESF and of equatorial ionospheric background condition on the E and valley region irregularities over Sanya are discussed.
A close link between the atmospheric Intertropical Convergence Zone (ITCZ) and ionospheric plasma... more A close link between the atmospheric Intertropical Convergence Zone (ITCZ) and ionospheric plasma bubble has been proposed since the last century. But this relationship has often appeared to be less than convincing due to the simultaneous roles played by several other factors in shaping the global distribution of ionospheric bubbles. From simultaneous collaborative radar multibeam steering measurements at Kototabang (0.2°S, 100.3°E) and Sanya (18.4°N, 109.6°E), conducted during September–October of 2012 and 2013, we find that the total numbers of nights with bubble (i.e., occurrence rates) at the two closely located longitudes (Kototabang and Sanya) are comparable. But interestingly, the total number of nights with locally generated bubble (i.e., generation rate) over Kototabang is clearly more than that over Sanya. Further analysis reveals that a more active ITCZ is situated around the longitude of Kototabang. We surmise that the enhanced ionospheric bubble generation at Kototabang longitude could be caused by a higher gravity wave activity associated with the more active ITCZ.
Equatorial plasma bubbles (EPBs) can cause rapid fluctuations in amplitude and phase of radio sig... more Equatorial plasma bubbles (EPBs) can cause rapid fluctuations in amplitude and phase of radio signals traversing the ionosphere and in turn produce serious ionospheric scintillations and disrupt satellite-based communication links. Whereas numerous studies on the generation and evolution of EPBs have been performed, the prediction of EPB and ionospheric scintillation occurrences still remains unresolved. The generalized Rayleigh–Taylor (R–T) instability has been widely accepted as the physical mechanism responsible for the generation of EPBs. But how the factors, which seed the development of R–T instability and control the dynamics of EPBs and resultant ionospheric scintillations, change on a short-term basis are not clear. In the East and Southeast Asia, there exist significant differences in the generation rates of EPBs at closely located stations, for example, Kototabang (0.2°S, 100.3°E) and Sanya (18.3°N, 109.6°E), indicating that the decorrelation distance of EPB generation is small (hundreds of kilometers) in longitude. In contrast, after the initial generation of EPBs at one longitude, they can drift zonally more than 2000 km and extend from the magnetic equator to middle latitudes of 40° or higher under some conditions. These features make it difficult to identify the possible seeding sources for the EPBs and to accurately predict their occurrence, especially when the onset locations of EPBs are far outside the observation sector. This paper presents a review on the current knowledge of EPBs and ionospheric scintillations in the East and Southeast Asia, including their generation mechanism and occurrence morphology, and discusses some unresolved issues related to their short-term forecasting, including (1) what factors control the generation of EPBs, its day-to-day variability and storm-time behavior, (2) what factors control the evolution and lifetime of EPBs, and (3) how to accurately determine ionospheric scintillation from EPB measurements. Special focus is given to the whole process of the EPB generation, development and disruption. The current observing capabilities, future new facilities and campaign observations in the East and Southeast Asia in helping to better understand the short-term variability of EPBs and ionospheric scintillations are outlined.
Equatorial plasma bubbles (EPBs) can cause rapid fluctuations in amplitude and phase of radio sig... more Equatorial plasma bubbles (EPBs) can cause rapid fluctuations in amplitude and phase of radio signals traversing the ionosphere and in turn produce serious ionospheric scintillations and disrupt satellite-based communication links. Whereas numerous studies on the generation and evolution of EPBs have been performed, the prediction of EPB and ionospheric scintillation occurrences still remains unresolved. The generalized Rayleigh-Taylor (R-T) instability has been widely accepted as the physical mechanism responsible for the generation of EPBs. But how the factors, which seed the development of R-T instability and control the dynamics of EPBs and resultant ionospheric scintillations, change on a short-term basis are not clear. In the East and Southeast Asia, there exist significant differences in the generation rates of EPBs at closely located stations, for example, Kototabang (0.2°S, 100.3°E) and Sanya (18.3°N, 109.6°E), indicating that the decorrelation distance of EPB generation is small (hundreds of kilometers) in longitude. In contrast, after the initial generation of EPBs at one longitude, they can drift zonally more than 2000 km and extend from the magnetic equator to middle latitudes of 40° or higher under some conditions. These features make it difficult to identify the possible seeding sources for the EPBs and to accurately predict their occurrence, especially when the onset locations of EPBs are far outside the observation sector. This paper presents a review on the current knowledge of EPBs and ionospheric scintillations in the East and Southeast Asia, including their generation mechanism and occurrence morphology, and discusses some unresolved issues related to their short-term forecasting, including (1) what factors control the generation of EPBs, its day-today variability and storm-time behavior, (2) what factors control the evolution and lifetime of EPBs, and (3) how to accurately determine ionospheric scintillation from EPB measurements. Special focus is given to the whole process of the EPB generation, development and disruption. The current observing capabilities, future new facilities and campaign observations in the East and Southeast Asia in helping to better understand the short-term variability of EPBs and ionospheric scintillations are outlined.
The ionospheric sporadic E (E s) layer is a thin metallic layer produced by wind shears in the he... more The ionospheric sporadic E (E s) layer is a thin metallic layer produced by wind shears in the height of 90-130 km (Whitehead, 1989). The density of E s is much higher than the background E region and can be even higher than the F layer of the ionosphere, thus density irregularities of various scales can be embedded in the E s structures and cause scintillation or even loss of lock of radio signals within various frequency bands (e.g., Chatterjee et al., 2013; Seif et al., 2015). It is demonstrated that the occurrence of E s is the highest during local summer (e.g., Chu et al., 2014). Physical mechanisms, such as the meteoric influxes, the electric
Understanding the day-today variability of equatorial plasma bubble (EPB) irregularities has been... more Understanding the day-today variability of equatorial plasma bubble (EPB) irregularities has been a major challenge for many decades that makes the deterministic prediction of EPBs remain elusive. In the present study, we report a case of intense and periodic EPBs observed during 8 and 9 April 2013 by the 47-MHz Equatorial Atmosphere Radar at Kototabang, Indonesia. The periodic EPBs separated by about 200-250 km were initiated before sunset. The presunset onset and development of these periodic EPBs were discussed in light of the gravity waves (GWs) excited in connection with the deep convection due to the tropical cyclone (TC) Victoria. The outgoing long-wave radiation measurement by very high-resolution radiometer (VHRR) onboard Indian meteorological satellite Kalpana-1 shows the occurrence of deep convective activity during these days. The presence of upward propagating GWs from the deep convective region associated with TC Victoria was confirmed using the GPS radio occultation (GPS-RO) observations in the troposphere and stratosphere. The GW signatures at ionospheric altitudes were also observed from the Ionosonde observations over magnetic equator, and medium-scale (~300 km) GWs were observed from the GPS-TEC (Total Electron Content) data near to the magnetic equator and cyclone center. From the GW parameters observed from GPS-TEC and GPS-RO, we surmise that the secondary GWs generated by the dissipation of primary GWs associated with TC Victoria could have served as a seeding source on the generation of periodic EPBs during these two consecutive days.
Ionospheric F-region irregularity backscatter plumes are commonly regarded as a nighttime phenome... more Ionospheric F-region irregularity backscatter plumes are commonly regarded as a nighttime phenomenon at equatorial and low latitudes. Using the Sanya (18.4°N, 109.6°E, dip lat. 12.8°N) VHF radar, F-region backscatter echoes were observed at daytime during 0700-1800 LT, with an unexpected high occurrence in June solstice of solar minimum. Radar interferometry and ICON satellite in situ results show that the daytime F-region echoes were from plume structures consisting of field-aligned irregularities. The daytime echoing structures appeared mostly above 350 km altitude, extending up to 650 km or more with apparent westward drifts at times. We surmise that the daytime F-region echoes were due to equatorial plasma bubble irregularities generated on the previous night around 100-125°E, where the irregularities could survive unexpectedly long time, beyond sunrise as vertically elongated fossil plumes. Under the ionospheric background dynamics, the fossil plumes could be elevated to high altitudes and drift zonally over Sanya. Plain Language Summary Equatorial plasma plumes, which are known as vertically elongated irregularity structures over the magnetic equator, have been widely accepted as being generated at nighttime. The plumes usually disappear before sunrise. At daytime, there are very few reported cases of F-region backscatter echoes. It is still not clear what caused the daytime echoes. During November 2016 to August 2020, the Sanya VHF radar was operated for observing F-region echoes at daytime. The observations show that the daytime F-region echoing structures could appear at any time during 0700-1800 LT, with a maximum occurrence around 0900 LT. Radar interferometry and ICON satellite in situ results reveal that these daytime echoes were from field-aligned irregularities, which are shown as plume structures in the topside ionosphere. It is suggested that the plume structures could be remnants of equatorial plasma bubble (EPB) irregularities generated on the previous night around 100-125°E. They rise to high altitudes and drift zonally together with background plasma, causing the daytime F-region backscattering structure over Sanya. Our results indicate that the EPBs could maintain their vertically elongated structures and meter-scale irregularities at F-region topside for much longer time than previously thought and have important implications for understanding their dynamics.
The occurrences of equatorial plasma bubble (EPB) irregularities over Asian and American sectors ... more The occurrences of equatorial plasma bubble (EPB) irregularities over Asian and American sectors often show a different behavior due to significantly different geometries of the geomagnetic field. By using the GPS total electron content observations over equatorial and low latitudes sectors of Asian (about 120°E) and American (about 60°W) during 1997-2018, we present a comparative study of long-term occurrences of EPB kilometer-scale irregularities over the two longitude sectors. The results show that post-sunset EPB irregularities over the Asian sector were positively correlated with solar activity. However, in the American sector, such a positive dependence was not apparent in 2000-2002, which could be partly induced by limited data available during the high occurrence season of solar maximum when the GPS receivers frequently lost lock. The geomagnetic disturbance tends to inhibit the generation of EPB irregularities over both sectors during post-sunset but enhance their generation during post-midnight. The latitudinal distribution of EPB irregularities exhibited a doublepeak structure centered within ± 10° (dip lat.), with the latitudinal limit below ± 25° at solar maximum in both sectors and below ± 20° and ± 10° at solar minimum in American and Asian sectors, respectively. Specifically, in the American sector, the occurrence rates were about two times higher than those in the Asian sector, and the occurrence rates in the northern hemisphere were apparently lower than those in the southern hemisphere. The enhanced ionization by particle precipitation over the South Atlantic Magnetic Anomaly region could cause the north-south asymmetry of occurrence rates of EPB irregularities. The results help in designing experiments to understand better the generation of EPB irregularities under the future international meridian circle project. Keywords Equatorial plasma bubble irregularity • Long-term occurrence characteristics • South American and East Asian low latitudes * Guozhu Li
A Meteor and ionospheric Irregularity Observation System (MIOS)-optical subsystem, which currentl... more A Meteor and ionospheric Irregularity Observation System (MIOS)-optical subsystem, which currently consists of 26 video cameras (4 spectrographs) at two stations Sanya (18.3°N, 109.6°E) and Ledong (18.4°N, 109°E) separated by about 70 km was developed. One of the major goals of MIOS is to study how the entry of meteoroids into the Earth's atmosphere changes the ionosphere through combining measurements of optical meteor with radar specular and non-specular meteor echoes and ionospheric irregularity echoes. This paper outlines the MIOS optical subsystem design for optical meteor detecting and methods for inferring physical and chemical properties. The spectrum and common volume optical trail observations from the two stations allow identifying emissions from meteor and atmospheric species, and calculating the meteor velocity, trajectory and orbital parameters. Preliminary results of meteors detected by the MIOS optical subsystem during July-August 2019 are presented and discussed.
The ionospheric sporadic E (E s) layers over middle and low latitudes are generally believed to b... more The ionospheric sporadic E (E s) layers over middle and low latitudes are generally believed to be produced by neutral wind shear (Whitehead, 1989). Based on Global Navigation Satellite System (GNSS) radio occultation technique, previous statistical studies on the global distribution of E s revealed that its occurrence always peaks in local summer (Chu et al., 2014). The planetary and tidal waves, and electric fields were proposed to modulate the occurrences and dynamics of E s (e.g., Abdu et al., 2014; Arras et al., 2009; Patra et al., 2009). The theory of gravity waves (GWs) has long been proposed to be responsible for modulating E s generation and morphology (Woodman et al., 1991), and is supported by a series of observational results from very high frequency (VHF) radars and ionosondes (e.g., Li et al., 2014; Ogawa et al., 2002). However, the horizontal shape of E s structures has remained unclear for decades. Although observations from radars and airglow imagers revealed the quasiperiodic and wave-like feature of E s (e.g., Djuth et al., 1999; Hysell Abstract Sporadic E (E s) structures have been observed occasionally covering a large horizontal scale of more than 1,000 km over China. Their onset locations and propagation features, and related generation mechanisms still remain unknown. In this study, a statistical analysis of large-scale E s structures is performed based on the ionospheric total electron content obtained from ground-based receiver networks, in combination with data from multiple ionosondes in China. The large-scale strong E s structures mainly occur during summer months, with dominant horizontal azimuth in the east-west and northwestsoutheast directions and dimensions of 1,000-3,000 km along the elongation. They predominantly drift southwestward at the speed of 30-210 m/s. The main onset region for the large-scale E s structures over China is identified for the first time, which is around 20°-45°N and 100°-125°E. Based on the morphological features of large-scale E s structures, and the observation of concurrent cases of traveling ionospheric disturbances, we surmise that gravity waves could play an important role in the generation of large-scale E s structures. Plain Language Summary The ionospheric sporadic E (E s) layer, which is of high occurrence over mid-to-low latitude regions of China, was observed simultaneously or sequentially at locations separated by thousands of kilometers at times. Based on the traditional observational techniques, e.g., ionosonde, it is hard to determine whether the E s layers observed at different locations belong to the same structure with a large horizontal scale or belong to regional E s patches developed independently. In this regard, the ground-based ionospheric total electron content measurements provide a good opportunity for characterizing large-scale strong E s structures, which were observed to elongate up to more than 1,000 km. Whereas the large-scale E s structures have been reported occasionally, their general features and onset locations remain unclear. This constrains further understanding on the generation mechanism for largescale strong E s occurrence. In this study, a statistical analysis of occurrences and dynamics of large-scale strong E s structures over China is performed. Most of the large-scale strong E s structures were initially observed around 20°-45°N and 100°-125°E. It was suggested that gravity waves could play an important role in the initial generation of large-scale strong E s structures. SUN ET AL.
Using total electron content (TEC) data from a chain of Beidou geostationary satellite TEC receiv... more Using total electron content (TEC) data from a chain of Beidou geostationary satellite TEC receivers along ∼110°E during 2016-2019, the local time, seasonal, and latitudinal occurrence characteristics of periodic TEC perturbation associated with medium scale traveling ionospheric disturbances (MSTIDs) are investigated. The results show that the daytime periodic ionospheric disturbances appear prominently in winter with dominant periods ∼30-40 min. An interesting feature is that the latitudinal variations of periodic ionospheric disturbances show two occurrence peaks at higher and lower latitudes, with a minimum centering around 30°N-33°N, where almost no periodic ionospheric disturbance was detected under the threshold of absolute TEC perturbation greater than 0.3 TECU. Further, using two short-baseline GNSS receiver arrays at middle and low latitudes, the propagation characteristics of the periodic ionospheric disturbances were derived. The median values of propagation azimuth, phase velocity, and period of the ionospheric disturbances at middle (low) latitude are 153.1° (157.4°), 36.7 min (33 min), and 97.2 m/s (62.5 m/s), respectively, showing typical characteristics of MSTIDs. We surmise that the perturbation source and background electron density could play important roles in causing the latitudinal variation of daytime periodic ionospheric disturbances along 110°E. HU ET AL.
Ionospheric scintillation is one of the main error sources that can reduce the quality of communi... more Ionospheric scintillation is one of the main error sources that can reduce the quality of communication and navigation. Ionospheric scintillations mainly occur at equatorial and low latitudes, and polar regions (e.g.,
The bottom-type irregularity scattering layer (BSL) that can appear in the ionospheric F region b... more The bottom-type irregularity scattering layer (BSL) that can appear in the ionospheric F region bottomside has been observed generally after sunset, serving as a possible telltale of equatorial spread F (ESF). Using simultaneous multibeam radar measurements over two low-latitude stations, Sanya (18.3°N, 109.6°E; dip latitude 13°N) and Fuke (19.3°N, 109.1°E; dip latitude 14°N) in China, we report, for the first time, a thin BSL that initially occurred at presunset (~1720 LT), much earlier than the occurrence of BSL generated from the equatorial plasma shear vortex-driven instability. The presunset BSL was situated around 225 km altitude and continued to exist until the appearance of ESF plumes after sunset (~1930 LT). Interestingly, the Doppler velocities of the presunset BSL echoes measured by the radar and the F layer virtual heights obtained from the collocated Digisonde measurements over Sanya both show oscillations with a period of about 1 h, suggesting a close link between the occurrences of the BSL and of F region plasma density large-scale wave structure before sunset. These observations could imply an important role of gravity waves in the generation of the presunset F region bottom-type irregularities.
Meteoroids entering the Earth's atmosphere can create meteor trail irregularity seriously disturb... more Meteoroids entering the Earth's atmosphere can create meteor trail irregularity seriously disturbing the background ionosphere. Although numerous observations of meteor trail irregularities were performed with VHF/UHF coherent scatter radars in the past, no simultaneous radar and optical instruments were employed to investigate the characteristics of meteor trail irregularity and its corresponding meteoroid. By installing multiple video cameras near the Sanya VHF radar site, an observational campaign was conducted during the period from November 2016 to February 2017. A total of 242 optical meteors with simultaneous non-specular echoes backscattered from the plasma irregularities generated in the corresponding meteor trails were identified. A good agreement between the angular positions of non-specular echoes derived from the Sanya radar interferometer and those of optical meteors was found, validating that the radar system phase offsets have been properly calibrated. The results also verify the interferometry capability of Sanya radar for meteor trail irregularity observation. The non-specular echoes with simultaneous optical meteors were detected at magnetic aspect angles greater than ~78°. Based on the meteor visual magnitude estimated from the optical data, it was found that the radar non-specular echoes corresponding to brighter meteors survived for longer duration. This could provide observational evidence for the significance of meteoroid mass on the duration of meteor trail irregularity. On the other hand, the simultaneous radar and video common-volume observations showed that there were some cases with optical meteors but without radar non-specular echoes. One possibility could be that some of the optical meteors appeared at extremely low altitudes where meteor trail irregularities rarely occur.. (2018). First results of optical meteor and meteor trail irregularity from simultaneous Sanya radar and video observations. Earth Planet. Phys., 2, 15-21. http://doi.
Unexpected daytime F-region irregularities following the appearance of an ionospheric hole have b... more Unexpected daytime F-region irregularities following the appearance of an ionospheric hole have been observed over low latitude. The irregularities developed initially above the F-region peak height (~ 360 km) with a thickness of about 30 km and an east-west extension of more than 200 km around 1057 LT and then expanded upward to 500 km altitude behaving like the equatorial spread-F (ESF) irregularities of the nighttime ionosphere. These daytime F-region irregularities cannot be explained on basis of an earlier suggestion that the F-region irregularities observed during daytime are the continuation of the irregularities initially generated on the previous night. Based on the coincidence, both in space and time, with the appearance of an ionospheric hole, which was generated after the passage of a rocket, we conclude that the daytime F-region irregularities must have been artificially generated locally through a manifestation of plasma instability triggered by the rocket exhaust-induced ionospheric hole over low latitude.
Storm time development of equatorial plasma bubbles (EPBs) around the meridian 120°E/60°W during ... more Storm time development of equatorial plasma bubbles (EPBs) around the meridian 120°E/60°W during early September 2017, when the Bz component of interplanetary magnetic field (IMF) experienced two large southward excursions, producing a strong geomagnetic storm that included two main phase decreases, was investigated. The observations from networks of Global Navigation Satellite Systems total electron content receivers, very high frequency radars, and ionosondes operated around the meridian reveal that in the American and Asian sectors, intense EPB irregularities developed and extended to dip latitudes of ~30°N and 46°N, respectively, following rapid sunset F layer height rises during two episodes of strong southward IMF Bz excursions. The storm-enhanced EPB irregularities, however, were not observed following the sunset terminator in the Pacific sector, where the sunset rise of F layer was not detected. More interestingly, the EPBs in the Asian sector were observed to drift toward the west, with velocity increasing from ~30 m/s at low latitude to ~95 m/s at middle latitude. The poleward increasing westward drifts drove the formation of west-titled structure of irregularities. For the EPBs in the American sector, no apparent west-tilted structure was detected. The results indicate that the prompt penetration undershielding electric fields (PPEF) of eastward polarity resulting from the two IMF Bz southward excursions dominated the generation of postsunset EPBs in the American and Asian sectors, respectively. The westward drifts of PPEF-induced EPBs in the Asian sector could be attributed dominantly to disturbance westward wind, with a possible contribution to it arising from the PPEF. Plain Language Summary The development and evolution of equatorial plasma bubbles (EPBs) exhibit complex global behavior during geomagnetic storms. In recent years, an international space weather meridian circle program, which aims to provide a global picture of unfolding space weather events by using diverse instruments along the approximate meridian 120°E/60°W, that is, the Asian and American longitude sectors, was launched. Considering the sunset interval (~12 hr) between the two longitudes, it is expected that the development of postsunset EPBs, if enhanced in one region by short-lived prompt penetration electric fields (PPEF), would be inhibited in the other region under the delayed and long duration effect of disturbance dynamo electric fields. Here we report a unique case of significantly enhanced postsunset EPBs developments by PPEF in both the American and Asian sectors, but their total absence by disturbance dynamo electric fields in the Pacific sector during the September 2017 geomagnetic storm sequence. Moreover, the PPEF-induced EPBs along the meridian show different characteristics, with apparent west-tilted structure in the Asian sector but not in the American sector. This sort of study based on the international space weather meridian circle program observations will strengthen our understanding on the generation and evolution characteristics of EPBs during geomagnetic storms.
A number of ionosondes have been operated in China to detect ionospheric disturbances for many ye... more A number of ionosondes have been operated in China to detect ionospheric disturbances for many years. These ionosondes, however, are not very suitable for the short-period (<15 min) disturbances due to their poor time resolutions (>5 min). During recent years, the Institute of Geology and Geophysics, Chinese Academy of Sciences, together with the South Central University for Nationalities has been developing a portable digital ionosonde (PDI) equipped with the capability to detect and characterize small-scale/short-period ionospheric disturbances and to be quickly assembled and set up at temporary field stations for low-latitude campaign coordinated observations. The PDI uses a set of technologies (e.g., code multiplexing and antenna/transmitter matching) that allow it to obtain quality Doppler ionograms at a good time resolution with small transmitting antennas. A preliminary analysis of observations by the PDI at Sanya (18.3°N, 109.6°E) shows the presence of ionospheric disturbances with periods ranging between several and tens of minutes. Interestingly, the disturbances (with different periods) were found to simultaneously occur at different F region altitudes, for example, with periods of ~5 and 20 min below and above ~180 km, respectively. The absence of shorter-period disturbance at higher altitude is consistent with acoustic gravity waves through the region with intrinsic periods above the Brunt-Väisälä period. The short-period disturbances observed in F region bottomside are not evident in total electron content. The results demonstrate the capability of PDI to detect ionospheric disturbances with temporal scales down to a few minutes in routine ionogram mode. Future prospects of PDI are outlined.
An Ionospheric Observation Network for Irregularity and Scintillation in East/Southeast Asia was ... more An Ionospheric Observation Network for Irregularity and Scintillation in East/Southeast Asia was recently deployed. Using ionospheric total electron content (TEC) from the two crossed Beidou geostationary satellite receiver chains of the network along 110°E and 23°N and Doppler velocity measurements from the Sanya (18.3°N, 109.6°E) portable digital ionosonde, we report first observations of low latitude TEC oscillations synchronized over a wide longitude range in East/Southeast Asia, which occur at nighttime, after the main phase of the geomagnetic storm on 20 April 2018. A comparison among TEC and Doppler velocity and interplanetary magnetic field (IMF) Bz component shows that the periodic TEC enhancements correlate with F region downward plasma drifts and IMF Bz southward turnings. The results suggest that the quasiperiodic southward turnings of IMF Bz could produce multiple short-lived westward prompt penetration electric fields, which contribute to driving the nighttime low latitude TEC oscillations simultaneously over the wide longitude range. Plain Language Summary Regional-scale positive/negative ionospheric storm effects have been widely investigated using GPS total electron content (TEC) and ionosonde observations. Here we report significant nighttime TEC oscillations detected at latitudes lower than the equatorial ionization anomaly crest in the longitudes of 95-120°E by the Ionospheric Observation Network for Irregularity and Scintillation in East/Southeast Asia during geomagnetic storm. Periodic TEC enhancements are nearly synchronized over the wide longitude range, in close association with downward plasma drifts at low latitude. The good consistence between the changes of plasma vertical drift and of interplanetary magnetic field Bz polarity indicates that multiple short-lived westward prompt penetration electric fields could drive the periodic downward plasma drifts, which are very likely to cause low-pressure region of topside ionosphere at low latitude and subsequent flowing of plasma from higher latitudes (equatorial ionization anomaly crest) to the low-pressure region. The extra plasma sourced from higher latitudes, which is against the loss of increased recombination due to F region height decrease, could contribute to the TEC oscillations. Compared with the TEC measurements by GPS receiver network that are affected by satellite motion, the Ionospheric Observation Network for Irregularity and Scintillation in East/Southeast Asia, which measures TEC at fixed ionospheric pierce points along the same longitude/latitude by receiving Beidou geostationary satellite signals, provides a unique means for fine-scale observations of ionospheric perturbations not previously possible.
The small-scale wave-like structure (SSWS) of F region bottomside plasma density was proposed to ... more The small-scale wave-like structure (SSWS) of F region bottomside plasma density was proposed to be an important seeding for equatorial plasma bubble (EPB) generation, and employed in theoretical simulations of EPBs in recent years. The seeding role of SSWS, however , is waiting to be demonstrated by observation. Here we present two cases of SSWS and EPB observed by the Fuke all-sky airglow imager (19.3°N, 109.1°E; dip latitude 14.3°N). For each case, the results show that two large-scale wave-like structures (LSWSs) initially appeared around sunset in the longitude regions separated by 3-4°, but EPB irregularities were only generated in one of the LSWSs where SSWSs were seen riding on LSWS. For the other LSWS, no SSWS and EPB irregularities were seen. Considering that the two LSWSs were situated closely in longitude where the amplitude of pre-reversal enhancement of background eastward electric field should be similar, the observation that EPB was only generated in the longitude with simultaneous LSWS and SSWS could provide supporting evidence for SSWS seeding of EPB.
All-sky meteor radars are primarily used for meteor observations. This paper reports the first ob... more All-sky meteor radars are primarily used for meteor observations. This paper reports the first observations of ionospheric E-region field-aligned irregularities (FAIs) from a conventional all-sky meteor radar located at Wuhan (31°N, 114°E) for the period of March-June 2018. E-region FAI echoes evident in range-time intensity (RTI) maps show quasiperiodic striations with positive and negative slopes, which are consistent with multiple FAI structures moving across the wide beam of the meteor radar. A statistical analysis shows that out of a total of 111 d, there are 73 d with E-region FAI echoes detected by the meteor radar. The FAI events correspond well with the presence of sporadic-E layers which provide the necessary plasma density gradient for the development of gradient drift instability producing FAIs. The results demonstrate the capability of conventional meteor radars to make simultaneous routine observations of meteors and ionospheric E-region FAIs through incorporating RTI and spectral analysis into the online realtime data processing. Meteor radar observations could potentially address the limitations of iono-spheric radars, which cannot provide simultaneous measurements of neutral winds and irregularity structures, and thereby contribute to better understanding of the dynamical processes producing E-region irregularities. meteor radar, ionospheric irregularities, sporadic E layer Citation: Xie H Y, Li G Z, Ning B Q, et al. The possibility of using all-sky meteor radar to observe ionospheric E-region field-aligned irregularities. Sci China
The coupling of equatorial and low-latitude ionosphere has been an active area of research for ma... more The coupling of equatorial and low-latitude ionosphere has been an active area of research for many years. At low latitude, the occurrences of E and valley region irregularities during postsunset hours are obviously different from those around the magnetic equator. In this work, we statistically investigated the occurrences of postsunset ionospheric E, valley, and F region irregularities, the postsunset rise of F layer, and their correlations by using the Sanya (18.4°N, 109.6°E, dip latitude 12.8°N) very high frequency radar and ionosonde observations during equinoctial months of 2012-2016. A statistically significant correlation between the presence of valley region irregularities and that of equatorial spread F (ESF) was found over Sanya. For the days without ESF, valley region irregularity was rarely observed. The Doppler velocities of valley region irregularities were mainly upward which resemble those of simultaneous ESF irregularities. On the other hand, an occurrence peak of E region irregularities was detected around sunset. After sunset, E region irregularity echo was frequently disrupted. The disruption can appear on both ESF and non-ESF days, when the amplitudes of F layer postsunset rise were significantly different. The effects of polarization electric field associated with ESF and of equatorial ionospheric background condition on the E and valley region irregularities over Sanya are discussed.
A close link between the atmospheric Intertropical Convergence Zone (ITCZ) and ionospheric plasma... more A close link between the atmospheric Intertropical Convergence Zone (ITCZ) and ionospheric plasma bubble has been proposed since the last century. But this relationship has often appeared to be less than convincing due to the simultaneous roles played by several other factors in shaping the global distribution of ionospheric bubbles. From simultaneous collaborative radar multibeam steering measurements at Kototabang (0.2°S, 100.3°E) and Sanya (18.4°N, 109.6°E), conducted during September–October of 2012 and 2013, we find that the total numbers of nights with bubble (i.e., occurrence rates) at the two closely located longitudes (Kototabang and Sanya) are comparable. But interestingly, the total number of nights with locally generated bubble (i.e., generation rate) over Kototabang is clearly more than that over Sanya. Further analysis reveals that a more active ITCZ is situated around the longitude of Kototabang. We surmise that the enhanced ionospheric bubble generation at Kototabang longitude could be caused by a higher gravity wave activity associated with the more active ITCZ.
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Papers by Guozhu Li