The dual frequency radio signals of the Global Positioning System (GPS) allow measurements of the total number of electrons, called total electron content (TEC), along a ray path from GPS satellite to receiver. We have developed a new technique to construct two-dimensional maps of absolute TEC over Japan by using GPS data from more than 1000 GPS receivers. A least squares fitting procedure is used to remove instrumental biases inherent in the GPS satellite and receiver. Two-dimensional maps of absolute vertical TEC are derived with time resolution of 30 seconds and spatial resolution of 0.15° × 0.15° in latitude and longitude. Our method is validated in two ways. First, TECs along ray paths from the GPS satellites are simulated using a model for electron contents based on the IRI-95 model. It is found that TEC from our method is underestimated by less than 3 TECU. Then, estimated vertical GPS TEC is compared with ionospheric TEC that is calculated from simultaneous electron density profile obtained with the MU radar. Diurnal and day-to-day variation of the GPS TEC follows the TEC behavior derived from MU radar observation but the GPS TEC is 2 TECU larger than the MU radar TEC on average. This difference can be attributed to the plasmaspheric electron content along the GPS ray path. This method is also applied to GPS data during a magnetic storm of September 25, 1998. An intense TEC enhancement, probably caused by a northward expansion of the equatorial anomaly, was observed in the southern part of Japan in the evening during the main phase of the storm.

A new technique for mapping of total electron content using GPS network in Japan

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2018JA025871

Storm‐Enhanced Development of Postsunset Equatorial Plasma Bubbles Around the Meridian 120°E/60°W on 7–8 September 2017

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.

Challenges to Equatorial Plasma Bubble and Ionospheric Scintillation Short-Term Forecasting and Future Aspects in East and Southeast Asia

Abstract A statistical study of L-band amplitude scintillations and zonal drift velocity of Fresnel-scale ionospheric irregularities is presented. Ground-based global positioning system (GPS) data acquired at the equatorial station of Sao Luis (2.33°S, 44.21°W, dip latitude 1.3°S), Brazil, during the solar maximum period from March 2001 to February 2002 are used in the analysis. The variation of scintillations and irregularity drift velocities with local time, season and magnetic activity are reported. The results reveal that for the near overhead ionosphere (satellite elevation angle >45°) a broad maximum in the occurrence of scintillation is seen from October to February. In general, weak scintillations (S 4 90%) during equinox (March–April; September–October) and December solstice (November–February) quiet time conditions and, many of the scintillations, occurred during pre-midnight hours. The mean zonal velocities of the irregularities are seen to be ∼30 m s −1 larger near December solstice, while during the equinoctial period the velocities decay faster and the scintillations tend to cease earlier. On geomagnetically disturbed nights, scintillation activity seems to be strongly affected by the prompt penetration of magnetospheric electric fields and disturbance dynamo effects. On disturbed days, during the equinox and December solstice seasons, the scintillations tend to be suppressed in the pre-midnight hours, whereas during June solstice months (May–August) the effect is opposite. In the post-midnight period, the mostly marked increase in the scintillation occurrence is observed during the equinox months. The results show that during disturbed conditions only one type of storm (when the main phase maximum takes place during the daytime hours) agrees with the Aarons’ description, that is the suppression of L-band scintillations in the first recovery phase night. The results also reveal that the storm-time irregularity drifts become more spread in velocity and occasionally may present westward drift. The present work is important to evaluate the behavior of the ionospheric irregularities at equatorial latitudes under geomagnetically quiet and disturbed conditions, which is one of the most relevant themes in the space weather studies.

Occurrence and zonal drifts of small-scale ionospheric irregularities over an equatorial station during solar maximum

. In this study the climatology of ionospheric scintillations and the zonal drift velocities of scintillation-producing irregularities are depicted for a station located under the southern crest of the equatorial ionization anomaly. Then, the α − μ ionospheric fading model is used for the first- and second-order statistical characterization of amplitude scintillations. In the statistical analyzes, data are used from single-frequency GPS receivers acquired during ∼ 17 years (September 1997–November 2014) at Cachoeira Paulista (22.4° S; 45.0° W), Brazil. The results reveal that the nocturnal occurrence of scintillations follows the seasonal distribution of plasma bubble irregularities observed in the longitudinal sector of eastern South America. In addition to the solar cycle dependence, the results suggest that the occurrence climatology of scintillations is also modulated by the secular variation in the dip latitude of Cachoeira Paulista, since the maximum occurrence of scintillations during the peak of solar cycle 24 was ∼ 20 % lower than that observed during the maximum of solar cycle 23. The dynamics of the irregularities throughout a solar cycle, as investigated from the estimates of the mean zonal drift velocities, presented a good correlation with the EUV and F10.7 cm solar fluxes. Meanwhile, the seasonal behavior showed that the magnitude of the zonal drift velocities is larger during the December solstice months than during the equinoxes. In terms of modeling, the results for the α − μ distribution fit quite well with the experimental data and with the temporal characteristics of fading events independently of the solar activity level.

/pdf/climatology-and-modeling-of-ionospheric-scintillations-and-2y0g9ohox8.pdf

Climatology and modeling of ionospheric scintillations and irregularity zonal drifts at the equatorial anomaly crest region

Statistical characteristics of low-latitude ionospheric scintillation over China

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.

Low Latitude Ionospheric TEC Oscillations Associated With Periodic Changes in IMF Bz Polarity

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2016JA023647

First observation of presunset ionospheric F region bottom-type scattering layer

/pdf/geomagnetically-conjugate-observations-of-equatorial-plasma-1af15qboo9.pdf

Kangkang Liu

Papers

Statistical characteristics of low-latitude ionospheric scintillation over China

Low Latitude Ionospheric TEC Oscillations Associated With Periodic Changes in IMF Bz Polarity

First observation of presunset ionospheric F region bottom-type scattering layer

Geomagnetically Conjugate Observations of Equatorial Plasma Irregularities From Swarm Constellation and Ground-Based GPS Stations

Possible evidence for small-scale wave seeding of equatorial plasma bubbles