Two-dimensional Total Electron Content Research Articles

Solar activity dependence of medium-scale traveling ionospheric disturbances using GPS receivers in Japan

In order to reveal solar activity dependence of the medium-scale traveling ionospheric disturbances (MSTIDs) at midlatitudes, total electron content (TEC) data obtained from a Global Positioning System (GPS) receiver network in Japan during 22 years from 1998 to 2019 were analyzed. We have calculated the detrended TEC by subtracting the 1-h running average from the original TEC data for each satellite and receiver pair, and made two-dimensional TEC maps of the detrended TEC with a spatial resolution of 0.15° × 0.15° in longitude and latitude. We have investigated MSTID activity, defined as delta I/overline{I}, where delta I and overline{I} are standard deviation of the detrended TEC and the average vertical TEC within the area of 133.0°–137.0° E and 33.0°–37.0° N for 1 h, respectively. From each 2-h time series of the detrended TEC data within the same area as the MSTID activity, auto-correlation functions (ACFs) of the detrended TEC were calculated to estimate the horizontal propagation velocity and direction of the MSTIDs. Statistical results of the MSTID activity and propagation direction of MSTIDs were consistent with previous studies and support the idea that daytime MSTIDs could be caused by atmospheric gravity waves, and that nighttime MSTIDs were caused by electro-dynamical forces, such as the Perkins instability. From the current long-term observations, we have found that the nighttime MSTID activity and occurrence rate increased with decreasing solar activity. For the daytime MSTID, the occurrence rate increased with decreasing solar activity, whereas the MSTID activity did not show distinct solar activity dependence. These results suggest that the secondary gravity waves generated by dissipation of the primary gravity waves propagating from below increase under low solar activity conditions. The mean horizontal phase velocity of the MSTIDs during nighttime did not show a distinct solar activity dependence, whereas that during daytime showed an anticorrelation with solar activity. The horizontal phase velocity of the daytime MSTIDs was widely distributed from 40 to 180 m/s under high solar activity conditions, whereas it ranged between 80 and 200 m/s, with a maximum occurrence at 130 m/s under low solar activity conditions, suggesting that gravity waves with low phase velocity could be dissipated by high viscosity in the thermosphere under low solar activity conditions.

Dynamic Changes of the Ionospheric Artificial Airglow Region Caused by High-Power Radio Waves Based on a Joint Analysis of Night-Sky Snapshots in the 630 nm Line and Total Electron Content Variation Maps

We describe a method for joint analysis of the night-sky snapshots and total electron content (TEC) variation maps and present a dynamic spatiotemporal configuration of the TEC variation intensity in the ionospheric airglow region in the red line of atomic oxygen (λ = 630 nm) stimulated by high-power radio waves from the Sura facility. The measurements were performed on the 29th of August, 2016. Two-dimensional TEC variation maps were plotted according to a network of 30 GNSS stations located within a radius of 700 km from the Sura heater. It is shown that the region of the maximum brightness of the artificial ionospheric airglow spot is spatially localized in the region of the minimum intensity of TEC variations, while the region of the maximum intensity of TEC variations corresponds to the region of the minimum brightness of the artificial airglow spot. Quantitative estimates for a cavity with reduced electron density in the artificial airglow region, which forms after the Sura pump wave is switched on, vary in the range ΔNe/Ne ≈ 4–9%, where Ne is the electron density and ΔNe is its variation in the cavity. The proposed experimental technique eliminates the limitations associated with the need for a navigation satellite to flyby directly over the main lobe of the Sura antenna pattern, which significantly increases the chances of successful simultaneous measurements of TEC variations and artificial ionospheric airglow in the 630 nm line.

Total Electron Content Observations by Dense Regional and Worldwide International Networks of GNSS

Two-dimensional ionospheric total electron content (TEC) maps have been derived from ground-based Global Navigation Satellite System (GNSS) receiver networks and applied to studies of various ionospheric disturbances since the mid-1990s. For the purpose of monitoring and researching ionospheric conditions and ionospheric space weather phenomena, we have developed TEC maps of areas over Japan using the dense GNSS network, GNSS Earth Observation NETwork (GEONET), which consists of about 1300 stations and is operated by the Geospatial Information Authority of Japan (GSI). Currently, we are providing high-resolution, two-dimensional maps of absolute TEC, detrended TEC, rate of TEC change index (ROTI), and loss-of-lock on GPS signal over Japan on a real-time basis. Such high-resolution TEC maps using dense GNSS receiver networks are one of the most effective ways to observe, on a scale of several 100 km to 1000 km, ionospheric variations caused by traveling ionospheric disturbances and/or equatorial plasma bubbles, which can degrade single-frequency and differential GNSS positioning/navigation. We have collected all the available GNSS receiver data in the world to expand the TEC observation area. Currently, however, dense GNSS receiver networks are available in only limited areas, such as Japan, North America, and Europe. To expand the two-dimensional TEC observation with high resolution, we have conducted the Dense Regional and Worldwide International GNSS TEC observation (DRAWING-TEC) project, which is engaged in three activities: (1) standardizing GNSS-TEC data, (2) developing a new high-resolution TEC mapping technique, and (3) sharing the standardized TEC data or the information of GNSS receiver network. We have developed a new standardized TEC format, GNSS-TEC EXchange (GTEX), which is included in the Formatted Tables of ITU-R SG 3 Databanks related to Recommendation ITU-R P.311. Sharing the GTEX TEC data would be easier than sharing the GPS/GNSS data among those in the international ionospheric researcher community. The DRAWING-TEC project would promote studies of medium-scale ionospheric variations and their effect on GNSS.

Morphology and dynamics of daytime mid-latitude sporadic-E patches revealed by GPS total electron content observations in Japan

Morphological characteristics of daytime mid-latitude sporadic-E (Es) patches are studied by two-dimensional total electron content (TEC) maps drawn using the Japanese dense network of Global Positioning System (GPS) receivers. By analyzing over 70 cases, we found that their horizontal shapes are characterized by frontal structure typically elongated in east-west by ~100 km. They are observed to migrate mainly northward in the morning and southward in the afternoon with speeds of 30–100 m/s. This may reflect the velocities of neutral winds controlled by the atmospheric tides. Such frontal structures are often found to include smaller scale structures.

Early warning from seismic ionospheric anomaly of the 24 May 2014, Mw = 6.4 Aegean-Sea earthquake: two-dimensional principal component analysis (2DPCA)

Two-dimensional principal component analysis (2DPCA) has been used to determine ionospheric two-dimensional total electron content (TEC) anomaly after Aegean-Sea earthquake at 09:25:03(UT) on 24 May, 2014 (Mw = 6.4). The TEC anomaly was more intense localized at 09:30 to 9:35 (UT) post the earthquake. Potential reason of the TEC anomaly, which might be a anomalous fluctuation e.g., electron density variation, is rising high speed acoustic shock wave with the speed of at least 1818.18 Km/h. m/s resulted by the mainshock of the earthquake. The duration time of the TEC anomaly was at least 5minutes. The anomalous fluctuation could be an early warning for the regions far from the epicenter when it began to propagate because the tsunami arrived at the far regions very slower than anomalous fluctuation if the tsunami was caused by earthquake with epicenter in the sea.

Possibility of tsunami early-warning from post-seismic ionospheric disturbance for 2 July, 2013, Mw = 6.1 Indonesia’s Bireun earthquake: Two-dimensional principal component analysis

Two-dimensional principal component analysis (2DPCA) has been used to determine ionospheric two-dimensional total electron content (TEC) anomaly after Indonesia’s Bireun earthquake at 07:37:02UT on 2 July, 2013 (Mw=6.1). The TEC data, from 07:35 to 07:50 (UT) during the earthquake, are examined. The TEC anomaly was more intense localized at 07:40–7:45 (UT) post earthquake. Potential reason of the TEC anomaly may be an anomalous fluctuation e.g., electron density variation which is a rising high speed acoustic shock wave resulted by the mainshock of the earthquake. The duration time of the TEC anomaly was at least 5min. The anomalous fluctuation could be an early warning for the regions far from the epicenter when it began to propagate because the tsunami arrived at the far regions much slower than the anomalous fluctuation if the tsunami was caused by an earthquake with the epicenter in the sea.

Possible reason of ionospheric anomaly post the April 20, 2013, Mw = 6.6 China' Lushan earthquake: Applying two-dimensional principal component analysis (2DPCA) to two-dimensional total electron content (TEC)

In this study, two-dimensional principal component analysis (2DPCA) was used to determine the reason for the ionospheric two-dimensional total electron content (TEC) anomaly after China's Lushan earthquake at 00:02:47 UT on April 20, 2013 (Mw = 6.6). TEC data from 00:00 to 00:15 (UT) on April 20, 2013 were examined. The TEC anomaly was very intense 00:05 to 00:10 (UT) after the earthquake. One potential cause of the TEC anomaly, which might have been a density variance, is a rising acoustic shock wave, with speed of at least 1127.82, ms−1, resulting from the mainshock. The duration of the TEC anomaly was at least 5 min.

Statistical study of large‐scale traveling ionospheric disturbances generated by the solar terminator over China

AbstractThis paper presents a statistical result of large‐scale traveling ionospheric disturbances (LSTIDs) associated with moving solar terminator in China during 12 months from February 2011 to January 2012. The LSTIDs are identified by the two‐dimensional total electron content (TEC) perturbation maps, which are built based on the observations of GPS network data from China. The GPS observations are combined with the observations from an ionosonde chain established by the Institute of Geology and Geophysics, Chinese Academy of Sciences. A total of 135 LSTID events are identified at dawn, while there is indiscernible LSTID at dusk. Meanwhile, these LSTIDs are captured by the ionosonde chain which shows that there are perturbations in the virtual heights during the passage of LSTIDs at the height between 200 and 700 km. The occurrence rate of LSTIDs shows a maximum in winter and a minimum in summer. The LSTIDs propagate across China with phase front widths larger than 1500 km. The propagation direction of LSTIDs is northwestward in winter, southwestward in summer, and quasi‐westward in equinoxes, respectively. The average period, horizontal phase velocity, and horizontal wavelength of LSTIDs are 79 ± 12 min, 288 ± 43 m/s, and 1503 ± 205 km, respectively. The relative TEC perturbations of the LSTIDs attenuate as the LSTIDs travel across China.

Monitoring traveling ionospheric disturbances using the GPS network around China during the geomagnetic storm on 28 May 2011

Larger-scale traveling ionospheric disturbances (LSTIDs) are studied using the total electron content (TEC) data observed from 246 GPS receivers in and around China during the medium storm on 28 May 2011. It is the first attempt to get the two-dimensional TEC perturbation maps in China. Two LSTID events are detected: one is in southwestern China before midnight propagating from low to middle latitude to the distance of about 1200 km with the phase front extending to about 500 km, and the other is in northeastern China after midnight propagating from middle to low latitudes to the distance of about 1200 km with the phase front extending to nearly 1400 km. By using the multichannel maximum-entropy method, we get the propagation parameters of these two LSTIDs. The LSTID that occurs before midnight has a higher horizontal phase velocity and a larger damping rate corresponding to the after midnight LSTID, and this may be caused by the relatively large vertical background TEC0 and high atmospheric temperature in the southwest of China before midnight. According to the variations of magnetic H component observed in high latitudes, the source region for the after midnight LSTID is likely to be located 1400–2600 km east of 140°E and north of 42°N; the before midnight LSTID is propably excited by the atmospheric gravity waves (AGWs) generated by the Joule heating of the equatorial electrojet. GPS, large scale traveling ionospheric disturbances, atmospheric gravity waves, total electron content

Ionospheric anomalies observed over South Korea preceding the Great Tohoku earthquake of 2011

We investigated the ionospheric anomalies observed before the Tohoku earthquake, which occurred near the northeast coast of Honshu, Japan on 11 March, 2011. Based on data from a ground-based Global Positioning System (GPS) network on the Korean Peninsula, ionospheric anomalies were detected in the total electron content (TEC) during the daytime a few days before earthquake. Ionospheric TEC anomalies appeared on 5, 8 and 11 March. In particular, the ionospheric disturbances on 8 March evidenced a remarkable increase in TEC. The GPS TEC variation associated with the Tohoku earthquake was an increase of approximately 20 total electron content units (TECU), observed simultaneously in local and global TEC measurements. To investigate these pre-earthquake ionospheric anomalies, space weather conditions such as the solar activity index (F10.7) and geomagnetic activity indices (the Kp and Dst indices) were examined. We also created two-dimensional TEC maps to visual the spatial variations in the ionospheric anomalies preceding the earthquake.

Global GPS Ionospheric Modelling Using Spherical Harmonic Expansion Approach

In this study, we developed a global ionosphere model based on measurements from a worldwide network of global positioning system (GPS). The total number of the international GPS reference stations for development of ionospheric model is about 100 and the spherical harmonic expansion approach as a mathematical method was used. In order to produce the ionospheric total electron content (TEC) based on grid form, we defined spatial resolution of 2.0 degree and 5.0 degree in latitude and longitude, respectively. Two-dimensional TEC maps were constructed within the interval of one hour, and have a high temporal resolution compared to global ionosphere maps which are produced by several analysis centers. As a result, we could detect the sudden increase of TEC by processing GPS observables on 29 October, 2003 when the massive solar flare took place.

A statistical study of large‐scale traveling ionospheric disturbances observed by GPS TEC during major magnetic storms over the years 2003–2005

In this paper, we plot two‐dimensional total electron content (TEC) perturbation maps and investigate the statistical characteristics of large‐scale traveling ionospheric disturbances (LSTIDs) during major magnetic storms from 2003 to 2005. The TEC data were obtained from more than 600 GPS receivers in North America within the geographical latitudes of 25°N–55°N. We found a total of 135 cases of LSTIDs, with amplitudes of up to 3.5 TECU and a maximum front width of ∼4000 km. The mean value of periods, horizontal velocities, and azimuths are 1.8 h, 300 m/s, and 187° (7° west of south), respectively. The mean velocity is obviously slower than that observed at lower latitudes such as Japan. Of all the 135 LSTID events, 35 cases (26%) occurred in the nighttime with their possible source within the region of North America, according to the variation of magnetic H component observed in this region. In addition, the occurrence of LSTIDs peaks at 1200 LT and at 1900 LT. It is also pointed out that the UT dependence of the occurrence of auroral geomagnetic disturbances plays a major role in the forming of UT and LT dependence of the occurrence of LSTIDs observed at midlatitudes.

The effect of total solar eclipse of October 3, 2005, on the total electron content over Europe

GPS observations from EUREF permanent GPS network were used to observe the response of TEC (Total Electron Content) to the total solar eclipse on October 3, 2005, under quiet geomagnetic conditions of the daytime ionosphere. The effect of the eclipse was detected in diurnal variations and more distinctly in the variations of TEC along individual satellite passes. The trough-like variations with a gradual decrease and followed by an increase of TEC at the time of the eclipse were observed over a large region. The depression of TEC amounted to 3–4 TECU. The maximum depression was observed over all stations located at the maximum path of the solar eclipse. The delay of a minimum level of TEC with respect to the maximum phase of the eclipse was about 20–30 min. The two-dimensional TEC maps constructed with high temporal resolution (5-min interval) show that the eclipse produced remarkable changes in the structure of the ionosphere. These TEC maps demonstrate also that the depression of TEC reached 20–30% compared to a quiet day (October 4, 2005). The complex pattern in the spatio-temporal TEC distribution presents the important role in the dynamic processes in the ionosphere during the eclipse.

Large‐scale traveling ionospheric disturbances observed by GPS total electron content during the magnetic storm of 29–30 October 2003

We have investigated the propagation of large‐scale traveling ionospheric disturbances (LSTIDs) during the super magnetic storm of 29–30 October 2003. Two‐dimensional total electron content (TEC) perturbation maps over North America were built using TEC data provided by the American GPS network and the International GNSS Service. Three LSTID events were observed in the range of 30°N–50°N, 60°W–110°W during this period. The first two LSTIDs occurred consecutively during 0620–0800 UT on 29 October at the local time of midnight, right after the onset of the big substorm; the third one was found at noon during the expansion phase of another substorm on 30 October. The phase fronts of these LSTIDs passed over the United States and traveled southwestward to the distance of ∼2000 km with the maximum front width of ∼4000 km and the duration of less than 2 hours. The maximum amplitude of TEC perturbations attained 3 total electron content units (TECUs). The results differ from the former observation of Afraimovich and Voeykov (2004) and Afraimovich et al. (2006), who reported a solitary LSTID propagating southwestward over the United States with the amplitudes of up to 14 TECU on 30 October 2003. We have checked the magnetic H component observed at the geomagnetic observatories in North America and found it is most likely that the auroral westward electrojet was the cause of the LSTIDs on 29 October. The source region for these TIDs was likely to be located several hundred kilometers north of 50°N. Cross‐spectral analysis was conducted to obtain the global propagation characteristics of LSTIDs during this superstorm. Equatorward LSTIDs were found in all the three sectors of North America, Europe, and Asia, showing high correlation with the occurrence of auroral substorms.

Regional reference total electron content model over Japan based on neural network mapping techniques

Abstract. A regional reference model of total electron content (TEC) was constructed using data from the GPS Earth Observation Network (GEONET), which consists of more than 1000 Global Positioning System (GPS) satellite receivers distributed over Japan. The data covered almost one solar activity period from April 1997 to June 2007. First, TECs were determined for 32 grid points, expanding from 27 to 45° N in latitude and from 127 to 145° E in longitude at 15-min intervals. Secondly, the time-latitude variation averaged over three days was determined by using the surface harmonic functional expansion. The coefficients of the expansion were then modeled by using a neural network technique with input parameters of the season (day of the year) and solar activity (F10.7 index and sunspot number). Thus, two-dimensional TEC maps (time vs. latitude) can be obtained for any given set of solar activity and day of the year.

Assessment of global TEC mapping using a three-dimensional electron density model

Dual-frequency transmissions from the Global Positioning System satellites can be used to measure and map ionospheric total electron content (TEC) on global scales. Using data exclusively from ground-based GPS networks, global ionosphere mapping has been successfully applied using either two- or three-dimensional techniques. Two-dimensional TEC maps retrieve a horizontally-varying distribution of total electron content, assuming a fixed vertical electron density profile. In three-dimensional mapping, both the horizontal and vertical distribution density are adjusted to fit the data. We describe a three-dimensional TEC mapping algorithm that uses three independent constant-density slabs stacked vertically to model the electron density, and compare with a more conventional two-dimensional approach using a single slab. One apparent benefit of the new method is reduction in a level error of the TEC maps, which decreased by 1.7 TECU using the three-dimensional retrieval on simulated data (1 TEC Unit corresponds to 10 16 electrons/m 2). Another benefit of the multilayer approach is improved slant TEC modeling. Using actual data from an equatorial site at Cocos Islands (96.8E, 12.2 S), three slab modeling improved estimates of slant TEC by a factor of 2 for elevation angles between 10 and 20° (9 versus 4.4 TECU, root-mean-square). However, the global structure of the vertical TEC retrievals we analyzed did not improve using three-dimensional modeling. This may be due to a critical approximation shared by both techniques that TEC persists unchanged at a given local time. This assumption is required to produce global maps from observations acquired from widely scattered ground receivers. Further improving the retrieval of global TEC structure with ground-based data probably requires improved dynamical models of TEC behavior. New data available from GPS receivers in low Earth orbit is also promising.

High resolution mapping of TEC perturbations with the GSI GPS Network over Japan

Two‐dimensional total electron content (TEC) perturbations over Japan are mapped with the Geographical Survey Institute (GSI) GPS network, GEONET (GPS Earth Observation Network). Its spatial resolution is 0.15° latitude × 0.15° longitude and temporal resolution is 30 seconds. Two‐dimensional TEC observations with these high resolutions revealed spatial structures and temporal evolutions of traveling ionospheric disturbances (TIDs) in the nighttime mid‐latitude ionosphere on July 03, 1997. A preliminary result of the TEC perturbation mapping indicates that it would be a strong tool to investigate the ionospheric structures in detail. Coordinated observations with other ionospheric observation techniques, such as incoherent scatter (IS) radars, airglow imagers, and ionosondes, are needed to clarify the vertical structure of the ionosphere.