Positive Total Electron Content Research Articles

Analysis of GNSS Data for Earthquake Precursor Studies Using IONOLAB-TEC in the Himalayan Region

Earthquake precursors are the indicators that appear before an earthquake. The release of radon gas, ionospheric disturbances, anomalous animal behavior, and so on are examples of seismic and aseismic events. Ionospheric perturbations can be proved to be a reliable method in earthquake prediction. The GNSS data detect changes in the ionosphere through the time lag of the transmitted GPS signals recorded at the Earth-based receivers. A negative TEC anomaly is caused by the stress released from the rocks before the earthquake, which elevates positive ions or p-holes in the atmosphere and decreases the ions in the ionosphere. A positive TEC anomaly follows this because of the increase in ions in the ionosphere. The ionospheric disruption in the Himalayan region is examined before five random earthquakes. For this, data from 15 separate GNSS stations are investigated using IONOLAB-TEC. A promising total electron content (TEC) data estimate with a temporal resolution of 30 s was analyzed. The results of the TEC data analysis depict the anomaly a month before the five earthquakes, followed by the later perturbation in the earthquake preparation zone. TEC anomalies are enhanced more by the uniform spatial distribution of GNSS stations in the epicentral region than by randomly distributed stations. The results of IONOLAB-TEC and the widely used GPS-TEC software were compared. Owing to its temporal resolution, IONOLAB-TEC has edge over the GPS-TEC software in that it can identify even the slightest negative anomalies before an earthquake.

Connection of total electron content disturbances with AE index of geomagnetic activity during geomagnetic storm in March 2015

Ionospheric response to the March 17, 2015 geomagnetic storm has been investigated using simulations of the Global Self-consistent Model of the Thermosphere, Ionosphere, Protonosphere (GSM TIP) [Dmitriev et al., 2017; Klimenko et al., 2018]. GSM TIP demonstrates results that do not contradict experimental data. This paper deals with GSM TIP simulated disturbances in the Total Electron Content (TEC) at different longitudes and zonal averages on March 17–23, 2015. At all longitudes, we can observe the existence of a band of TEC positive disturbances, located over the geomagnetic equator, and the formation of an after-storm ionospheric effect that appeared as positive TEC disturbances at midlatitude 3–5 days after the geomagnetic storm main phase. We have analyzed the dependence of disturbances of the thermosphere-ionosphere system (total electron content, n(N2), n(O), zonal electric field, meridional component of the thermospheric wind at a height of 300 km, and electron temperature at a height of 1000 km), calculated by GSM TIP from variations in the geomagnetic activity index AE. The analysis is based on Pearson’s correlation coefficients, presented as maps of the dependence of the correlation coefficient on UT and latitude for selected longitudes and for zonal averaged values. The results suggest that at high latitudes of the Northern and Southern hemispheres the correlation coefficient of TEC disturbances and AE variations is close to 1 at all longitudes in the period from 12 UT to 23 UT. From 9 UT to 12 UT, the minimum value of the correlation coefficient is observed at all latitudes and longitudes. The time intervals of the correlation values are associated with the features of a particular geomagnetic storm, for which, for example, the interval from 12 UT to 23 UT on March 17, 2015 corresponds to the geomagnetic storm main phase. We discuss possible mechanisms for the formation of such a relationship between simulated TEC disturbances and the AE index.

Связь возмущений полного электронного содержания с AE-индексом геомагнитной активности во время геомагнитной бури в марте 2015 г.

Ionospheric response to the March 17, 2015 geomagnetic storm has been investigated using simulations of the Global Self-consistent Model of the Thermosphere, Ionosphere, Protonosphere (GSM TIP) [Dmitriev et al., 2017; Klimenko et al., 2018]. GSM TIP demonstrates results that do not contradict experimental data. This paper deals with GSM TIP simulated disturbances in the Total Electron Content (TEC) at different longitudes and zonal averages on March 17–23, 2015. At all longitudes, we can observe the existence of a band of TEC positive disturbances, located over the geomagnetic equator, and the formation of an after-storm ionospheric effect that appeared as positive TEC disturbances at midlatitude 3–5 days after the geomagnetic storm main phase. We have analyzed the dependence of disturbances of the thermosphere-ionosphere system (total electron content, n(N2), n(O), zonal electric field, meridional component of the thermospheric wind at a height of 300 km, and electron temperature at a height of 1000 km), calculated by GSM TIP from variations in the geomagnetic activity index AE. The analysis is based on Pearson’s correlation coefficients, presented as maps of the dependence of the correlation coefficient on UT and latitude for selected longitudes and for zonal averaged values. The results suggest that at high latitudes of the Northern and Southern hemispheres the correlation coefficient of TEC disturbances and AE variations is close to 1 at all longitudes in the period from 12 UT to 23 UT. From 9 UT to 12 UT, the minimum value of the correlation coefficient is observed at all latitudes and longitudes. The time intervals of the correlation values are associated with the features of a particular geomagnetic storm, for which, for example, the interval from 12 UT to 23 UT on March 17, 2015 corresponds to the geomagnetic storm main phase. We discuss possible mechanisms for the formation of such a relationship between simulated TEC disturbances and the AE index.

Potential utilization of air temperature, total electron content, and air relative humidity as possible earthquake precursors: A case study of Mexico M7.4 earthquake

This study examines ionospheric total electron content (TEC) perturbations from six International Global Navigation Satellite System stations (GNSS) including GUAT-Guatemala, SSIA-El Salvador, INEG-Mexico, MANA-Nicaragua, MDO1-United States of America, and BOGT- Colombia for several days around the occurrence of a major earthquake (M 7.4 and depth 20.0 km) in Mexico, June 23, 2020 at 10:29 Local Time (LT). The INEG station in the North-Northwest of the epicenter at a distance of about 936 km indicated a positive TEC anomaly on June 18, 2020, which can be possibly viewed as an earthquake precursor due to its occurrence during a quiet geomagnetic storm and inactive solar activity. Study findings reveal that other TEC perturbations may not be related to the earthquake, because they appeared during geomagnetic activities. Moreover, the atmospheric parameters have significant and synchronous deviations from the earthquake epicenter on June 13, 2020. The highest atmospheric chemical potential (ACP) is about 0.010 eV, atmospheric air temperature has positive deviation of 3.937 °C at 15:00 LT, the lowest atmospheric relative humidity has negative deviation of 25.387% at 13:00 LT and outgoing longwave radiation (OLR) 27.58 W m−2. Observations validate that in the earthquake preparation zone, variations in atmospheric air temperature and relative humidity reach at peak value during 10 days prior to the impending earthquake event with the later perturbations in the ionosphere.

The Conjugated Ionospheric Anomalies Preceding the 2011 Tohoku-Oki Earthquake

We present the conjugated ionospheric total electron content (TEC) anomalies prior to the 2011 Mw9.0 Tohoku-oki earthquake, Japan, observed by the Global Navigation Satellite System (GNSS) stations in northern Australia. The onset time of the anomaly, determined by the Akaike’s information criterion, is 41.5 min before the earthquake in Australia, which is very close to the time observed in Japan. The positive TEC anomalies in Australia emerged on the same longitude as the land area of NE Japan. This supports the model that electric fields within the ionosphere redistributed the electrons immediately before large earthquakes. However, the observed anomaly is shifted equatorward by ∼500 km reflecting the difference in physical mechanisms between the two hemispheres. We also found that the geomagnetic declination near the conjugate point simultaneously started to change ∼40 min before the earthquake, but its physical implication is yet to be explored.

African and American Equatorial Ionization Anomaly (EIA) Responses to 2013 SSW Event

AbstractThis study investigates the responses of the African and American Equatorial Ionization Anomaly (EIA) regions to 2013 Sudden Stratospheric Warming (SSW) event. The Total Electron Content (TEC) data obtained from chains of Global Positioning System receivers within ±40° geomagnetic latitudes in the African and American sectors were used to construct the EIA structures for both longitudinal sectors. The responses of the EIA structures, constructed from the TEC, ΔTEC, and ionospheric irregularities data to the 2013 SSW event were investigated. During the SSW peak phase, EIA structures in both longitudinal sectors responded significantly, with the pole‐ward flow of plasma from the equator to higher crests' locations (strengthening of the EIA). Furthermore, a clear asymmetry in plasma distribution in the northern and southern crests of the EIA was observed. Generally, for the entire data span, TEC enhancements and ionospheric irregularities occurrences during SSW were more in the American sector than the African sector. The geomagnetic activity of 17 January 2013 caused negative TEC response in the African sector and positive TEC response in the American sector. Moderate storm‐induced TEC enhancements were generally lower than SSW‐induced TEC enhancements. Furthermore, solar flux‐induced TEC of 10 January 2013 was lesser than the SSW‐induced TEC of 15–16 January 2013.

Ionospheric Response at Conjugate Locations During the 7–8 September 2017 Geomagnetic Storm Over the Europe‐African Longitude Sector

AbstractThis paper focuses on unique aspects of the ionospheric response at conjugate locations over Europe and South Africa during the 7–8 September 2017 geomagnetic storm including the role of the bottomside and topside ionosphere and plasmasphere in influencing electron density changes. Analysis of total electron content (TEC) on 7 September 2017 shows that for a pair of geomagnetically conjugate locations, positive storm effect was observed reaching about 65% when benchmarked on the monthly median TEC variability in the Northern Hemisphere, while the Southern Hemisphere remained within the quiet time variability threshold of ±40%. Over the investigated locations, the Southern Hemisphere midlatitudes showed positive TEC deviations that were in most cases twice the comparative response level in the Northern Hemisphere on the 8 September 2017. During the storm main phase on 8 September 2017, we have obtained an interesting result of ionosonde maximum electron density of the F2 layer and TEC derived from Global Navigation Satellite System (GNSS) observations showing different ionospheric responses over the same midlatitude location in the Northern Hemisphere. In situ electron density measurements from SWARM satellite aided by bottomside ionosonde‐derived TEC up to the maximum height of the F2 layer (hmF2) revealed that the bottomside and topside ionosphere as well as plasmasphere electron content contributions to overall GNSS‐derived TEC were different in both hemispheres especially for 8 September 2017 during the storm main phase. The differences in hemispheric response at conjugate locations and on a regional scale have been explained in terms of seasonal influence on the background electron density coupled with the presence of large‐scale traveling ionospheric disturbances and low‐latitude‐associated processes. The major highlight of this study is the simultaneous confirmation of most of the previously observed features and their underlying physical mechanisms during geomagnetic storms through a multi–data set examination of hemispheric differences.

Global Distribution of Persistence of Total Electron Content Anomaly

To better understand the ionospheric morphology response to lithospheric activities, we study the global location preference of the positive and negative total electron content (TEC) anomalies persisting continuously for longer than 24 h at middle and low latitudes (within ±60° N geomagnetic latitudes). The TEC is obtained from the global ionospheric map (GIM) of Center for Orbit Determination in Europe (CODE) under the geomagnetic quiet condition of Kp ≤ 3o during the period of 2005 to 2018. There are a few (less than 4%) TEC anomalies that can persist over 24 h. The conjugate phenomenon is most significant in the eastern Asia to Australia longitudinal sector. The result shows the persistence of the positive TEC anomaly along the ring of fire on the western edge of the Pacific Ocean. The high persistence of the TEC anomalies at midlatitudes suggests that thermospheric neutral wind contributes to the anomaly formation.

TEC behavior over the Mexican region

With the advent of the Navigation Satellites the Total Electron Content (TEC) has become one of the main parameters of the ionosphere. This is the result of a continuous TEC monitoring and rather dense network of GPS receivers. For Mexican region having no ionosondes the use of TEC for ionospheric conditions studies and monitoring has a special value. To study the behaviour of TEC and for its applied aspects two types of source-files are used worldwide: IONEX (global maps) and RINEX (local data) depending on the task solved. Magnetometer and satellite data from CHAMP and DMSP were involved in the analysis. First, benefits and limitations of TEC derived from both types of files are discussed in regard to the estimation of the ionosphere state in the Mexican region. Second, using both methods the specific features of diurnal, seasonal and annual patterns in TEC behaviour over Mexico were revealed, among which are the shift of the diurnal maximum to 14 LT, dependence on solar activity, high probability of night-time enhancements, presence of annual and winter anomalies. Third, it was revealed that the positive short-lived TEC enhancements are characteristic for Mexican region. They may occur even under quiet conditions. The answer is given what part of the ionosphere is responsible for TEC change during these positive disturbances. The results for Mexico were compared to the neighboring regions and South-East zone.

Aerosols and seismo-ionosphere coupling: A review

The role of atmosphere aerosols in the global electric circuit, particularly during earthquakes preparation periods, is discussed in this review paper. Aerosols participate in production and transport of electric charges as well as in clouds formation. Satellite imagery shows increased aerosol optical depth over the tectonic faults and formation of the anomalous clouds aligned with the faults shortly before the earthquake shocks. At the same time variations of the ionospheric electric field and total electron content (TEC) are observed. We assume that the vertical electric current is generated over the fault due to the separation and vertical transport of charges with different masses and polarities. This charges the ionosphere positively relative to the Earth in the same way as the thunderstorm currents do. The resulting electric field in the ionosphere drives F2-layer plasma via the electromagnetic [E→×B→] drift and decreases or increases electron density depending on the configuration of the electric field, thus, creating observed negative or positive TEC disturbances. The important role of the electric dynamo effect in these processes is underlined.

Long‐term trends in the total electron content

AbstractThe total electron content (TEC) is important among others for Global Navigation Satellite Systems/GPS signal propagation and applications. However, there is only one comprehensive analysis of TEC trends, and the resulted trends are not consistent with trends in other ionospheric parameters. Here we use the TEC data of Lean et al. (2011) and the JPL35 homogeneous TEC data series derived by Emmert et al. (2017). This analysis results in three main conclusions: (1) Too positive TEC trends by Lean et al. (2011) are caused by data problems in 1995–2001, particularly by too low Center for Orbit Determination data. (2) TEC reveals a weak negative trend at the edge of reliability; no trend or trend break is also possible; longer data series than 1994–2015 is required. (3) About 99% of the total variance of yearly average global TEC values is explained by variability of solar activity.

Features of TEC behaviour over the low-latitude North-American region during the period of medium solar activity

The study is focused on Total Electron Content (TEC) behavior in low-latitude North-American region within the twelve-year period from 2005 to 2016. The obtained systematic (regular) TEC variations (diurnal, seasonal patterns) conform to the known tendencies described by climatological TEC models. TEC variations during disturbances have a number of distinctive features. The majority of positive TEC enhancements occur already with Dst <−30nT. The intensity of TEC change does not depend on Dst-index minimum value. Almost all cases of day-time TEC disturbances are accompanied by night-time disturbances with the possibility of relative night-time deviations being larger that is confirmed by data of low-orbit satellites CHAMP and DMSP. The simultaneity of changes of critical frequencies of the ionosphere with changes of TEC during disturbances was observed at Ramey (Puerto Rico) and Eglin (U.S.A.) stations when ionosonde data was available. This proves that TEC reflects the character of ionospheric disturbances. Spatial non-synchronism of TEC change that was sometimes observed can be explained by local character of the area covered with positive TEC disturbance. It was shown the potential provided by local GNSS networks for the detailed analysis of TEC variations over the considered region and estimation of the ionosphere state in near-real time.

Investigation of ionospheric precursors related to deep and intermediate earthquakes based on spectral and statistical analysis

Ionospheric TEC (Total Electron Content) variations prior to the deep (≈600km) earthquake doublet close to magnetic equator in Peru (M=7.6) and to the intermediate (≈200km) earthquake in Afghanistan (M=7.5) during 2015 were investigated using measurements from Global Navigation Satellite System (GNSS) network with the aim to detect possible ionospheric precursors of these events. For this we applied both statistical and spectral analysis. Ionospheric anomalies related to both earthquakes were observed few hours and few days prior to the earthquakes during daytime localized mainly near the epicenter. These were large-scale positive TEC anomalies and small-scale TEC oscillations with periods of 20min and duration around 2–4h appearing at the same local time each day. Several days prior to the earthquake in Peru a significant phenomenon was observed during afternoon time related to the modification of the Equatorial Ionization Anomaly (EIA) structure. During nighttime, however, it was not possible to identify any ionospheric earthquake precursor due to the concurrence of various phenomena, such as Equatorial Plasma Bubbles and pre- and post-midnight TEC peaks prior to Peru earthquake, and solar terminator transition prior to both earthquakes which could induce resembling ionospheric anomalies.

Statistical discrimination of global post-seismic ionosphere effects under geomagnetic quiet and storm conditions

ABSTRACTThe retrospective statistical analysis of total electron content (TEC) is carried out using global ionospheric maps (GIM) for 1999–2015. TEC anomalies are analysed for 2670 earthquakes (EQ) from M6.0 to M10.0 classified into 2205 ‘non-storm’ EQs and 465 ‘storm’ EQs during geomagnetic storms. The geomagnetic storms are specified by relevant thresholds of geomagnetic indices AE, aa, ap, ap(τ) and Dst. Using sliding-window statistical analysis, moving daily–hourly TEC median μ for 15 preceding days with estimated variance bounds is obtained for each grid pixel of GIM-TEC maps. The derived ionosphere variability index, Vσ, is expressed in terms of ΔTEC deviation from the median normalized by the standard deviation σ. Vσ index segmentation is introduced specifying TEC anomaly if an instant TEC is outside the bound of μ ± 1σ. Efficiency of EQ impact on the ionosphere (Eσ) is growing with EQ magnitude and depth representing relative density of TEC anomalies within area of 1000 km radius around EQ hypocentre. Positive TEC ‘storm’ anomalies are twice as much as those of non-storm values. This observation supports dominant post-EQ TEC enhancement with Eσ peak decreasing during 12 h for daytime but growing by nighttime during 6 h after EQ followed by gradual recovery afterwards.

Three‐dimensional distribution of ionospheric anomalies prior to three large earthquakes in Chile

AbstractUsing regional Global Positioning System (GPS) networks, we studied three‐dimensional spatial structure of ionospheric total electron content (TEC) anomalies preceding three recent large earthquakes in Chile, South America, i.e., the 2010 Maule (Mw 8.8), the 2014 Iquique (Mw 8.2), and the 2015 Illapel (Mw 8.3) earthquakes. Both positive and negative TEC anomalies, with areal extent dependent on the earthquake magnitudes, appeared simultaneously 20–40 min before the earthquakes. For the two midlatitude earthquakes (2010 Maule and 2015 Illapel), positive anomalies occurred to the north of the epicenters at altitudes 150–250 km. The negative anomalies occurred farther to the north at higher altitudes 200–500 km. This lets the epicenter, the positive and negative anomalies align parallel with the local geomagnetic field, which is a typical structure of ionospheric anomalies occurring in response to positive surface electric charges.

Seasonal variations of GPS derived TEC at three different latitudes of the southern hemisphere during geomagnetic storms

Data from the archive of the International GNSS Services (IGS) were used to study the seasonal variations of Total Electron Content (TEC) over three stations located at different latitudes in the southern hemisphere during the geomagnetic storms of 11 January, 6 April, 8 June, and 13 October 2000, representing storms that occurred in summer, autumn equinox, winter and spring equinox, respectively. The percentage TEC deviation with respect to reference values differs substantially from season to season. A strong seasonal anomaly and clear equinoctial asymmetry in TEC response to the storms were observed. Weak and short-lived positive TEC deviations as well as strong and long-lasting negative trends were observed in summer storm during the main and recovery phases respectively over the high and low latitudes whereas in winter storm, the highest positive TEC deviations was recorded during the main phase over the entire latitudes. TEC enhancement dominated all the stations during the autumn (March) equinox storm while TEC depletion was majorly observed during the spring (September) equinox. All these variations find their explanations in the thermospheric composition change and circulation. Future work with direct or modeled measurement of atomic Oxygen to molecular Nitrogen ratio (O/N2), large number of storms and other possible factors such as variations in storm’s intensity and local time dependence of the storm onset is expected to validate the observations in this study.

Two-dimensional observations of midlatitude sporadicEirregularities with a dense GPS array in Japan

[1] We observed two-dimensional structure and time evolution of ionospheric irregularities caused by midlatitude sporadic E (Es) over Japan as positive anomalies of total electron content (TEC) by analyzing the data from the nationwide Global Positioning System (GPS) array. In this paper we report a case study of strong Es observed in the local evening of 21 May 2010, over Tokyo, Japan. In the slant TEC time series, Es showed a characteristic pulse-like enhancement of ~1.5 TEC units lasting for ~10 min. We plotted these positive TEC anomalies on the subionospheric points of station-satellite pairs to study the horizontal structure of the Es irregularity. We confirmed that the irregularity existed at the height of ~106 km by comparing the data of multiple GPS satellites, which is consistent with the local ionosonde observations. The horizontal shapes of the Es irregularity showed frontal structures elongated in E-W, spanning ~150 km in length and ~30 km in width, composed of small patches. The frontal structure appears to consist of at least two parts propagating in different directions: one moved eastward by ~60 m s−1, and the other moved southwestward by ~80 m s−1. Similar TEC signatures of Es were detected by other GPS satellites, except one satellite that had line of sight in the N-S direction which dips by 40–50° toward north, which indicates the direction of plasma transportation responsible for the Es formation. We also present a few additional observation results of strong Es irregularities.

Formation mechanism of great positive TEC disturbances prior to Wenchuan earthquake on May 12, 2008

The problems of physical explanation and possible mechanisms of the seismo-ionospheric effects formation are under discussion now. There are proposed different mechanisms of such effects, for example, large- and small-scale internal gravity waves (IGWs), atmospheric electric field, electromagnetic fields and emissions. However, the appearance of local large-scale seismo-ionospheric anomalies in Total Electron Content ( TEC) is possible to explain only by two mechanisms: an atmospheric electric field and/or small-scale IGWs. In this paper, the simulation results for reproduction of the observed seismo-ionospheric great positive effects in TEC prior to strong Wenchuan earthquake are presented. The obtained results confirm the proposed mechanism of seismo-ionospheric effects formation by the penetration of the seismogenic electric field from the atmosphere into the ionosphere. It is suggested that so great TEC enhancement observed 3 days prior to Wenchuan earthquake could be explained by combined action of seismogenic vertical electric field and IGWs generated by the solar terminator.

Vertical TEC over seismically active region during low solar activity

Using data of Japanese dense array of GPS receivers (GEONET) we analyzed variations of total electron content (TEC) over Japan during a period of low solar and geomagnetic activity in April–September 2008. During that time, five large earthquakes with magnitude M>6.8 occurred around the Japanese Islands. We calculated vertical TEC with time resolution of 1 h and we compared the TEC over Japan with TEC over southwestern USA. In order to detect abnormal signals, we processed daily TEC based on median and quartile of preceding 15 days. Knowing that the decisive role in the ionosphere state is performed by space weather effects, we compared the estimated TEC values with time series of the B z component of the interplanetary magnetic field (IMF), UV solar radiation, index of geomagnetic activity Dst and planetary index Kp. We have found that though the analyzed time period falls on the minimum of solar activity, the observed positive anomalies in the regional TEC correspond to fluctuations of IMF B z and to the following increase in Kp and Dst indices. Other case of ∼25% TEC increase in the duration of four days (DOY: 140–143) is, most likely, caused by increase in the solar UV radiation and coincides with growth of the global TEC. Other positive or negative abnormal TEC variations coincide with fluctuations of the global mean TEC or are of large spatial scale, i.e. much larger than the earthquake preparation zone. Therefore, it is rather difficult to find correlation between the observed anomalies and the five large earthquakes. Our results do not disprove the possibility of precursory phenomena but show the difficulties in identifying earthquake precursors in the ionosphere TEC on the background of TEC changes produced by space weather effects, even during solar minimum.

Ionospheric anomalies possibly associated with M ⩾ 6.0 earthquakes in the Japan area during 1998–2010: Case studies and statistical study

In this paper, we examine pre-earthquake ionospheric anomalies in time series and perform a statistical test by using total electron content (TEC) derived from global ionosphere maps (GIM) around the Japan area for the first time. The normalized GIM-TEC (GIM-TEC *), which is computed based on 15 days backward running mean of GIM-TEC, have been investigated for minimizing possible confounding effects of consecutive earthquakes and identify the abnormal signals. Meanwhile, to reduce the effect of strong geomagnetic activities such as geomagnetic storms, the criterion for removing the GIM-TEC data have been adapted; that is when Dst index exceeds −60 nT. Temporal variations of GIM-TEC * for large and destructive earthquakes in Japan have been studied; which are the 2004 mid-Niigata Prefecture Earthquake (M6.8), its aftershock (M6.1), the 2007 offshore mid-Niigata Earthquake (M6.8), and the 2008 Iwate–Miyagi Nairiku Earthquake (M7.2). Although there are some positive and negative TEC anomalies before and after the four earthquakes, there is a tendency that positive TEC anomalies appear 1–5 days before all the above earthquakes even during the quiet geomagnetic condition. Superposed epoch analyses have been performed for the statistical analysis of TEC anomalies associated with M ⩾ 6.0 earthquakes during the 12-year period of May 1998–May 2010. The statistical result indicates the significance of the positive TEC anomalies 1–5 days before earthquakes within 1000 km from the epicenter around Japan.