Showing papers on "TEC published in 2015"

SHPTS: towards a new method for generating precise global ionospheric TEC map based on spherical harmonic and generalized trigonometric series functions

Abstract: To take maximum advantage of the increasing Global Navigation Satellite Systems (GNSS) data to improve the accuracy and resolution of global ionospheric TEC map (GIM), an approach, named Spherical Harmonic plus generalized Trigonometric Series functions (SHPTS), is proposed by integrating the spherical harmonic and the generalized trigonometric series functions on global and local scales, respectively. The SHPTS-based GIM from January 1st, 2001 to December 31st, 2011 (about one solar cycle) is validated by the ionospheric TEC from raw global GPS data, the GIM released by the current Ionospheric Associate Analysis Center (IAAC), the TOPEX/Poseidon satellite and the DORIS. The present results show that the SHPTS-based GIM over the area where no real data are available has the same accuracy level (approximately 2–6 TECu) to that released by the current IAAC. However, the ionospheric TEC in the SHPTS-based GIM over the area covered by real data is more accurate (approximately 1.5 TECu) than that of the GIM (approximately 3.0 TECu) released by the current IAAC. The external accuracy of the SHPTS-based GIM validated by the TOPEX/Poseidon and DORIS is approximately 2.5–5.5 and 1.5–4.5 TECu, respectively. In particular, the SHPTS-based GIM is the best or almost the best ranked, along with those of JPL and UPC, when they are compared with TOPEX/Poseidon measurements, and the best (in addition to UPC) when they are validated with DORIS data. With the increase in the number of GNSS satellites and contributing stations, the performance of the SHPTS-based GIM can be further improved. The SHPTS-based GIM routinely calculated using global GPS, GLONASS and BDS data will be found at the website http://www.gipp.org.cn.

SHPTS: Towards a new method for generating precise global ionospheric TEC maps based on spherical harmonic and generalized trigonometric series functions

Abstract: To take maximum advantage of the increasing Global Navigation Satellite Systems (GNSS) data to improve the accuracy and resolution of global ionospheric TEC map (GIM), an approach, named Spherical Harmonic plus generalized Trigonometric Series functions (SHPTS), is proposed by integrating the spherical harmonic and the generalized trigonometric series functions on global and local scales, respectively. The SHPTS-based GIM from January 1st, 2001 to December 31st, 2011 (about one solar cycle) is validated by the ionospheric TEC from raw global GPS data, the GIM released by the current Ionospheric Associate Analysis Center (IAAC), the TOPEX/Poseidon satellite and the DORIS. The present results show that the SHPTS-based GIM over the area where no real data are available has the same accuracy level (approximately 2–6 TECu) to that released by the current IAAC. However, the ionospheric TEC in the SHPTS-based GIM over the area covered by real data is more accurate (approximately 1.5 TECu) than that of the GIM (approximately 3.0 TECu) released by the current IAAC. The external accuracy of the SHPTS-based GIM validated by the TOPEX/Poseidon and DORIS is approximately 2.5–5.5 and 1.5–4.5 TECu, respectively. In particular, the SHPTS-based GIM is the best or almost the best ranked, along with those of JPL and UPC, when they are compared with TOPEX/Poseidon measurements, and the best (in addition to UPC) when they are validated with DORIS data. With the increase in the number of GNSS satellites and contributing stations, the performance of the SHPTS-based GIM can be further improved. The SHPTS-based GIM routinely calculated using global GPS, GLONASS and BDS data will be found at the website http://www.gipp.org.cn.

Ionospheric Response to Infrasonic-Acoustic Waves Generated By Natural Hazard Events

Abstract: Recent measurements of GPS-derived total electron content (TEC) reveal acoustic wave periods of ∼1–4 min in the F region ionosphere following natural hazard events, such as earthquakes, severe weather, and volcanoes. Here we simulate the ionospheric responses to infrasonic-acoustic waves, generated by vertical accelerations at the Earth's surface or within the lower atmosphere, using a compressible atmospheric dynamics model to perturb a multifluid ionospheric model. Response dependencies on wave source geometry and spectrum are investigated at middle, low, and equatorial latitudes. Results suggest constraints on wave amplitudes that are consistent with observations and that provide insight on the geographical variability of TEC signatures and their dependence on the geometry of wave velocity field perturbations relative to the ambient geomagnetic field. Asymmetries of responses poleward and equatorward from the wave sources indicate that electron perturbations are enhanced on the equatorward side while field aligned currents are driven principally on the poleward side, due to alignments of acoustic wave velocities parallel and perpendicular to field lines, respectively. Acoustic-wave-driven TEC perturbations are shown to have periods of ∼3–4 min, which are consistent with the fraction of the spectrum that remains following strong dissipation throughout the thermosphere. Furthermore, thermospheric acoustic waves couple with ion sound waves throughout the F region and topside ionosphere, driving plasma disturbances with similar periods and faster phase speeds. The associated magnetic perturbations of the simulated waves are calculated to be observable and may provide new observational insight in addition to that provided by GPS TEC measurements.

Coseismic ionospheric disturbance of the large strike-slip earthquakes in North Sumatra in 2012: Mw dependence of the disturbance amplitudes

Abstract: We studied ionospheric responses to the 2012 April 11 Mw8.6 North Sumatra earthquake using total electron content (TEC) measurements with the regional Global Navigation Satellite System (GNSS) network. This earthquake ruptured the oceanic lithosphere off the Indian Ocean coast of North Sumatra, and is known as the largest strike-slip earthquake ever recorded. Coseismic ionospheric disturbances (CID) with rapid TEC enhancement of a few TEC units propagated northward with a speed of acoustic waves (~1 km/s). Resonant atmospheric oscillation with a frequency ~4 mHz have been found as monochromatic oscillation of TEC lasting for an hour after the main shock and the largest aftershock. We compared CID amplitudes of 21 earthquakes worldwide with moment magnitudes (Mw) 6.6-9.2. They roughly obeyed a law such that CID amplitude increases by two orders of magnitude for the Mw increase of three. The 2012 North Sumatra earthquakes slightly deviated negatively from the trend possibly reflecting their strike-slip mechanisms, i.e. small vertical crustal movements for their magnitudes.

L-band scintillations and calibrated total electron content gradients over Brazil during the last solar maximum

Abstract: This work presents a contribution to the understanding of the ionospheric triggering of L-band scintillation in the region over Sao Paulo state in Brazil, under high solar activity. In particular, a climatological analysis of Global Navigation Satellite Systems (GNSS) data acquired in 2012 is presented to highlight the relationship between intensity and variability of the total electron content (TEC) gradients and the occurrence of ionospheric scintillation. The analysis is based on the GNSS data acquired by a dense distribution of receivers and exploits the integration of a dedicated TEC calibration technique into the Ground Based Scintillation Climatology (GBSC), previously developed at the Istituto Nazionale di Geofisica e Vulcanologia. Such integration enables representing the local ionospheric features through climatological maps of calibrated TEC and TEC gradients and of amplitude scintillation occurrence. The disentanglement of the contribution to the TEC variations due to zonal and meridional gradients conveys insight into the relation between the scintillation occurrence and the morphology of the TEC variability. The importance of the information provided by the TEC gradients variability and the role of the meridional TEC gradients in driving scintillation are critically described.

Temporal-Spatial Variation of Global GPS-Derived Total Electron Content, 1999-2013.

Abstract: To investigate the temporal-spatial distribution and evolutions of global Total Electron Content (TEC), we estimate the global TEC data from 1999 to 2013 by processing the GPS data collected by the International Global Navigation Satellite System (GNSS) Service (IGS) stations, and robustly constructed the TEC time series at each of the global 5°×2.5° grids. We found that the spatial distribution of the global TEC has a pattern where the number of TECs diminishes gradually from a low-latitude region to high-latitude region, and anomalies appear in the equatorial crest and Greenland. Temporal variations show that the peak TEC appears in equinoctial months, and this corresponds to the semiannual variation of TEC. Furthermore, the winter anomaly is also observed in the equatorial area of the northern hemisphere and high latitudes of the southern hemisphere. Morlet wavelet analysis is used to determine periods of TEC variations and results indicate that the 1-day, 26.5-day, semi-annual and annual cycles are the major significant periods. The fitting results of a quadratic polynomial show that the effect of solar activity on TEC is stronger in low latitudes than in mid-high latitudes, and stronger in the southern hemisphere than in the northern hemisphere. But the effect in low latitudes in the northern hemisphere is stronger than that in low latitudes in the southern hemisphere. The effect of solar activity on TECs was analyzed with the cross wavelet analysis and the wavelet coherence transformation, and we found that there appears to be a strong coherence in the period of about 27 days. So the sunspot as one index of solar activity seriously affects the TEC variations with the sun’s rotation. We fit the TEC data with the least squares spectral analysis to study the periodic variations of TEC. The changing trend of TEC is generally -0.08 TECu per year from 1999 to 2013. So TECs decrease over most areas year by year, but TECs over the Arctic around Greenland maintained a rising trend during these 15 years.

A regional ionospheric TEC mapping technique over China and adjacent areas on the basis of data assimilation

Abstract: In this paper, a regional total electron content (TEC) mapping technique over China and adjacent areas (70 degrees E-140 degrees E and 15 degrees N-55 degrees N) is developed on the basis of a Kalman filter data assimilation scheme driven by Global Navigation Satellite Systems (GNSS) data from the Crustal Movement Observation Network of China and International GNSS Service. The regional TEC maps can be generated accordingly with the spatial and temporal resolution being 1 degrees x1 degrees and 5min, respectively. The accuracy and quality of the TEC mapping technique have been validated through the comparison with GNSS observations, the International Reference Ionosphere model values, the global ionosphere maps from Center for Orbit Determination of Europe, and the Massachusetts Institute of Technology Automated Processing of GPS TEC data from Madrigal database. The verification results indicate that great systematic improvements can be obtained when data are assimilated into the background model, which demonstrates the effectiveness of this technique in providing accurate regional specification of the ionospheric TEC over China and adjacent areas.

M-w dependence of the preseismic ionospheric electron enhancements

Abstract: Ionospheric electron enhancement was reported to have occurred ~40 min before the 2011 Tohoku-oki (Mw9.0) earthquake, Japan, by observing total electron content (TEC) with Global Navigation Satellite Systems receivers. Their reality has been repeatedly questioned due mainly to the ambiguity in the derivation of the reference TEC curves from which anomalies are defined. Here we propose a numerical approach, based on Akaike's information criterion, to detect positive breaks (sudden increase of TEC rate) in the vertical TEC time series without using reference curves. We demonstrate that such breaks are detected 25–80 min before the eight recent large earthquakes with moment magnitudes (Mw) of 8.2–9.2. The amounts of precursory rate changes were found to depend upon background TEC as well as Mw. The precursor times also showed Mw dependence, and the precursors of intraplate earthquakes tend to start earlier than interplate earthquakes. We also performed the same analyses during periods without earthquakes to evaluate the usefulness of TEC observations for short-term earthquake prediction.

Interplay between Raman shift and thermal expansion in graphene: temperature-dependent measurements and analysis of substrate corrections

Abstract: Measurements and calculations have shown significant disagreement regarding the sign and temperature variations of the thermal expansion coefficient (TEC) of graphene $\ensuremath{\alpha}(T)$. Here we report dedicated Raman scattering experiments conducted for graphene monolayers deposited on silicon nitride substrates and over a broad temperature range extending over 150--800 K. The relation between those measurements for the $G$ band and the graphene TEC, which involves correcting the measured signal from the mismatch contribution of the substrate, is analyzed based on different theoretical candidates for $\ensuremath{\alpha}(T)$. Contrary to calculations in the quasiharmonic approximation, a many-body potential reparametrized for graphene correctly reproduces experimental data, suggesting that the TEC is more likely to be positive above room temperature.

Impacts of solar activity on performance of the IRI-2012 model predictions from low to mid latitudes

Abstract: This study investigates the impacts of solar activity on the performance of the latest release of International Reference Ionosphere (IRI) model version 2012 (IRI-2012) predictions during the ascending phase of solar activity from 2009 to 2013. The study is based on the data of total electron content (TEC) retrieved from the Global Positioning System (GPS) at Singapore (NTUS) (geographic latitude 01.34°N, longitude 103.67°E, geomagnetic latitude 8.4°S), Thailand (CUSV) (geographic latitude 13.73°N, longitude 100.54°E, geomagnetic latitude 3.96°N), China (KUNM) (geographic latitude 25.02°N, longitude 102.79°E, geomagnetic latitude 15.15°N), Mongolia (ULAB) (geographic latitude 47.67°N, longitude 107.05°E, geomagnetic latitude 37.73°S), and Russia (IRKM) (geographic latitude 52.21°N, 104.31°E, geomagnetic latitude 42.28°S). The GPS-TEC has been compared with the IRI-2012 model TEC for three different options, namely, IRI-NeQ, IRI01-corr, and IRI-2001, for topside Ne over all the above five stations lying at different latitudes from equatorial-equatorial ionization anomaly (EIA) to mid-latitude regions but at around the same longitude line (104° ± 3°E). The study showed that the IRI model predictions for different topside options are different and significant in low-latitude region but insignificant in mid-latitude regions (except during winter season of high solar activity year 2012). During the period from 2009 to 2013, upon moving from low to high solar activity, the prediction nature (overestimation/underestimation) of IRI-2012 model changes significantly at EIA station KUNM of low-latitude region. The discrepancy in IRI-2012 model TEC as compared to GPS-TEC in low-latitude region is found to be larger and significant than in mid-latitude region (Mongolia and Russia). The discrepancy in the IRI-2012 model TEC with IRI-2001 topside is found to be maximum at equatorial station CUSV (RMSD 99%) during the solar minimum year 2009 and decreases moving towards high solar activity year. This suggests that significant improvements to the IRI-2012 model (hmF2 model) are required particularly in the equatorial regions taking the impacts of solar minimum year into account.

Ionospheric assimilation of radio occultation and ground-based GPS data using non-stationary background model error covariance

Abstract: . Ionospheric data assimilation is a powerful approach to reconstruct the 3-D distribution of the ionospheric electron density from various types of observations. We present a data assimilation model for the ionosphere, based on the Gauss–Markov Kalman filter with the International Reference Ionosphere (IRI) as the background model, to assimilate two different types of slant total electron content (TEC) observations from ground-based GPS and space-based FORMOSAT-3/COSMIC (F3/C) radio occultation. Covariance models for the background model error and observational error play important roles in data assimilation. The objective of this study is to investigate impacts of stationary (location-independent) and non-stationary (location-dependent) classes of the background model error covariance on the quality of assimilation analyses. Location-dependent correlations are modeled using empirical orthogonal functions computed from an ensemble of the IRI outputs, while location-independent correlations are modeled using a Gaussian function. Observing system simulation experiments suggest that assimilation of slant TEC data facilitated by the location-dependent background model error covariance yields considerably higher quality assimilation analyses. Results from assimilation of real ground-based GPS and F3/C radio occultation observations over the continental United States are presented as TEC and electron density profiles. Validation with the Millstone Hill incoherent scatter radar data and comparison with the Abel inversion results are also presented. Our new ionospheric data assimilation model that employs the location-dependent background model error covariance outperforms the earlier assimilation model with the location-independent background model error covariance, and can reconstruct the 3-D ionospheric electron density distribution satisfactorily from both ground- and space-based GPS observations.

Influence of GPS/GLONASS differential code biases on the determination accuracy of the absolute total electron content in the ionosphere

Abstract: Systematic error arises when the total electron content (TEC) is estimated with the simultaneous use of phase and code GPS/GLONASS measurements. This is related to the different signal propagation times at L1 and L2 frequencies in the radio frequency path of the transmitting and receiving equipment, the so-called differential code biases. A differential code bias of 1 ns results in an error of ~2.9 TECU when TEC is determined. Differential code bias variations on a long time interval, which were obtained at the CODE laboratory, were analyzed. It has been found that the systematic variation in these biases and considerable seasonal variations apparently caused by the environmental state (temperature and humidity), which sometimes reach 20 TECU (in TEC units), are observed for several stations. The algorithm for determining differential code biases at an individual station and the results of correction for absolute slant TEC are also presented. Presented results show algorithm effectiveness for various geographical regions and solar activity.

Ionospheric storms—A challenge for empirical forecast of the total electron content

Abstract: Since the last decades, the functioning of society depends more and more on well-functioning communication and navigation systems. As the availability and reliability of most of these satellite-based systems can be severely impacted by ionospheric storms, the accurate forecast of these events becomes a required task for mitigating social and economic risks. Here we aim to make initial steps toward an empirical model for ionospheric perturbations related to space weather events that are observable in the total electron content (TEC). The perturbation TEC forecast model will be a fast and robust approach, improving TEC forecasts based on climatological models during storm conditions. The derivation of such a model is a challenging task, because although a general dependence of the storm features (enhancement or depletion of electron density) on the storm onset time, local time, season and geomagnetic latitude is well known, there is a large deviation from the mean behavior. For a better understanding of storm conditions, this paper presents analyses of ionospheric storms observed in the TEC, broken down into diverse classes of storms. It provides a detailed characterization of the typical ionospheric storm behavior over Europe from high to midlatitudes, beyond case studies. Generally, the typical clear strong TEC enhancement starting in high latitudes and propagating equatorward is found to be strongest for storms starting in the morning hours independent of the season. In midlatitudes, it is strongest during noon. In addition, a clear difference between summer and winter storms is reported. While only winter storms develop high-latitude TEC enhancements, only summer storms typically exhibit TEC depletions during the storm recovery phase. During winter storms TEC enhancements can also occur the day following the storm onset, in contrast to summer storms. Strong correlation of TEC perturbation amplitudes to the Bz component of the interplanetary magnetic field and to a proxy of the polar cap potential are shown especially for summer midlatitude TEC enhancements during storms with and onset in the morning hours (6 to 12 UT over Europe) and for winter high-latitude TEC enhancements (around 60∘N). The results indicate the potential to derive improved predictions of maximum TEC deviations during space weather events, based on solar wind measurements.

Estimating the total electron content absolute value from the GPS/GLONASS data

Abstract: When determining the absolute oblique total electron content (TEC) of the ionosphere using both GLONASS/GPS code and phase measurements, there occurs a systematic error associated with the differential code biases (DCBs). A 1-ns DCB leads to the ∼2.9 TECU error when determining L1-L2 dual-frequency oblique TEC. We have developed an algorithm for DCB estimation from the data of a single GPS/GLONASS station. Presented are the results of the algorithm operation compared with the oblique TEC correction by using CODE laboratory DCB data.

A pilot study comparing the use of Thiel- and formalin-embalmed cadavers in the teaching of human anatomy

Abstract: Formalin had traditionally been used to preserve human material to teach gross anatomy. In 2008 the Centre for Anatomy and Human Identification (CAHID) at the University of Dundee embarked on the use of the Thiel method of embalming. The aim of this pilot study was to assess the difference between formalin-embalmed cadavers (FEC) and Thiel-embalmed cadavers (TEC) used for teaching and surgical training. Three different questionnaires were prepared for data collection from undergraduate and postgraduate students and clinical staff. All undergraduate and postgraduate students as well as clinical staff commented on the appearance of the TEC. There was no overall consensus concerning the use of TEC, some respondents preferred TEC for the entire dissection, some only for certain areas such as the musculoskeletal system. On a technical level TEC were considered less hazardous then FEC by one-third of participants with fewer than 10% regarding TEC as more irritating than FEC. Psychologically, 32.7% of undergraduate students expressed the view that TEC made them feel more uncomfortable compared with FEC because of their life-like appearance. However, 57.1% of undergraduate students encountered the same uncomfortable feelings when viewing both TEC and FEC. The use of Thiel-embalmed cadavers to teach anatomy has an added value, though further research is required over longer periods of time to identify its best usage.

Day-to-night transport in the Martian ionosphere: Implications from total electron content measurements

Abstract: The nightside Martian ionosphere is thought to be contributed by day-to-night transport and electron precipitation, of which the former has not been well studied. In this work, we evaluate the role of day-to-night transport based on the total electron content (TEC) measurements made by the Mars Advanced Radar for Subsurface and Ionospheric Sounding on board Mars Express. This is accomplished by an examination of the variation of nightside TEC in the time domain rather than the traditional solar zenith angle domain. Our analyses here, being constrained to the Northern Hemisphere where the effects of crustal magnetic fields can be neglected, reveal that day-to-night transport serves as the dominant source for the nightside Martian ionosphere from terminator crossing up to time in darkness of ≈5.3 × 103 s, beyond which it is surpassed by electron precipitation. The observations are compared with predictions from a simplified time-dependent ionosphere model. We conclude that the solid body rotation of Mars is insufficient to account for the observed depletion of nightside TEC but the data could be reasonably reproduced by a zonal electron flow velocity of ≈1.9 km s−1.

Two-mode ionospheric response and Rayleigh wave group velocity distribution reckoned from GPS measurement following Mw 7.8 Nepal earthquake on 25 April 2015

Abstract: The coseismic-induced ionospheric total electron content (TEC) perturbations were analyzed following the Mw 7.8 Nepal earthquake (28.147°N, 84.708°E; depth ~15 km) that occurred on 25 April 2015 at 06:11:26 UTC. The ionospheric response is due to both the modes, i.e., shock acoustic waves (slow mode) and Rayleigh wave induced (fast mode). The continuous Global Positioning System (GPS) data at about 60 sites from various GPS networks have been used in the present study. All the sites within epicentral distance of ~2400 km and 70°–170° azimuth recorded the Rayleigh wave-induced TEC response, while the sites within ~400–2200 km in the same azimuth recorded the response from both the modes. The maximum coseismic-induced peak-to-peak TEC amplitude is ~1.2 total electron content unit, 1 TECU = 1016 el m−2. From Hodochron plot, the apparent Rayleigh wave velocity has been determined as ~2400 m/s on the average and the acoustic wave velocity as 1180 m/s, both these waves being discernible beyond ~1200 km of epicentral distance as also evident from Hodochron plot and wavelet spectrographs. We reckoned the Rayleigh wave group velocities using ionospheric response at selected radial pairs of stations and validated. The ionospheric response distribution seen mainly depending on the epicentral distance, satellite geometry, directivity of radiation pattern, and the upper crustal heterogeneity. This study highlights the characteristics of ionospheric response consequent to the 2015 Nepal earthquake.

Variability of GPS/GLONASS differential code biases

Abstract: While estimating ionospheric total electron content (TEC) using both pseudorange and phase GPS/GLONASS data, there occurs a systematic error caused by the difference in processing times of L1 and L2 signals through radio frequency paths of satellites and receivers, known as differential code biases (DCBs). A 1-ns DCB causes an ∼2.9 TECU error in TEC estimation. Along with systematic DCB variations, seasonal variations, most likely related to variations in the receiver environment (temperature, humidity), also exist for some receivers and can reach in some cases up to ∼20 TECU.

EOF analysis and modeling of GPS TEC climatology over North America

Abstract: An empirical ionospheric model of the total electron content (TEC) over North America (20°–60°N, 40°–140°W) is constructed using the GPS TEC data collected by Massachusetts Institute of Technology (MIT) Haystack Observatory during the years 2001–2012. This model is based on an analysis of quiet time monthly averages using the empirical orthogonal function (EOF) decomposition technique, allowing for separation of spatial and temporal variations. The importance of different types of spatial-temporal variations to the overall TEC variability can be well represented by the characteristics of EOF basis functions and associated principal components coefficients, with various modes. The mode one EOF decomposition constitutes 97.5% of the total variance and therefore represents the essential feature of North America spatial and diurnal variation of the TEC. The mode two EOF, as reported in an earlier study, reveals a large and significant symmetric longitudinal variation of the ionosphere, organized with respect to magnetic declination. The mode three EOF decomposition shows midlatitude latitudinal structure that varies with season in a manner very similar to the so-called winter anomaly. Because of the quick convergence of EOF decomposition modes, the first four EOF modes are utilized for constructing a TEC empirical model. For each of the EOF modes, the temporal variations are expressed analytically in terms of local time, season, and solar activity, and the spatial variations by cubic-spline functions. An analysis of accuracy and quality indicates that this regional empirical TEC model can reflect the majority of the quiet time monthly means, and represent characteristic temporal-spatial variations in the North America.

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

Abstract: 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

On the onset of ionospheric precursors 40 min before strong earthquakes

Abstract: Heki (2011) and Heki and Enomoto (2013) claimed that anomalous, yet similar, increases of ionospheric total electron content (TEC) started ~40 min prior to the 2011 Tohoku-Oki, as well as before other Mw > 8 earthquakes. The authors concluded that the reported TEC anomalies were likely related to the pending earthquakes, suggesting also that TEC monitoring may be useful for future earthquake prediction. Here we carefully examine the findings of Heki (2011) and Heki and Enomoto (2013) by performing new analyses of the same TEC data. Our interpretation is that the 40 min onset of the ionospheric precursors is an artifact induced by the definition of the reference line adopted in analyzing TEC variations. We also discuss this repeatability in the tectonic and geodynamic context of the earthquakes. By performing a Superimposed Epoch Analysis of TEC data, we show that, however, the TEC increase reported by Heki (2011) was not particularly anomalous. We conclude that the TEC precursors reported by Heki (2011) and Heki and Enomoto (2013) are not useful for developing short-term earthquake prediction capabilities.