Background Total Electron Content Research Articles

IonosphericTotal Electron Content Changes during the 15 February 2018 and 30 April 2022 Solar Eclipses over South America and Antarctica

This is one of the first papers to study the ionospheric effects of two solar eclipses that occurred in South America and Antarctica under geomagnetic activity in different seasons (summer and autumn) and their impact on the equatorial ionization anomaly (EIA). The changes in total electron content (TEC) during the 15 February 2018 and 30 April 2022 partial solar eclipses will be analyzed. The study is based on more than 390 GPS stations, Swarm-A, and DMSP F18 satellite measurements, such as TEC, electron density, and electron temperature. The ionospheric behaviors over the two-fifth days on both sides of each eclipse were used as a reference for estimating TEC changes. Regional TEC maps were created for the analysis. Background TEC levels were significantly higher during the 2022 eclipse than during the 2018 eclipse because ionospheric levels depend on solar index parameters. On the days of the 2018 and 2022 eclipses, the ionospheric enhancement was noticeable due to levels of geomagnetic activity. Although geomagnetic forcing impacted the ionosphere, both eclipses had evident depletions under the penumbra, wherein differential vertical TEC (DVTEC) reached values <−40%. The duration of the ionospheric effects persisted after 24 UT. Also, while a noticeable TEC depletion (DVTEC ∼−50%) of the southern EIA crest was observed during the 2018 eclipse (hemisphere summer), an evident TEC enhancement (DVTEC > 30%) at the same crest was seen during the eclipse of 2022 (hemisphere autumn). Swarm-A and DMSP F18 satellite measurements and analysis of other solar eclipses in the sector under quiet conditions supported the ionospheric behavior.

Nighttime Midlatitude E‐F Coupling in Geomagnetic Conjugate Ionospheres: A Double Thin Shell Model and a Multi‐Source Data Investigation

AbstractGeomagnetic conjugate mid‐latitude nighttime ionospheres are frequently simultaneously populated with electrified nighttime medium‐scale traveling ionospheric disturbances (MSTIDs). Earlier observations and theoretical analysis have underscored the ionosphere E‐F coupling and the postulation of coupled conjugate hemispheres, playing a pivotal role in the formation of electrified MSTIDs. In this paper, the conjugate MSTIDs are studied to elucidate the causes and effects of E‐F coupling in the interhemispheric coupled ionosphere. The hemisphere‐coupled ionospheres over Japan and Australia are observed and analyzed using total electron content (TEC) measurements, supplemented with multi‐source observations from ionosondes, Ionospheric Connection Explorer (neutral wind), Constellation Observing System for Meteorology, Ionosphere, and Climate (electron density), and Swarm (magnetic field). A double‐thin‐shell model is introduced to analyze the ionospheric responses in E and F regions during the coupling process. For the first time, observation results provide the evidence that F‐region geomagnetic conjugate irregularities in both hemispheres are mainly driven by the Es layers in the summer hemisphere. The Es layer in the summer hemisphere subsequently triggers local E‐F coupling and inter‐hemispheric coupling. In the winter hemisphere, Es layers show amplitude reduction or even dissipation during the interhemispheric coupling process. Furthermore, thermospheric winds, non‐equipotential magnetic field lines, and background TEC are presumed candidates for the inter‐hemispheric asymmetry in MSTIDs amplitudes and growth rates.

Storm Time Electrified MSTIDs Observed Over Mid‐Latitude North America

AbstractMedium‐scale Traveling Ionospheric Disturbances (MSTIDs) are prominent and ubiquitous features of the mid‐latitude ionosphere, and are observed in Super Dual Auroral Radar Network (SuperDARN) and high‐resolution Global Navigational Satellite Service (GNSS) Total Electron Content (TEC) data. The mechanisms driving these MSTIDs are an open area of research, especially during geomagnetic storms. Previous studies have demonstrated that nightside MSTIDs are associated with an electrodynamic instability mechanism like Perkins, especially during geomagnetically quiet conditions. However, dayside MSTIDs are often associated with atmospheric gravity waves. Very few studies have analyzed the mechanisms driving MSTIDs during strong geomagnetic storms at mid‐latitudes. In this study, we present mid‐latitude MSTIDs observed in de‐trended GNSS TEC data and SuperDARN radars over the North American sector, during a geomagnetic storm (peak Kp reaching 9) on 7–8 September 2017. In SuperDARN, MSTIDs were observed in ionospheric backscatter with Line of Sight (LOS) velocities exceeding 800 m/s. Additionally, radar LOS velocities oscillated with amplitudes reaching ±500 m/s as the MSTIDs passed through the fields‐of‐view. In detrended TEC, these MSTIDs produced perturbations reaching ∼50 percent of background TEC magnitude. The MSTIDs were observed to propagate in the westward/south‐westward direction with a time period of ∼15 min. Projecting de‐trended GNSS TEC data along SuperDARN beams showed that enhancements in TEC were correlated with enhancements in SuperDARN SNR and positive LOS velocities. Finally, SuperDARN LOS velocities systematically switched polarities between the crests and the troughs of the MSTIDs, indicating the presence of polarization electric fields and an electrodynamic instability process during these MSTIDs.

Ionospheric Disturbances Triggered by China's Long March 2D Rocket

Using the data of 382 ground global navigation satellite system (GNSS) network stations in Western China, we studied and analyzed the ionospheric disturbances triggered by the “Long March” 2D rocket launch in Jiuquan, China on December 3, 2017. As compared with previous research, a higher sampling resolution for GNSS data (with a frequency of 1 Hz) was used to obtain more accurate occurrence times and propagation velocities of ionospheric disturbances. By using a method based on a quadratic function of time to fit a raw total electron content (TEC) series, a filtered TEC series was calculated using carrier observations, and a two-dimensional disturbances map was drawn. A new method, which accounts for the flight time of the rocket, was used to calculate the velocity of the shock wave. Ionospheric depletions and the shock wave were observed after the launch of the rocket. The depletion was observed within 100 to 1000 km south of the launch site along the rocket trajectory, which had a maximum amplitude of ∼3.8 TEC units (TECU), reaching ∼56% of the background TEC. A shock wave of V-shaped disturbances with amplitudes of ∼0.67 TECU was detected on both sides of the rocket trajectory. The shock wave moved southeast at an average velocity of ∼1861 m/s at a location 2200 km away from the launch site. Ionospheric disturbances with distances of more than ∼3000 km from the launch site were also observed.

Understanding the Total Electron Content Variability Over Europe During 2009 and 2019 SSWs

AbstractThe nature of the variability of the Total Electron Content (TEC) over Europe is investigated during 2009 and 2019 Northern Hemisphere (NH) SSW events in this study by using a combination of Global Navigation Satellite System (GNSS) based TEC observations and Thermosphere‐Ionosphere Electrodynamics General Circulation Model (TIE‐GCM) simulations. To simulate the SSW effects in TIE‐GCM, the dynamical fields from the Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension (WACCM‐X) simulations of 2009 and 2019 SSWs are specified at the TIE‐GCM lower boundary. The observed and simulated TEC are in overall good agreement and therefore the simulations are used to understand the sources of mid‐latitude TEC variability during both SSWs. Through comparison of TIE‐GCM simulations with and without geomagnetic forcing, we find that the TEC variability during the 2019 SSW event, was predominantly geomagnetically forced, while for the 2009 SSW, the major variability in TEC was accounted for by the changes in vertically propagating migrating semidiurnal solar (SW2) and lunar (M2) tides. By comparing the TIE‐GCM simulations with and without the SW2 and M2 tides, we find that these semidiurnal tides contribute to 20%–25% increase in the quiet background TEC.

On the Properties of and Ionospheric Conditions Associated With a Mid‐Latitude Scintillation Event Observed Over Southern United States

AbstractWhile low and high‐latitude ionospheric scintillation have been extensively reported, significantly less information is available about the properties of and conditions leading to mid‐latitude scintillations. Here, we report and discuss scintillation observations made in the Southern United States (UT Dallas, 32.99°N, 96.76°W, 43.2°N dip latitude) on June 1st, 2013. The measurements were made by a specialized dual‐frequency GPS‐based scintillation monitor which allowed us to determine main properties of this mid‐latitude scintillation event. Additionally, simultaneous airglow observations and ionospheric total electron content (TEC) maps provided insight on the conditions leading to observed scintillations. Moderate amplitude scintillations (S4>∼0.4) occurred in both L1 and L2C signals, and severe (S4 > ∼0.8) events occurred in L2C signals at low (<30°) elevation angles. Phase scintillation accompanied amplitude fadings, with maximum σϕ values exceeding 0.5 radians in L2C. We also show that the observed phase scintillation magnitudes increased with amplitude scintillation severity. Decorrelation times were mostly between 0.25 and 1.25 s, with mean value around 0.65 s for both L1 and L2C. Frequency scaling of S4 matched fairly well the predictions of weak scattering theory but held for observations of moderate and strong amplitude scintillation as well. Scintillation occurred during the main phase of a modest magnetic storm that, nevertheless, prompted an extreme equatorward movement of the mid‐latitude trough and large background TEC enhancements over the US. Scintillations, however, occurred within TEC and airglow depletions observed over Texas. Finally, scintillation properties including severity and rapidity, and associated TEC signatures are comparable to those associated with equatorial spread F.

The TEC Enhancement Before Seismic Events Is an Artifact

AbstractSince 2011 there has been an ongoing debate about the possibility of short‐term earthquake prediction using total electron content (TEC) ionospheric monitoring by the Global Navigation Satellite System (GNSS). Heki (2011), https://doi.org/10.1029/2011gl047908 initiated this debate when he published results for the 2011 Tohoku event reporting a TEC enhancement 40 min before the earthquake; several later papers by Heki and coworkers have made similar claims for other earthquakes. If correct, Heki's methods might contribute to short‐term earthquake prediction. However, Heki's claims have been strongly criticized as being due to a decrease in the background TEC after earthquakes–the so called ionospheric hole–rather than an enhancement before. Depending on the choice of reference curve to be subtracted from the raw data to infer the “anomaly,” the data analysis can produce either a hole or an enhancement. We show that the choice of reference curve ‐calculated by Heki with a polynomial fit‐is strongly affected by the degree of the polynomial, as well as by the selection of the time window. We also show using synthetic examples that even if there is actually no signal before the event, Heki's methods can lead to spurious precursory signals (i.e., signals with non‐zero amplitude before the event) after the reference curve is subtracted. It thus appears likely that the reported TEC enhancements are artifacts.

Plasma depletions lasting into daytime during the recovery phase of a geomagnetic storm in May 2017: Analysis and simulation of GPS total electron content observations

This paper reports that plasma density depletions appearing at middle latitudes near sunrise survived until afternoon on 29 May 2017 during the recovery phase of a geomagnetic storm. By analyzing GPS data collected in Japan, we investigate temporal variations in the horizontal two-dimensional distribution of total electron content (TEC) during the geomagnetic storm. The SYM-H index reached −142 nT around 08 UT on 28 May 2017. TEC depletions extending up to approximately 38°N along the meridional direction appeared over Japan around 05 LT (LT = UT + 9 hours) on 29 May 2017, when TEC rapidly increased at sunrise due to the solar extreme ultraviolet (EUV) radiation. The TEC depletions appeared sequentially over Japan for approximately 8 hours in sunlit conditions. At 06 LT on 29 May, when the plasma depletions first appeared over Japan, the background TEC was enhanced to approximately 17 TECU, and then decreased to approximately 80% of the TEC typical of magnetically quiet conditions. We conclude that this temporal variation of background plasma density in the ionosphere was responsible for the persistence of these plasma depletions for so long in daytime. By using the Naval Research Laboratory: Sami2 is Another Model of the Ionosphere (SAMI2), we have evaluated how plasma production and ambipolar diffusion along the magnetic field may affect the rate of plasma depletion disappearance. Simulation shows that the plasma density increases at the time of plasma depletion appearance; subsequent decreases in the plasma density appear to be responsible for the long-lasting persistence of plasma depletions during daytime. The plasma density depletion in the top side ionosphere is not filled by the plasma generated by the solar EUV productions because plasma production occurs mainly at the bottom side of the ionosphere.

Ionospheric Diurnal Double‐Maxima Patterns Observed by the TEC From Beidou Geostationary Satellites in the Asian‐Australian Sector During 2016–2018

AbstractStatistical analysis was conducted to investigate the ionospheric diurnal double‐maxima (DDM) patterns using the total electron content (TEC) from the Beidou geostationary (GEO) satellites. The data were used over the Asian‐Australian sector during 2016–2018. It was found that the ionospheric DDM had the obvious seasonal and latitudinal variations as well as hemispheric differences. The occurrence rate of ionospheric DDM presented a semi‐annual variation with a main peak around the June solstice (local summer) and a secondary peak around the December solstice in the northern hemisphere and equator region, whereas an annual variation with the highest occurrence rate around the June solstice (local winter) was observed in the southern hemisphere. Furthermore, the occurrence rate of ionospheric DDM around the equatorial ionization anomaly crest region was lower than that at other latitudes. There was an anti‐correlation between the occurrence rate of ionospheric DDM and the background TEC, indicating that the drivers, which determine the variations of the background TEC, possibly play an important role in producing the latitudinal and seasonal variations of ionospheric DDM as well. Nevertheless, the causative mechanisms could be different for daytime and post‐sunset DDMs.

Ionospheric Interhemispheric Asymmetry Induced by Planetary Wave Under Solar Minimum Condition

AbstractThe westward quasi‐6‐day planetary wave with zonal wave number 1 (Q6DW) is important in the Earth's atmospheric coupling. The International Global Navigation Satellite System Service total electron content (TEC) maps show that the Q6DW‐related ionospheric perturbations exhibit clear interhemispheric asymmetry, which do not always agree with the interhemispheric asymmetry feature of the background TEC. The National Center for Atmospheric Research Thermosphere‐Ionosphere‐Mesosphere Electrodynamic General Circulation Model is then used to investigate the reasons for planetary wave‐induced interhemispheric asymmetry by imposing Q6DW forcing at the model lower boundary at vernal equinox under solar minimum condition. The interhemispheric asymmetry features of the ionospheric responses to Q6DW are well captured by our model simulations. For example, the southern branch of the Q6DW perturbation in F region is ~26% larger than that in the Northern Hemisphere at 30°E, while the background Model TEC is~4.7% stronger in the Northern Hemisphere. We found that the vertical plasma transportation induced by Q6DW perturbations in the neutral wind fields in F region show the same asymmetric features with TEC, which leads to corresponding ionospheric perturbations by changing the local recombination effect. Our further simulations show that the vertical plasma drifts induced by Q6DW through E × B process can also cause interhemispheric asymmetry in Model TEC. It is also found that the zonal component of Q6DW perturbations contributes more to the asymmetry than the meridional component through both neutral wind direct transportation effects and E region wind dynamo effects. Moreover, the ionospheric asymmetry of the Q6DW‐related oscillations also exhibit clear longitudinal variabilities.

Climatology of the Quasi‐6‐Day Wave in the Mesopause Region and Its Modulations on Total Electron Content During 2003–2017

AbstractThe westward quasi‐6‐day planetary wave (Q6DW) with zonal wave number 1 is a prominent oscillation in the mesosphere and lower thermosphere region, which causes significant variabilities in the ionosphere through E region wind dynamo. In this paper, the neutral wind observations from the Thermosphere Ionosphere and Mesosphere Electric Dynamics Doppler Imager experiment, the kinetic temperature observations from the Sounding of the Atmosphere using Broadband Emission Radiometry instrument, and the total electron content (TEC) maps from globally distributed Global Positioning System (GPS) receivers during 2003–2017 are utilized to statistically study the Q6DW in both the neutral atmosphere and the ionosphere. Our statistical results show that (1) the Q6DW is most likely to occur during May and September, exhibiting a quasi‐semiannual oscillation. (2) The periods of the Q6DW show two statistical crests at ~6 and ~7 days separately. (3) The ionospheric oscillations due to the Q6DW are positively related to both the 10.7‐cm solar flux index and the neutral wave amplitudes in the mesosphere and lower thermosphere region. The stronger oscillations in TEC under higher solar activity conditions are due to the larger ionization rate and thus larger background TEC. A stronger Q6DW results in stronger wind dynamos in the E region, which leads to stronger F region responses indirectly. (4) The interhemispheric asymmetries of the ionospheric responses to Q6DW show weak negative correlations with both solar activities and neutral wave perturbations, the mechanism of which needs our further investigation.

Complex analysis of the ionospheric response to operation of “Progress” cargo spacecraft according to the data of GNSS receivers in Baikal region

As part of the Plasma–Progress and Radar–Progress space experiments conducted from 2006 to 2014, effects of the Progress spacecraft engines on the ionosphere have been studied using data from Global Navigation Satellite System (GNSS) receivers. 72 exper-iments have been carried out. All these experiments were based on data from the International GNSS Service (IGS) to record ionospheric plasma irregularities caused by engine operation. 35 experiments used data from the ISTP SB RAS network SibNet. The analysis of the spatio-temporal structure of total electron content (TEC) variations has shown that the problem of identifying the TEC response to engine operation is complicated by a number of factors: 1) the engine effect on ionospheric plasma is strongly localized in space and has a relatively low intensity; 2) a small number of satellite—receiver radio rays due to the limited number of GNSS stations, particularly before 2013; 3) a potential TEC response is masked with background ionospheric disturbances of various intensi-ties. However, TEC responses are identified with certainty when a satellite—receiver radio ray crosses a disturbed region within minutes after the impact. TEC responses have been registered in 7 experiments (10 % of cases). The amplitude of ionospheric response (0.3–0.16 TECU) exceeded the background TEC variations (~0.25 TECU) several times. The TEC data indicate that the ionospheric irregularity lifetime is from 4 to 10 minutes. According to the estimates we made, the transverse size of irregularities is from 12 to 30 km.

Комплексный анализ реакции ионосферы на работу двигательных установок ТГК «Прогресс» по данным ГНСС-приемников в Байкальском регионе

As part of the Plasma—Progress and Radar—Progress space experiments performed from 2006 to 2014, effects of Progress spacecraft engines on the ionosphere have been studied using data from Global Navigation Satellite System (GNSS) receivers. 72 experiments have been carried out. All these experiments were based on data from the worldwide IGS network of GNSS receivers to record ionospheric plasma irregularities caused by engine operation. 35 experiments used data from the ISTP SB RAS SibNet network. The analysis of the spatio-temporal pattern of total electron content (TEC) variations has shown that the problem of identifying the TEC response to engine operation is complicated by a number of factors: 1) the engine effect on ionospheric plasma is strongly localized in space and has a relatively low intensity; 2) a small number of satellite—receiver radio rays due to the small number of GNSS stations, particularly before 2013; 3) a potential TEC response is masked with background ionospheric disturbances of various intensity. However, TEC responses are identified with certainty when a satellite—receiver radio ray crosses a disturbed region within minutes after exposure. TEC responses have been registered in 7 experiments (10 % of cases). The amplitude of ionospheric response (0.3–0.16 TECU) exceeded the background TEC variations (~0.25 TECU) several times. The TEC data indicate that the ionospheric irregularity lifetime is from 4 to 10 minutes. According to the estimates we made, the transverse size of irregularities is from 12 to 30 km.

The Modulation of the Quasi‐Two‐Day Wave on Total Electron Content as Revealed by BeiDou GEO and Meteor Radar Observations Over Central China

AbstractThe ground‐based total electron content (TEC) observations from BeiDou Geostationary Orbit (GEO) satellites provide more accurate temporal ionospheric variations nearly at the fixed ionospheric pierce points as compared with the non‐GEO TEC. BeiDou GEO TEC measurements during 2016, along with the horizontal wind observations from a collocated meteor radar, are utilized to quantitatively study the ionospheric response to the quasi‐two‐day wave (QTDW) in the neutral atmosphere at Meng Cheng (116.6°E, 33.3°N) over central China. The QTDW peaks during July with amplitudes of ~25–30 m/s for both zonal and meridional winds. It is suggested that the nonlinear interactions between the QTDW and tides and their influences on TEC are insignificant for this QTDW event. Nevertheless, the diurnal tide decreases by ~40–60% during the QTDW episode, which is most likely related to the change of background wind induced by the QTDW. Correspondingly, the 24 h oscillations in TEC decrease by ~18%. In addition, the wind perturbations of the QTDW can also possibly modulate the midlatitude electric dynamo and result in a quasi‐two‐day oscillation (QTDO) of ~1.2 total electron content unit (TECU), about ~10% of the background TEC. Our analysis also shows that the background TEC decreases by ~1 TECU (~8%) when the QTDW reaches maximum amplitude. This is probably related to the thermospheric composition changes (e.g., O and N2) induced by the QTDW dissipation.

One sliding PCA method to detect ionospheric anomalies before strong Earthquakes: Cases study of Qinghai, Honshu, Hotan and Nepal earthquakes

A sliding principal component analysis (PCA) method is proposed to detect pre-earthquake ionospheric anomalies. We analyzed the precision of this new method with different length of time window in detecting the reference background total electron content (TEC). The results showed that the most suitable window length is 27days which is consistent with the solar rotation period. We compared the precision of this new method with the sliding inter quartile range (IQR) method and the sliding average method in detecting the background TECs, and found that the precision of sliding PCA is better than those of the traditional methods in the middle and low latitudes because the detected background TEC residual errors by the sliding PCA method are less than that by the traditional methods. We adopted a more reasonable method to calculate the background value’s upper and lower bounds and then take four strong earthquakes (Qinghai earthquake, Honshu earthquake, Hotan earthquake and Nepal earthquake) as examples to prove the effectiveness of the sliding PCA method. From the detection results of the sliding PCA we found that obvious ionospheric anomalies appeared on April 1, 2010, March 8, 2011, February 2, 2014, April 11, 23, year 2015. After the further analysis of the solar-terrestrial environment and the distribution of TEC anomaly area, it can be considered that the pre-earthquake anomalies on these days before the earthquake may have a large correlation with the following earthquake. In addition, the TEC anomaly area spans largely in the longitude direction along the bound of equator anomalous zone, and the anomalous morphology has the conjugate structure. This regular can provide a valuable reference for the short-impending earthquake prediction in the future.

Large ionospheric TEC depletion induced by the 2016 North Korea rocket

A rocket called Kwangmyongsong-4 was launched from North Korea at 00:30UT on February 7, 2016. We investigated ionospheric total electron content (TEC) depletions induced by the rocket using the Global Navigation Satellite System (GNSS) stations in South Korea. A sudden depletion in TEC variations appeared ∼6min after the rocket launch. The drops in slant TEC exceeded 17 TEC unit (TECU) and those in vertical TEC were approximately 7 TECU. It is remarkable that the TEC drop by the 2016 Kwangmyongsong-4 rocket is larger (almost by three times) than that by the 2012 Unha-3 rocket. There are the differences of the background TEC values at the 2012andthe 2016 cases. These results suggest that the difference of the background electron density affects the magnitude of TEC depletion. The horizonal velocity of the rocket was 1.6km/s, which was estimated from horizontal distances with an initial time of TEC disturbances. However, the 2012 Unha-3 rocket (∼2.5km/s) moved faster horizontally than the 2016 Kwangmyongsong-4 rocket. Furthermore, when the rocket moved from high latitudes to low latitudes, TEC disturbances reduced gradually, and then, the depletion persisted for a longer time at the west side (the right side of southern direction).

Mw dependence of the preseismic ionospheric electron enhancements

AbstractIonospheric 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.

Observation of the neutral‐ion coupling through 6 day planetary wave

AbstractA westward 6 day oscillation with zonal wave number 1 is identified in the global maps of the ionospheric total electron content (TEC) during May 2003. This signature coincides with the wave activities in the mesosphere and lower thermosphere (MLT) region with a similar zonal structure and period, deduced from the temperature and wind observations from the Thermosphere Ionosphere and Mesosphere Electric Dynamics (TIMED) satellite. The vertical wavelength of this 6 day wave is estimated to be ~65 km, which enables it to propagate up into the lower thermosphere and therefore modulate the ionosphere through E region wind dynamo. The zonal wind perturbations of the 6 day wave maximize in the equatorial region with an amplitude of ~30 m/s and the meridional wind perturbations peak at middle latitudes with an amplitude of 15–20 m/s. The 6 day oscillation in TEC peaks at the geomagnetic equatorial ionization anomaly (EIA) crests in both hemispheres with a deep minimum at the equator. The absolute TEC perturbations maximize at ~1400–1800 LT with an amplitude of ~9 (~7) total electron content unit (TECU; 1 TECU = 1016 m−2) in the southern (northern) hemisphere, which account for ~16% (~10%) of the background TEC. Larger relative TEC perturbations of ~20% are found at 0200–0400 LT. The similar wave number‐period spectra and the consistent temporal variations of the 6 day periodical signatures in both neutral atmosphere and ionosphere suggest a strong neutral‐ion coupling through planetary wave.

Ionospheric precursor for a deep earthquake (∼378 km) near Papua New Guinea occurred on 7 July 2013, Mw = 7.2 in the environment of geomagnetic storm using two-dimensional principal component analysis

Two-dimensional ionospheric total electron content (TEC) data during the time period from 00:00 on 2 July to 12:00 UT on 8 July 2013, which was 5 days before to 1 day after a deep earthquake at 18:35:30 on 7 July 2013 UT (Mw = 7.2) with a depth at about 378.8 km in Papua New Guinea, were examined by two-dimensional principal component analysis (2DPCA) to detect TEC precursor related to the earthquake because TEC precursors usually have shown up in earlier time periods. A TEC precursor was highly localized around the epicenter from 06:00 to 06:05 on 6 July, where its duration time was at least 5 minutes. Ionizing radiation radon gas release should be a possibility to cause the anomalous TEC fluctuation, e.g., electron density variation. The plasma might have large damping at that time to cause TEC fluctuation of short time, and the gas released with small amount in short time period, and 2DPCA could identify short time TEC fluctuation while the fluctuation lasted for a long time. Other background TEC anomalies caused by the geomagnetic storm, small earthquakes and non-earthquake activities, e.g., equatorial ionization anomaly resulted in the small principal eigenvalues, therefore the detection of TEC precursor was regardless of these background TEC anomalies.

Global TEC maps based on GNSS data: 1. Empirical background TEC model

AbstractA global background total electron content (TEC) model is built by using the Center for Orbit Determination of Europe (CODE) TEC data for full 13 years, 1999–2011. It describes the climatological behavior of the ionosphere under both its primary external driver, i.e., the direct photo‐ionization by incident solar radiation, and regular wave particularly tidal forcing from the lower atmosphere. The model construction is based on the very different time scales of the solar cycle, seasonal, and diurnal TEC variabilities (at least an order of magnitude); this leads to modulations of shorter‐period variabilities with periods of the longer ones. Then the TEC spatial‐temporal variability is presented as a multiplication of three separable functions. The solar activity is described by both parameters: F10.7 and its linear rate of change KF while the seasonal variability is presented by sine functions including four subharmonics of the year. The diurnal variability of the TEC model is described by 2D (longitude‐time) sine functions with zonal wave numbers up to 4 and 4 subharmonics of the solar day. The model offers TEC maps which depend on geographic coordinates (5°×5° in latitude and longitude) and UT at given solar activity and day of the year. The presented background model fits to the CODE TEC input data with a zero systematic error and an RMS error of 3.387 TECU. It is able to reproduce the well‐known ionospheric structures as Weddell Sea Anomaly and some longitudinal wave‐like structures.