Total Electron Content Anomalies Research Articles

Effects of Electric Field and Neutral Wind on the Asymmetry of Equatorial Ionization Anomaly

AbstractThe zonal electric field and the meridional neutral wind are the principal drivers that define the geometry and characteristics of the equatorial ionization anomaly (EIA). Here we present the response of the EIA to the variability of the zonal electric field based on measurements of the equatorial electrojet (EEJ) currents and trans‐equatorial neutral winds for the generation and control of the asymmetries of the EIA crests of total electron content (TEC) in the western side of the South American continent. The EEJ strengths are determined using a pair of magnetometers. The 24‐hr trans‐equatorial neutral wind profile is measured using the Second‐Generation, Optimized, Fabry‐Perot Doppler Imager (SOFDI) located near the geomagnetic equator. The EIA is evaluated using TEC data measured by Global Positioning System (GPS) receivers from the Low‐Latitude Ionospheric Sensor Network and several other networks in South America. A physics‐based numerical model, Low‐Latitude Ionospheric Sector, and SOFDI data are used to study the effects of daytime meridional neutral winds on the consequent evolution of an asymmetry in equatorial TEC anomalies during the afternoon and onward for the first time. We find that the configuration parameters such as strength, shape, amplitude, and latitudinal width of the EIAs are affected by the eastward electric field associated with the EEJ under undisturbed conditions. The asymmetries of EIA crests are observed more frequently during solstices and the September equinox than in the March equinox season. Importantly, this study indicates that the meridional neutral wind plays a very significant role in the development of the EIA asymmetry by transporting the plasma up the field lines. This result suggests that a precise observation of the latitudinal TEC profile at low latitudes can be used to derive the meridional wind.

Statistical Analysis of TEC Anomalies Prior to M6.0+ Earthquakes During 2003–2014

There are many studies on the anomalous variations of the ionospheric TEC prior to large earthquakes. However, whether or not the morphological characteristics of the TEC anomalies in the daytime and at night are different is rarely studied. In the present paper, based on the total electron content (TEC) data from the global ionosphere map (GIM), we carry out a statistical survey on the spatial–temporal distribution of TEC anomalies before 1339 global M6.0+ earthquakes during 2003–2014. After excluding the interference of geomagnetic disturbance, the temporal and spatial distributions of ionospheric TEC anomalies prior to the earthquakes in the daytime and at night are investigated and compared. Except that the nighttime occurrence rates of the pre-earthquake ionospheric anomalies (PEIAs) are higher than those in the daytime, our analysis has not found any statistically significant difference in the spatial–temporal distribution of PEIAs in the daytime and at night. Moreover, the occurrence rates of pre-earthquake ionospheric TEC both positive anomalies and negative anomalies at night tend to increase slightly with the earthquake magnitude. Thus, we suggest that monitoring the ionospheric TEC changes at night might be a clue to reveal the relation between ionospheric disturbances and seismic activities.

Pre-seismic ionospheric anomalies of the 2013 Mw = 7.7 Pakistan earthquake from GPS and COSMIC observations

The seismo-ionospheric anomalies may provide some insights about the earthquake. However, pre-seismic ionospheric anomalies are still challenging. In this paper, seismo-ionospheric anomalies are investigated before the September 24, 2013 (Mw = 7.7) Awaran (Pakistan) earthquake from GPS TEC (Total Electron Content) and COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate) data. The TEC data are showing anomalies on September 21, 2013. The abnormality detected in the temporal data is about 10 TECU beyond the 30-day running median. The percentage deviation of the TEC anomaly on September 21, is 30% above the upper confidence interval. The anomalies prevail 5° in Latitude and 10° in Longitude over the epicenter. The spatial and temporal data of TEC showed anomalies in TEC from UT = 08 to UT = 12. In addition, the enhancement on September 21, 2013 is also very significant in COSMIC data. The results of COSMIC completely agreed with GPS TEC anomalies on September 21, 2013. The percentage deviation of the peak plasma frequency on September 21 is 5% of the normal distribution. The storm indices are quiet before and after the earthquake. The pre-sesimic ionospheric anomalies are most probably associated with the 2013 Mw = 7.7 Awaran (Pakistan) earthquake.

Ionospheric response to Huge Gas Explosion in Kaohsiung City, Taiwan, on July 31, 2014

In this paper the two-dimensional Principal Component Analysis (2DPCA) was used to determine the total electron content (TEC) anomaly using the principal eigenvalue of 2DPCA as a mathematical indicator in the ionos-pheric response to the huge gas explosion in Kaohsiung city (22°36′39″N 120°19′8″E), Taiwan at 15: 59 (UT) on July 31, 2014. 2DPCA has been shown to be sensitive to detect weak TEC anomalies. 2DPCA gives a careful data handling description. Results have shown that a TEC anomaly over Taiwan was observed from 16:00 to 16:05 (UT). The acoustic shock waves have caused a TEC anomaly detectable without considering the atmosphere situation and traveling of acoustic shock waves during this explosion. The duration time of the TEC anomaly was at least 5 minutes.

Statistical seismo-ionospheric precursors of M7.0+ earthquakes in Circum-Pacific seismic belt by GPS TEC measurements

The Circum-Pacific seismic belt is the region heavily affected by earthquakes in the world. The relationship between earthquake (e.g., the geographic location, occurrence time, magnitude, and focal depth) and ionospheric anomalies in the belt was investigated using 100 M7.0+ earthquakes during 2006–2015. The ground-based GPS measurements and global ionosphere map (GIM) data were used for the analyses of the ionospheric variations preceding the earthquakes. The results indicated that the occurrence rate of total electron content (TEC) anomalies was proportional to the magnitude and inversely proportional to the focal depth to a certain degree, and the occurrence frequency of anomalies had a rising trend with the days getting close to the main shock. The occurrence rate of TEC anomalies in the Southern hemisphere was larger than that in the Northern hemisphere. Besides, the spatial characteristics of TEC anomalies showed that the anomalies in low-middle latitudes did not coincide with the epicenter, sometimes the anomalies were also observed in the corresponding conjugated region. However, the TEC anomalies in the high latitude usually appeared around the epicenter and within the seismogenic zone while no TEC anomalies appeared in the conjugated area. These results may have potential applications to the earthquake prediction in the Circum-Pacific seismic belt.

Global navigation satellite system detection of preseismic ionospheric total electron content anomalies for strong magnitude (Mw>6) Himalayan earthquakes

Electron content in the ionosphere is very sensitive to temporary disturbances of the Earth’s magnetosphere (geomagnetic storm), solar flares, and seismic activities. The Global Navigation Satellite System (GNSS)-based total electron content (TEC) measurement has emerged as an important technique for computations of earthquake precursor signals. We examined the pre-earthquake signatures for eight strong magnitude (Mw>6: 6.1 to 7.8) earthquakes with the aid of GNSS-based TEC measurement in the tectonically active Himalayan region using International GNSS Service (IGS) stations as well as local GNSS-based continuously operating reference stations (CORS). The results indicate very significant ionospheric anomalies in the vertical total electron content (vTEC) a few days before the main shock for all of the events. Geomagnetic activities were also studied during the TEC observation window to ascertain their role in ionospheric perturbations. It was also inferred that TEC variation due to low magnitude events could also be monitored if the epicenter lies closer to the GNSS or IGS station. Therefore, the study has confirmed TEC anomalies before major Himalayan earthquakes, thereby making it imperative to set up a much denser network of IGS/CORS for real-time data analysis and forewarning.

Midlatitude sporadic-E episodes viewed by L-band split-spectrum InSAR

Sporadic-E (Es) is a layer of ionization that irregularly appears within the E region of the ionosphere and is known to generate an unusual propagation of very high frequency waves over long distances. The detailed spatial structure of Es remains unclear due to the limited spatial resolution in the conventional ionosonde observations. We detect midlatitude Es by interferometric synthetic aperture radar (InSAR), which can clarify the spatial structure of Es with unprecedented resolution. Moreover, we use the range split-spectrum method (SSM) to separate dispersive and nondispersive components in the InSAR image. While InSAR SSM largely succeeds in decomposing into dispersive and nondispersive signals, our results indicate that small-scale dispersive signals due to the total electron content anomalies are accompanied by nondispersive signals with similar spatial scale at the same locations. We also examine the effects of higher-order terms in the refractive index for dispersive media. Both of these detected Es episodes indicate that smaller-scale dispersive effects originate from higher-order effects. We interpret that the smaller-scale nondispersive signals could indicate the emergence of nitric oxide (NO) generated by the reactions of metals, Mg and Fe, with nitric oxide ion (NO+) during the Es.

Empirical model of TEC response to geomagnetic and solar forcing over Balkan Peninsula

An empirical total electron content (TEC) model response to external forcing over Balkan Peninsula (35°N-50°N; 15°E-30°E) is built by using the Center for Orbit Determination of Europe (CODE) TEC data for full 17 years, January 1999 - December 2015. The external forcing includes geomagnetic activity described by the Kp-index and solar activity described by the solar radio flux F10.7. The model describes the most probable spatial distribution and temporal variability of the externally forced TEC anomalies assuming that they depend mainly on latitude, Kp-index, F10.7 and LT. The anomalies are expressed by the relative deviation of the TEC from its 15-day mean, rTEC, as the mean value is calculated from the 15 preceding days. The approach for building this regional model is similar to that of the global TEC model reported by Mukhtarov et al. (2013a) however it includes two important improvements related to short-term variability of the solar activity and amended geomagnetic forcing by using a “modified” Kp index. The quality assessment of the new constructing model procedure in terms of modeling error calculated for the period of 1999–2015 indicates significant improvement in accordance with the global TEC model (Mukhtarov et al., 2013a). The short-term prediction capabilities of the model based on the error calculations for 2016 are improved as well. In order to demonstrate how the model is able to reproduce the rTEC response to external forcing three geomagnetic storms, accompanied also with short-term solar activity variations, which occur at different seasons and solar activity conditions are presented.

INVESTIGATION OF THE RELATIONSHIP BETWEEN IONOSPHERIC TEC ANOMALY VARIATIONS AND FAULT TYPES BEFORE THE EARTHQUAKES

Abstract. Earthquakes are natural phenomena that shake the earth and cause many damage. Since the time of arrival of the earthquakes cannot be determined directly, some signs before the earthquake should be examined and interpreted by examining the environmental changes. One of the methods used for this is monitoring the ionospheric total electron content (TEC) changes in total electron content unit (TECU). GPS satellites have begun to be used as a means of monitoring ionospheric TEC anomalies before earthquakes since they began to be used as sensors around the world. In this study, three fault type (normal, thrust and strike-slip faulting) of 28 earthquakes with a magnitude greater than 7 (Mw) and the percentage changes of TEC anomalies before the earthquakes were investigated. The ionospheric TEC anomalies before the earthquake were calculated according to the 15-day running median statistical analysis method. Different solar and geomagnetic indices have been investigated to determine the active space weather conditions and quiet days before and after the earthquake. The TEC anomalies were determined during the quiet days before the earthquake by comparing the ionospheric anomalies that occurred before the earthquake after the determination of quiet days with the indices of the space weather conditions. The results show that there is a relationship between fault type and the earthquake precursor percentage changes and were determined as 47.6 % TECU for regions where normal faulting, 50.4 % TECU for regions where thrust faulting, and 44.2 % TECU for regions where strike-slip faulting occurred, respectively.

Atmospheric-ionospheric disturbances following the April 2015 Calbuco volcano from GPS and OMI observations

The atmospheric-ionospheric anomalies triggered by eruptions of the Calbuco volcano on 22th and 23th, April 2015 are investigated from 50 continuous GPS stations observations. Prominent total electron content (TEC) disturbances with the amplitude of 0.1–0.4 TECU are found about one hour after the eruptions. The disturbance lasts more than one hour for the first eruption and about half hour for the second eruption. The disturbance features are extracted from PRN03 and PRN23 for the first eruption and PRN13 and PRN28 for the second eruption. The propagation velocities from the travel time diagrams are around 800m/s and 900m/s for the two eruptions, respectively. The spectrogram of filtered TEC time series is centered at 3.7mHz, which is in the frequency range of infrasonic waves. The distribution and characteristics of the TEC anomalies are closely associated with the position relationship between SIP tracks and volcano. The results of SO2 and temperature anomalies around the eruptions obtained from Aura/Ozone Monitoring Instrument (OMI) and Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) vertical profiles are consistent with results from GPS TEC.

Analysis of ionospheric disturbances associated with powerful cyclones in East Asia and North America

East Asia and North America are the regions most heavily affected by powerful cyclones. In this paper we investigate the morphological characteristics of ionospheric disturbances induced by cyclones in different continents. The global ionosphere map supplied by the Center for Orbit Determination in Europe (CODE), International Reference Ionosphere Model (IRI) 2012, and Wallops Island ionosonde station data are used to analyse the ionospheric variations during powerful typhoons/hurricanes in East Asia and North America, respectively. After eliminating the ionospheric anomalies due to the solar-terrestrial environment, the total electron content (TEC) time series over the point with maximum wind speed is detected by the sliding interquartile range method. The results indicate that significant ionospheric disturbances are observed during powerful tropical cyclones in East Asia and North America, respectively, and that all the ionospheric anomalies are positive. In addition, the extent and magnitude of travelling ionospheric disturbances are associated with the category of tropical cyclone, and the extent of TEC anomalies in longitude is more pronounced than that in latitude. Furthermore, the maximum ionospheric anomaly does not coincide with the eye of the storm, but appears in the region adjacent to the centre. This implies that ionospheric disturbances at the edges of cyclones are larger than those in the eye of the winds. The phenomenon may be associated with the gravity waves which are generated by strong convective cells that occur in the spiral arms of tropical cyclones. This comprehensive analysis suggests that the presence of powerful typhoons/hurricanes may be a possible source mechanism for ionospheric anomalies.

Seismo-ionospheric anomalies in ionospheric TEC and plasma density before the 17 July 2006 <i>M</i>7.7 south of Java earthquake

Abstract. In this paper, we report significant evidence for preseismic ionospheric anomalies in total electron content (TEC) of the global ionosphere map (GIM) and plasma density appearing on day 2 before the 17 July 2006 M7.7 south of Java earthquake. After distinguishing other anomalies related to the geomagnetic activities, we found a temporal precursor around the epicenter on day 2 before the earthquake (15 July 2006), which agrees well with the spatial variations in latitude–longitude–time (LLT) maps. Meanwhile, the sequences of latitude–time–TEC (LTT) plots reveal that the TECs on epicenter side anomalously decrease and lead to an anomalous asymmetric structure with respect to the magnetic equator in the daytime from day 2 before the earthquake. This anomalous asymmetric structure disappears after the earthquake. To further confirm these anomalies, we studied the plasma data from DEMETER satellite in the earthquake preparation zone (2046.4 km in radius) during the period from day 45 before to day 10 after the earthquake, and also found that the densities of both electron and total ion in the daytime significantly increase on day 2 before the earthquake. Very interestingly, O+ density increases significantly and H+ density decreases, while He+ remains relatively stable. These results indicate that there exists a distinct preseismic signal (preseismic ionospheric anomaly) over the epicenter.

Preseismic ionospheric anomalies detected before the 2016 Kumamoto earthquake

AbstractOn 15 April 2016, the Kumamoto earthquake (Mw 7.3) occurred in Japan with no warning signals. Global Navigation Satellite System (GNSS) receivers provide useful information on disturbances in the ionosphere by calculating the changes in total electron content (TEC), which is the number of electrons in the ionosphere. Here we show our recently proposed correlation analysis of TEC data which can detect the preseismic ionospheric anomalies from the public GNSS data. Our method detected the ionospheric anomaly several tens of minutes before the 2016 Kumamoto earthquake near its epicenter. Furthermore, we gave an indicator to distinguish between the preseismic TEC anomalies and the medium‐scale traveling ionospheric disturbances (MSTIDs) by calculating the anomalous area rates. These results support the hypothesis for existence of the preceding phenomena before large earthquakes.

Applying two-dimensional principal component analysis (2DPCA) to the early detection of a weak earthquake-induced tsunami: a case study

ABSTRACTTwo-dimensional principal component analysis (2DPCA) was performed to detect the two-dimensional ionospheric total electron content (TEC) anomalies associated with an earthquake in Japan and its associated weak earthquake-induced tsunami. TEC data from a time period of five days before the Mw 6.7 earthquake in Miyako, Japan that occurred at 23:06:27 (UTC) on 16 February 2015 were processed. Another TEC data set from the six-hour time period after this earthquake were processed to detect an associated earthquake-induced tsunami. A TEC precursor to this earthquake indicated by the largest principal eigenvalue of 2DPCA was detectable over the epicentre from 04:40:00 to 04:50:00 (UTC) on 15 February 2015. The duration was approximately 10 minutes. A weak earthquake-induced tsunami TEC anomaly was detectable from 23:15:00 to 23:20:00 (UTC) on 16 February 2015. The tsunami TEC anomaly occurred approximately nine minutes after the earthquake with a duration of approximately five minutes. This study confi...

Total Electron Content Anomalies Before Mw 6.0+ Earthquakes in the Seismic Zone of Southwest China Between 2001 and 2013

Abstract To investigate the coupling relationship between ionospheric variations and great earthquakes in the seismic zone of southwest China, the total electron content (TEC) anomalies before Mw 6.0+ earthquakes in that area between 2001 and 2013 were analyzed to study the precursors of great earthquakes. To analyze the lithosphere-atmosphere-ionosphere effect, TEC estimated by the Center for Orbit Determination in Europe was used to determine anomalies prior to 18 moderate-to-great earthquakes with magnitudes of ≥ 6.0. Spatial and temporal TEC variations in the seismic zone were calculated and examined to study the relationship between the identified TEC anomalies and subsequent earthquakes. The results showed that ionospheric anomalies were observed before 72 % of earthquakes and among those anomalies, positive and negative anomalies accounted for approximately 58 % and 42 % of anomalies, respectively. Most of the ionospheric anomalies could be observed within 1 to 6 days prior to the earthquakes and approximately 82 % of ionospheric anomalies usually occurred between 13:00 and 18:00 in the region's local time. Furthermore, the anomaly amplitudes and occurrence time were proportional to the magnitude of the earthquake but the relationship was not linear. After eliminating the effect of solar activities and geomagnetic storms, some obvious and regular anomalous behavior in TEC could be detected on the 19th day before several earthquakes and the peak of anomaly enhancement coincided with the vertical projection of the epicenter. Our results indicate that these TEC anomalies may be the ionospheric precursors of forthcoming earthquakes in the seismic belt of southwest China.

Ionospheric TEC disturbance study over seismically region in China

Using data of Crustal Movement Observation Network of China (CMONOC) and IGS (International GNSS Service), we attempt to give a statistical behavior of the total electron content (TEC) perturbation before 30Mw6.0+ earthquakes from 2000 to 2010 in China. Results show that TEC anomalies are detected before 20 earthquakes, nearly 67%. The anomalies represent positive before most events and occur mostly within 2–6days before the shocks, significantly during the afternoon period, 12–20LT. To verify the reliability of the anomalous behavior of TEC detected in the statistical study, we analyze variations of TEC over two regions during a period of low solar and geomagnetic activity in April-October 2008. In southwest China, during that time, six large earthquakes with magnitude M≧6.0 occurred. And a ‘check-region’ with same geomagnetic latitudes as the epicentral area is selected. Known that the decisive role in the ionosphere state is performed by space weather effects, we compare the TEC variations with time series of EUV solar radiation, Bz component of the interplanetary magnetic field (IMF), index of geomagnetic activity Dst and planetary index Kp, respectively. It is found that, both in the two regions, the observed anomalies in the regional TEC are in good correlation with the variations of above index. We detect different levels of TEC anomalies before the 6 earthquakes, and most of them are amost certainly dominated by space weather expect for Wenchuan case. The work illustrates the difficulties of searching precursors of earthquake in the ionosphere, but also, it reminds the importance for considering the effect of atmospheric waves on the ionosphere as well as the applicability of statistical approach.

Statistical analysis of ionospheric TEC anomalies before global M w ≥ 7.0 earthquakes using data of CODE GIM

Based on Center of Orbit Determination in Europe (CODE) global ionospheric map (GIM) data, a statistical analysis of local total electron content (TEC) anomalies before 121 low-depth (D ≤ 100 km) strong (M w ≥ 7.0) earthquakes has been made using the sliding median differential calculation method combining with a new approach of image processing technique. The results show that significant local TEC anomalies could be observed 0–6 days before 80 earthquakes, about 66.1% out of the total. The positive anomalies occur more often than negative ones. For 26 cases, both positive and negative anomalies are observed before the shock. The pre-earthquake TEC anomalies show local time recurrence for 38 earthquakes, which occur around the same local time on different days. The local time distribution of the pre-earthquake TEC anomalies mainly concentrates between 19 and 06 LT, roughly from the sunset to sunrise. Most of the pre-earthquake TEC anomalies do not locate above the epicenter but shift to the south. The pre-earthquake TEC anomalies could be extracted near the magnetic conjugate point of the epicenter for 40 events, which is 50% out of the total 80 cases with significant local TEC anomalies. In general, the signs of the anomalies around epicenter and its conjugate point are the same, but the abnormal magnitude and lasting time are not.

Detection of ionospheric anomalies during intense space weather over a low-latitude GNSS station

The operational availability of Global Navigation Satellite System is affected by large-scale irregularities of the ionosphere. The space weather events induce several intense irregularities and cause the non-linear distribution of ionospheric electron density. Monitoring of ionospheric responses due to extreme space weather events plays a key role in trans-ionospheric radio wave propagation. In the present analysis, a novel technique based on wavelet transform has been implemented for the analysis and detection of ionospheric anomalies during two intense space weather events that occurred in 2013. The investigations have been carried out using the ionospheric observable, Total Electron Content (TEC), derived from the Global Positioning System (GPS) receiver located at an Equatorial Ionisation Anomaly region, KL University, Guntur, India (Geographic Lat.16.37°N, Geographic Long. 80.37°E). The effects of geomagnetic storms (Sym-H ≤ −100 nT) on the perturbations of ionospheric TEC have been investigated. The algorithm of Continuous wavelet transform has been used to study and characterise the presence of ionospheric anomalies in the local time-scale plane. It can detect spatial and temporal details of ionospheric anomaly intensity during strong solar-terrestrial and geophysical events. It is observed that during the main phase of the geomagnetic storm that occurred during 17 March 2013, TEC enhanced by 7 TECU, while a suppression of 10 TECU in the GPS-TEC can be noticed during the main phase of the 29 June 2013 storm. The variation in the intensity of ionospheric TEC anomalies during storm time has been detected and compared to the intensity of the space weather events measured through solar and geomagnetic indices (F10.7, Sym-H, IMF Bz and IEF Ey).

국외 대규모 지진과 최근 발생한 국내지진에 의한 이온층 총 전자수 변화

대규모 지진 전/후에 이온층에서 발생하는 총 전자수(TEC)의 이상 변화를 재 확인하고, 국내에서 발생하는 지진에서도 이와 유사한 현상이 발견되는지를 조사하였다. 먼저 최근 지진관련 이온층 변화가 보고되었던 국외 대규모 지진들과, 근래에 발생한 규모 7.8 에콰도르 강진에 대해 TEC를 계산하고, 지진 발생 전 후의 특성을 확인하였다. TEC 계산 결과, 선행 연구들과 유사한 TEC 변화 시계열을 얻을 수 있었으나, 시계열들이 후처리되는 방법에 따라 전조현상 또는 후지진 현상으로 해석될 수 있다는 사실을 확인하였다. 최근 국내에서 발생한 지진들에 대하여 지진에 의한 TEC 변화 가능성을 조사하였으나, 2016년 경주지진을 포함한 가장 높은 규모의 국내지진들과 관련하여 지진 전/후에 TEC의 이상 변화를 찾아볼 수 없었다. 이는 판의 경계에서 발생하는 국외의 대규모 지진들에 비해, 국내 지진이 대기 중으로 방출하는 탄성파(음파) 에너지가 작기 때문이라고 여겨진다. We investigated pre- and post- seismic total electron content (TEC) anomalies in ionosphere caused by recent large-scale earthquakes around the globe and additionally examined whether the similar phenomena are detected in connection with the earthquakes around the Korean Penisula. TEC anomalies associated with the large-scale earthquakes showed the similar results to previous studies. In addition, we newly found the similar TEC changes from the recent 2016 Ecuador earthquake (M7.8). However, the post-seismic TEC changes would be falsely interpreted as the pre-seismic TEC changes dependent on the post-processing of TEC observation. We also investigated the possibility of TEC responses from the recent domestic earthquakes including 2016 Gyeongju earthquake but could not find any anomalous TEC changes. This is probably because the domestic earthquakes release significantly smaller acoustic wave energy than that of large-scale earthquakes occurring in plate boundaries.

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.