Pre-earthquake ionospheric anomalies before three major earthquakes by GPS-TEC and GIM-TEC data during 2015–2017

The dual-frequency satellite-based measurements from Global Positioning System (GPS) may provide feasible ways of studying and potentially detecting of earthquake (EQ) related anomalies in the ionosphere. In this paper, GPS based Total Electron Content (TEC) data are studied for three major M?>?7.0 EQs in Nepal and Iran-Iraq border during 2015–2017 by implementing statistical procedures on temporal and spatial scale. Previous studies presented different time interval of pre-seismic ionospheric anomalies, however, this study showed that EQs ionospheric precursors may occur within 10?days. Furthermore, the ionospheric anomalies on the suspected day occurred during UT?=?08:00–12:00?h before the main shock. The Global Ionospheric Map TEC (GIM-TEC) data retrieved over the epicenter of M7.8 (Nepal EQ) showed a significant increase of 6 TECU on April 24, 2015 (one day before the main shock), which is recorded by the ground GPS station data of Islamabad (station lies within the EQ preparation zone). Furthermore, the spatial GIM-TEC result imply significant anomalies over the epicenter during the time interval between UT?=?08:00–12:00?h (LT?=?13:00–17:00). For M7.3 (Nepal EQ), the TEC anomalies were detected on May 10, 2015 (2?days before the event) in the temporal data. The spatial TEC data imply the huge clouds over the epicenter at about UT?=?08:00–12:00?h on May 10, 2015, that may be associated with this EQ in the quiet geomagnetic storm conditions. Similarly, temporal and spatial TEC showed anomaly on November 3, 2017, during UT?=?08:00–12:00 (9?days before the Iran-Iraq border EQ) after implementing the statistical method on it. Conversely, there exists a short-term but low magnitude TEC anomaly synchronized with a geomagnetic storm on November 7–8, 2017 (4 to 5?days prior to M7.3 Iran-Iraq border EQ). The diurnal and hourly GIM-TEC and VTEC data also imply the execution of ionospheric anomalies within 10?days prior to all events. All these positive anomalies in TEC may be due to the existence of a huge energy from the epicenter during the EQ preparation period.     

Response of low latitude D-region ionosphere to the total solar eclipse of 22 July 2009 deduced from ELF/VLF analysis

Response of the D-region of the ionosphere to the total solar eclipse of 22 July 2009 at low latitude, Varanasi (Geog. lat., 25.27° N; Geog. long., 82.98° E; Geomag. lat. = 14° 55’ N) was investigated using ELF/VLF radio signal. Tweeks, a naturally occurring VLF signal and radio signals from various VLF navigational transmitters are first time used simultaneously to study the effect of total solar eclipse (TSE). Tweeks occurrence is a nighttime phenomena but the obscuration of solar disc during TSE in early morning leads to tweek occurrence. The changes in D-region ionospheric VLF reflection heights (h) and electron density (n_e: 22.6–24.6 cm⁻³) during eclipse have been estimated from tweek analysis. The reflection height increased from ∼89 km from the first occurrence of tweek to about ∼93 km at the totality and then decreased to ∼88 km at the end of the eclipse, suggesting significant increase in tweek reflection height of about 5.5 km during the eclipse. The reflection heights at the time of totality during TSE are found to be less by 2–3 km as compared to the usual nighttime tweek reflection heights. This is due to partial nighttime condition created by TSE. A significant increase of 3 dB in the strength of the amplitude of VLF signal of 22.2 kHz transmitted from JJI-Japan is observed around the time of the total solar eclipse (TSE) as compared to a normal day. The modeled electron density height profile of the lower ionosphere depicts linear variation in the electron density with respect to solar radiation as observed by tweek analysis also. These low latitude ionospheric perturbations on the eclipse day are discussed and compared with other normal days.     

Seismo ionospheric anomalies in Turkey associated with Mw ≥ 6.0 earthquakes detected by GPS stations and GIM TEC

Earthquake (EQ) anomalies in the form of enhancement and depletion in ionospheric Total Electron Content (TEC) from Global Positioning System (GPS) may considerably alarm about short and long term precursors of the impending main shock. In this paper, TEC anomalies are investigated from permanent GPS ground-stations in Turkey associated to M_w ≥ 6.0 EQs occurred in 2011–2012. Temporal and spatial analyses of TEC at 2 h sampling have shown significant evidences about EQ induced ionospheric anomalies during 10–14 h of UT (Universal Time) within 5 days before M_w 6.0 Greece, and M_w 7.1, Turkish EQ. Spatial analyses have manifested arrival of TEC anomalies at UT = 10 h to epicenter of both EQs, which linger above epicenter during UT = 12–14 h and left seismogenic zone after UT = 14 h before every EQ during K_p < 3 and Dst = 0 nT. Meanwhile, a geomagnetic storm (Dst < -100 nT) induce perturbation two days after the M_w 7.1 Turkish EQ, showing no relation with epicenter during spatial analysis. It also shows that TEC can be useful to distinguish geomagnetic storm variations to successfully detect EQ precursors. These anomalies during quiet storm (K_p < 3; Dst = 0 nT) conditions may be effective to link the lithosphere and ionosphere in severe seismic zones to detect EQ precursors before future EQs. Interpretation of TEC anomalies and it enhancements over EQ epicenters during UT = 12–14 h for both EQs have shown that EQs anomalies only occurred in particular time. Whereas, geomagnetic storm effect occurred during whole abnormal day over the Earth.     

Seismogenic ionospheric anomalies associated with the strong Indonesian earthquake occurred on 11 April 2012 (M = 8.5)

Ionospheric perturbations in possible association with a major earthquake (EQ) (M?=?8.5) which occurred in India-Oceania region are investigated by monitoring subionospheric propagation of VLF signals transmitted from the NWC transmitter (F?=?19.8?kHz), Australia to a receiving station at Varanasi (geographic lat. 25.3°N, long 82.99°E), India. The EQ occurred on 11 April 2012 at 08:38:35?h UT (magnitude?≈?8.5, depth?=?10?km, and lat.?=?2.3°N, long.?=?93.0°E). A significant increase of few days before the EQ has been observed by using the VLF nighttime amplitude fluctuation method (fixed frequency transmitter signal). The analysis of total electron contents (TEC) derived from the global positioning system (GPS) at three different stations namely, Hyderabad (latitude 17.38°N, longitude 78.48°E), Singapore (latitude 1.37°N, longitude 103.84°E) and Port Blair (latitude 11.62°N, longitude 92.72°E) due to this EQ has also been presented. Significant perturbation in TEC data (enhancements and depletion) is noted before and after the main shock of the EQ. The possible mechanisms behind these perturbations due to EQ have also been discussed.     

Ground-based and satellite DC-ULF electric field anomalies around Wenchuan M8.0 earthquake

The ground-based and satellite DC-ULF electric field data were analyzed around Wenchuan M8.0 earthquake on May 12, 2008 in China. The results show that ground electric field anomalies occurred at 3 stations located to the north and south of the epicenter with the amplitude of 3–100 mV/km. The change shapes and their amplitude of ground electric field anomalies are different largely due to their individual underground layer conductivity, water level and so on. The analysis of long time series illustrates that the abnormal geoelectric field started since March 2008. Onboard the DEMETER satellite, the ULF waveforms of electric field were collected and processed by wavelet transform method. The disturbances in the ionosphere were about 3–5 mV/m at a frequency band lower than 0.5 Hz. When the ground and space electric field anomalies were compared, their occurrence time and spatial distribution points are consistent with each other, including the long time anomalies from March 2008 and the short term ones 1–2 days before the Wenchuan earthquake. Finally, the coupling mechanism was discussed.     

Response of the mid-latitude D-region ionosphere to the total solar eclipse of 22 July 2009 studied using VLF signals in South Korean peninsula

In this paper, we analyze VLF signals received at Busan to study the the D-region changes linked with the solar eclipse event of 22 July 2009 for very short (∼390 km) transmitter–receiver great circle path (TRGCP) during local noon time 00:36–03:13 UT (09:36–12:13 KST). The eclipse crossed south of Busan with a maximum obscuration of ∼84%. Observations clearly show a reduction of ∼6.2 dB in the VLF signal strength at the time of maximum solar obscuration (84% at 01:53 UT) as compared to those observed on the control days. Estimated values of change in Wait ionospheric parameters: reflection height (h′) in km and inverse scale height parameter (β) in km⁻¹ from Long Wave Propagation Capability (LWPC) model during the maximum eclipse phase as compared to unperturbed ionosphere are 7 km and 0.055 km⁻¹, respectively. Moreover, the D-region electron density estimated from model computation shows 95% depletion in electron density at the height of ∼71 km. The reflection height is found to increase by ∼7 km in the D-region during the eclipse as compared to those on the control days, implying a depletion in the Lyman-α flux by a factor of ∼7. The present observations are discussed in the light of current understanding on the solar eclipse induced D-region dynamics.     

Ionospheric perturbations in plasma parameters before global strong earthquakes

Based on the electron density (Ne) and temperature (Te) data from DEMETER, the ionospheric perturbations before 82 Ms ? 7.0 earthquakes (EQs) during 2005–2010 were studied, using moving median and space difference methods within 10 days before and 2 days after these events in local nighttime. It was found that the plasma parameters disturbances appeared before 49 EQs, in which more disturbances were detected before shallow-focus earthquakes than deep ones, and there was little difference between continental and oceanic ones, both exceeding 1/2 percentage. For the disturbed time, more perturbations were seen in 1, 3, 5, 6, 8 days before EQs and 1 day after EQs. For the spatial distribution, the anomalies before EQs were not just above the epicenters, but shifted equatorward with several degrees to almost twenty degrees. Most of the abnormities were positive ones, which demonstrate that Ne increases before EQs at the altitude of 670 km of DEMETER. Perturbations of Ne were more than that of Te, which illustrates that Ne is much more sensitive to seismic activity than Te.     

VLF波渗透电离层传播计算研究进展

3~30kHz的甚低频（Very-Low-Frequency,VLF）电磁波对近地空间的高能粒子分布具有非常重要的作用.闪电和地面VLF通信台等VLF波主要辐射源产生的VLF波能够渗透进入电离层,并以哨声波模式继续传播至磁层与高能粒子发生相互作用.本文从VLF电磁波渗透电离层传播计算方法的发展、计算模型验证以及模型在电离层现象研究中的应用等方面对VLF波渗透进电离层之后的传播计算的研究进展进行综述,并对未来研究进行初步展望.      

Ionospheric perturbations due to earthquakes as determined from VLF and GPS-TEC data analysis at Agra,India

Employing SoftPAL receiver, amplitude variations of VLF transmitter signals NWC (19.8?kHz) and NPM (21.4?kHz) are analyzed at Agra station in India (Geograph. lat. 27.2°N, long. 78°E) ±15?days from five major earthquakes of magnitude M?=?6.9–8.5 occurred in Indian subcontinent during the years 2011–2013. We apply nighttime fluctuation (NF) method and show that in almost all cases the trend decreases and dispersion and NF increase on the same days corresponding to each earthquake about 11–15?days prior to the main shock. Assuming that the ionospheric perturbations are caused by atmospheric gravity waves (AGW), we also calculate AGW modulation index for each case and find its values increased on the days amplitude fluctuations take place. Its value is decreased in one case only where the perturbations may be attributed to penetration of seismogenic electric field.In order to support the above results we also present GPS-TEC data analyzed by us corresponding to three of the above earthquakes. We study the TEC anomalies (unusual enhancements) and find that in one case the precursory period is almost the same as that found in NF method.     

Peculiarities of the nighttime winter foF2 increase over Irkutsk

An analysis of properties and peculiarities of the nighttime winter foF2 increases (NWI) in the East Siberia is made on data of ionospheric station Irkutsk in the periods 1958–1992 and 2002–2009 and the empirical model of the F2 layer critical frequency under the geomagnetic quiet conditions deduced from these data (model Q-F2). It is revealed, that the NWI is the stable regularity of the quiet ionosphere over Irkutsk. The amplitude of the NWI (the difference between maximum and minimum foF2 values at night hours) is the greatest in December–January and nearly the same at low and middle solar activity. It is a peculiarity of the quiet ionosphere in the East Siberia. Maximum in night foF2 under quiet geomagnetic conditions is observed mainly after midnight (02-04 LT) and is shifted to predawn hours as solar activity increases. At low solar activity the quiet ionosphere at ∼02–04 LT shows the following properties: (a) the fluctuations of foF2 and hmF2 are in the reverse correlation but this dependence is weak; (b) very strong fluctuations of foF2 (|δfoF2| > 30%) occur seldom (∼4% of events) and almost all of them are positive; an example of very strong fluctuations of foF2 up to 60% can be an extreme increase in the foF2 on 19.12.2008; (c) the very strong enhancements of foF2 in the NWI maximum can be observed at the low geomagnetic activity, they occur more often during substorms but very seldom during geomagnetic storms. Possible reasons of these properties of NWI are discussed.     

Study of the 2013 Lushan M7.0 earthquake coseismic ionospheric disturbances

On April 20, 2013, an earthquake of M7.0 occurred in Lushan, Sichuan province, China. This paper investigates the coseismic ionospheric anomalies using GPS (Global Positioning System) data from 23 reference stations in Sichuan province that are a part of the Crustal Movement Observation Network of China (CMONOC). The recorded results show that a clear ionospheric anomaly occurred within 15 min after the earthquake near the epicenter, and the occurrence time of the anomalies recorded by various stations is related to the distance from the epicenter. The maximum anomaly is 0.25 TECu, with a 2 min duration and the distance of the recording station to the epicenter is 83 km. Acoustic waves generated by the crustal vertical movement during the earthquake propagate up to the height of the ionosphere lead to the ionospheric anomaly, and the propagation speed of the acoustic wave is calculated as 0.72 ± 0.04 km/s based on the propagation time and propagation distance, consistent with the average speed of sound waves within a 0–450 km atmospheric height.     

Observations of wave activity in the ionosphere over South Africa in geomagnetically quiet and disturbed periods

The present paper deals with observations of wave activity in the period range 1–60 min at ionospheric heights over the Western Cape, South Africa from May 2010 to July 2010. The study is based on the Doppler type sounding of the ionosphere. The Doppler frequency shift measurements are supplemented with measurements of collocated Digisonde DPS-4D at SANSA Space Sciences, Hermanus. Nine geomagnetically quiet days and nine geomagnetically active days were included in the study. Waves of periods 4–30 min were observed during the daytime independent of the level of geomagnetic activity. Amplitudes of 10–30 min waves always increased between 14:00 and 16:15 UT (16:00–18:15 LT). Secondary maxima were observed between 06:00 and 07:00 UT (08:00–09:00 LT). The maximum wave amplitudes occurred close to the time of passage of the solar terminator in the studied region which is known to act as a source of gravity waves.     

Simultaneous observations of some unusual whistlers and VLF hiss emission at a low L-value (L = 1.17)

The purpose of this paper is to describe some unusual whistlers (doublets and triplets) and VLF hiss emission recorded simultaneously on February 18, 1998 during nighttime at a low latitude Indian ground station Jammu (geomag. lat., 22° 26′ N; L = 1.17), and to make some discussions about their origin. The detailed structures of the observed VLF hiss emission clearly show that these emissions confined to a narrow continuous frequency band. Some times the frequency of hiss band oscillates and subsequently touches the upper edge of the first whistler component of the doublet. Detailed structures of the dynamic spectra of whistler and VLF hiss emission are briefly presented. From the dispersion analysis of the whistler doublets and triplets, it is found that the individual whistlers of the doublets and triplets simultaneously observed on the same day are one-hop whistlers having propagation path along higher and closely spaced L-values. Our result also shows that VLF hiss has been generated in the equatorial region of higher L-values. Generation and propagation mechanisms are briefly discussed.     

Response of Hainan GPS ionospheric scintillations to the different strong magnetic storm conditions

Using the GPS ionospheric scintillation data at Hainan station (19.5°N, 109.1°E) in the eastern Asia equatorial regions and relevant ionospheric and geomagnetic data from July 2003 to June 2005, we investigate the response of L-band ionospheric scintillation activity over this region to different strong magnetic storm conditions (Dst < −100 nT) during the descending phase of the solar cycle. These strong storms and corresponding scintillations mainly took place in winter and summer seasons. When the main phase developed rapidly and reached the maximum near 20–21 LT (LT = UT + 8) after sunset, scintillations might occur in the following recovery phase. When the main phase maximum occurred shortly after midnight near 01–02 LT, following the strong scintillations in the pre-midnight main phase, scintillations might also occur in the post-midnight recovery phase. When the main phase maximum took place after 03 LT to the early morning hours no any scintillation could be observed in the latter of the night. Moreover, when the main phase maximum occurred during the daytime hours, scintillations could also hardly be observed in the following nighttime recovery phase, which might last until the end of recovery phase. Occasionally, scintillations also took place in the initial phase of the storm. During those scintillations associated with the nighttime magnetic storms, the height of F layer base (h’F) was evidently increased. However, the increase of F layer base height does not always cause the occurrence of scintillations, which indicates the complex interaction of various disturbance processes in ionosphere and thermosphere systems during the storms.     

Sensing the Earth’s low ionosphere during solar flares using VLF signals and goes solar X-ray data

An analysis of D-region electron density height profile variations, induced by four isolated solar X-ray flares during period from September 2005 to December 2006, based on the amplitude and the phase delay perturbation of 22.1 kHz signal trace from Skelton (54.72 N, 2.88 W) to Belgrade (44.85 N, 20.38 E), coded GQD, was carried out. Solar flare data were taken from NOAA GOES12 satellite one-minute listings. For VLF data acquisition and recordings at the Institute of Physics, Belgrade, Serbia, the AbsPAL system was used. Starting from LWPCv21 code (Ferguson, 1998), the variations of the Earth-ionosphere waveguide characteristic parameters, sharpness and reflection height, were estimated during the flare conditions. It was found that solar flare events affected the VLF wave propagation in the Earth-ionosphere waveguide by changing the lower ionosphere electron density height profile, in a different way, for different solar flare events.     

Use of principal component analysis in the identification of the spatial pattern of an ionospheric total electron content anomalies after China’s May 12, 2008, M = 7.9 Wenchuan earthquake

This paper uses principal component analysis (PCA) to determine the spatial pattern of total electron content (TEC) anomalies in the ionosphere post the China’s Wenchuan Earthquake of 12 May, 2008 (UTC) (M_w = 7.9). PCA and image processing are applied to the global ionospheric map (GIM) with transforms conducted for the time period 08:00–10:00 UT on 12 May 2008. Results show that at a height of approximately 200 km the anomaly is widespread and less intense; however, it becomes more localized with height reaching maximum intensity and localization at an altitude of 300 km. The spatial distribution is remarkably similar to that reported for a TEC anomaly previously identified as a precursor anomaly on May 9, 2008 for the same time period. Potential causes of the results are discussed with emphasis given to vertical acoustic gravity waves based on the spatial pattern identified.     

Low-latitude ionospheric scintillations and total electron content obtained with the CITRIS instrument on STPSat1 using radio transmissions from DORIS ground beacons

The primary objective of the Scintillation and Tomography Receiver in Space (CITRIS) is to detect ionospheric irregularities from space at low latitude. For this purpose, the satellite receiver uses the UHF and S-Band transmissions of the ground network of Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) beacons. CITRIS, developed at the Naval Research Laboratory, differs from the normal DORIS receiver by being able to capture and store the complex amplitude of the 401.25 and 2036.25 MHz transmissions at 200 Hz sample rate. Ground processing of the CITRIS data yields total electron content (TEC) and both phase and amplitude scintillations. With CITRIS flying on the US Space Test Program (STP) satellite STPSat1, 2 years of data were collected and processed to determine the fluctuations in ionospheric TEC and radio scintillations associated with equatorial irregularities. CITRIS flights over DORIS transmitters yield direct measurements of the horizontal plasma density fluctuations associated with equatorial plasma bubbles. Future flights of CITRIS can provide valuable complements to other satellite instruments such as GPS occultation receivers used to estimate vertical electron density profiles in the ionosphere.     

Study of long path VLF signal propagation characteristics as observed from Indian Antarctic station,Maitri

To examine the quality and propagation characteristics of the Very Low Frequency (VLF) radio waves in a very long propagation path, Indian Centre for Space Physics, Kolkata, participated in the 27th Indian scientific expedition to Antarctica during 2007–2008. One Stanford University made AWESOME VLF receiving system was installed at the Indian Antarctic station Maitri and about five weeks of data were recorded successfully from the Indian transmitter VTX and several other transmitting stations worldwide. The quality of the signal from the VTX transmitter was found to be very good, consistent and highly stable in day and night. The signal shows the evidences of the presence of the 24 h solar radiation in the Antarctic region during local summer. Here we report the both narrow band and broadband VLF observations from this site. The diurnal variations of VTX signal (18.2 kHz) are presented systematically for Antarctica path and also compared the same with the variations for a short propagation path (VTX-Kolkata). We compute the spatial distribution of the VTX signal along the VTX-Antarctica path using the most well-known LWPC model for an all-day and all-night propagation conditions. The calculated signal amplitudes corresponding to those conditions relatively corroborate the observations. We also present the attenuation rate of the dominant waveguide modes corresponding to those propagation conditions where the effects of the Antarctic polar ice on the attenuation of different propagating waveguide modes are visible.     

Ionospheric precursors observed at low latitudes around the time of koyna earthquake

On December 11, 1967 at 05:21 LT, an immense earthquake of magnitude 6.7 struck Koyna, the Indian province of Maharashtra. Its epicenter was located at geographic latitude 17.37°N and longitude 73.75°E with depth of about 3 km. Ground based measurements show variation in the critical frequency of ionospheric F2 layer (foF2) before and after the shock. In the present study the behavior of F2-region of ionosphere has been examined over the equatorial and low latitudinal region ionosphere during the month of December 1967 around the time of Koyna earthquake. For this purpose, the ionospheric data collected with the help of ground-based ionosondes installed at Hyderabad (located close to the earthquake epicenter) Ahmedabad, Trichirapulli, Kodaikanal and Trivendrum have been utilized. The upper and lower bound of Interquartile range (IRQ) are constructed to monitor the variations in foF2 other than day-to-day and diurnal pattern for finding the seismo-ionospheric precursors. Some anomalous electron density variations are observed between post midnight hours to local pre-noon hours at each station. These anomalies are strongly time dependent and appeared a couple of days before the main shock. The period considered in this study comes under the quiet geomagnetic conditions. Hence, the observed anomalies (which are more than the usual day-to-day variability) over all stations are likely to be associated with this imminent earthquake. The possible mechanism to explain these anomalies is the effect of seismogenic electric field generated just above the surface of earth within the earthquake preparation zone well before the earthquake due to emission of radioactive particles and then propagated upward, which perturbs the F-region ionosphere.     

The asymmetrical features in electron density during extreme solar minimum

The variations of plasma density in topside ionosphere during 23rd/24th solar cycle minimum attract more attentions in recently years. In this analysis, we use the data of electron density (Ne) from DEMETER (Detection of Electromagnetic Emissions Transmitted from Earthquake Regions) satellite at the altitude of 660–710 km to investigate the solstitial and equinoctial asymmetry under geomagnetic coordinate system at LT (local time) 1030 and 2230 during 2005–2010, especially in solar minimum years of 2008–2009. The results reveal that ΔNe (December–June) is always positive over Southern Hemisphere and negative over northern part whatever at LT 1030 or 2230, only at 0–10°N the winter anomaly occurs with ΔNe (December–June) > 0, and its amplitude becomes smaller with the declining of solar flux from 2005 to 2009. The ΔNe between September and March is completely negative during 2005–2008, but in 2009, it turns to be positive at latitudes of 20°S–40°N at LT 1030 and 10°S–20°N at LT 2230. Furthermore, the solstitial and equinoctial asymmetry index (AI) are calculated and studied respectively, which all depends on local time, latitude and longitude. The notable differences occur at higher latitudes in solar minimum year of 2009 with those in 2005–2008. The equinoctial AI at LT 2230 is quite consistent with the variational trend of solar flux with the lowest absolute AI occurring in 2009, the extreme solar minimum, but the solstitial AI exhibits abnormal enhancement during 2008 and 2009 with bigger AI than those in 2005–2007. Compared with the neutral compositions at 500 km altitude, it illustrates that [O/N₂] and [O] play some roles in daytime and nighttime asymmetry of Ne at topside ionosphere.