Ionospheric Perturbations Research Articles

Criticality Analysis of the Lower Ionosphere Perturbations Prior to the 2016 Kumamoto (Japan) Earthquakes as Based on VLF Electromagnetic Wave Propagation Data Observed at Multiple Stations

The perturbations of the ionosphere which are observed prior to significant earthquakes (EQs) have long been investigated and could be considered promising for short-term EQ prediction. One way to monitor ionospheric perturbations is by studying VLF/LF electromagnetic wave propagation through the lower ionosphere between specific transmitters and receivers. For this purpose, a network of eight receivers has been deployed throughout Japan which receive subionospheric signals from different transmitters located both in the same and other countries. In this study we analyze, in terms of the recently proposed natural time analysis, the data recorded by the above-mentioned network prior to the catastrophic 2016 Kumamoto fault-type EQs, which were as huge as the former 1995 Kobe EQ. These EQs occurred within a two-day period (14 April: and , 15 April: ) at shallow depths (~10 km), while their epicenters were adjacent. Our results show that lower ionospheric perturbations present critical dynamics from two weeks up to two days before the main shock occurrence. The results are compared to those by the conventional nighttime fluctuation method obtained for the same dataset and exhibit consistency. Finally, the temporal evolutions of criticality in ionospheric parameters and those in the lithosphere as seen from the ULF electromagnetic emissions are discussed in the context of the lithosphere-atmosphere-ionosphere coupling.

Signature of gravity waves and ionospheric perturbations over European and North African region during the solar eclipse of 1 August 2008

Solar eclipses provide unique chances to study the ionospheric/thermospheric response to solar radiation changes. The solar eclipse of 1 August 2008 was an outstanding event since it has occurred during the deep minimum of the solar cycle 23. The ionospheric behavior during that event has been investigated using global GPS-Total Electron Content (TEC) maps, COSMIC satellite measurements and Coherent Ionospheric Doppler Radar (CIDR). We analyzed the behavior of total ionospheric ionization over European and North African region by reconstructing TEC maps and differential TEC maps. TEC estimation showed that the total ionization reduces up to 30% at high latitude region during the eclipse as a function of obscuration. COSMIC observations showed a height profile of percentage reduction in electron density with a maximum reduction at the F2 layer of about 30%. The CIDR system allowed us to study the spatial changes of the ionospheric F2 layer during the passage of the solar eclipse which revealed many interesting features of horizontally-propagating gravity waves with wavelength (λ) about 752.39 km and velocity ∼ 151.28 km/min associated with the solar eclipse. The wavelet analysis displays the existence of Wave-like Structures (WLS) of ∼5 min period at F-region altitude.

Coseismic Contortion and Coupled Nocturnal Ionospheric Perturbations During 2016 Kaikoura, Mw 7.8 New Zealand Earthquake

AbstractThe oblique‐thrust Kaikoura earthquake of Mw 7.8 that struck New Zealand on 13 November 2016 at 11:02:56 UTC (local time at 00:02:56 a.m. on 14 November 2016) was one of the most geometrically and tectonically complex earthquakes recorded onshore in modern seismology. The event ruptured in the region of multisegmented faults and propagated unilaterally northeastward for more than 170 km from the epicenter. The GPS derived coseismic surface displacements reveal a larger widespread horizontal and vertical coseismic surface offsets of ~6 m and ~2 m, respectively, with two distinct tectonic thrust zones. We study the characteristics of coseismic ionospheric perturbations based on tectonic and nontectonic forcing mechanisms and demonstrate that these perturbations are linked to two distinct surface thrust zones with rotating horizontal reinforcement trending the rupture, rather than merely to the displacements oriented along the rupture propagation direction.

Surface waves magnitude estimation from ionospheric signature of Rayleigh waves measured by Doppler sounder and OTH radar

Surface waves emitted after large earthquakes are known to induce atmospheric infrasonic waves detectable at ionospheric heights using a variety of techniques, such as high frequency (HF) Doppler, global positioning system (GPS), and recently over-the-horizon (OTH) radar. The HF Doppler and OTH radar are particularly sensitive to the ionospheric signature of Rayleigh waves and are used here to show ionospheric perturbations consistent with the propagation of Rayleigh waves related to 28 and 10 events, with a magnitude larger than 6.2, detected by HF Doppler and OTH radar respectively. A transfer function is introduced to convert the ionospheric measurement into the correspondent ground displacement in order to compare it with classic seismometers. The ground vertical displacement, measured at the ground by seismometers, and measured at the ionospheric altitude by HF Doppler and OTH radar, is used here to compute surface wave magnitude. The ionospheric surface wave magnitude (Msiono) proposed here introduces a new way to characterize earthquakes observing the signature of surface Rayleigh waves in the ionosphere. This work proves that ionospheric observations are useful seismological data to better cover the Earth and to explore the seismology of the Solar system bodies observing the ionosphere of other planets.

Simulating the dependence of seismo-ionospheric coupling on the magnetic field inclination

Abstract. Infrasound generated during a seismic event upon reaching the ionospheric heights possesses the ability to perturb the ionosphere. Detailed modelling investigation considering 1-D dissipative linear dynamics, however, indicates that the magnitude of ionospheric perturbation strongly depends on the magnetic field inclination. Physics-based SAMI2 model codes have been utilized to simulate the ionosphere perturbations that are generated due to the action of the vertical wind perturbations associated with the seismic infrasound. The propagation of the seismic energy and the vertical wind perturbations associated with the infrasound in the model has been considered to be symmetric about the epicentre in the north–south directions. Ionospheric response to the infrasound wind, however, has been highly asymmetric in the model simulation in the north–south directions. This strong asymmetry is related to the variation in the inclination of the Earth's magnetic field north and south of the epicentre. Ionospheric monitoring generally provides an efficient tool to infer the crustal propagation of the seismic energy. However, the results presented in this paper indicate that the mapping between the crustal process and the ionospheric response is not a linear one. These results also imply that the lithospheric behaviour during a seismic event over a wide zone in low latitudes can be estimated through ionospheric imaging only after factoring in the magnetic field geometry. Keywords. Atmospheric composition and structure (pressure, density, and temperature) – history of geophysics (atmospheric sciences) – ionosphere (ionosphere–atmosphere interactions)

On the Tempo-Spatial Evolution of the Lower Ionospheric Perturbation for the 2016 Kumamoto Earthquakes from Comparisons of VLF Propagation Data Observed at Multiple Stations with Wave-Hop Theoretical Computations

There have been published many papers on VLF (very low frequency) characteristics to study seismo-ionospheric perturbations. Usually VLF records (amplitude and/or phase) are used to investigate mainly the temporal evolution of VLF propagation anomalies with special attention to one particular propagation path. The most important advantage of this paper is the simultaneous use of several propagation paths. A succession of earthquakes (EQs) happened in the Kumamoto area in Kyusyu Island; two strong foreshocks with magnitude of 6.5 and 6.4 on 14 April (UT) and the main shock with magnitude 7.3 on 15 April (UT). Because the EQ epicenters are not far from the VLF transmitter (with the call sign of JJI in Miyazaki prefecture), we can utilize simultaneously 8 observing stations of our network all over Japan. Together with the use of theoretical computations based on wave-hop theory, we try to trace both the temporal and spatial evolutions of the ionospheric perturbation associated with this succession of EQs. It is found that the ionospheric perturbation begins to appear about two weeks before the EQs, and this perturbation becomes most developed 5 - 3 days before the main shock. When the perturbation is most disturbed, the maximum change in vertical direction is depletion in the VLF effective ionospheric height of the order of 10 km, and its horizontal scale (or its radius) is about 1000 km. These spatio-temporal changes of the seismo-ionospheric perturbation will be investigated in details in the discus-sion, a comparison has made with the VLF characteristics of the 1995 Kobe with the same magnitude and of the same fault-type, and a brief discussion on the generation mechanism of seismo-ionospheric perturbation is finally made.

Examples of unusual ionospheric observations by the CSES prior to earthquakes

The CSES (China seismic electromagnetic satellite) was launched on February 2, 2018 in a circular polar orbit at an altitude of ~507 km. One of the main objectives of CSES is to search for and characterize ionospheric perturbations that can be associated with seismic activities, to better understand the generation mechanism of such perturbations. Its scientific payload can measure a broad frequency range of electromagnetic waves and some important plasma parameters. This paper is a first-hand study of unusual observations recorded by the CSES over seismic regions prior to four earthquakes with M >7.0 since the satellite's launch. CSES detectors measured irregularities near the epicenter of these four earthquakes. It is already clear that data from instruments onboard the CSES will be of significant help in studies of characteristics of ionospheric perturbations related to earthquakes and their generation mechanisms.

The Results of Measurements of Features of Artificial Electromagnetic and Plasma Perturbations in the Outer Ionosphere of the Earth Using the DEMETER Satellite

The results of the DEMETER satellite mission for 2005–2010 are summarized. The characteristics of electromagnetic and plasma perturbations induced in the F2 layer of the Earth’s ionosphere by the highpower HF radio waves from the SURA heating facility are analyzed.

Space Weather Nowcasting for Area‐Denied Locations: Testing All‐Sky Imaging Applications at Geomagnetic Conjugate Points

AbstractWe explore the concept of using an all‐sky imager (ASI) in one hemisphere to provide nowcasting of ionospheric perturbations in the opposite hemisphere. The specific example deals with low‐latitude plasma instabilities known as equatorial spread F (ESF) that largely depend on geomagnetic field controlled electrodynamics. ASI observations of 630.0 nm airglow from ~300 km exhibit regions of low emission (“airglow depletions”) that correlate highly with ESF patterns of radio wave disruptions, for example, from GPS satellites. For both oceanographic and geopolitical reasons, there are vast regions of the globe that cannot be used for ground‐based nowcasting of local ESF effects. For such area‐denied locations, it is possible to use observations of airglow depletions from the opposite hemisphere to specify both local and conjugate location environmental impacts. We use 15 months of ASI observations from the El Leoncito Observatory (Argentina) to predict simultaneous conditions at its trans‐equatorial geomagnetic conjugate point in Villa de Leyva (Colombia)—validated by independent ASI observations at that location. We find the success rate of conjugate point nowcasting to be greater than 95% for large‐scale ESF occurrence patterns. For a different pair of stations at higher magnetic latitudes, 3 years of observations from the Arecibo Observatory (Puerto Rico) were used to make ESF nowcasting at its conjugate point in Mercedes (Argentina) with a similar high success rate.

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.

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.

Finite-Difference Modeling of Very-Low-Frequency Propagation in the Earth-Ionosphere Waveguide

Simulations of Very-low-frequency (VLF) transmitter signals are conducted using three models: the long-wave propagation capability, a finite-difference (FD) time-domain model, and an FD frequency-domain model. The FD models are corrected using Richardson extrapolation to minimize the numerical dispersion inherent in these models. Using identical ionosphere and ground parameters, the three models are shown to agree very well in their simulated VLF signal amplitude and phase, to within 1 dB of amplitude and a few degrees of phase, for a number of different simulation paths and transmitter frequencies. Furthermore, the three models are shown to produce comparable phase changes for the same ionosphere perturbations, again to within a few degrees. Finally, we show that the models reproduce the phase data of existing VLF transmitter-receiver pairs reasonably well, although the nighttime variation in the measured phase data is not captured by the simplified characterization of the ionosphere.

Longitudinal Variation of the Lunar Tide in the Equatorial Electrojet

AbstractThe atmospheric lunar tide is one known source of ionospheric variability. The subject received renewed attention as recent studies found a link between stratospheric sudden warmings and amplified lunar tidal perturbations in the equatorial ionosphere. There is increasing evidence from ground observations that the lunar tidal influence on the ionosphere depends on longitude. We use magnetic field measurements from the CHAMP satellite during July 2000 to September 2010 and from the two Swarm satellites during November 2013 to February 2017 to determine, for the first time, the complete seasonal‐longitudinal climatology of the semidiurnal lunar tidal variation in the equatorial electrojet intensity. Significant longitudinal variability is found in the amplitude of the lunar tidal variation, while the longitudinal variability in the phase is small. The amplitude peaks in the Peruvian sector (∼285°E) during the Northern Hemisphere winter and equinoxes, and in the Brazilian sector (∼325°E) during the Northern Hemisphere summer. There are also local amplitude maxima at ∼55°E and ∼120°E. The longitudinal variation is partly due to the modulation of ionospheric conductivities by the inhomogeneous geomagnetic field. Another possible cause of the longitudinal variability is neutral wind forcing by nonmigrating lunar tides. A tidal spectrum analysis of the semidiurnal lunar tidal variation in the equatorial electrojet reveals the dominance of the westward propagating mode with zonal wave number 2 (SW2), with secondary contributions by westward propagating modes with zonal wave numbers 3 (SW3) and 4 (SW4). Eastward propagating waves are largely absent from the tidal spectrum. Further study will be required for the relative importance of ionospheric conductivities and nonmigrating lunar tides.

Statistical Study on Variations of the Ionospheric Ion Density Observed by DEMETER and Related to Seismic Activities

AbstractIn this paper, we present the result of a statistical study performed on the ionospheric ion density variations above areas of seismic activity. The ion density was observed by the low altitude satellite DEMETER between 2004 and 2010. In the statistical analysis a superposed epoch method is used where the observed ionospheric ion density close to the epicenters both in space and in time is compared to background values recorded at the same location and in the same conditions. Data associated with aftershocks have been carefully removed from the database to prevent spurious effects on the statistics. It is shown that, during nighttime, anomalous ionospheric perturbations related to earthquakes with magnitudes larger than 5 are evidenced. At the time of these perturbations the background ion fluctuation departs from a normal distribution. They occur up to 200 km from the epicenters and mainly 5 days before the earthquakes. As expected, an ion density perturbation occurring just after the earthquakes and close to the epicenters is also evidenced.

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.

Intermittency-induced criticality in the lower ionosphere prior to the 2016 Kumamoto earthquakes as embedded in the VLF propagation data observed at multiple stations

A network of 8 VLF/LF receivers has recently been established across Japan, receiving subionospheric signals from different transmitters located both in the same and other countries. The primary purpose of this network is to study disturbances in the VLF/LF propagation through the lower ionosphere in possible relation to earthquake (EQ) preparation processes. Ionospheric perturbations of possible seismic origin have long been investigated and considered very promising for short-term EQ prediction. The raw amplitude data on reception of the above-mentioned network, after being appropriately filtered, were analyzed by means of the method of critical fluctuations (MCF) in analogy to thermal critical systems. The MCF analysis of the VLF/LF propagation revealed that intermittency-induced criticality was reached in the lower ionosphere from ~1week to 3days prior to the catastrophic 2016 Kumamoto EQs. These fault-type EQs occurred within a two-day period (14 April: MW=6.2 and MW=6.0, 15 April: MW=7.0) at shallow depths (~10km) and very close epicenters, while the main event was as huge as the former 1995 Kobe EQ. MCF analysis results are compared to those by the conventional nighttime fluctuation method as well as to those by natural time analysis method obtained for the same dataset, and are found to exhibit remarkable consistency.

Statistical analysis of the ionospheric ion density recorded by DEMETER in the epicenter areas of earthquakes as well as in their magnetically conjugate point areas

Results of a statistical variation of total ion density observed in the vicinity of epicenters as well as around magnetically conjugated points of earthquakes are presented in this paper. Two data sets are used: the ion density measured by DEMETER during about 6.5 years and the list of strong earthquakes (MW ≥ 4.8) occurring globally during this period (14,764 earthquakes in total). First of all, ionospheric perturbations with 23–120 s observation time corresponding to spatial scales of 160–840 km are automatically detected by a software (64,287 anomalies in total). Second, it is checked if a perturbation could be associated either with the epicenter of an earthquake or with its magnetically conjugated point (distance < 1500 km and time < 15 days before the earthquake). The index Kp < 3 is also considered in order to reduce the effect of the geomagnetic activity on the ionosphere during this period. The results show that it is possible to detect variations of the ionospheric parameters above the epicenter areas as well as above their conjugated points. About one third of the earthquakes are detected with ionospheric influence on both sides of the Earth. There is a trend showing that the perturbation length increases as the magnitude of the detected EQs but it is more obvious for large magnitude. The probability that a perturbation appears is higher on the day of the earthquake and then gradually decreases when the time before the earthquake increases. The spatial distribution of perturbations shows that the probability of perturbations appearing southeast of the epicenter before an earthquake is a little bit higher and that there is an obvious trend because perturbations appear west of the conjugated point of an earthquake.

On precursors to the Italy earthquake M.6.2 using VLF signal of NSY(45.9 kHz) received at Kiel Longwave Monitor

On August 24, 2016, an earthquake of magnitude 6.2 struck Central Italy (42.706°N and 13.223°E) at 1:36 UT. We present the results obtained from the US Navy VLF Transmitter’s NSY signal of 45.9 kHz transmitted from Niscemi, in the province of Sicily, Italy and received at the Kiel Longwave Monitor, Germany for 2016. We analysed the terminator times and their individual differences. We also analysed trends, dispersion and night time fluctuation which gave us a possible precursor the Italy earthquake. We found ionospheric perturbations in these parameters on 14th, 19th and 22nd of August, few days prior. Moreover, we filtered the possible effects due to lightning, storms and auroras if any.

Analysis framework for systematically studying ionospheric response to impulsive events from below

AbstractImpulsive phenomena in the Earth's atmosphere produce acoustic and gravity waves which perturb the ionosphere. Such perturbations are often measured using total electron content fluctuations (TEC), derived from ground‐based Global Positioning System data. Using TEC data from the Japanese GEONET ground network after the Tōhoku earthquake on 11 March 2011, we demonstrate capabilities of a new framework of methodologies for analyzing ionospheric perturbations. The framework consists of several new techniques: calculating velocity along a single direction to reduce error due to anisotropic propagation, producing normalized bidirectional band‐pass spectra that preserve relative timing between various frequencies and allowing a more systematic determination of broadband pulses, and utilizing a wavelet‐based technique that considers instantaneous wave phase changes, rather than best fit time differences, to evaluate wave characteristics (speed, direction, and wavelength) within spectral ranges of interest. Using these techniques together decreases subjectivity and reduces errors in attributing fluctuations to given sources. In validating this framework using the Tōhoku case, we consistently identify three kinds of waves: a broad‐band pulse (speed: &gt;2000 m/s, max range: &gt;1400 km) arriving in the ionosphere 10–15 min after the quake, acoustic waves following the pulse (period: 3–5 min, speed: 700–1000 m/s, max range: &lt;500 km), and gravity waves (period: 10–15 min, speed: 150–500 m/s, max range: &gt;1400 km) propagating away from the epicenter, consistent with theory and demonstrated in previous studies. This framework also can be applied to other impulsive events in the atmosphere that are more difficult to detect and attribute to sources.

Thunderstorm‐/lightning‐induced ionospheric perturbation: An observation from equatorial and low‐latitude stations around Hong Kong

AbstractTotal electron content (TEC) computed from the network of Global Positioning System over Hong Kong area known as Hong Kong Sat‐Ref‐network has been used to study perturbation in the ionosphere from thunder storm activity. Data for geomagnetic quiet day (Kp &lt; 4, on 1 April 2014) have been analyzed. The lightning activity was measured from Total Lightning sensor LS8000 over/around the Hong Kong region. Deviation in vertical TEC (DTEC) and the rate of change of TEC index (ROTI) have been derived and compared for lightning day of 1 April 2014 and nonlightning day of 7 April 2014. An analysis showed reduction in TEC during evening hour (up to 1245 UT), whereas an enhancement during nighttime hour on the lightning day is observed. The variations in DTEC during nonlightning day are found to be insignificant in comparison to that during the lightning day. The ionospheric perturbation in TEC has been noticed up to a distance around ~500 km and more from the lightning center. ROTI is found to vary from 3 to 60 total electron content unit (TECU)/min (1 TECU = 1016 el m−2) on the day of thunderstorm activity, whereas ROTI is insignificant on nonlightning days. Signature of density bubbles in slant TEC data and periodicities (10–100 min) in DTEC data are observed. For the same pseudorandom numbers (1, 10, 13, 23, and 28) strong amplitude scintillations are also observed at a close by station. Amplitude scintillations are proposed to be caused by plasma bubbles. The results are tentatively explained by thunderstorm‐induced electric fields and gravity waves.