Total Electron Content Fluctuations Research Articles

The 2009 Samoa and 2010 Chile tsunamis as observed in the ionosphere using GPS total electron content

[1] Ground-based Global Positioning System (GPS) measurements of ionospheric total electron content (TEC) show variations consistent with atmospheric internal gravity waves caused by ocean tsunamis following two recent seismic events: the Samoa earthquake of 29 September 2009 and the Chile earthquake of 27 February 2010. Both earthquakes produced ocean tsunamis that were destructive to coastal communities near the epicenters, and both were observed in tidal gauge and buoy measurements throughout the Pacific Ocean. We observe fluctuations in TEC correlated in time, space, and wave properties with these tsunamis using the Jet Propulsion Laboratory's Global Ionospheric Mapping software. These TEC measurements were band-pass filtered to remove ionospheric TEC variations with wavelengths and periods outside the typical range for tsunamis. Observable variations in TEC appear correlated with the tsunamis in some locations (Hawaii and Japan), but not in others (Southern California or near the epicenters). Where variations are observed, the typical amplitude tends to be ∼0.1–0.2 TEC units for these events, on the order of ∼1% of the background TEC value. These observations are compared to estimates of expected tsunami-driven TEC variations produced by Embry Riddle Aeronautical University's Spectral Full Wave Model, an atmosphere-ionosphere coupled model, and are found to be in good agreement. Significant TEC variations are not always seen when a tsunami is present, but in these two events the regions where a strong ocean tsunami was observed coincided with clear TEC observations, while a lack of clear TEC observations coincided with smaller sea surface height amplitudes. There exists the potential to apply these detection techniques to real-time GPS TEC data, providing estimates of tsunami speed and amplitude that may be useful for early warning systems.

On the occurrence of postmidnight equatorialFregion irregularities during the June solstice

[1] Postmidnight equatorial F region irregularities (EFIs) are known to develop mainly during the solstitial months. However, it is not well understood whether they occur at all longitudes and what process causes their occurrence at different longitude sectors. In this study, we use the GPS total electron content (TEC) fluctuations obtained from a global GPS network and spread F in ionograms from Jicamarca (283°E, 12°S, Dip 1°N) in the American longitude sector and Kwajalein (167°E, 9°N, Dip 4°N), Bac Lieu (106°E, 9°N, Dip 2°N), and Chumphon (99°E, 11°N, Dip 3°N) in the Pacific and Asian longitude sectors during 2000–2009, to investigate the EFI characteristics during June solstice. Results from global TEC fluctuations show that at solar maximum, the occurrence rate of postmidnight EFIs is high in African and Pacific regions, moderate in the Southeast Asian region, and low in the Peruvian region and that most postmidnight EFIs are the continuation of postsunset EFIs. During solar minimum the postmidnight EFIs were rarely observed in TEC but were very frequent in ionograms. The latter had more frequent postmidnight onsets over Peru, whereas they were initiated during late postsunset hours in Pacific and Southeast Asian longitudes. In both longitudes the postsunset layer rise occurred with some delay. The layer rise was more prominent on spread F nights over Jicamarca and less so over Pacific longitudes. The results showing different degrees of association at the different longitudes between the postsunset/postmidnight EFIs and F layer heights highlight the influence of other factors in the late-night EFI development. Perturbation seeds and plasma drifts/neutral winds, in particular, are discussed as strong candidates for causing these irregularities in the June solstitial months of solar minimum years.

Vertical TEC over seismically active region during low solar activity

Using data of Japanese dense array of GPS receivers (GEONET) we analyzed variations of total electron content (TEC) over Japan during a period of low solar and geomagnetic activity in April–September 2008. During that time, five large earthquakes with magnitude M>6.8 occurred around the Japanese Islands. We calculated vertical TEC with time resolution of 1 h and we compared the TEC over Japan with TEC over southwestern USA. In order to detect abnormal signals, we processed daily TEC based on median and quartile of preceding 15 days. Knowing that the decisive role in the ionosphere state is performed by space weather effects, we compared the estimated TEC values with time series of the B z component of the interplanetary magnetic field (IMF), UV solar radiation, index of geomagnetic activity Dst and planetary index Kp. We have found that though the analyzed time period falls on the minimum of solar activity, the observed positive anomalies in the regional TEC correspond to fluctuations of IMF B z and to the following increase in Kp and Dst indices. Other case of ∼25% TEC increase in the duration of four days (DOY: 140–143) is, most likely, caused by increase in the solar UV radiation and coincides with growth of the global TEC. Other positive or negative abnormal TEC variations coincide with fluctuations of the global mean TEC or are of large spatial scale, i.e. much larger than the earthquake preparation zone. Therefore, it is rather difficult to find correlation between the observed anomalies and the five large earthquakes. Our results do not disprove the possibility of precursory phenomena but show the difficulties in identifying earthquake precursors in the ionosphere TEC on the background of TEC changes produced by space weather effects, even during solar minimum.

Fluctuations of total electron content in the ionosphere as deduced from measurements by single-frequency GPS receivers

Using an ionospheric diagnostic technique developed by the authors and single-frequency GPS receivers, we obtained the first results of these diagnostics. In particular, we show that the ionosphere is characterized by large-scale periodic processes with frequencies of 3–5 mHz.

Atmospheric airglow fluctuations due to a tsunami‐driven gravity wave disturbance

A spectral full‐wave model is used to study the upward propagation of a gravity wave disturbance and its effect on atmospheric nightglow emissions. Gravity waves are generated by a surface displacement that mimics a tsunami having a maximum amplitude of 0.5 m, a characteristic horizontal wavelength of 400 km, and a horizontal phase speed of 200 m/s. The gravity wave disturbance can reach F region altitudes before significant viscous dissipation occurs. The response of the OH Meinel nightglow in the mesopause region (∼87 km altitude) produces relative brightness fluctuations, which are ∼1% of the mean for overhead viewing. The wave amplitudes grow as the wave disturbance propagates upward, which causes the thermospheric nightglow emission responses to be large. For overhead viewing, the brightness fluctuations are ∼50% and 43% of the mean for the OI 6300 Å and O 1356 Å emissions, respectively. The total electron content fluctuation is ∼33% of the mean for overhead viewing. For oblique viewing, the relative brightness fluctuations are slightly smaller than those obtained for overhead viewing. In spite of this, the thermospheric nightglow brightness fluctuations are large enough that oblique viewing could provide early warning of an approaching tsunami. Thus, the monitoring of thermospheric nightglow emissions may be a useful augmentation to other observational techniques of tsunami effects in the thermosphere/ionosphere system.

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.

A study of GPS ionospheric scintillations observed at Guilin

The occurrence of strong ionospheric scintillations with S4≥0.2 was studied using global positioning system (GPS) measurements at Guilin (25.29°N, 110.33°E; geomagnetic: 15.04°N, 181.98°E), a station located near the northern crest of equatorial anomaly in China. The results are presented for data collected from January 2007 to December 2008. The results show that amplitude scintillations occurred only during the first five months of the considered years. Nighttime amplitude scintillations, observed mainly in the south of Guilin, always occurred with phase scintillations, total electron content (TEC) depletions, and Rate Of change of TEC (ROT) fluctuations. However, TEC depletions and ROT fluctuations were weak during daytime amplitude scintillations, and daytime amplitude scintillations usually occurred in most of the azimuth directions. GPS scintillation/TEC observations recorded at Guilin and signal-to-noise-ratio measurements obtained from GPS-COSMIC radio occultation indicate that nighttime and daytime scintillations are very likely caused by ionospheric F region irregularities and sporadic E, respectively.

Effect of TEC Variation on GPS Precise Point at Low Latitude

The ionosphere is a dispersive medium of charged particles between the satellite and the user on Earth. These dispersive ionized media play a vital role in the various applications of GPS (Global Positioning Systems) because the ionosphere directly influences transionospheric radio waves propagating from the satellite to the receiver. Solar flares affect the ionization state of the ionosphere with their high intensity. Sometimes the intensity is so severe that it accelerates the rate of ionization, resulting in ionospheric storms; during the ionospheric storms the concentration of charged particles varies. Among the various phenomena in the ionosphere, TEC (Total Electron Content) is responsible for range error which produces a time delay in the radio signal. The rate of change of TEC with respect to time is abbreviated as ROT. It is one of the parameters that express the ionospheric irregularities with respect to time. This work investigates the effect of ROT fluctuation on the precise positioning of GPS receivers during low solar activity periods in the equatorial anomaly region. Good geometry and a sufficient number of locked satellites provide more accuracy within the centimeter level, but the case may be different when there are any ionospheric storms. Even a few satellite signals passing through the ionospheric irregularities can cause a significant error in positioning. Thus, it is important to understand the ionospheric irregularities observed by GPS receivers in order to correct them. The ROT (TEC/Minute) parameter is used here to study the occurrence of TEC fluctuation and its potential effect on GPS, such as a horizontal positional error or the satellite geometry of the GPS receiver. This investigation is based on the analysis of a one-year observation of a fixed GPS receiver installed at Bhopal (23.2020N, 77.4520E), India during low solar active period in 2005. The GPS receiver used here is a GISTM-based dual frequency NovAtel OEM4 GPS receiver.

Simultaneous observations of ESF irregularities over Indian region using radar and GPS

Abstract. In this paper, we present simultaneous observations of temporal and spatial variability of total electron content (TEC) and GPS amplitude scintillations on L1 frequency (1.575 GHz) during the time of equatorial spread F (ESF) while the MST radar (53 MHz) located at Gadanki (13.5° N, 79.2° E, Dip latitude 6.3° N), a low latitude station, made simultaneous observations. In particular, the latitudinal and longitudinal extent of TEC and L-band scintillations was studied in the Indian region for different types of ESF structures observed using the MST radar during the low solar activity period of 2004 and 2005. Simultaneous radar and GPS observations during severe ESF events in the pre-midnight hour reveal that significant GPS L band scintillations, depletions in TEC, and the double derivative of the TEC index (DROTI), which is a measure of fluctuations in TEC, obtained at low latitudes coincide with the appearance of radar echoes at Gadanki. As expected, when the irregularities reach higher altitudes as seen in the radar map during pre-midnight periods, strong scintillations on an L-band signal are observed at higher latitudes. Conversely, when radar echoes are confined to only lower altitudes, weak scintillations are found and their latitudinal extent is small. During magnetically quiet periods, we have recorded plume type radar echoes during a post-midnight period that is devoid of L-band scintillations. Using spectral slopes and cross-correlation index of the VHF scintillation observations, we suggest that these irregularities could be "dead" or "fossil" bubbles which are just drifting in from west. This scenario is consistent with the observations where suppression of pre-reversal enhancement (PRE) in the eastward electric field is indicated by ionosonde observations of the height of equatorial F layer and also occurrence of low spectral width in the radar observations relative to pre-midnight period. However, absence of L-band scintillations during post-midnight event, when radar observed plume like structures and scintillations were recorded on VHF band, raises questions about the process of evolution of the irregularities. A possible explanation is that whereas small scale (~3 m) irregularities are generated through secondary waves that grow on the walls of km scale size irregularities, in this case evolution of the Rayleigh-Taylor instability itself did not extend to irregularities of scale sizes of a few hundred meters that produce scintillation on a L-band signal.

Space Weather and the Global Positioning System

Space Weather and the Global Positioning System

GPS Observations of Ionospheric Effects of the Major Storm of Nov. 7–10, 2004

AbstractThe TEC and amplitude scintillation measurements have been carried out at Wuhan (114.36°E, 30.53°N, magnetic latitude 19.4°) since 2003 using a GPS ionospheric total electron content (TEC) and scintillation monitor to investigate the ionospheric irregularities and TEC disturbances at the region of the equatorial anomaly. In this paper ionospheric effects of the magnetic storm of November, 2004 are analyzed by using GPS observations. Results of data analysis show that the ionospheric storm during this magnetic storm mainly exhibits the long duration positive storm phase around Wuhan, and the positive deviations of TEC occur during two main phases of the storm on November 8 and 10 with the largest deviation of 50 TECU. Another primary effect of the storm on the ionosphere is the remarkably enhanced activity and intensity of amplitude scintillations during the later main phase. The maximum value of the S4 index exceeded 1.0. There occurred the plasma bubbles and TEC fluctuations initiated from the equatorial region at the same time when strong amplitude scintillations occurred. A comparison between amplitude scintillations and TEC fluctuations was made. The statistic analysis shows there is a remarkable positive correlation between standard deviation of the change rate of TEC, the ROTI index, and the S4 index with correlation coefficient of 0.97.

The occurrence of polar cap patches in TEC fluctuations detected using GPS measurements in southern hemisphere

GPS technique is widely used to study the global structure and dynamics of the ionosphere. This paper presents data on the occurrence of large-scale ionospheric irregularities observed at Antarctic stations McMurdo-MCM4 (−78N, 167E, Φ = 80°), Casey-CAS1 (−66N, 110E, Φ = 80°), Mawson-MAW1 (−68N, 63E, Φ = 71°), and Davis-DAV1 (−69N, 78E, Φ = 75°) in 2001. The irregularities caused strong Total Electron Content fluctuations. The intensity of TEC fluctuations was estimated with the ROT parameter and rate of TEC index (ROTI) expressed in TECU/min. Storm-time development of TEC fluctuations are presented for two geomagnetic active periods: June and September 2001. Data on temporal variations in TEC along individual satellites passes were also analyzed. Polar patches were associated with deep TEC fluctuations as the TEC enhancements were 2–10 times larger than the background, whereas the TEC increased by 10–50 TECU in about 5–10 min. The duration of such structures is 10–20 min. At 4–5 h interval, 3–5 strong and some weak patches were detected. The activity of patches depends to a low degree only on the magnetic activity index Kp. The intensity of TEC fluctuations increases during substorm activity. Diurnal, seasonal dependence, and longitudinal features of the occurrence of polar patches are also presented in the report.

Analysis of the fluctuations of the total electron content (TEC) measured at Goose Bay using tools of nonlinear methods

In this paper we report the evidence of a low‐dimensional chaos in a set of data observed outside laboratories. The dynamic behavior of the time series of the fluctuations of the total electron content (TEC) measured at Goose Bay, which is a high‐latitude station, is analyzed in detail using the tools of nonlinear dynamics. The low‐dimensional character of the dynamics is evident from the estimated value of the fraction of false neighbors for various dimensions and the correlation dimension. The deterministic nature of the dynamics is investigated using recurrence plots and spatiotemporal entropy. The chaotic nature of the underlying dynamics of the fluctuations of TEC is shown by the power spectrum indicating exponential decay and the calculated positive value of Lyapunov exponent. This is also supported by the results of the comparison of the chaotic characteristics of the time series of variations of TEC with the pseudochaotic characteristic of the colored noise time series. The results of the tests based on the prediction error and the time reversal asymmetry statistic reject the hypothesis that TEC belongs to the family of linear stochastic signals. The nonlinear non‐Gaussian nature of the oscillations of variations of TEC is further investigated by the surrogate data test based on several geometrical and dynamical characteristics of the variations of TEC such as mutual information, the fraction of the false nearest neighbours, the local slopes of the correlation sums, the curves giving Lyapunov exponents, and finally, the value of Lyapunov exponents. The results of this analysis show that low‐dimensional chaotic dynamics could be a possible and fruitful concept which can be utilized to study the disturbance in the ionosphere as in the case of magnetospheric dynamics. We feel that the dynamical invariants like Lyapunov exponents and correlation dimension can describe the disturbance in the variations of TEC and thus the disturbance in the ionosphere. Hence the techniques of nonlinear and chaos theory and the measure of the dynamical invariants could be used for the characterization and thus for the modeling of variations of the total electron content.

Traveling wave packets of total electron content disturbances as deduced from global GPS network data

We identified a specific class of mid-latitude medium-scale traveling ionospheric disturbances (MSTIDs), namely traveling wave packets (TWPs) of total electron content (TEC) disturbances. For the first time, we present the TWP morphology for 105 days 1998–2001. A total number of the TEC series, with a duration of each series of about 2.3 h (2h 18m), exceeded 700,000. The data were obtained using the technology GLOBDET of global detection of ionospheric disturbances using a global network of GPS receivers, and the technique of GPS interferometry of TIDs, developed at the ISTP SD RAS. It was found that TWPs are observed no more than in 0.1–0.4% of all TEC series, most commonly during the daytime in winter and autumn. TWPs are quasi-periodic oscillations of TEC with a period of around 10– 20 min , and a time duration of the order of 1 h . The TWP amplitudes exceed the amplitudes of “background” TEC fluctuations by one order of magnitude, as a minimum. The radius of spatial correlation of TWPs does not exceed 500– 600 km (3–5 wavelengths). We carried out a detailed analysis of the spatial–temporal properties of TWPs by considering an example of the most conspicuous manifestation of TWPs on October 18, 2001 over California, USA. The velocity and direction of TWP displacement correspond to those of mid-latitude MSTIDs.

Irregularities of the topside ionosphere observed by GPS/MET radio occultation

Large‐scale fluctuations (vertical scales ∼40–60 km) of total electron content (TEC) are retrieved from phase path fluctuations of GPS radio occultation links of the GPS/MET experiment. The large‐scale TEC fluctuations at observation heights 400–600 km are maximal at local times from 20:00 to 24:00 h. The fluctuations mainly occur on the summer hemisphere at low magnetic latitudes 0–30°N and seem to be concentrated over Africa and Arabia. The derived global distribution of TEC fluctuations around solar minimum (June/July 1995) is assumed to be closely related to the occurrence of spread F and radio scintillations. The meridional distribution of the background electron density (averaged for 19:00–23:00 LT) has a maximum in the topside ionosphere at around 10°N, beyond an anomaly structure of the F2 peak layer. The meridional location of the anomaly structure agrees with the location of enhanced TEC fluctuations. Both phenomena are possibly related to the enhancement of upward ion velocity in the postsunset ionosphere at equatorial latitudes. Large TEC depletions of 1–20 TEC‐units are present at observation heights 200–400 km in many TEC profiles of the equatorial ionosphere after sunset. The TEC depletions possibly indicate plasma bubbles on their way from the lower ionosphere to the topside ionosphere. GPS navigation signal loss due to spread F or high‐latitude irregularities is not found in the GPS/MET data of June/July 1995.

DYNAMICAL CHARACTERISTICS OF TRAVELING WAVE PACKETS OF TOTAL ELECTRON CONTENT DISTURBANCES

Using the COPHASE method and the GPS interferometry method for travelling ionospheric disturbances, we analyze in detail the spatio-temporal properties of travelling wave packets (TWP) of total electron content (TEC) disturbances. The analysis is performed on the example of a clearest TWP manifestation observed in California, USA, in October 18, 2001, using the GLOBDET technique, developed at the Institute of Solar-Terrestrial Physics of the Siberian Branch of RAS for global detection and monitoring of natural and technogenic ionospheric disturbances on the basis of TEC variations retrieved from the global network of GPS receivers. In the time domain, TWPs are quasi-periodic TEC oscillations of duration about 1 h, period of 10–20 min, and amplitude exceeding that of the background TEC fluctuations by at least one order of magnitude. The velocity and direction of TWP motion are similar to those of mid-latitude mesoscale travelling ionospheric disturbances, as obtained earlier from the analysis of phase parameters of HF radio signals and the signals of geostationary satellites and discrete space radio sources.

Ionospheric effects of major magnetic storms during the International Space Weather Period of September and October 1999: GPS observations, VHF/UHF scintillations, and in situ density structures at middle and equatorial latitudes

In this paper we present a study of the ionospheric effects of a halo coronal mass ejection (CME) initiated on the Sun on September 20, 1999, and causing the largest magnetic storm during this month on September 22–23, 1999, with the hourly Dst index being −167 nT at ∼2400 UT on September 22. The recurrent CME on October 18 caused an even larger magnetic storm on October 22, 1999, with Dst of −231 nT at ∼0700 UT. The ionospheric effects of these two major magnetic storms are studied through their effects on a prototype of a Global Positioning System (GPS)‐based navigation system called Wide Area Augmentation System (WAAS) being developed by the Federal Aviation Administration for use in the continental United States and their impact on global VHF/UHF communication systems. It is shown that the penetration of transient magnetospheric electric fields equatorward of the shielding region at midlatitudes, which have been well‐correlated in the past with rapid changes in the well‐known Dst index (or through its recently available high resolution 1‐min counterpart the SYM‐H index), can cause large increases of total electron content (TEC), TEC fluctuations, and saturated 250‐MHz scintillation, and these, in turn, may have significant impacts on WAAS. The local time of Dst changes (and not just Dst magnitude) was found to be very important for WAAS, since the largest effects on TEC are seen near dusk. The prompt penetration of these magnetospheric electric fields all the way to the magnetic equator causes augmentation or inhibition of equatorial spread F. The global ionospheric response to these storms has been obtained from ground‐based TEC observations with a GPS network and space‐based in situ density and electric field measurements using the Republic of China Satellite‐1 (ROCSAT‐I) and several Defense Meteorological Satellite Program satellites. These prompt penetration electric fields cause VHF/UHF scintillations and GPS TEC variations at low latitudes in the specific longitude sector for which the early evening period corresponds to the time of rapid Dst variations and maximum Dst phase. The effects of the delayed ionospheric disturbance dynamo and those of decreased magnetospheric convection on postmidnight irregularity generation are shown to be confined to a part of the same longitude range that actively responded to the prompt penetration of electric fields in the early evening sector.

Ionospheric effects of the solar flares of September 23, 1998 and July 29, 1999 as deduced from global GPS network data

This paper presents data from first global positioning system (GPS) measurements of global response of the ionosphere to solar flares of September 23, 1998 and July 29, 1999. The analysis used novel technology of a global detection of ionospheric effects from solar flares as developed by one of the authors (Afraimovich, Radio Sci. 35 (2000) 1417). The essence of the method is that use is made of appropriate filtering and a coherent processing of variations in total electron content (TEC) in the ionosphere which is determined from GPS data, simultaneously for the entire set of visible (over a given time interval) GPS satellites at all stations used in the analysis. It was found that fluctuations of TEC, obtained by removing the linear trend of TEC with a time window of about 5 min, are coherent for all stations and the line-of-sight to the GPS satellites on the dayside of the Earth. The time profile of TEC responses is similar to the time behavior of hard X-ray emission variations during flares if the relaxation time of electron density disturbances in the ionosphere of order 50–100 s is introduced. No such effect on the nightside of the Earth has been detected yet.

Ultralow frequency waves and total electron content fluctuations

Low frequency electromagnetic waves in the ionosphere are examined, and we calculate the refraction angles and other properties as these waves propagate through the ionosphere. The correlation to fluctuations in the total electron content is derived, and the results are compared to observed results. © 2001 American Institute of Physics.

The shock-acoustic waves generated by earthquakes

Abstract. We investigate the form and dynamics of shock-acoustic waves generated by earthquakes. We use the method for detecting and locating the sources of ionospheric impulsive disturbances, based on using data from a global network of receivers of the GPS navigation system, and require no a priori information about the place and time of the associated effects. The practical implementation of the method is illustrated by a case study of earthquake effects in Turkey (17 August and 12 November 1999), in Southern Sumatra (4 June 2000), and off the coast of Central America (13 January 2001). It was found that in all instances the time period of the ionospheric response is 180–390 s, and the amplitude exceeds, by a factor of two as a minimum, the standard deviation of background fluctuations in total electron content in this range of periods under quiet and moderate geomagnetic conditions. The elevation of the wave vector varies through a range of 20–44°, and the phase velocity (1100–1300 m/s) approaches the sound velocity at the heights of the ionospheric F-region maximum. The calculated (by neglecting refraction corrections) location of the source roughly corresponds to the earthquake epicenter. Our data are consistent with the present views that shock-acoustic waves are caused by a piston-like movement of the Earth’s surface in the zone of an earthquake epicenter.Key words. Ionosphere (ionospheric disturbances; wave propagation) – Radio science (ionospheric propagation)