Increase In Total Electron Content Research Articles

Multi-instrument observations of plasma features in the Arctic ionosphere during the main phase of a geomagnetic storm in December 2006

Arctic ionospheric variations during the main phase of a magnetic storm on 14–15 December, 2006 were investigated to characterize the high energy particle precipitation caused effects, based on multi-instrument observations. These include electron density observations provided by the Global Positioning System (GPS) total electron content (TEC) measurements, European Incoherent Scatter (EISCAT) radar, the radio occultation (RO) from both the CHAMP satellite and the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellite, as well as the ionospheric absorption of cosmic radio noise measured by the Imaging Riometer for Ionospheric Studies (IRIS) at Kilpisjärvi in the northern Finland (69.05°N, 20.79°E). Significant increases in the electron density for these different observations were found in the Arctic ionosphere during the main phase of the magnetic storm. These increase occurred in Scandinavian, Northwest part of Russia and Svalbard (SNRS) region, primarily at an altitude of about 110km. These results are first reported for the SNRS region, and our study contributes to a more complete description of this space weather event during 14–15 December, 2006. Our observations also provide direct evidence that the stormtime E-layer electron density enhancement (e.g., the sporadic E) can form a nearly dominant portion in the observed TEC increase. These increases were accompanied by the ionospheric absorption enhancement at the altitude of about 90km. The Y-component of magnetic field to the south of SNRS decreased, indicating strong upward field aligned electric current in the Arctic ionosphere. These features are interpreted as the effect of the high energy electron precipitation during the magnetic storm, which is caused by the sub-storm reflected on AL index and the measurements of IMAGE (International Monitor for Auroral Geomagnetic Effects) chain. The average energy of the precipitation electrons reached to about 10keV and the boundary of the high energy electron precipitation was also found to move poleward with a speed of about 800m/s.

Ionospheric anomalies of local earthquakes detected by GPS TEC measurements using data from Tashkent and Kitab stations

This paper reports the ionospheric anomalies observed during strong local earthquakes (M⩾5.0) which occurred mostly in and around Uzbekistan in seismically active zones, during years 2006 to 2009 within approximately 1000km distance from the observing GPS stations located in Tashkent and Kitab, Uzbekistan. The solar and geomagnetic conditions were quiet during occurrence of the selected strong earthquakes. We produce Total Electron Content (TEC) time series over both sites and apply them to detect anomalous TEC signals preceding or accompanying the local earthquakes. The results show anomalous increase or decrease of TEC before or during the earthquakes. In general the anomalies occurred 1–7days before the earthquakes as ionospheric electromagnetic precursors. To identify the anomalous values of TEC we calculated differential TEC (dTEC). dTEC is obtained by subtracting monthly averaged diurnal vTEC from the values of observed vTEC at each epoch. This procedure removes normal diurnal variations of vTEC. The present results are in good agreement with the previous observations on ionospheric earthquake precursors reported by various researchers.

Time series modeling and analysis of trends of daily averaged ionospheric total electron content

A 10.7cm solar radio flux F10.7, geomagnetic planetary equivalent amplitude (Ap index), and period variations were considered in this paper to construct a linear model for daily averaged ionospheric total electron content (TEC). The correlation coefficient of the modeled results and International GNSS Service (IGS) observables was approximately 0.97, which implied that the model could accurately reflect the realistic variation characteristics of the daily averaged TEC. The influences of the different factors on TEC and its characteristics at different latitudes were examined with this model. Results show that solar activity, annual and semiannual cycles are the three most important factors that affect daily averaged TEC. Solar activity is the primary determinant of TEC during periods with high solar activity, whereas periodic factors primarily contribute to TEC during periods with minimum solar activity. The extent of the influences of the different factors on TEC exhibits obvious differences at varying latitudes. The magnitude of the semiannual variation becomes less significant with the increase in latitude. Furthermore, a geomagnetic storm causes an increase in TEC at low latitudes and a decrease at high latitudes.

Response of the EIA ionosphere to the 7-8 May 2005 geomagnetic storm

In this paper, response of low latitude ionosphere to a moderate geomagnetic storm of 7–8 May 2005 (SSC: 1920UT on 7 May with Sym-H minimum, ∼−112nT around 1600UT on 8 May) has been investigated using the GPS measurements from a near EIA crest region, Rajkot (Geog. 22.29°N, 70.74°E, Geomag.14°), India. We found a decrease in total electron content (TEC) in 12h after the onset of the storm, an increase during and after 6h of Sym-H deep minimum with a decrease below its usual-day level on the second day during the recovery phase of the storm. On 8 May, an increase of TEC is observed after sunset and during post-midnight hours (maximum up to 170%) with the formation of ionospheric plasma bubbles followed by a nearly simultaneous onset of scintillations at L-band frequencies following the time of rapid decrease in Sym-H index (−30nT/h around 1300UT).

Study of ionospheric disturbances during solar flare events using the SuperDARN Hokkaido radar

It is well known that many types of ionospheric disturbances occur during solar flare events. The sudden increase in total electron content (SITEC) has been studied for several decades, but total electron content (TEC) data do not provide information on the altitudinal distribution of electron density changes. Previous studies used HF Doppler system data to investigate the contributions of the D-region and F-region ionospheric electron density changes by examining the HF radio wave frequency dependence on the Doppler shift values. In this study we examined the dependence of the elevation angle of the Doppler shift of ground scatter echoes using the SuperDARN Hokkaido radar. We analyzed solar flare events from Dec 2006 to Mar 2012. A sudden fade-out of echoes was observed in almost all the events we analyzed, which was the result of the radio absorption associated with a significant increase in electron density within the D-region ionosphere. In addition, we discovered positive Doppler shifts just before the sudden fade-out of echoes. The Doppler shift is negatively correlated with the elevation angle of received radar waves. It indicates that variation of electron density in the D-region ionosphere is dominant during solar flare events. This result is consistent with a previous study. We also compared the irradiation by X-ray and extreme ultra violet rays observed by the GOES-14 and GOES-15 satellites, which generated Doppler shifts. A positive Doppler shift is consistent with a change of X-ray flux.

A New Method to Correlate a Possible Coupling between the Upper and the Lower Atmosphere

Problem statement: Sun-earth coupling studies are one of the methods of projection to explain certain physical mechanisms of how solar activity can influence weather/climate changes. Approach: To gain insight of the above challenge, analysis of propagation delays in GPS-transmitted signals in the upper and lower atmosphere during transient events associated with intense geomagnetic storms was conducted within the Antarctic polar cap. Results: An increase in Total Electron Content (TEC) indicates an increase in the density of energetic particles in the atmosphere and major changes in the signals would affect the water vapor phase transition in the troposphere. Conclusion/Recommendations: From the correlation between the ionospheric TEC and atmospheric Precipitable Water Vapor (PWV) variations, the Global Electric Circuit (GEC) is identified as a possible physical mechanism for the coupling process.

The Low‐Latitude Ionosphere Sensor Network: Initial results

The Low‐Latitude Ionospheric Sensor Network (LISN) is a distributed observatory designed to nowcast the state and dynamics of the low‐latitude ionosphere and to develop forecasts of the electric fields, densities, and equatorial spread F over the South American continent. The LISN observatory consists of three different types of instruments: GPS receivers, fluxgate magnetometers, and vertical incidence pulsed ionospheric radar (VIPIR) ionosondes. This report provides a succinct summary of recent observations obtained using the LISN GPS receivers and complemented with measurements from other instruments and GPS receivers that operate in South America. More specifically, the following are shown here: (1) observations of total electron content (TEC) enhancements that occur near local midnight, (2) maps of TEC perturbations associated with the passage of traveling ionospheric disturbances over South America, and (3) statistics of TEC depletions for 2 years of low solar activity. Near‐midnight TEC enhancements consist of sudden increases in TEC that occur after sunset at low latitudes on 30% of the days. These TEC enhancements last for several hours and can have amplitudes between 1 and 50 TEC units. On 11–12 March 2011 the largest TEC enhancement was observed in South America at times when the Jicamarca incoherent scatter radar operated and observed peak densities above 106 el/cc at 300 km altitude. It is suggested that a combination of zonal electric fields and meridional neutral winds are able to redistribute the plasma along the field lines and create regions of enhanced TEC. Maps of TEC perturbations associated with the passage of gravity waves (GWs) over South America have been used to measure the phase velocity and direction of propagation of GWs. The large number of GPS receivers over South America has allowed us to record bubble events for every day during 2008 and 2009. It was found that the number of TEC depletion detections varies with a periodicity of 28 days. It is mentioned how these new observations and the installation of the last four VIPIR ionosondes will lead to new discoveries in the near future.

Ionospheric anomalies observed over South Korea preceding the Great Tohoku earthquake of 2011

We investigated the ionospheric anomalies observed before the Tohoku earthquake, which occurred near the northeast coast of Honshu, Japan on 11 March, 2011. Based on data from a ground-based Global Positioning System (GPS) network on the Korean Peninsula, ionospheric anomalies were detected in the total electron content (TEC) during the daytime a few days before earthquake. Ionospheric TEC anomalies appeared on 5, 8 and 11 March. In particular, the ionospheric disturbances on 8 March evidenced a remarkable increase in TEC. The GPS TEC variation associated with the Tohoku earthquake was an increase of approximately 20 total electron content units (TECU), observed simultaneously in local and global TEC measurements. To investigate these pre-earthquake ionospheric anomalies, space weather conditions such as the solar activity index (F10.7) and geomagnetic activity indices (the Kp and Dst indices) were examined. We also created two-dimensional TEC maps to visual the spatial variations in the ionospheric anomalies preceding the earthquake.

Studying characteristics of the large-scale ionospheric irregularities induced by high-power HF radio emission with GPS diagnosis

UDC 533.9 We present the results of experimental studies of artificial large-scale irregularities of the ionospheric electron number density with dual-frequency GPS diagnosis. The total electron content was analyzed in the GPS signal trajectory when the satellites passed over the heated region. Spectral composition of the observed variations was determined by wavelet analysis. Characteristic scales of artificial irregularities of the electron number density in the F layer are estimated. It is experimentally proved that the irregularities remain for at least 15–20 min after the transmitter is switched off. A more confident excitation and increased intensity of the irregularities were also confirmed when the beam was inclined south of the magnetic-zenith direction. When the ionosphere is affected by high-power HF radio emission, electron-density irregularities with sizes from tens of centimeters to tens of kilometers are formed in the region of its reflection [1]. Diagnosis of the large-scale (more than 0.5 km) structure of the ionospheric region disturbed by a high-power radio wave was based on raying the ionosphere by decametric radio emission from space radio sources, aspect scattering of HF and UHF radio waves, raying the heated region by UHF signals of low-orbit and geostationary satellites, satellite radio tomography, and electron density in-situ measurements during the flyby of spacecraft and geophysical rockets through and over the heated region (see [1, 2] and the references cited therein). Large-scale irregularities with scales 5–50 km can also be effectively studied using dual-frequency raying by signals of the Navstar (GPS) or GLONASS microwave satellite systems. During propagation through the heated region, such signals acquire an additional phase increment stipulated by the dispersion of radio waves in the ionospheric plasma and linearly related to the total electron content (TEC) on the propagation trajectory [3, 4]. Since the early eighties, this method has been used to determine the TEC in the ionosphere under natural conditions, and since 2007 the Navstar satellites have intensely been used to determine the total electron content in the ionosphere during the experiments on ionosphere modification by high-power HF radio emission. Such studies were based on the HAARP [5] and Sura [2, 6] facilities. A TEC increase correlated with the ionospheric heating was observed in the daytime experiments [2] and [5]. In [6], where the observations were performed mainly after the sunset with the beam of the Sura facility inclined 12 ◦ south in the geomagnetic meridian plane, both a notable TEC increase in the heated region determined by the angular sizes of the antenna pattern and long-period (10–15 min) TEC variations relative to the average value were recorded. On the basis of obtained results, the authors of [6] made a statement about the manifestation of the magnetic-zenith effect as the most effective impact on the ionosphere in the direction along the geomagnetic field.

Role of solar X-ray flares and CME in the E region ionosphere of Mars: MGS observations

The relationship of the solar flare and the Coronal Mass Ejection (CME) with the ionosphere is a key problem in all planetary atmospheres. Earth is the only planet where ground based observations can be used to study this relationship on a continuous basis. In this paper we have used radio occultation data obtained from Mars Global Surveyor (MGS) for the period of 12–18 May, 2005 to address the responses of X-ray flare and CME in Total Electron Content (TEC) of E region ionosphere of Mars during a violent solar event that occurred on 13th May. It was found that the TEC increased by factors of 6 in the E region ionosphere of Mars during this flare. The response of mass ejections from the Sun's corona is registered in the E region ionosphere of Mars after ∼38h. Our results suggest that TEC can increase by factor of ∼3.5–4.5 during the arrival of CME on Mars. This increase in TEC can be attributed to ionization by energetic particles associated with magnetic storms. We report that shocked waves driven by CME compressed the magnetosheath of Mars by 15km during the magnetic storms.

An ionospheric precursor to the Tohoku earthquake

Most scientists believe that earthquakes are inherently unpredictable, and reports of various kinds of earthquake precursor signals have been difficult to verify. However, in a new study, Heki reports a possible ionospheric precursor to the devastating 11 March 2011 magnitude 9 Tohoku earthquake in Japan. Analyzing data from the Japanese GPS network, he detected an increase in the total electron content (TEC) in the ionosphere above the focal region of the earthquake beginning about 40 minutes before the quake. The TEC enhancement reached about 8% above the background electron content. The increase in TEC was greatest above the earthquake epicenter and diminished with distance from the epicenter. The author also analyzed GPS records from previous earthquakes and found that similar ionospheric anomalies occurred before the 2010 Mw 8.8 Chile earthquake and possibly the 2004 Sumatra Mw 9.2 earthquake and the 1994 Mw 8.3 Hokkaido earthquake, but TEC enhancements were not seen before smaller earthquakes.

Meteor impact perturbation in the lower ionosphere of Mars: MGS observations

We have analyzed 1500 electron density profiles obtained from radio occultation experiment onboard Mars Global Surveyor to study the physical characteristics of meteoric plasma layers over Mars during the months of January–June, 2005. It is found that 65 electron density profiles were strongly perturbed with peak densities ∼0.5–1.4×1010m−3 at altitude between 80km and 105km probably due to ablation of meteoroids and subsequent ionization of meteoric atoms. These profiles are used to estimate Total Electron Content (TEC) in the lower atmosphere of Mars. Our analysis suggests that meteoroids ablation caused enhancements in TEC by a factor of ∼1.5–3.0. We also report that maximum TEC were observed on 21 January and 23 May, 2005, when comets 2007 PL42 and 4015 Wilson-Harrington intersected the orbit of Mars from a close distance 1.49AU and 1.17AU, respectively. TEC were increased by a factor of ∼5–7 on these days. This is a significant increase in TEC, which might be associated with the meteor showers that were produced when Mars crossed the dust stream left along the orbit of these comets.

Nighttime enhancements in TEC near the crest of northern equatorial ionization anomaly during low solar activity period

Enhancement in nighttime total electron content (TEC) near the crest of equatorial ionization anomaly at Bhopal (Geog. 23.2°N, 77.4°E, and MLAT 14.2°N) has been studied for the solar minimum period March 2005- November 2006. TEC data recorded by GPS Ionospheric Scintillation and TEC Monitor (GISTM) GSV4004A receiver is used for the study and results are presented in the paper. More than 10% increase in TEC with respect to background content is considered for analysis. Out of total 138 enhancements, 65 observed during pre midnight hours and 73 during post midnight hours. It is observed that nighttime enhancement in TEC at Bhopal occurs in all seasons; it is more frequent during summer, less during equinox and least during winter months. The enhancement in nighttime TEC can be observed both in geomagnetic disturbed time and in quiet time. We found that mean peak amplitude for pre-midnight TEC enhancement are more in equinox and less in winter, while in post-midnight TEC enhancement it is highest in summer and less in winter. Post-midnight enhancements have smaller peak amplitude as compared to pre-midnight. Also the most probable values for pre- and post-midnights are 4.4 TECU and 2.17 TECU respectively. The percentage occurrence of nighttime TEC enhancement does not show any dependence on solar activity whereas the peak amplitude depends on solar activity. The percentage occurrence of nighttime TEC enhancement decreases as the magnetic activity increases whereas there is no such dependency found with peak amplitude. It is also observed that majority of nighttime TEC enhancements are occurred without scintillation. The localised nighttime TEC enhancements near the crest of equatorial ionization anomaly region have been observed most of the nights during the period of study.

GPS total electron content and surface latent heat flux variations before the 11 March 2011 M9.0 Sendai earthquake

The present paper describes the variations of the GPS total electron content (TEC) from the International GNSS service network and surface latent heat flux (SLHF) from the Scientific Computing Division of the National Center for Atmospheric Research (NCAR) before the 11 March 2011 M9.0 Sendai earthquake, respectively. The analysis shows pronounced enhancements in the GPS TEC and SLHF a few days prior to the earthquake event. The maximum increase in the GPS TEC was about 30 TECu with an extended spatial distribution on a geomagnetically quiet day (Dst ⩾ −20 nT, between two moderate geomagnetic storms), 8 March, 3 days prior to the earthquake. This giant positive disturbance was possibly associated with the impending disastrous earthquake and contributed from the enhanced solar radiation. Moreover, there were several anomalous regions of SLHF on the global map, but an area of enhanced SLHF very close to the epicenter. The purpose of this paper is to report the existence of the changes in surface and ionosphere, and show the potential application of multi-source data to identify seismic precursors.

Monitoring the ionospheric total electron content variations over the Korean Peninsula using a GPS network during geomagnetic storms

We have established a regional ionospheric model (RIM) for investigating changes in the total electron content (TEC) over South Korea using 38 Korean GPS reference stations. The inverse distance weighted (IDW) interpolation method was applied to create a two-dimensional ionospheric map of vertical TEC units (TECU) based on a grid. To examine the diurnal patterns of ionospheric TEC over South Korea, we first processed the GPS data from a geomagnetically quiet period of 10 days. In a second step, we compared the estimated GPS-TEC variations with the changes in geomagnetic activity indices (the Kp and Dst indices) and the auroral electrojet index (AE) as a function of universal time (UT) on 4 and 20 November, 2003. The GPS-TEC responses for those storm events were proportional to the geomagnetic activity at this mid-latitude location. The sudden increases in ionospheric TEC (SITEC) caused by the geomagnetic storms were detected. The variations in GPS-TEC may help reveal the processes of ionospheric disturbances caused by geomagnetic storms.

Ionospheric disturbances triggered by the 11 March 2011M9.0 Tohoku earthquake

[1] An earthquake of magnitude 9.0 occurred near the east coast of Honshu (Tohoku area), Japan, producing overwhelming Earth surface motions and inducing devastating tsunamis, which then traveled into the ionosphere and significantly disturbed the electron density within it (hereafter referred to as seismotraveling ionospheric disturbances (STIDs)). The total electron content (TEC) derived from nationwide GPS receiving networks in Japan and Taiwan is employed to monitor STIDs triggered by seismic and tsunami waves of the Tohoku earthquake. The STIDs first appear as a disk-shaped TEC increase about 7 min after the earthquake occurrence centered at about 200 km east of the epicenter, near the west edge of the Japan Trench. Fast propagating disturbances related to Rayleigh waves quickly travel away from the epicenter along the main island of Japan with a speed of 2.3–3.3 km/s, accompanied by sequences of concentric circular TEC wavefronts and followed by circular ripples (close to a tsunami speed of about 720–800 km/h) that travel away from the STID center. These are the most remarkable STIDs ever observed where signatures of Rayleigh waves, tsunami waves, etc., simultaneously appear in the ionosphere.

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.

GPS Ionospheric Total Electron Content and Scintillation Measurements during the October 2003 Magnetic Storm

Problem Statement: Ionospheric scintillations, cause significant effects on satellite signals for communication and navigation in equatorial region and polar regions mainly during sever magnetic storms periods. This phenomenon is not fully understood due to few studies performed. The study investigates variability of Total Electron Content (TEC) and ionospheric scintillation during October 2003 magnetic storm over Antarctica using ground based GPS technique. Approach: The TEC/scintillation measuring system at Scott Base station, consists of Trimble TS5700 24-channel (a high-precision dual-frequency GPS receiver), a Trimble Zephyr Geodetic antenna and a notebook computer for data logging. The absolute GPS TEC was calculated from differential phase advance GPS observables (1-L2). The GPS signal-to-noise ratios (C/No) and 1/L2 carrier frequencies were employed to determine the scintillation index S4 every 60 s, amplitude scintillation (in dB-Hz) and phase scintillation. Results: The GPS measurements during storm periods at Scott Base show pronounced phase and amplitude scintillation activities, sudden increase in TEC followed by trough-like figure depletions. The maximum value of phase scintillation during the main phase of third episode was 8.3 times the value during Sudden Storm Commencement (SSC) period. Measured amplitude scintillation and S4 index on both 1 and L2 signals are >15dB-Hz and >0.4dB-Hz respectively. Conclusion/Recommendation: The timing and intensity of TEC and scintillation measurements during the storm event were are in a good agreement with WDC measurements. For this particular event, the duration of enhanced periods were approximately 12 h while periods of TEC depletions were more than 30 h. This value implies better understanding of the polar ionospheric response to magnetic storm and eases efforts for better space weather prediction in this region.

Global GPS Ionospheric Modelling Using Spherical Harmonic Expansion Approach

In this study, we developed a global ionosphere model based on measurements from a worldwide network of global positioning system (GPS). The total number of the international GPS reference stations for development of ionospheric model is about 100 and the spherical harmonic expansion approach as a mathematical method was used. In order to produce the ionospheric total electron content (TEC) based on grid form, we defined spatial resolution of 2.0 degree and 5.0 degree in latitude and longitude, respectively. Two-dimensional TEC maps were constructed within the interval of one hour, and have a high temporal resolution compared to global ionosphere maps which are produced by several analysis centers. As a result, we could detect the sudden increase of TEC by processing GPS observables on 29 October, 2003 when the massive solar flare took place.

Total electron content in the Mars ionosphere: Temporal studies and dependence on solar EUV flux

Total electron content (TEC) derived from radar signal distortions is a useful tool in probing the ionosphere of Mars. We consider 26 months of data from the subsurface mode of the Mars Express MARSIS instrument and confirm that the TEC dependence on solar zenith angle (SZA) approximately matches Chapman theory. After detrending this dependence, we find no clear trend with Martian season or dust activity but find that disturbed solar and space weather conditions can produce prolonged higher TEC values and that isolated solar energetic particle events are coincident with short‐lived increases in TEC of ∼1015 m−2 at all SZAs. We present the first comparison between TEC and directly measured solar EUV flux in the 30.4 nm He‐II line. We find that the relationship between TEC and both He‐II line irradiance and F10.7 solar radio flux (a long‐used EUV proxy) can be expressed as power laws with exponents of 0.54 and 0.44, respectively, in approximate agreement with Chapman theory.