Showing papers on "Total electron content published in 2013"

The influence of atmospheric tide and planetary wave variability during sudden stratosphere warmings on the low latitude ionosphere

Abstract: [1] Numerical simulations are performed for a sudden stratosphere warming (SSW) under different atmospheric tide and planetary wave forcing conditions to investigate the tidal variability in the mesosphere and lower thermosphere (MLT). The influence of variability of different tides in the MLT on generating perturbations to the low latitude ionosphere is also investigated. Significant changes are found to occur in the migrating semidiurnal solar (SW2) and lunar (M2) tides as well as in the westward propagating nonmigrating semidiurnal tide with zonal wave number 1 (SW1). The changes in the zonal mean atmosphere that occur during SSWs lead to an enhancement in the SW2 and M2 tides. The vertical wavelength of the SW2 is also changed, resulting in phase variability in the SW2 at a constant altitude. Significant enhancements in the SW1 are found to occur only in the presence of additional planetary wave forcing, and this demonstrates that nonlinear planetary wave‒tide interactions lead to the enhanced SW1 during SSWs. The amplitude and phase variability of the SW2 is found to be capable of producing temporal variability in the vertical plasma drift velocity that is similar to the observed variability. Changes in the M2 during SSWs can contribute up to an additional ∼30% of the total ionosphere variability; however, the overall influence of the lunar tide is found to be dependent upon the phase of the moon relative to the timing of the SSW. Although the influence is relatively minor, the SW1 also contributes to the low latitude ionosphere variability during SSWs. The simulation results for the vertical plasma drift velocity and total electron content (TEC) further illustrate that significant longitude variability occurs in the ionosphere response to SSWs.

Ionospheric Prediction and Forecasting

Abstract: Ionospheric Structure.- Ionospheric measurements for prediction and forecasting. Ionospheric prediction for radio propagation purposes.- Total electron content modelling and mapping.- Ionospheric forecasting.- Prediction and forecasting user manual.

Ionospheric response to earthquakes of different magnitudes: Larger quakes perturb the ionosphere stronger and longer

Abstract: Recently, it has been shown that the ionosphere is capable of showing images of seismic fault shortly after an earthquake. This gives rise to the idea of retrieval of seismic information from ionospheric observations. As the first step toward such inversion, here we study distinctive features of ionospheric response to shallow earthquakes, both submarine and inland, of moment magnitudes Mw7.2–9.1. Using GPS measurements of the ionospheric total electron content, we show that: (1) the amplitude of coseismic total electron content variations in the near-field is larger after more powerful earthquakes, and (2) stronger earthquakes (M > 7.9) are in general characterized by a longer negative phase in coseismic perturbations.

Ionospheric Effects of Sudden Stratospheric Warming During Moderate-to-High Solar Activity: Case Study of January 2013

Abstract: [1] A major sudden stratospheric warming (SSW) occurred in January 2013 during moderate-to-high solar activity conditions. Observations during the winter of 2012/2013 reveal strong ionospheric disturbances associated with this event. Anomalous variations in vertical ion drift measured at the geomagnetic equator at Jicamarca (12°S, 77°W) are observed for over 40 days. We report strong perturbations in the total electron content (TEC) that maximize in the crests of equatorial ionization anomaly, reach 100% of the background value, exhibit significant longitudinal and hemispheric asymmetry, and last for over 40 days. The magnitude of ionospheric anomalies in both vertical drifts and TEC is comparable to the anomalies observed during the record-strong SSW of January 2009 that coincided with the extreme solar minimum. This observation contrasts with results of numerical simulations that predict weaker ionospheric response to the tidal forcing during high solar activity.

Online, automatic, near‐real time estimation of GPS‐TEC: IONOLAB‐TEC

Abstract: [1] The variability of space weather can best be captured using total electron content (TEC), which corresponds to total number of electrons on a ray path. The dual-frequency ground based GPS receivers provide a cost-effective means for monitoring TEC. Computation of TEC for a single GPS station is a challenge due to various unknowns and ambiguities such as inter-frequency receiver bias and satellite bias, choice of mapping function, and peak height of ionosphere for ionospheric piercing point. In this study, IONOLAB group introduces a robust, automatic, online computation routine near-real time TEC, IONOLAB-TEC, for IGS and/or EUREF stations from www.ionolab.org. The user can choose online one station or multiple stations, date or dates for the computation. The IONOLAB-TEC values can be compared with TEC estimates from IGS analysis centers. The output can be obtained either in graphical form, or IONOLAB-TEC estimates can be provided in an excel file. The service is easy to use with a graphical user interface. This unique and original space weather application is provided online, and IONOLAB-TEC estimates are downloaded automatically to the user defined directories under user defined filenames.

Comparative analysis of spread-F signature and GPS scintillation occurrences at Tucumán, Argentina

Abstract: [1] We analyze data recorded from October 2010 to September 2011, during the ascending phase of the 24th solar cycle, from an Advanced Ionospheric Sounder-Istituto Nazionale di Geofisica e Vulcanologia ionosonde and a GPS Ionospheric Scintillation and total electron content (TEC) monitor scintillation receiver, colocated at low latitude in the Southern American longitudinal sector (Tucuman, 26.9°S, 294.6°E, magnetic latitude 15.5°S, Argentina). The site offers the opportunity to perform spread-F and GPS scintillation statistics of occurrence under the southern crest of the equatorial ionospheric anomaly. Spread-F signatures, classified into four types (strong range spread-F (SSF), range spread-F, frequency spread-F (FSF), and mixed spread-F), the phase and amplitude scintillation index (σΦ and S4, respectively), the TEC, and the rate of TEC parameter, marker of the TEC gradients, that can cause scintillations, are considered. The seasonal behavior results as follows: the occurrence of all four types of spread-F is higher in summer and lower in winter, while the occurrence of scintillations peaks at equinoxes in the postsunset sector and shows a minimum in winter. The correspondence between SSF and scintillations seems to be systematic, and a possible correlation between S4 and FSF peaks is envisaged at the terminator. The investigation focused also on two particular periods, from 12 to 16 March 2011 and from 23 to 29 September 2011, both characterized by the simultaneous presence of SSF signatures and scintillation phenomena, allowing to discuss the role of traveling ionospheric disturbances as a strong candidate causing ionospheric irregularities.

Solar activity impact on the Earth’s upper atmosphere

Abstract: The paper describes results of the studies devoted to the solar activity impact on the Earth’s upper atmosphere and ionosphere, conducted within the frame of COST ES0803 Action.Aim : The aim of the paper is to represent results coming from different research groups in a unified form, aligning their specific topics into the general context of the subject.Methods : The methods used in the paper are based on data-driven analysis. Specific databases are used for spectrum analysis, empirical modeling, electron density profile reconstruction, and forecasting techniques.Results: Results are grouped in three sections: Medium- and long-term ionospheric response to the changes in solar and geomagnetic activity, storm-time ionospheric response to the solar and geomagnetic forcing, and modeling and forecasting techniques.Section 1 contains five subsections with results on 27-day response of low-latitude ionosphere to solar extreme-ultraviolet (EUV) radiation, response to the recurrent geomagnetic storms, long-term trends in the upper atmosphere, latitudinal dependence of total electron content on EUV changes, and statistical analysis of ionospheric behavior during prolonged period of solar activity.Section 2 contains a study of ionospheric variations induced by recurrent CIR-driven storm, a case-study of polar cap absorption due to an intense CME, and a statistical study of geographic distribution of so-called E-layer dominated ionosphere.Section 3 comprises empirical models for describing and forecasting TEC, the F-layer critical frequency foF 2, and the height of maximum plasma density. A study evaluates the usefulness of effective sunspot number in specifying the ionosphere state. An original method is presented, which retrieves the basic thermospheric parameters from ionospheric sounding data.

Accuracy of GPS total electron content: GPS receiver bias temperature dependence

Abstract: [1] Having an accurate method to estimate and remove ionospheric effects is a major issue for low-frequency radio astronomy arrays, as the ionosphere is one of their largest error terms. One way to estimate the ionosphere is to measure total electron content (TEC) using dual frequency global positioning system (GPS) signals. This technique uses the dispersive nature of the ionosphere, as both group and phase velocities are (to first order) dependent on the inverse square of the frequency and on TEC. Using these properties, TEC can be measured to a high degree of accuracy by computing the delay difference between signals at GPS's two frequencies (L1 = 1575.42 and L2 = 1227.6 MHz). Unfortunately, effects other than ionospheric dispersion also introduce differential delay differences. These additional differences, called biases, can be separated into those introduced by the satellite and those by the receiver. Receiver biases show the most significant variations, sometimes over intervals of hours. Changing temperature conditions at the receiver antenna, along the cable, or in the internal receiver hardware are thought to be responsible for some of these variations. We report here on an investigation of the temperature dependence of the GPS receiver bias. Our results show that for our particular receiver, antenna, and cable set-up, a temperature-dependent bias is clearly evident, and that this temperature dependence varies from receiver to receiver. When the receiver bias temperature dependence is removed, a noise level of 1–3 TEC units still remains in the bias estimation.

Characteristics of global plasmaspheric TEC in comparison with the ionosphere simultaneously observed by Jason-1 satellite

Abstract: [1] We compared the global plasmaspheric total electron content (pTEC) with the ionospheric TEC (iTEC) simultaneously measured by Jason-1 satellite during the declining phase of solar cycle 23 (2002–2009) to investigate the global morphology of the plasmaspheric density in relation to the ionosphere. Our study showed that the plasmaspheric density structures fundamentally follow the ionosphere, but there are also significant differences between them. Although the diurnal variations are very similar to each region, the plasmasphere shows much weaker variations, only approximately 1 TECU day-night difference. By analyzing the day-night differences in the plasmasphere, we found that the plasmaspheric contribution to the nighttime ionosphere does not increase with solar activity and the largest contribution occurs during June solstice. The plasmasphere shows similar seasonal variations to the ionosphere, except for the semiannual variation, which is essentially absent in the plasmasphere. There is also an important difference in the annual variation: although the annual variation in the ionosphere exists regardless of longitude, it occurs only at American sector in the plasmasphere. As solar activity increases to moderate level, the pTEC substantially enhances from approximately 2 to 4 TECU at the initial increase of solar activity below F10.7p = 100 and then quickly slows down while the iTEC almost linearly enhances. Although it is well known that magnetic storms are the major source of plasmaspheric density depletion, pTEC does not show this aspect of the plasmasphere probably due to the relatively small Kp values for high magnetic activity (Kp > 2.5) in the current study.

Is an ionospheric electron enhancement preceding the 2011 Tohoku-Oki earthquake a precursor?

Abstract: [1] Heki [2011] reported that the TEC gradually enhanced from 40 minutes before the 2011 M9.0 off the Pacific coast of Tohoku earthquake (Tohoku EQ) to the time when the co-seismic acoustic wave reached the ionosphere and the TEC immediately recovered at the normal state. This paper shows an alternative interpretation of total electron content (TEC) variation in the ionosphere associated with the Tohoku EQ. Our interpretation is that a tsunamigenic ionospheric hole, a wide depletion of the TEC, occurred after the co-seismic acoustic wave reached the ionosphere and gradually recovered at the normal state with several tens of minutes [Kakinami et al., 2012]. The difference between Heki [2011] and Kakinami et al. [2012] is attributed to the reference curves of the TEC to extract the ionospheric variations. The former is given by the least-squares fitting curve of the EQ day data excluding an expected precursor period, while the latter is given by the data of the similar orbit of global positioning system (GPS) satellite on another day. The results suggest that variation of slant TEC is explained by the depletion of TEC due to tsunami rather than the precursory enhancement.

Stationary planetary wave and nonmigrating tidal signatures in ionospheric wave 3 and wave 4 variations in 2007-2011 FORMOSAT-3/COSMIC observations

Abstract: [1] The wave 3 and wave 4 modulations of the Equatorial Ionization Anomalies are a robust feature of the low-latitude ionosphere, when viewed at constant local time. Although initially associated, respectively, with DE2 and DE3, nonmigrating diurnal tides in the mesosphere and lower thermosphere region, recent results have suggested that the wave 3 and wave 4 may also have significant contributions from other tidal and stationary planetary wave (SPW) signatures. We present observations of total electron content (TEC) variations associated with tidal and SPW signatures comprising the ionospheric wave 3 and wave 4 structures from FORMOSAT-3/COSMIC from 2007 to 2011. We find that the wave 3 (wave 4) feature is comprised predominately by DE2 (DE3) and SPW3 (SPW4) signatures in TEC throughout all 5 years, with contributions from SE1 (SE2) being less significant. The wave 3 component also has recurring contributions from DW4 during December/January. The absolute amplitudes of all the aforementioned tidal and SPW signatures are directly related to the level of solar activity and the semiannual variation in zonal mean TEC. After normalizing by the zonal mean, the relative amplitudes of the wave 4 signatures are inversely related to solar activity through 2010, which is not seen with the wave 3-related signatures. The seasonal variation and phases of the main constituents of wave 3 and wave 4 are consistent from year to year, as evidenced by the interannual recurrence in the peak and trough locations of wave 3 and wave 4.

System and methods for risk prediction and assessment

Abstract: A system and methods for predicting and assessing risk before an event occurs More particularly, the present invention predicts and assesses risk such as the occurrence of an earthquake prior to the event by collecting and analyzing changes in Total Electron Content (“TEC”) in the ionosphere

Variability of ionospheric TEC during solar and geomagnetic minima (2008 and 2009): external high speed stream drivers

Abstract: . We study solar wind–ionosphere coupling through the late declining phase/solar minimum and geomagnetic minimum phases during the last solar cycle (SC23) – 2008 and 2009. This interval was characterized by sequences of high-speed solar wind streams (HSSs). The concomitant geomagnetic response was moderate geomagnetic storms and high-intensity, long-duration continuous auroral activity (HILDCAA) events. The JPL Global Ionospheric Map (GIM) software and the GPS total electron content (TEC) database were used to calculate the vertical TEC (VTEC) and estimate daily averaged values in separate latitude and local time ranges. Our results show distinct low- and mid-latitude VTEC responses to HSSs during this interval, with the low-latitude daytime daily averaged values increasing by up to 33 TECU (annual average of ~20 TECU) near local noon (12:00 to 14:00 LT) in 2008. In 2009 during the minimum geomagnetic activity (MGA) interval, the response to HSSs was a maximum of ~30 TECU increases with a slightly lower average value than in 2008. There was a weak nighttime ionospheric response to the HSSs. A well-studied solar cycle declining phase interval, 10–22 October 2003, was analyzed for comparative purposes, with daytime low-latitude VTEC peak values of up to ~58 TECU (event average of ~55 TECU). The ionospheric VTEC changes during 2008–2009 were similar but ~60% less intense on average. There is an evidence of correlations of filtered daily averaged VTEC data with Ap index and solar wind speed. We use the infrared NO and CO2 emission data obtained with SABER on TIMED as a proxy for the radiation balance of the thermosphere. It is shown that infrared emissions increase during HSS events possibly due to increased energy input into the auroral region associated with HILDCAAs. The 2008–2009 HSS intervals were ~85% less intense than the 2003 early declining phase event, with annual averages of daily infrared NO emission power of ~ 3.3 × 1010 W and 2.7 × 1010 W in 2008 and 2009, respectively. The roles of disturbance dynamos caused by high-latitude winds (due to particle precipitation and Joule heating in the auroral zones) and of prompt penetrating electric fields (PPEFs) in the solar wind–ionosphere coupling during these intervals are discussed. A correlation between geoeffective interplanetary electric field components and HSS intervals is shown. Both PPEF and disturbance dynamo mechanisms could play important roles in solar wind–ionosphere coupling during prolonged (up to days) external driving within HILDCAA intervals.

Global TEC maps based on GNSS data: 1. Empirical background TEC model

Abstract: [1] A global background total electron content (TEC) model is built by using the Center for Orbit Determination of Europe (CODE) TEC data for full 13 years, 1999–2011 It describes the climatological behavior of the ionosphere under both its primary external driver, ie, the direct photo-ionization by incident solar radiation, and regular wave particularly tidal forcing from the lower atmosphere The model construction is based on the very different time scales of the solar cycle, seasonal, and diurnal TEC variabilities (at least an order of magnitude); this leads to modulations of shorter-period variabilities with periods of the longer ones Then the TEC spatial-temporal variability is presented as a multiplication of three separable functions The solar activity is described by both parameters: F107 and its linear rate of change KF while the seasonal variability is presented by sine functions including four subharmonics of the year The diurnal variability of the TEC model is described by 2D (longitude-time) sine functions with zonal wave numbers up to 4 and 4 subharmonics of the solar day The model offers TEC maps which depend on geographic coordinates (5°×5° in latitude and longitude) and UT at given solar activity and day of the year The presented background model fits to the CODE TEC input data with a zero systematic error and an RMS error of 3387 TECU It is able to reproduce the well-known ionospheric structures as Weddell Sea Anomaly and some longitudinal wave-like structures

Statistical analysis of ionospheric responses to solar flares in the solar cycle 23

Abstract: [1] In this study, we studied the ionospheric responses to solar flares during 1999–2006 by using GOES 0.1–0.8 nm X-ray, 26–34 nm EUV, and GPS/total electron content (TEC). The solar zenith angle (SZA) dependence was quantitatively investigated by analyzing global TEC enhancements during about 100 X-class flares. The mean ratio of ΔTEC at SZA = 90° to ΔTEC at SZA = 0° is about 0.39. The statistical results show that a limb flare has less effect on the ionosphere than a central flare does because the main ionization source of the ionosphere, solar EUV radiation, can be absorbed by thick solar gas due to large central meridian distance (CMD), which is called the CMD effect. Furthermore, the CMD effect decreases with decreasing flare X-ray class. The results show that TEC responses are not highly related to solar X-ray flux enhancement with correlation coefficient of 0.6, but more closely related to solar EUV flux enhancement with correlation coefficient of 0.91 for 26–34 nm EUV. The combination of X-ray flux and flare location is also a good indicator for TEC response: The correlation coefficient of ∆X-ray*cos(CMD) and ∆TEC is as high as 0.95. The seasonal dependence in TEC response is also investigated. There are larger responses in equinoxes than in solstices. The seasonal variation of neutral density is considered to be a main cause of the season dependence of TEC response.

Two‐dimensional simulation of ionospheric variations in the vicinity of the epicenter of the Tohoku‐oki earthquake on 11 March 2011

Abstract: [1] Unusual ionospheric variations were observed in the M9.0 Tohoku-oki earthquake on 11 March 2011. Among various kinds of features in the ionosphere, significant depletion of total electron content (TEC) near the epicenter was observed after the earthquake. Although previous studies have suggested that the coseismic ionospheric variations are associated with atmospheric perturbation caused by vertical displacement of the sea surface, the mechanism of the TEC depletion has not been fully understood. In this paper, a two-dimensional nonlinear nonhydrostatic compressible atmosphere-ionosphere model is employed to investigate the ionospheric variations in the vicinity of the epicenter. The simulation results reveal that an impulsive pressure pulse produced by a sudden uplift of the sea surface leads to local atmospheric expansion in the thermosphere and that the expansion of the thermosphere combined with the effect of inclined magnetic field lines in the ionosphere causes the sudden TEC depletion above the epicenter region.

Variation in total electron content above large thunderstorms

Abstract: [1] Total electron content (TEC) measured by Global Positioning System (GPS) receivers in the United States Great Plains is examined for three nights with large thunderstorms and for one night with little thunderstorm activity. The GPS TEC data are fit with a polynomial, and the variations are estimated by subtracting this fit from the data. We found that anomalous TEC variations are closely associated in time and space to the large underlying thunderstorms. The largest storm-related TEC variation is observed to be ~1.4 total electron content unit (TECU) over a typical nighttime background value of several TECUs. The variations near the storm appear to have more high-frequency content than those away from the storm, with periods of minutes to tens of minutes. No detectable localized TEC variation is observed for the thunderstorm-quiet night.

Seasonal and local time variation of ionospheric migrating tides in 2007-2011 FORMOSAT-3/COSMIC and TIE-GCM total electron content

Abstract: [1] This study examines the seasonal and interannual variation of the major migrating tidal components in midlatitude to low-latitude total electron content (TEC) observations from the FORMOSAT-3/COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) satellite constellation from 2007 to 2011. Although the absolute amplitudes of the TEC zonal mean and migrating tidal components show a strong positive relation to the increasing and decreasing phases of the solar cycle, the relative tidal amplitudes following normalization by maximum background values show a more varied response to solar activity levels. Features of ionospheric local time variation produced by individual migrating tidal components are consistent from year to year, with DW1 forming the equatorial daytime peak in TEC, SW2 corresponding to the generation of the equatorial ionization anomaly (EIA) crests, and TW3 contributing to the TEC trough between the EIA crests. Numerical experiments using Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM) are also performed to determine the sensitivity of the ionospheric migrating tides to upward propagating migrating tidal components from the neutral mesosphere and lower thermosphere (MLT). Zonal mean TECs decrease when MLT tidal forcing is applied and are particularly sensitive to the MLT DW1. Most of the ionospheric SW2 response is attributable to MLT SW2 forcing, enhancing the EIA crests by amplifying the equatorial fountain. TW3 in the model is generated through both in situ photoionization and nonlinear interaction between DW1 and SW2.

Wavenumber broadening of the quasi 2 day planetary wave in the ionosphere

Abstract: [1] Using the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model, we investigate the observed zonal wavenumber broadening phenomena in the ionospheric quasi 2 day oscillation (QTDO) that is associated with westward zonal wavenumber 3 (W3) quasi 2 day wave (QTDW) perturbations in the mesosphere and lower thermosphere (MLT). We aim to explain why the observed longitudinal structures of the QTDOs in the ionosphere are different from those of the QTDWs in the MLT. We find that large QTDOs in the ionosphere with zonal wavenumbers other than W3 occur in the model run with the true magnetic field, but not in the model run with an aligned dipole field. These numerical experiments suggest that the occurrence of the additional zonal wavenumbers in ionospheric QTDOs is related to the longitudinal variations of the Earth's magnetic field configuration, strength, and dip angle, which have distinct stationary zonal wavenumbers. We also find that when the specified W3 QTDW winds drive ionospheric plasma motion in the magnetic field, the resultant QTDOs in ionospheric parameters, such as the dynamo electric field, ion vertical drifts, plasma densities, and total electron content, have more complicated longitudinal variations than simply W3, corresponding to a zonal wavenumber broadening effect. Additionally, we find that the wavenumber broadening effect in the ionosphere can be fed back onto the neutrals through ion drag, to produce small QTDW winds with new wavenumbers in the thermosphere.

Coupling between mesosphere and ionosphere over Beijing through semidiurnal tides during the 2009 sudden stratospheric warming

Abstract: [1] Sudden stratospheric warming (SSW) in the winter of 2008/2009 is the strongest recorded SSW event. The enhancement in semidiurnal variation of ionospheric TEC (total electron content) with phase shift forward is shown during 22 to 27 January 2009, based on the TEC observations in Beijing (40.30°N, 116.19°E geographic, 39.73°N dip latitude). We focus on finding the reason for the TEC variation. Winds observed by an all-sky meteor radar in the same observatory are used to study mesospheric variation. The semidiurnal solar tide in the mesosphere starts to increase before the SSW and maintains oscillation with period 16–20 days during the SSW. The semidiurnal lunar tides in TEC and wind start to increase on 17 and 15 January, respectively. Although the semidiurnal lunar tide in TEC over Beijing almost dies out on 1 February, that over equatorial ionospheric anomaly crest does not vanish until 15 February when lunar tide in wind tends to be very weak. The maximum of lunar tide in wind appears on 2 February at 96 km with amplitudes of 15 m/s and 21 m/s for zonal and meridional winds. The phase comparison shows that lunar tides in TEC and zonal wind reach their maxima at almost the same time, which is 2–4 h lag behind the meridional wind. The coupling between the mesosphere and ionosphere contributes to the semidiurnal variation of TEC through both solar and semidiurnal lunar tides. The enhancement in semidiurnal lunar tide is responsible for the TEC peak shift forward during the SSW.

Reliability of the TEC Computed Using GPS Measurements — The Problem of Hardware Biases

Abstract: In this paper, we outline the procedure used at the Royal Observatory of Belgium in order to compute the TEC using GPS measurements with a precision of 2–3 TECU. This procedure requires the determination of the so-called receiver and satellite differential group delays. The combined biases (receiver + satellite) are determined on a daily basis (i.e. one solution a day) using the geometry-free combination of code observations. The method is applied to a network of 7 permanently operating Turbo Rogue receivers; the reliability of these computed biases is discussed.