Showing papers on "Total electron content published in 2010"

Ionospheric variability due to planetary waves and tides for solar minimum conditions

Abstract: [1] Large ionospheric variability is found at low to middle latitudes when a quasi-stationary planetary wave is specified in the winter stratosphere in the National Center for Atmospheric Research thermosphere-ionosphere-mesosphere electrodynamics general circulation model for solar minimum conditions. The variability includes change of electric field/ion drift, F2 peak density and height, and the total electron content. The electric field/ion drift change is the largest near dawn in the numerical experiments. Analysis of model results suggests that, although the quasi-stationary planetary wave does not propagate deep into the ionosphere or to low latitudes due to the presence of critical layers and strong molecular dissipation, the planetary wave and tidal interaction leads to large changes in tides, which can strongly impact the ionosphere at low and middle latitudes through the E region wind dynamo. Large zonal gradients of zonal and meridional winds from the tidal components and the zonal gradient of electric conductivities at dawn can produce large convergence/divergence of Hall and Pedersen currents, which in turn produces a polarization electric field. The ionospheric changes are dependent on both the longitude and local time, and are determined by the amplitudes and phases of the superposing wave components. The model results are consistent with observed ionospheric changes at low and middle latitudes during stratospheric sudden warming events, when quasi-stationary planetary waves become large.

Error analysis of Abel retrieved electron density profiles from radio occultation measurements

Abstract: . This letter reports for the first time the simulated error distribution of radio occultation (RO) electron density profiles (EDPs) from the Abel inversion in a systematic way. Occultation events observed by the COSMIC satellites are simulated during the spring equinox of 2008 by calculating the integrated total electron content (TEC) along the COSMIC occultation paths with the "true" electron density from an empirical model. The retrieval errors are computed by comparing the retrieved EDPs with the "true" EDPs. The results show that the retrieved NmF2 and hmF2 are generally in good agreement with the true values, but the reliability of the retrieved electron density degrades in low latitude regions and at low altitudes. Specifically, the Abel retrieval method overestimates electron density to the north and south of the crests of the equatorial ionization anomaly (EIA), and introduces artificial plasma caves underneath the EIA crests. At lower altitudes (E- and F1-regions), it results in three pseudo peaks in daytime electron densities along the magnetic latitude and a pseudo trough in nighttime equatorial electron densities.

A three-dimensional time-dependent algorithm for ionospheric imaging using GPS

Abstract: Global Positioning System (GPS) satellite receivers provide a world-wide network of phase and group delay measurements. The combination of two-frequency measurements can be used to derive the integral of the electron concentration along each satellite-to-receiver path, a parameter known as the Total Electron Content (TEC). At this stage these slant TEC data are diffi cult to interpret as they originate from a combination of a temporally changing ionosphere and spatially changing observation geometry. In this paper TEC data are inverted to evaluate the underlying distribution and time evolution of electron concentration. Accordingly, a new three-dimensional, time-dependent algorithm is presented here for imaging ionospheric electron concentration using GPS signals. The inversion results in a three-dimensional movie rather than a static image of the electron-concentration distribution. The technique is demonstrated using simulated ground-based GPS data from actual measurement geometry over Europe.

Electron and ion density variations before strong earthquakes (M>6.0) using DEMETER and GPS data

Abstract: . Using IAP (plasma analyzer) and ISL (Langmuir probe) experiments onboard DEMETER (Detection of Electromagnetic Emissions Transmitted from Earthquake Regions) satellite and GPS (Global Positioning System) measurements, we have statistically analyzed the variations of the electron and ion densities to search for disturbances in the vicinity of four large earthquakes prior to events. The indices Dst and Kp were used to distinguish pre-earthquake anomalies from the other anomalies related to the geomagnetic activities. For each studied case, a very good agreement was found between the different parameters estimated by DEMETER and GPS data in the detection of pre-seismic anomalies. Our statistics results show that the anomalous deviations prior to earthquakes have different sign from case to case, and that their amplitude depends on the magnitude of the earthquake. It has also been found that the electron density measured by the ISL experiment at night detects anomalous variations significantly before the earthquakes. The appearance of positive and negative anomalies in both of DEMETER and TEC (Total Electron Content) data during 1 to 5 days before all studied earthquakes during quiet geomagnetic conditions indicates that these anomalous behaviors are highly regarded as seismo-ionospheric precursors.

GPS TEC, scintillation and cycle slips observed at high latitudes during solar minimum

Abstract: . High-latitude irregularities can impair the operation of GPS-based devices by causing fluctuations of GPS signal amplitude and phase, also known as scintillation. Severe scintillation events lead to losses of phase lock, which result in cycle slips. We have used data from the Canadian High Arctic Ionospheric Network (CHAIN) to measure amplitude and phase scintillation from L1 GPS signals and total electron content (TEC) from L1 and L2 GPS signals to study the relative role that various high-latitude irregularity generation mechanisms have in producing scintillation. In the first year of operation during the current solar minimum the amplitude scintillation has remained very low but events of strong phase scintillation have been observed. We have found, as expected, that auroral arc and substorm intensifications as well as cusp region dynamics are strong sources of phase scintillation and potential cycle slips. In addition, we have found clear seasonal and universal time dependencies of TEC and phase scintillation over the polar cap region. A comparison with radio instruments from the Canadian GeoSpace Monitoring (CGSM) network strongly suggests that the polar cap scintillation and TEC variations are associated with polar cap patches which we therefore infer to be main contributors to scintillation-causing irregularities in the polar cap.

Ionospheric gravity waves detected offshore Hawaii after tsunamis

Abstract: [1] On-board satellites techniques provide global coverage and could play an important role in the continuous oceanic survey to prevent the damage produced by powerful tsunamis We report here new ionospheric observations related to three significant transpacific tsunami events triggered by the 2006 Kuril earthquake, the 2009 Samoa earthquake and the 2010 Chile earthquake Total Electron Content (TEC) variations extracted from data recorded by a dense Global Positioning System (GPS) network based in Hawaii show ionospheric disturbances within the hours following the tsunami wave passage at sea-level For each event, we observe ionospheric gravity waves propagating with velocity, direction and arrival time coherent with the tsunami The tsunamigenic signature in the ionosphere is also compared to in-situ sea-level measurements These observations provide new examples of the sensitivity of the ionosphere to tsunamigenic gravity waves and confirm that ionospheric monitoring by GPS can provide complementary information on tsunami propagation

Quiet time ionospheric variability over Arecibo during sudden stratospheric warming events

Abstract: [1] We present observations of the F-region ionosphere over Arecibo, Puerto Rico (18.34°N, 66.75°W), during the January–February 2008 and January–February 2009 sudden stratospheric warming (SSW) events. For the first period (2008), we have used incoherent scatter radar (ISR) electron density and temperature measurements from the Arecibo Observatory (AO), as well as relative total electron content (TEC) derived from a dual-frequency GPS receiver. For the second event (2009), during which we observed the largest recorded stratospheric warming, we have used the relative GPS TEC. Our analysis indicates that the ionosphere over Arecibo exhibits perturbations after the SSW, the effects are most visible during the daytime. The strongest signatures are observed in the TEC measurements, represented by large enhancements (with respect to non SSW days), particularly during daytime hours. However, the local time dependence of these enhancements is not the same in the two events. In addition, the data show that our results are consistent with the larger than normal daytime vertical drift differences observed at the magnetic equator over Jicamarca. The electron temperature is also affected during the daytime due to changes in electron density, indicating that the electron temperatures is influenced, indirectly, by changes in planetary wave activity in the lower altitudes.

Assessment of GPS global ionosphere maps (GIM) by comparison between CODE GIM and TOPEX/Jason TEC data: Ionospheric perspective

Abstract: [1] We performed a comprehensive comparison between GPS global ionosphere map (GIM) and TOPEX/Jason (T-J) total electron content (TEC) data for the periods of 1998–2009 in order to assess the performance of GIM over the global ocean where the GPS ground stations are very sparse. Using the GIM model constructed by the Center for Orbit Determination in Europe at the University of Bern, the GIM TEC values were obtained along the T-J satellite orbit at specific locations and times of measurements and then binned into various geophysical conditions for direct comparison with the T-J TEC. On the whole, the GIM model was able to reproduce the spatial and temporal variations of the global ionosphere as well as the seasonal variations. However, the GIM model was not accurate enough to represent the well-known ionospheric structures such as the equatorial anomaly, the Weddell Sea Anomaly, and the longitudinal wave structure. Furthermore, a fundamental limitation of the model seems to be evident in the unexpected negative differences (i.e., GPS T-J) at night represent the plasmaspheric contribution to GPS TEC, which is maximized, reaching up to 100% of the corresponding T-J TEC values in the early morning sector. In particular, the relative differences decreased with increasing solar activity, and this may indicate that the plasmaspheric contribution to the maintenance of the nighttime ionosphere does not increase with solar activity, which is different from what we normally anticipate.

Global ionospheric response observed by COSMIC satellites during the January 2009 stratospheric sudden warming event

Abstract: [1] This paper investigates the global ionospheric response during the January 2009 stratospheric sudden warming (SSW) event by using electron density profiles derived from GPS radio occultation measurements of the COSMIC satellites. The peak density (NmF2), peak height (hmF2), and ionospheric total electron content (ITEC) increase in the morning hours and decrease in the afternoon globally for 75% of the cases, in which electron density profiles during SSW and non-SSW days are available around the same location and local time bins. NmF2, hmF2, and ITEC during SSW days, on average, increase 19%, 12 km, and 17% in the morning and decrease 23%, 19 km, and 25% in the afternoon, respectively, in comparison with those during non-SSW days from global COSMIC observations. These results agree well with previous results from total electron content observations in low-latitude and equatorial regions. Interestingly, the unique COSMIC observations also revealed that during this SSW event the ionosphere responds globally, not only in the equatorial regions but also at the high and middle latitudes. The high-latitude ionosphere shows increased NmF2 and ITEC and decreased hmF2 in either the morning or afternoon sector. Thus, these results indicate that the ionospheric response in low-middle latitude and equatorial regions during SSW can be explained by either the modulated vertical drift resulting from the interaction between the planetary waves and tides through E region dynamo or the possible direct propagation of tides from the lower atmosphere, whereas the ionospheric variations at the middle and high latitude during the SSW might be attributed to the neutral background changes due to the direct propagation of tides from the lower atmosphere to the ionospheric F2 region. The competitive effects of different physical processes, such as the electric field, neutral wind, and composition, might cause the complex features of ionospheric variations during this SSW event.

Ionospheric Threat Parameterization for Local Area Global-Positioning-System-Based Aircraft Landing Systems

Abstract: Observations of extreme spatial rates of change of ionospheric electron content and the characterization strategy for mitigation applied by the US local area augmentation system are shown During extreme ionospheric activity, the gradient suffered by a global navigation satellite system user a few kilometers away from a ground reference station may reach as high as 425 mm of delay (at the GPS L1frequency) per km of user separation The method of data analysis that produced these results is described, and a threat space that parameterizes these possible threats to user integrity is defined Certain configurations of user, reference station, global navigation satellite system satellite, and ionospheric storm-enhanced density may inhibit detection of the anomalous ionosphere by the reference station

Coseismic ionospheric disturbances triggered by the Chi-Chi earthquake

Abstract: [1] At 17:47 UT on 20 September 1999, a large earthquake of magnitude Mw 7.6 struck the central Taiwan near a small town of Chi-Chi. The ground-based receivers of the global positioning system (GPS) in the Taiwan area detected coseismic ionospheric disturbances (CIDs) in the total electron content (TEC) triggered by the Chi-Chi earthquake. When the CIDs travel away from the origin on the Earth surface and then propagate into the ionosphere, their amplitudes and periods generally become smaller and longer, respectively. Moreover, two global grid searches, adapting the ray-tracing and the beam-forming techniques, have been used to analyze the observed GPS TEC. We have not only estimated the average speed of the CIDs propagating in the atmosphere and ionosphere but also determined the location of CID origin on the Earth surface by using the two techniques. The results show that the observed CIDs result from shock-acoustic waves triggered by sudden and large vertical motions of the Chi-Chi earthquake.

A global positioning system–based climatology for the total electron content in the ionosphere

Abstract: [1] The total electron content (TEC) in the ionosphere is an important factor in the propagation of radio waves. Since 1998 the coverage global positioning system (GPS) observations has been sufficient to monitor the TEC globally. We have used the global ionosphere maps provided by the Jet Propulsion Laboratory to devise a new ionosphere climatology (NIC09). The climatology fits the GPS maps to 4.5 TECU, more than twice as well as the IRI2007 climatology. The use of the global mean TEC as the input parameter reduces scaling errors and long-term mean errors in the model. For climatic studies of sea level measured by satellite radar altimeters, it is necessary to go back before 1998. During the earlier years of radar altimetry, we use TOPEX dual-frequency altimeter data to reconstruct the global mean TEC or use the solar radio flux (F10.7) as a proxy. The comparison of dual-frequency altimeter data with the GPS maps demonstrated that about 8% of the TEC extends above 1350 km and about 14% above 800 km. The root mean square error of the NIC09 climatology was found to be approximately 18% of the TEC, compared to 14% for the GPS TEC maps, and 35% for IRI2007.

GPS slant total electron content accuracy using the single layer model under different geomagnetic regions and ionospheric conditions

Abstract: The use of observations from the Global Positioning System (GPS) has significantly impacted the study of the ionosphere As it is widely known, dual-frequency GPS observations can provide very precise estimation of the slant Total Electron Content (sTEC—the linear integral of the electron density along a ray-path) and that the precision level is bounded by the carrier-phase noise and multi-path effects on both frequencies Despite its precision, GPS sTEC estimations can be systematically affected by errors in the estimation of the satellites and receivers by Inter-Frequency Biases (IFB) that are simultaneously determined with the sTEC Thus, the ultimate accuracy of the GPS sTEC estimation is determined by the errors with which the IFBs are estimated This contribution attempts to assess the accuracy of IFBs estimation techniques based on the single layer model for different ionospheric regions (low, mid and high magnetic latitude); different seasons (summer and winter solstices and spring and autumn equinoxes); different solar activity levels (high and low); and different geomagnetic conditions (quiet and very disturbed) The followed strategy relies upon the generation of a synthetic data set free of IFB, multi-path, measurement noise and of any other error source Therefore, when a data set with such properties is used as the input of the IFB estimation algorithms, any deviation from zero on the estimated IFBs should be taken as indications of the errors introduced by the estimation technique The truthfulness of this assessment work is warranted by the fact that the synthetic data sets resemble, as realistically as possible, the different conditions that may happen in the real ionosphere The results of this work show that during the high solar activity period the accuracy for the estimated sTEC is approximately of ±10 TECu for the low geomagnetic region and of ±22 TECu for the mid-latitude During low solar activity the accuracy can be assumed to be in the order of ±2 TECu For the geomagnetic high-disturbed period, the results show that the accuracy is degraded for those stations located over the region where the storm has the strongest impact, but for those stations over regions where the storm has a moderate effect, the accuracy is comparable to that obtained in the quiet period

Ionospheric response to the initial phase of geomagnetic storms: Common features

Abstract: [1] Ionospheric responses to the initial phases of three geomagnetic storms: 2–5 April 2004, 7–9 November 2004, and 13–16 December 2006, were compared using both ground-based GPS total electron content (TEC) data and coupled magnetosphere ionosphere thermosphere (CMIT) model simulations. The onset times for these storms all occurred at local daytime in the North American sector. This similarity of onset times and other factors resulted in some common features in their ionospheric response. These common features include (1) enhanced TEC (positive response) at low and middle latitudes in the daytime, (2) depleted TEC (negative response) around the geomagnetic equator in the daytime, (3) a north-south asymmetry in the positive response as the northern hemispheric response appeared to be more pronounced, and (4) negative response at high latitudes as the storms progressed. The CMIT model captured most of these features. Analysis of model results showed that storm-time enhancements in the daytime eastward electric field were the primary cause of the observed positive storm effects at low and middle latitudes as well as the negative response around the geomagnetic equator in the daytime. These eastward electric field enhancements were caused by the penetration of high latitude electric fields to low latitudes during southward interplanetary magnetic field (IMF) periods, when IMF Bz oscillated between southward and northward direction in the initial, shock phase of the storms. Consequently, the ionosphere was lifted up at low and middle latitudes to heights where recombination was weak allowing the plasma to exist for a long period resulting in higher densities. In addition, the CMIT model showed that high-latitude negative storm responses were related to the enhancements of molecular nitrogen seen in TIMED/Global Ultraviolet Imager observations, whereas the negative storm effects around the geomagnetic equator were not associated with thermospheric composition changes; they were the result of plasma transport processes.

Temporal and spatial precursors in the ionospheric global positioning system (GPS) total electron content observed before the 26 December 2004 M9.3 Sumatra–Andaman Earthquake

Abstract: [1] We report seismo-ionospheric precursors of anomalous decreases in the total electron content (TEC) appearing day 5 prior to an M9.3 earthquake, the largest one in the last five decades, which occurred in Sumatra-Andaman, Indonesia on 26 December 2004. Sequences of global ionosphere maps of the TEC derived from worldwide ground-based receivers of the global positioning system (GPS) are used to statistically study the temporal and spatial precursors of the earthquake. It was found that the temporal precursor of the GPS TEC around the epicenter was significantly reduced during the afternoon period on d 5 before the earthquake. The spatial precursors prominently, persistently, and simultaneously appear around the epicenter and its conjugate areas of the Sumatra-Andaman earthquake.

Flare location on the solar disk: Modeling the thermosphere and ionosphere response

Abstract: [1] Solar flare enhancements to the soft X-ray (XUV) and extreme ultraviolet (EUV) spectral irradiance depend on the location of the flare on the solar disk. Most emission lines in the XUV region (∼0.1 to ∼25 nm) are optically thin and are weakly dependent on the location of the flare, but in the EUV region (∼25 to ∼120 nm), many important lines and continua are optically thick, so enhancements are relatively smaller for flares located near the solar limb, due to absorption by the solar atmosphere. The flare irradiance spectral model (FISM) was used to illustrate these location effects, assuming two X17 flares that are identical except that one occurs near disk center and the other near the limb. FISM spectra of these two flares were used as solar input to the National Center for Atmospheric Research (NCAR) thermosphere-ionosphere-mesosphere electrodynamics general circulation model (TIME-GCM) to investigate the ionosphere/thermosphere (I/T) response. Model simulations showed that in the E region ionosphere, where XUV dominates ionization, flare location does not affect I/T response. However, flare-driven changes in the F region ionosphere, total electron content (TEC), and neutral density in the upper thermosphere, are 2–3 times stronger for a disk-center flare than for a limb flare, due to the importance of EUV enhancement. Flare location did not affect the timing of the ionospheric response, but the thermospheric response was ∼20 min faster for the disk-center flare. Model simulations of I/T responses to an X17 flare on 28 October 2003 were consistent with measurements of TEC and neutral density changes.

Ionospheric holes made by ballistic missiles from North Korea detected with a Japanese dense GPS array

Abstract: [1] A dense array of global positioning system (GPS) receivers is a useful tool to study ionospheric disturbances. Here we report observations by a Japanese GPS array of ionospheric holes, i.e., localized electron depletion. They were made by neutral molecules in exhaust plumes (e.g., water) of ballistic missiles from North Korea, Taepodong-1 and -2, launched on 31 August, 1998, and 5 April, 2009, respectively. Negative anomaly of electron density emerged ∼6 min after the launches in the middle of the Japan Sea, and extended eastward along the missile tracks. By comparing the numerical simulation of electron depletion and the observed change in ionospheric total electron content, we suggest that the exhaust plumes from the Taepodong-2 second stage effused up to ∼1.5 × 1026 water molecules per second. The ionospheric hole signature was used to constrain the Taepodong-2 trajectory together with other information, e.g., coordinates of the launch pad, time and coordinates of the first stage splashdown, and height and time of the second stage passage over Japan. The Taepodong-2 is considered to have reached the ionospheric F region in ∼6 min, flown above northeastern Japan ∼7 min after the launch, and crashed to the Pacific Ocean without attaining the first astronautical velocity. The ionospheric hole in the 1998 Taepodong-1 launch was much less in size, but it is difficult to compare directly the thrusts of the two missiles due to uncertainty of the Taepodong-1 trajectory.

Latitudinal variations in Saturn's ionosphere: Cassini measurements and model comparisons

Abstract: [1] We present a study of latitudinal variations in Saturn’s ionosphere using Cassini Radio Science Subsystem (RSS) measurements and Saturn‐Thermosphere‐Ionosphere‐Model (STIM) simulations. On the basis of Cassini RSS observations, the peak electron density (NMAX) and the total electron content (TEC) both exhibit a clear increase with latitude, with a minimum at Saturn’s equator. When compared with these RSS trends, current model simulations overestimate NMAX and TEC at low latitudes and underestimate those parameters at middle and high latitudes. STIM is able to reproduce the RSS values for NMAX and TEC at low latitude when an additional low‐latitude loss process, such as a water influx, is introduced near Saturn’s equator. The lack of auroral precipitation processes in the model likely explains some model/data discrepancies at high latitude; however, most of the high‐latitude RSS data are from latitudes outside of Saturn’s typical main auroral oval. Using Cassini RSS electron density altitude profiles combined with ion density fractions and neutral background parameters calculated in STIM, we also present estimates of the latitudinal variations of Saturn’s Pedersen conductance, SP. We find SP to be driven by ion densities in Saturn’s lower ionosphere and to exhibit a latitudinal trend with a peak at mid‐latitude. Model calculations are able to reproduce low‐latitude conductances when an additional loss process is introduced, as before, but consistently underestimate most of the mid‐ and high‐latitude conductances derived from Cassini observations, perhaps indicating a missing ionization source within the model.

Verification of the concept of seismoionospheric coupling under quiet heliogeomagnetic conditions, using the Wenchuan (China) earthquake of May 12, 2008, as an example

Abstract: The variability of the ionosphere during April–May, 2008, has been analyzed in detail in order to reveal anomalous variations related to seismic activity, initiated by the strongest Wenchuan earthquake (M = 7.9) in the Sichuan province on May 12, 2008. Information about the total electron content (TEC) from the network of GPS receivers in the earthquake region, the global IONEX TEC maps, and the reconstructed vertical profiles of electron density according to the data of GPS receivers were used as a data source. The spatial and time localization of the observed anomalies, their morphological features, and the absence of geomagnetic disturbances during the observation period undoubtedly demonstrate that the observed variations were caused by seismic activity.

Time delay and duration of ionospheric total electron content responses to geomagnetic disturbances

Abstract: . Although positive and negative signatures of ionospheric storms have been reported many times, global characteristics such as the time of occurrence, time delay and duration as well as their relations to the intensity of the ionospheric storms have not received enough attention. The 10 years of global ionosphere maps (GIMs) of total electron content (TEC) retrieved at Jet Propulsion Laboratory (JPL) were used to conduct a statistical study of the time delay of the ionospheric responses to geomagnetic disturbances. Our results show that the time delays between geomagnetic disturbances and TEC responses depend on season, magnetic local time and magnetic latitude. In the summer hemisphere at mid- and high latitudes, the negative storm effects can propagate to the low latitudes at post-midnight to the morning sector with a time delay of 4–7 h. As the earth rotates to the sunlight, negative phase retreats to higher latitudes and starts to extend to the lower latitude toward midnight sector. In the winter hemisphere during the daytime and after sunset at mid- and low latitudes, the negative phase appearance time is delayed from 1–10 h depending on the local time, latitude and storm intensity compared to the same area in the summer hemisphere. The quick response of positive phase can be observed at the auroral area in the night-side of the winter hemisphere. At the low latitudes during the dawn-noon sector, the ionospheric negative phase responses quickly with time delays of 5–7 h in both equinoctial and solsticial months. Our results also manifest that there is a positive correlation between the intensity of geomagnetic disturbances and the time duration of both the positive phase and negative phase. The durations of both negative phase and positive phase have clear latitudinal, seasonal and magnetic local time (MLT) dependence. In the winter hemisphere, long durations for the positive phase are 8–11 h and 12–14 h during the daytime at middle and high latitudes for 20≤Ap

Response of the thermosphere and ionosphere to an ultra fast Kelvin wave

Abstract: [1] Ultra Fast Kelvin (UFK) waves are eastward propagating planetary waves with periods between 3 and 5 days, which are capable of penetrating into the thermosphere and ionosphere where they may modulate phenomena occurring in this region. A sensitivity study has been conducted to examine the effect of an Ultra Fast Kelvin wave on the thermosphere and ionosphere using the NCAR Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIME-GCM) under June solstice solar minimum conditions. It is found that realistic ultra fast Kelvin waves with amplitudes in the MLT region of approximately 20–40 m s−1 in zonal wind fields and 10–20 K in temperature fields, can result in approximately 8–12% perturbations in hourly neutral density at 350 km, as well as hourly total electron content (TEC) perturbations of 25–50% in regions corresponding to the equatorial ionization anomalies (EIAs), with the largest relative changes resolved during the nighttime due to the lower electron densities. The electrodynamical calculations in the model were then disabled to identify the relative importance of ionospheric electrodynamics and direct wave propagation in generating the aforementioned changes. The subsequent results show that changes in thermospheric neutral density are relatively insensitive to the presence of the dynamo electric field, while UFK wave modulation of the dynamo accounts for most of the TEC perturbations due to changes of ionospheric vertical plasma drift.

Correlation between the ionospheric WN4 signature and the upper atmospheric DE3 tide

Abstract: [1] The present work studies the correlation relationship between the longitudinal ionospheric structure of wave number 4 (WN4) and the upper atmospheric tide of nonmigrating tidal mode DE3 (diurnal eastward wave number 3). Global ionospheric maps produced by the Jet Propulsion Laboratory were used to deduce the latitudinal integration of total electron content in the low-latitude ionosphere, and TIDI/TIMED observations were used to retrieve the atmospheric zonal and meridional winds. By applying Fourier filtering and fitting techniques, the WN4 wave and DE3 tidal components are derived from the ionospheric and upper atmospheric observations, respectively. We found that the observed WN4 wave and DE3 zonal wind components experience very similar annual and interannual variations, but the DE3 meridional wind component behaves in a quite different manner. Both WN4 and DE3 zonal winds are very intense during northern summer and autumn; they also appear in the later spring, but tend to vanish in winter. Their amplitudes increase as the solar activity decreases, and both are stronger in the quasi-biennial oscillation (QBO) eastward wind phase than in the westward phase. At the same time, the DE3 meridional wind likes to occur only in winter and seems not change with solar activity and QBO phase. We further studied the correlation between the WN4 wave and the two wind components of the DE3 tide. We found that the cross-correlation coefficient between the WN4 wave and the DE3 zonal wind is much larger, while that between the WN4 wave and the DE3 meridional wind is relatively smaller. Such different correlations are attributed to the different latitudinal symmetry of different DE3 wind components. The DE3 zonal wind is likely in latitudinally symmetric tidal mode; hence, it can efficiently affect the F region ion drifts. In contrast, the meridional wind is mainly in antisymmetric mode and thus seldom affects the ionospheric drifts. The present results support the suggestion that the longitudinal WN4 structure in the ionospheric F region originates from the symmetric modes, mainly the zonal wind component, of the upper atmospheric nonmigrating tidal mode DE3 in the ionospheric E region.

Solar proxies pertaining to empirical ionospheric total electron content models

Abstract: [1] Solar proxies and indices exhibiting extreme ultraviolet (EUV) irradiance that affects the ionospheric total electron content (TEC) were examined through training an artificial neural network (ANN). A TEC database was constructed from a dense GPS receiver network over Japan from April 1997 to March 2008, covering an entire 11 year solar activity period. In empirical models of upper atmospheric parameters, such as the International Reference Ionosphere model and the Mass Spectrometer and Incoherent Scatter thermospheric model, the 10.7 cm solar radio flux (F10.7) or the sunspot number (R) is used as a proxy for determining the solar activity. In the present study, ANN training for predicting TEC as a target parameter was done by including new solar proxies/indices in the input space that were based on direct measurements of solar EUV/UV flux, SOHO_SEM26–34 (the integrated 26–34 nm EUV emission), and Mg II cwr (the core-to-wing ratio of Mg II 280 nm line), as well as the traditional indices F10.7 and R. Root mean square errors (RMSEs) of TEC were compared after the training was completed using a variety of combinations of solar proxies. When a single proxy was used, SOHO_SEM26–34 yielded the smallest RMSE, or it was the best proxy for modeling ionospheric TEC. Further, general improvements were obtained by combining different types of proxies and short- and long-term means of them. The best combination was the 3 day smoothed daily, 7 day and 27 day backward mean values of Mg II cwr, SOHO_SEM26–34, and the 10.7 cm radio flux.

Scintillation‐producing Fresnel‐scale irregularities associated with the regions of steepest TEC gradients adjacent to the equatorial ionization anomaly

Abstract: [1] Using ground-based GPS and digital ionosonde instruments, we have built up at latitudes of the equatorial ionization anomaly (EIA), in the Brazilian sector, a time-evolving picture of total electron content (TEC), L-band amplitude scintillations, and F region heights, and we have investigated likely reasons for the occurrence or suppression of equatorial scintillations during the disturbed period of 18–23 November 2003. During the prestorm quiet nights, scintillations are occurring postsunset, as expected; however, during the storm time period, their spatial-temporal characteristics and intensity modify significantly owing to the dramatic changes in the ionospheric plasma density distribution and in the temporal evolution of TEC. The two-dimensional maps showing both TEC and amplitude scintillations revealed strong evidence of turbulences at the Fresnel length (causing scintillations) concurrent with those regions of steepest TEC gradients adjacent to the crests of the EIA. The largest density gradients have been found to occur in an environment of increased background electron density, and their spatial distribution and location during the disturbed period may differ significantly from the magnetic quiet night pattern. However, in terms of magnitude the gradients at equatorial and low latitudes appear to not change during both magnetic quiet and disturbed conditions. The scenarios for the formation or suppression of scintillation-producing Fresnel-scale irregularities during the prestorm quiet nights and disturbed nights are discussed in view of different competing effects computed from numerical simulation techniques.

Accuracy analysis of the GPS instrumental bias estimated from observations in middle and low latitudes

Abstract: With one bias estimation method, the latitude-related error distribution of instrumental biases estimated from the GPS observations in Chinese middle and low latitude region in 2004 is analyzed statistically It is found that the error of GPS instrumental biases estimated under the assumption of a quiet ionosphere has an increasing tendency with the latitude decreasing Besides the asymmetrical distribution of the plasmaspheric electron content, the obvious spatial gradient of the ionospheric total electron content (TEC) along the meridional line that related to the Equatorial Ionospheric Anomaly (EIA) is also considered to be responsible for this error increasing The RMS of satellite instrumental biases estimated from mid-latitude GPS observations in 2004 is around 1 TECU (1 TECU = 1016/m2), and the RMS of the receiver's is around 2 TECU Nevertheless, the RMS of satellite instrumental biases estimated from GPS observations near the EIA region is around 2 TECU, and the RMS of the receiver's is around 3–4 TECU The results demonstrate that the accuracy of the instrumental bias estimated using ionospheric condition is related to the receiver's latitude with which ionosphere behaves a little differently For the study of ionospheric morphology using the TEC derived from GPS data, in particular for the study of the weak ionospheric disturbance during some special geo-related natural hazards, such as the earthquake and severe meteorological disasters, the difference in the TEC accuracy over different latitude regions should be paid much attention

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

Abstract: [1] 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.

Ionosphere response to recurrent geomagnetic activity: Local time dependency

Abstract: [1] Observations of global positioning system total electron content (TEC) and in situ electron densities at altitudes of ~350-370 km from the CHAMP satellite are used to illustrate the local time and latitude dependence of 9 day periodicities in the ionosphere due to recurring high-speed solar wind streams and geomagnetic activity during 2005. A local time dependence is found, with nighttime TEC oscillations concentrated at high latitudes and close to ±40% of background levels. The largest oscillations in daytime TEC occur at midlatitudes and are ±25% of background levels. Furthermore, the daytime response is generally symmetric about the geomagnetic equator with anticorrelation between high and low latitudes, whereas at night the high-latitude Northern Hemisphere is generally in-phase with low latitudes and anticorrelated with the high-latitude Southern Hemisphere. A combination of enhanced equatorward neutral winds and changes in neutral composition are thought to be the primary mechanisms responsible for the observed ionospheric response. Although similar mechanisms are driving the response, the local time dependency arises because of the presence (lack) of photoionization during the daytime (nighttime). Similar trends are observed in CHAMP in situ electron densities; however, the oscillations at a near-constant altitude are ~10-15% larger than the TEC oscillations. Additionally, the CHAMP observations reveal possible variations in the strength of the equatorial ionization anomaly, indicating that disturbance dynamo electric fields may also contribute to the ionospheric response to recurrent geomagnetic activity. The results presented are the first to reveal the significant differences between the daytime and nighttime response of the ionosphere to periodic forcing from solar wind high-speed streams.