[show abstract][hide abstract] ABSTRACT: The problem of ionospheric variability is regarded from the point of view of spatial and temporal correlations between the GPS TEC variations derived from the neighboring GPS receiver records. It is demonstrated that the technique of the spatial correlation coefficient developed earlier to reveal the ionospheric variations induced by seismic activity sometimes is not very reliable. The new index of the local ionospheric variability is proposed describing the spread of GPS TEC within the given area. It is tested for periods of geomagnetic disturbances and periods of several days preceding the strong (M ⩾ 6) earthquakes occuring within the area of GPS receiver’s network. It is shown that the new index is a promising indicator of the earthquake preparation process, it increases few (5–10) days before the seismic shocks and comes back to normal state after the earthquake. During the periods of increased geomagnetic activity the index does not show such variability.
Advances in Space Research 05/2013; · 1.18 Impact Factor
[show abstract][hide abstract] ABSTRACT: NeQuick 2 ionospheric empirical model depends on global ionospheric
coefficients that are estimated from unevenly distributed ionosonde
measurements. In regions, like Africa, where very few observational data
were available until recently, the model estimated the ionospheric peak
parameters by interpolation. When one wants to employ the model to
specify the ionosphere where very few data have been used for model
development, the performances of the model need careful validation. This
study investigates the performances of NeQuick 2 in the East African
region by assisting the model with measurements from a single Global
Positioning System (GPS) receiver, which has been deployed recently.
This can be done by first calculating an effective ionization level that
drives NeQuick 2 to compute slant total electron content (sTEC) which
fits, in the least square sense, with the measurements taken from a
single GPS receiver. We then quantify the performances of NeQuick 2 in
reproducing the measured TEC by running the model at four other
locations, where GPS stations are available, using the same effective
ionization level that we calculated from a single GPS station as a
driver of the model. Finally, the performances of the model before and
after data ingestion have been investigated by comparing the model
results with the experimental sTEC and vertical TEC (vTEC) obtained from
the four test stations. Three months data during low solar activity
conditions have been used for this study. We have shown that the
capability of NeQuick 2, in describing the East African region of the
ionosphere, can be improved substantially by data ingestion. We found
that the model after ingestion reproduces the experimental TEC better as
far as about 620 km away from the reference station than that before
adaptation. The statistical comparisons of the performances of the model
in reproducing sTEC before and after ingestion are also discussed in
Radio Science 10/2012; 47(5):5002-. · 1.00 Impact Factor
[show abstract][hide abstract] ABSTRACT: We studied six different parameters derived from three different radiophysical techniques used to monitor area over the central Italy around the time of L'Aquila M6.3 earthquake of 6 April 2009. Namely VLF monitoring, ground based ionospheric sounding and GPS vertical TEC. The parameters derived from these data are: variations of amplitude of VLF signal on different propagation passes, critical frequency foF2 variations, cross-correlation coefficient for different pairs of ground based ionosondes, vertical GPS TEC time series, local variability index for the network of GPS receivers, and GPS TEC map. High congruence of techniques in time and space is found.
General Assembly and Scientific Symposium, 2011 XXXth URSI; 09/2011
[show abstract][hide abstract] ABSTRACT: Latest studies related to Abruzzo earthquake (Italy) (Fall AGU 2009), have reported precursory signals observed on the ground and in space associated with earthquake of April 6, 2009. Further questions, that are still been debating in the science community are: (1) whether such signals systematically precede most of the earthquakes in the region; and (2) what is the physical link between the earthquake processes and the atmospheric/ ionospheric signals ? To address some of these we present four different data sets of continuous observations for 5 years period (2005-2009) and their temporal and spatial dynamics several days before the Abruzzo earthquake. This earthquake was in the middle point between three radon ground stations. Beginning March 30th, 2009 radon enhancement coincides (with some delay) with an increase in air temperature in the epicentral area. And subsequently from April 1 to 3, 2009 an increase of outgoing infra-red radiation was observed from satellite data. The GPS/Total Electron Content (TEC) data indicate an increase of electron concentration reaching a maximum on April 5, 2009. We have found a significant relationship between radon emanation and atmospheric and ionospheric anomalies associated with Abruzzo earthquake. This study conforms: (1) Our initial findings of atmospheric awakening prior to the Abruzzo earthquake; and (2) Demonstrate the presence of related variations of several parameters implying their connection to the earthquake preparation process. This study also can help to understand different short-term earthquake precursors and their association with earthquakes described by Lithosphere-Atmosphere-Ionosphere Coupling (LAIC) model.
[show abstract][hide abstract] ABSTRACT: Recent studies indicate an enhanced coupling between the atmospheric boundary layer and the ionosphere, which have been proposed to be related with large (>M6) earthquakes. We present observations data from January to April 2009 of five physical parameters and their temporal and spatial variations several days before the onset of the Abruzzo earthquake. This earthquake was situated in the middle of three ground stations monitoring radon. These data show an increase prior to the main earthquake beginning on March 30th this enhancement of radon coincides (with some delay) with an increase in the air temperature (measured on the ground) in the epicentral area. And subsequently from April 1 to 3 there was an association with the acceleration of outgoing infra-red radiation observed on the top of the atmosphere from satellite. The GPS/Total Electron Content data indicate an increase of electron concentration in ionosphere from April 3 to 5, reaching a maximum on April 5. We have found a significant relationship between radon emanation and atmospheric and ionospheric anomalies associated with Abruzzo earthquake. This relationship has been studied using an integrated set of observations of several physical and environmental parameters (radon, seismicity, temperature of the atmosphere boundary layer, outgoing earth infra-red radiation and GPS/TEC). Our results suggest for an existence of coupling process between radon activity and atmosphere, several days in advance to the April 6th Abruzzo earthquake.
[show abstract][hide abstract] ABSTRACT: The Total Electron Content (TEC) of the ionosphere is a key parameter for describing the ionospheric state. This paper deals with the large scale behaviour of TEC under low and high solar activity conditions. Large scale structures of the plasma density are formed by fundamental ionospheric processes mainly driven by solar radiation input, neutral winds and electric fields. The monitoring of large scale structures contributes to a comprehensive understanding of these coupling mechanisms which are rather complex particularly under perturbed geomagnetic conditions. The paper addresses techniques to monitor TEC with sufficient accuracy of a few TEC units (1016m-2) to measure large scale structures over Europe and over the polar areas. The availability of GPS data from global GPS receiver networks as e.g., those from the International GPS Service (IGS) is dense enough to generate TEC maps on a continuous base. A model assisted technique is briefly described for mapping TEC over the European and polar areas. A statistical estimation of horizontal TEC gradients reveals large scale gradients of up to about 6 TECU/1000 km under high solar activity conditions at an occurrence probability level of about 1%. Occasionally, during severe ionospheric storms this value may increase by a factor of 10 or even more. A close correlation of large scale gradients and the geomagnetic activity has been found giving the chance to forecast TEC gradient amplitudes by using predicted geomagnetic indices. Since TEC is proportional to first-order range errors in Global Satellite Navigation Systems (GNSS) such as the US GPS and the Russian GLONASS the study of the behaviour of this parameter has a practical meaning in GNSS based navigation and positioning. The paper addresses the close relationship between TEC and ranging errors in GNSS. Having in view Galileo, the planned Europe?s own global satellite navigation system, some aspects related to the mitigation of ionospheric propagation errors within the European Geostationary Navigation Overlay System (EGNOS) are discussed. Since EGNOS will augment the two above mentioned satellite navigation systems and make them suitable for safety critical applications such as flying aircraft or navigating ships through narrow channels the ionospheric propagation errors have to be mitigated as much as possible.
[show abstract][hide abstract] ABSTRACT: The IRI model offers a choice of options for the computation of the electron density profile and electron content (TEC). Recently new options for the topside electron density profile have been developed, which have a strong impact on TEC. Therefore it is important to test massively the IRI and the new options with experimental data. A large number of permanent stations record dual frequency GPS data from which it is possible to obtain TEC values. Thirty-one worldwide distributed stations have been selected to investigate the capabilities of the IRI to reproduce experimental TEC. Data for years 2000 (high solar activity) and 2004 (medium solar activity) have been analyzed computing modeled values with the IRI-2001 and the IRI-2007-NeQuick topside options. It is found that IRI-2007-NeQuick option generally improves the estimate of the slant TEC, especially in the case of high latitudes stations during high solar activity.
Advances in Space Research 08/2008; · 1.18 Impact Factor
[show abstract][hide abstract] ABSTRACT: Equatorial plasma bubbles (EPBs) are field-aligned depletions of F-region ionospheric plasma density that grow from irregularities caused by the generalized Rayleigh–Taylor instability mechanism in the postsunset equatorial sector. Although they have been studied for some decades, they continue to be an important subject of both experimental and theoretical investigations because of their effects on trans-ionospheric radio communications.In this work, calibrated data of slant total electron content (sTEC) taken every 10 min from EGNOS System Test Bed Brazzaville (Congo), Douala (Cameroon), Lome (Togo) and N’Djamena (Chad), and International GNSS Service Ascension Island, Malindi (Kenya), and Libreville (Gabon), stations are used to detect plasma bubbles in the African equatorial region during the first 6 months of 2004. To identify these irregularities, the trend of every curve of sTEC against time is subtracted from the original data. The size of the EPBs is estimated by measuring its amplitude in the de-trended time variation of sTEC.
Journal of Atmospheric and Solar-Terrestrial Physics 01/2008; · 1.42 Impact Factor
[show abstract][hide abstract] ABSTRACT: The Global Positioning System (GPS) has become a powerful tool for ionospheric studies. In addition, ionospheric corrections
are necessary for the augmentation systems required for Global Navigation Satellite Systems (GNSS) use. Dual-frequency carrier-phase
and code-delay GPS observations are combined to obtain ionospheric observables related to the slant total electron content
(sTEC) along the satellite-receiver line-of-sight (LoS). This observable is affected by inter-frequency biases [IFB; often
called differential code biases (DCB)] due to the transmitting and the receiving hardware. These biases must be estimated
and eliminated from the data in order to calibrate the experimental sTEC obtained from GPS observations. Based on the analysis
of single differences of the ionospheric observations obtained from pairs of co-located dual-frequency GPS receivers, this
research addresses two major issues: (1) assessing the errors translated from the code-delay to the carrier-phase ionospheric
observable by the so-called levelling process, applied to reduce carrier-phase ambiguities from the data; and (2) assessing
the short-term stability of receiver IFB. The conclusions achieved are: (1) the levelled carrier-phase ionospheric observable
is affected by a systematic error, produced by code-delay multi-path through the levelling procedure; and (2) receiver IFB
may experience significant changes during 1day. The magnitude of both effects depends on the receiver/antenna configuration.
Levelling errors found in this research vary from 1.4 total electron content units (TECU) to 5.3 TECU. In addition, intra-day
vaiations of code-delay receiver IFB ranging from 1.4 to 8.8TECU were detected.
Journal of Geodesy 01/2007; 81(2):111-120. · 2.81 Impact Factor
[show abstract][hide abstract] ABSTRACT: The NeQuick ionospheric electron density model has a very simple topside formulation, which allows taking into account the electron content up to 20,000km. In the present work, the Gallagher model has been used in connection with NeQuick to provide a more realistic representation of the electron concentration distribution in the plasmasphere. An analysis of the impact of the proposed modification in low geomagnetic latitudes (±40°) on modeled vertical total electron content has been performed for different levels of solar activity, season, and universal time.In addition, GPS vertical total electron content values obtained from seven IGS (International GPS Service) station data have been compared with the NeQuick and NeQuick+Gallagher vertical total electron content values, confirming the importance of including a plasmaspheric formulation in the NeQuick model.
Advances in Space Research 01/2007; 39(5):739-743. · 1.18 Impact Factor
[show abstract][hide abstract] ABSTRACT: This work describes results from our analysis of: (1) GPS Total Electron
Content (TEC) measurements collected from ground based stations; (2)
ionospheric electromagnetic plasma measurements from the DEMETER
satellite; (3) thermal infrared (TIR) data (NOAA/AVHRR, MODIS); (4)
emitted long-wavelength radiation (OLR) and; (5) surface latent heat
flux (SLHF) from NOAA/NCEP. We found atmospheric and ionospheric
anomalies approximately one week prior for both the Dec 26, 2004, M 9.0
Sumatra-Andaman Islands and the March 28, 2005, M 8.7 Northern Sumatra
earthquakes. There were 125 earthquakes (M>5.5) in the 118 days of
data (December 1, 2004 through April 1, 2005) we analyzed in this region
(0°-10°, N and 90°-100° E). Ionospheric variations
(GPS/TEC), associated with the Northern Sumatra events, were determined
from data of both the regional SuGAr array (13 stations in Indonesia)
and four IGS stations. For each station a time series of the vertical
TEC (VTEC) data were computed and compared with the magnetic Dst index,
to see if these signals were the result of external magnetic field
activity. On December 22, four days prior to the M9.0 quake, GPS/TEC
data reached a monthly maximum with low Dst values. For the M 8.7 March
28 earthquake values of GPS/TEC were observed to increase four days
(March 22-25) prior to the event again with low Dst values. The VLF/ICE
instrument on DEMETER was operational during the M 8.7 earthquake of
March 28, 2005. Our analysis shows intensive ELF emissions within the 0
-200 Hz window at the conjugated points on 22 of March, 2005, 6 days
before the earthquake during DEMETERs close proximity to the epicenter.
In the case of the Dec 26, 2004, M 9.0 quake, OLR data were >80 W/m2,
corresponding to approximately two sigma above the mean for the months
of December, between 2001 and 2004, were found on December 21, 2004,
five days before the event. Anomalous values of SLHF associated with the
M9.0 Sumatra-Andaman Islands earthquake event were found on Dec 7, 2005
(above five sigma) and Dec 22, 2005 (SLHF +280Wm2). Smaller anomalies
are also seen on Mar 23, 2005 (SLHF +180Wm2). The anomalies consistently
occur over regions of maximum stress (along continental boundaries), and
appear not to be atmospheric due to the long persistence over the same
[show abstract][hide abstract] ABSTRACT: Global Positioning Satellite Systems GPS is an essential element for a series of technological applications in different fields of human activity In addition they have greatly contributed to scientific research in the field of geodesy as well as in remote sensing the Earth s ionosphere Nowadays ionospheric information that can be derived from GPS observations represents the richest source of data for the study of ionospheric processes that define the climate and weather of the ionosphere under the effects of the varying solar radiation The reliability of such data is of fundamental importance to guarantee the validity of their use It is also evident that every improvement on the knowledge of the ionosphere s behavior can be used to advance the quality of GPS operation and GPS assisted services After processing the GPS observations the Total Electron Content TEC which is the integral of the electron density over the ray path that links the satellite with the ground receiver can be calculated GPS TEC determinations are the result of a calibration process that includes models and assumptions that are widely accepted and used within the scientific community Based on the GPS TECs obtained from pairs of co-located GPS receivers this contribution shows GPS TEC single differences of more than 10 TECu exceeding several times the expected value derived from the calibration models The analysis focalizes on two major issues the errors translated from the code-delay to the carrier-phase ionospheric observable by the so-called levelling process and the stability of
[show abstract][hide abstract] ABSTRACT: The problem of ionospheric variability is regarded from the point of view of spatial and temporal correlation between the GPS TEC variations derived from the neighbor GPS receivers records. It is demonstrated that the technique of the spatial correlation coefficient developed earlier to reveal the ionospheric variations induced by seismic activity sometimes fails. The new index of the local ionospheric variability is proposed describing the spread of GPS TEC within the given area. It is tested for periods of geomagnetic disturbances and periods of several days preceding the strong (M ≥ 6) earthquakes happened within the area of GPS receiver’s network. It is shown that the new index is a good indicator of the earthquake preparation process, it increases few (3-7) days before the seismic shocks and comes to the normal state after the earthquake. During the periods of increased geomagnetic activity the index does mot show such variability.
Proceedings of the 6th International Conference “PROBLEMS OF GEOCOSMOS”, St. Petersburg, Petrodvorets May 23–27; 01/2006
[show abstract][hide abstract] ABSTRACT: The paper examines the possible relationship of anomalous variations of different atmospheric and ionospheric parameters observed around the time of a strong earthquake (M<sub> w </sub> 7.8) which occurred in Mexico (state of Colima) on 21 January 2003. These variations are interpreted within the framework of the developed model of the Lithosphere-Atmosphere-Ionosphere coupling. The main attention is focused on the processes in the near ground layer of the atmosphere involving the ionization of air by radon, the water molecules' attachment to the formed ions, and the corresponding changes in the latent heat. Model considerations are supported by experimental measurements showing the local diminution of air humidity one week prior to the earthquake, accompanied by the anomalous thermal infrared (TIR) signals and surface latent heat flux (SLHF) and anomalous variations of the total electron content (TEC) registered over the epicenter of the impending earthquake three days prior to the main earthquake event. Statistical processing of the data of the GPS receivers network, together with various other atmospheric parameters demonstrate the possibility of an early warning of an impending strong earthquake.
[show abstract][hide abstract] ABSTRACT: Anomalous ionospheric variations associated with the process of strong earthquake (M>5) preparation has been fairly well established. To check possible ionospheric variations connected with the recent Colima earthquake, M=7.8, of 21 January 2003, the data of five stationary GPS receivers of the National Institute of Statistics, Geography and Informatics (INEGI) of Mexico network were analyzed. It was found that the vertical total electron content showed anomalies two to three days before the seismic shock, while the daily cross correlation coefficient calculated for the different pairs of GPS receivers presented a drop within an interval of one to five days before the seismic shock. Also, the horizontal spatial (latitude-longitude) distribution of the TEC deviation had its peak of deviation in a point close to the vertical projection on the ionosphere of the impending earthquake epicenter, thus revealing the local character of the observed anomaly. We conclude, that the observed variations in the ionosphere can be regarded as a possible short-term earthquake precursors.
[show abstract][hide abstract] ABSTRACT: In this paper we focus on the variability of electron concentration in the ionosphere measured by ground based ionosondes and GPS receivers around the time of strong earthquakes. It has been detected and statistically proven that several days before the seismic shock the level of this variability increases at the station closest to the epicenter, a fact which can be regarded as precursory phenomenon. More precisely the localness of this specific kind of ionospheric variability is used for the correlation analysis of data of several observation points. The similarity of geographical location of the observation points leads to the similarity of ionospheric variations registered at these sites during both quiet and disturbed geomagnetic conditions, except in the case of those located at the seismoactive zone. As a rule, the local anomalies in the F2 layer and TEC accompanying the preparation of strong earthquakes show themselves in the breaking of the mutual correlation of the critical frequencies f<sub>o</sub>F2 or TEC between stations situated in and outside the seismic zone. The precursory phenomenon appears 1 to 7 days before the time of the seismic shock.
[show abstract][hide abstract] ABSTRACT: The present work analyzes results coming from global maps of ionospheric
total electron content (TEC) obtained from observations and from
different empirical models like the International Reference Ionosphere
(IRI), the model family developed at Trieste and Graz (NeQuick,
COSTprof, and NeUoG-plas), and the GPS operational model (formulation by
J. A. Klobuchar). Since they still appear in the context of assessment
studies we have also included "old" models like the Bent model. The
attention is focused on situations which occurred in the present period
of high solar activity, pointing out features like the equatorial
anomaly and polar regions, which are crucial test regions for
ionospheric TEC models. Experimental estimates of slant TEC from
International GPS Service (IGS) stations have been compared particularly
with the predictions of the NeQuick and GPS models. A very simple
picture of the ionosphere like the one given by the GPS operational
model appears to be insufficient to reproduce the global complex
behavior of the ionosphere, as it is needed for assessment studies or
for modern operational real-time corrections of transionospheric
propagation errors. The IRI estimates of TEC still present serious
problems, essentially owing to the topside under high solar activity
conditions, and the model cannot be integrated to heights above 2000 km.
With processing resources suitable for real-time operation, it seems
that the NeQuick model can give a more widely reliable picture of the
TEC estimated from GPS measurements. Computing times for this model are
considerably smaller than for more complex models like NeUoG-plas.
Radio Science 01/2004; 39(2). · 1.00 Impact Factor
[show abstract][hide abstract] ABSTRACT: The evaluation of Total Electron Content from Differential Delay data requires proper programs, which have been set up at IROE to process measurements from Navy Navigation Satellite System and Global Positioning System. Problems inherent to this analysis are: phase ambiguities and unknown hardware delays, the mapping function used to estimate vertical Total Electron Content from the slant one, and, for Global Positioning System only, the plasmaspheric content. The use of a “truth” ionosphere, as the one provided by International Reference Ionosphere, enables the production of artificial data. The Total Electron Content obtained processing these data can be compared with the “truth” Total Electron Content, helping in insulating the various problems, to give a deeper insight into the modeling of the analysis.
Advances in Space Research 01/2002; 29(6):959-966. · 1.18 Impact Factor