Feza Arikan

North‐south components of the annual asymmetry in the ionosphere

A retrospective study of the asymmetry in the ionosphere during the solstices is made using the different geospace parameters in the North and South magnetic hemispheres. Data of total electron content (TEC) and global electron content (GEC) produced from global ionospheric maps, GIM-TEC for 1999-2013, the ionospheric electron content (IEC) measured by TOPEX-Jason 1 and 2 satellites for 2001-2012, the F-2 layer critical frequency and peak height measured on board ISIS 1, ISIS 2, and IK19 satellites during 1969-1982, and the earthquakes M5+ occurrences for 1999-2013 are analyzed. Annual asymmetry is observed with GEC and IEC for the years of observation with asymmetry index, AI, showing January > July excess from 0.02 to 0.25. The coincident pattern of January-to-July asymmetry ratio of TEC and IEC colocated along the magnetic longitude sector of 270 degrees +/- 5 degrees E in the Pacific Ocean is obtained varying with local time and magnetic latitude. The sea/land differences in the F-2 layer peak electron density, NmF2, and the peak height, h(m)F(2), gathered with topside sounding data exhibit tilted ionosphere along the seashores with denser electron population at greater peak heights over the sea. The topside peak electron density NmF2, TEC, IEC, and the hemisphere part of GEC are dominant in the South hemisphere which resembles the pattern for seismic activity with dominant earthquake occurrence in the South magnetic hemisphere. Though the study is made for the hemispheric and annual asymmetry during solstices in the ionosphere, the conclusions seem valid for other aspects of seismic-ionospheric associations with tectonic plate boundaries representing zones of enhanced risk for space weather.

Estimation of

F2-layer is the most important and characteristic layer of the ionosphere in the propagation of high frequency (HF) waves due to the highest level of conductivity in the propagation path. In this study, the relation of Total Electron Content (TEC) with the maximum ionization height (hmF2) and the critical frequency (foF2) of F2-layer are investigated within their defined parametric range using the IRI model extended towards the plasmasphere (IRI-Plas). These two parameters are optimized using daily observed GPS-TEC (IONOLAB-TEC) in an iterational loop through Non-Linear Least Squares (NLSQ) optimization while keeping the physical correlation between hmF2 and foF2 parameters. Optimization performance is examined for daily (24-hour) and hourly TEC optimizations separately. It is observed that hourly TEC optimization produces results with much smaller estimation errors. As a result of the hourly optimization, we obtain the hourly hmF2 and foF2 estimates as they are the optimization parameters. Obtained hmF2 and foF2 estimates are compared with the ionosonde estimates for various low, middle and high latitude locations for both quite and disturbed days of ionosphere. The results show that hmF2 and foF2 estimates obtained from IRI-Plas optimization (IRI-Plas-Opt) and ionosonde are very much in agreement with each other. These results also signify that IRI-Plas provides a reliable background model for ionosphere. With the proposed method, it is possible to build a virtual ionosonde via optimization of IRI-Plas model using the observed TEC values.

hmF2

On 23 October 2011, a very strong earthquake with a magnitude of Mw = 7.2 shook Eastern Anatolia, and tremors were felt up to 500 km from the epicentre. In this study, we present an early analysis of ionospheric disturbance due to this earthquake using Global Positioning Satellite-Total Electron Content (GPS-TEC). The variability with respect to average quiet day TEC (AQDT) and variability between the consecutive days are measured with symmetric Kullback–Leibler divergence (SKLD). A significant variability in total electron content (TEC) is observed from the GPS stations in the 150 km neighbourhood of the epicentre eight and nine days prior to the earthquake. An ionospheric disturbance is observed from GPS stations even more than 1,000 km to the epicentre, especially those on the North Anatolian fault (NAF). The present results support the existence of lithosphere–atmosphere–ionosphere coupling (LAIC) associated with Van, Turkey earthquake.

and

In this paper, a statistical analysis approach is proposed to characterize the variability of HF channel response to single-tone signals by using only the amplitude information of the received signal. By the proposed methodology, robust estimates of the time varying mean and variance of the channel response can be obtained. For this purpose, we use sliding window statistics of the available data. On the basis of the estimated variance of the obtained results, a detailed justification of the proper window size is given. In order to obtain more reliable estimates, the data are median filtered prior to statistical analysis. A robust way of choosing the length of the median filter is presented. We applied the statistical analysis approach to a set of available data obtained from a measurement campaign between England and Turkey conducted from April 1992 to February 1993. The results of the statistical analysis confirmed the expectations of the physical behavior of the ionospheric channel. It was found that the midlatitude single-frequency channel is slowly time varying and locally stationary in a sliding window of 22 s. Also, it was observed that the amplitude of the received signal exhibits a significant diurnal variation. In addition, during early morning hours and night hours, the channel is considerably more stable for communication purposes compared with day and early evening hours.

foF2

Slant total electron content (STEC), the total number of free electrons on a ray path, is an important space weather observable. STEC is the main input for computerized ionospheric tomography (CIT). STEC can be estimated using the dual-frequency GPS receivers. GPS-STEC contains the space weather variability, yet the estimates are prone to measurement and instrument errors that are not related to the physical structure of the ionosphere. International Reference Ionosphere Extended to Plasmasphere (IRI-Plas) is the international standard climatic model of ionosphere and plasmasphere, providing vertical electron density profiles for a desired date, time, and location. IRI-Plas is used as the background model in CIT. Computation of STEC from IRI-Plas is a tedious task for researchers due to extensive geodetic calculations and IRI-Plas runs. In this study, IONOLAB group introduces a new space weather service to facilitate the computation of STEC from IRI-Plas (IRI-Plas-STEC) at www.ionolab.org. The IRI-Plas-STEC can be computed online for a desired location, date, hour, elevation, and azimuth angle. The user-friendly interface also provides means for computation of IRI-STEC for a desired location and date to indicate the variability in hour of the day, elevation, or azimuth angles. The desired location can be chosen as a GPS receiver in International GNSS Service (IGS) or EUREF Permanent Network (EPN). Also instead of specifying elevation and azimuth angles, the user can directly choose from the GPS satellites and obtain IRI-Plas-STEC for a desired date and/or hour. The computed IRI-Plas-STEC values are presented directly on the screen or via e-mail as both text and plots.

Communication Parameters of Ionosphere

This study investigates the efiects of incorporating Doppler velocity measurements directly into track association and maintenance parts for single and multiple target tracking unit in a multi function phased array radar (MFPAR). Since Doppler velocity is the major discriminant of clutter from a desired target, the measurement set has been expanded from range, azimuth and elevation angles to include Doppler velocity measurements. We have developed data association and maintenance part of a well known tracking method, Interacting Multiple Model Probabilistic Data Association

F2

Spatiotemporal variations in the ionosphere affects the HF and satellite communications and navigation systems. Total Electron Content (TEC) is an important parameter since it can be used to analyze the spatial and temporal variability of the ionosphere. In this study, the performance of the two widely used Kriging algorithms, namely Ordinary Kriging (OrK) and Universal Kriging (UnK), is compared over the synthetic data set. In order to represent various ionospheric states, such as quiet and disturbed days, spatially correlated residual synthetic TEC data with different variances is generated and added to trend functions. Synthetic data sampled with various type of sampling patterns and for a wide range of sampling point numbers. It is observed that for small sampling numbers and with higher variability, OrK gives smaller errors. As the sample number increases, UnK errors decrease faster. For smaller variances in the synthetic surfaces, UnK gives better results. For increasing variance and decreasing range values, usually, the errors increase for both OrK and UnK.

-Layer Using GPS Data and IRI-Plas Model

Three-dimensional imaging of the electron density distribution in the ionosphere is a crucial task for investigating the ionospheric effects. Dual-frequency Global Positioning System (GPS) satellite signals can be used to estimate the slant total electron content (STEC) along the propagation path between a GPS satellite and ground-based receiver station. However, the estimated GPS-STEC is very sparse and highly nonuniformly distributed for obtaining reliable 3-D electron density distributions derived from the measurements alone. Standard tomographic reconstruction techniques are not accurate or reliable enough to represent the full complexity of variable ionosphere. On the other hand, model-based electron density distributions are produced according to the general trends of ionosphere, and these distributions do not agree with measurements, especially for geomagnetically active hours. In this study, a regional 3-D electron density distribution reconstruction method, namely, IONOLAB-CIT, is proposed to assimilate GPS-STEC into physical ionospheric models. The proposed method is based on an iterative optimization framework that tracks the deviations from the ionospheric model in terms of F2 layer critical frequency and maximum ionization height resulting from the comparison of International Reference Ionosphere extended to Plasmasphere (IRI-Plas) model-generated STEC and GPS-STEC. The suggested tomography algorithm is applied successfully for the reconstruction of electron density profiles over Turkey, during quiet and disturbed hours of ionosphere using Turkish National Permanent GPS Network.

Observed Ionospheric Effects of 23 October 2011 Van, Turkey Earthquake

This paper presents results from a study of GPS total electron content (TEC) grid maps and ionospheric electron content (IEC) over the oceans delivered by the TOPEX/Jason satellites during half a solar cycle (July 2001 to December 2008). The IEC data are averaged and binned at latitudes from 60°S to 60°N in steps of 5°±2.5°, at longitudes from 180°W to 180°E in steps of 15°±7.5°, and for 0–23 h UT in steps of 1±0.5 h UT. The ratio of monthly averaged TEC/IEC over the oceans from the observations was compared to the reference model ratio of TECm/IECm obtained using the plasmaspheric model augmented with the International Reference Ionosphere. By definition, TEC should exceed IEC by the plasmaspheric electron content (PEC) contribution at the altitude range from 1336 km (TOPEX orbit) to 20,200 km (GPS orbit). However, as solar activity tends to the minimum, we found that IEC estimates systematically exceed those of GPS TEC. An empirical scale factor was derived in terms of the smoothed sunspot number, and this factor reduced the systematic excess of the TOPEX/Jason-derived IEC over the GPS TEC by a factor of 1.5 towards the solar minimum. This factor was tested with observations made at the solar minimum and revealed that the plasmaspheric electron content to be a residual of the GPS TEC and modified TOPEX/Jason IEC.

Statistical characterization of time variability in midlatitude single‐tone HF channel response

Computerized Ionospheric Tomography (CIT) is a method to reconstruct ionospheric electron density images by using the Global Positioning System data collected by the earth based receivers. In this study, Total Electron Content values obtained from a model based ionosphere and tomographic reconstruction techniques are used together to obtain ionospheric electron density distribution. Algebraic Reconstruction Technique (ART) is one of the most commonly used reconstruction method in medical tomography due to its simplicity in implementation. The performance of ART is independent of basis functions and very sensitive to the initial state. Total Least Squares (TLS) algorithm assumes no regularization and produces the lowest error for Haar basis for a given latitude interval. The performance of TLS is improved with the number of receivers. If only one receiver is used, TLS algorithm together with Haar basis functions produces a low computational complexity and has a lower reconstruction error compared to Regularized Least Squares Algorithm. When the estimation by TLS is input as the initial state of ART, the overall reconstruction error reduces significantly compared to the reconstruction error of ART only or TLS with Haar basis only.