Lung-Chih Tsai

Comparisons of GPS/MET retrieved ionospheric electron density and ground based ionosonde data

The Global Positioning System/Meteorology (GPS/MET) mission has been the first experiment to use a low Earth orbiting (LEO) satellite (the MicroLab-1) to receive multi-channel Global Positioning System (GPS) carrier phase signals and demonstrate active limb sounding of the Earth’s atmosphere and ionosphere by radio occultation technique. Under the assumption of spherical symmetry at the locality of the occultation, the dual-band phase data have been processed to yield ray-path bending angle profiles, which have then been used to yield profiles of refractive index via the Abel integral transform. The refractivity profiles can then, in turn, yield profiles of ionospheric electron density and other atmospheric variables such as neutral atmospheric density, pressure, and temperature in the stratosphere and upper troposphere, and water vapor in the lower troposphere with the aid of independent temperature data. To approach a near real-time process, electron density profiles can also be derived by the Abel transform through the computation of total electron content (TEC) assuming straight-line propagation (neglecting bending). In order to assess the accuracy of the GPS/MET ionospheric electron density retrievals, coincidences of ionosonde data with GPS/MET occultations have been examined. The retrieved electron density profiles from GPS/MET TEC observations have been compared with ionogram inversion results derived from digital ionospheric sounders operated by the National Central University (the Chung-Li digisonde; 24.6°N, 121.0°E) and by Utah State University (the Bear-Lake dynasonde; 41.9°N, 111.4°W). A fuzzy classification method for the automatic identification and scaling of ionogram traces has been applied to recorded ionograms, and then bottomside ionospheric electron density profiles are determined from true-height analysis. The comparison results show better agreement for both of the derived electron density profiles and the F2-layer critical frequency ( foF2) at mid-latitude observations than at low-latitude observations. The rms foF2 differences from the GPS/MET retrievals are 0.61 MHz to the Bear-Lake dynasonde measurements and 1.62 MHz to the Chung-Li digisonde measurements.

Vertical phase and group velocities of internalgravity waves derived from ionograms during the solareclipse of 24 October 1995

Abstract A procedure is developed to derive the vertical phase and group velocities of wavesfrom measurements of ionograms. We apply the developed procedure to a sequence of ionogramsrecorded by the digisonde portable sounder in Taiwan and find numerous waves occuring in theionosphere during the solar eclipse of 24 October 1995. A detailed analysis of an 87-min periodwave shows that the vertical phase velocities of the wave below and above the F1 ledgeare about 100 m/s in the downward and upward directions, respectively. The associated groupvelocities below and above the ledge are found to be about 10 m/s in the downward and upwarddirections, respectively, which indicates that during the solar eclipse the wave source is near the F1-ledge.

Ionogram analysis using fuzzy segmentation and connectedness techniques

We present a new procedure for the analysis of ionograms that evolves from methods developed for image analysis and utilizes techniques based on the concepts of fuzzy segmentation and connectedness. Ionogram traces are often not “crisply” defined, and we demonstrate that it is possible to approximate them as fuzzy subsets within the two-dimensional space defined by the time-of-flight and the radio frequency. A real number between 0 and 1 is assigned to each pixel in an ionogram, thereby defining the membership of that pixel to each of the fuzzy subsets, effectively creating a “gray scale” ionogram. In this context, ionogram analysis becomes a problem in fuzzy geometry, and various geometrical properties, including the topological concepts of connectedness, adjacency, height, width, and major axis, can be defined. It is shown that not only does the fuzzy segmentation process separate signals from the chaotic noise background that often characterizes ionograms, but that it can also be applied to classify ionospheric echoes according to standard nomenclature, e.g., normal E, F, or Es layers. Furthermore, in reference to the skeleton or thinning extraction procedures employed in imaging processing, the fuzzy connectedness between echoes in selected segments can be used to determine the primary layers that are characteristic of vertical incidence ionospheric reflection. This information can be provided as input to automatic scaling or true-height inversion routines, which can then be used to derive either the standard URSI set of ionospheric parameters or the electron density distribution in the overhead ionosphere, or both. This fuzzy algorithm approach has been successfully applied to midlatitude ionogram data from advanced digital ionospheric sounders operated by the National Central University and Utah State University.

Tomographic imaging of the ionosphere using the GPS/MET and NNSS data

Abstract The earlier experiments of ionospheric tomography were conducted by receiving satellite signals from ground-based stations and then reconstructing electron density distribution from measures of the total electron content (TEC). In June 1994, National Central University built up the low-latitude ionospheric tomography network (LITN) including six ground stations spanning a range of 16.7° (from 14.6°N to 31.3°N) in latitude within 1° of 121°E longitude to receive the naval navigation satellite system (NNSS) signals (150 and 400 MHz ). In the study of tomographic imaging of the ionosphere, TEC data from a network of ground-based stations can provide detailed information on the horizontal structure, but are of restricted utility in sensing vertical structure. However, an occultation observation mission termed the global positioning system/meteorology (GPS/MET) program used a low Earth orbiting (LEO) satellite (the MicroLab-1) to receive multi-channel GPS carrier phase signals (1.5 and 1.2 GHz ) and demonstrate active limb sounding of the Earths atmosphere and ionosphere. In this paper, we have implemented the multiplicative algebraic reconstruction technique (MART) to reconstruct and compare two-dimensional ionospheric structures from measured TECs through the receptions of the GPS signals, the NNSS signals, and/or both of the systems. We have also concluded the profiles retrieved from tomographic reconstruction showing much reasonable electron density results than the original vertical profiles retrieved by the Abel transformation and being in more agreement in peak electron density to nearby ionosonde measurements.

Ionospheric electron content anomalies detected by a FORMOSAT-3/COSMIC empirical model before and after the Wenchuan Earthquake

An empirical model of ionospheric electron content (IEC), based on FORMOSAT3/COSMIC (F3/C) data, is constructed in order to detect pre-earthquake anomalies. The empirical model provides IEC with four parameters of local time, season, longitude and latitude. For the first time we try to detect anomalies in the F3/C IEC by comparing the model values with observations during the 12 May 2008 Wenchuan Earthquake period. It is found that around the epicentre an IEC enhancement appears on day 3 (9 May) and sequential IEC reductions occur on day 6 to day 1 (6 to 11 May) before the earthquake.

The effect of geomagnetic storm on ionospheric total electron content at the equatorial anomaly region

Abstract The effects of geomagnetic storm on ionospheric total electron content (TEC) have been investigated by using the Global Positioning System (GPS) data during two observations on January 10 and May 15, 1997, respectively. It is found that after the onset of sudden storm commencement (SSC), the equatorial anomaly crests move poleward, and the daytime TEC is significantly reduced one day after SSC. Some possible mechanisms are proposed to explain the above phenomena.

On the derivation of an improved parameter configuration for the Dynasonde

The inherent precision and ambiguity in the measurement of ionospherically reflected echoes by digital ionosondes, which utilize interferometric receiving arrays, depends on the transmitted pulse set pattern, the receiving array configuration, and the data analysis scheme. Building on earlier work carried out for the National Oceanic and Atmospheric Administration HF radar [Grubb, 1979] by Pitteway and Wright [1992], we use six phase parameters (Φo, Φx, Φy, Φl, Φp, and Φf) to derive echo location, Doppler shift, and wave polarization. We have applied the method of least squares to determine the precision and a “zero-freedom” technique to derive the ambiguity associated with each of the phase parameters. Three criteria can be specified which lead to an optimum design of the system parameters: (1) the phase parameters must have aliasing values equal to 2π; (2) the relative confidence limit factors of the derived phase parameters should be as small as possible; and (3) there must be no aliasing of echo location. By varying the array configuration, frequency pattern, the number of pulses per pulse set, and the number of parallel receivers and receiving dipoles, various designs have been analyzed, and an improved configuration has been obtained. We have shown that various arrangements of the four-pulse, two-receiver configuration can reduce the sum of the relative uncertainties in the phase parameters found for the WERPOL array by up to 33% and improve the ambiguity in each of the derived phase parameters to 2π.

A comparison of a model using the FORMOSAT-3/COSMIC data with the IRI model

In this study, an empirical model constructed using data of FORMOSAT3/COSMIC (F3/C) from 29 June, 2006, to 17 October, 2009, retrieves altitude profiles of electron density (Ne). The model derives global Ne profiles from 150 to 590 km altitude as functions of the solar EUV flux, day of year, local time and location under geomagnetically quiet conditions (Kp < 4). Ne profiles derived by the model are further compared with those of the International Reference Ionosphere (IRI). Results show that the F2 peak altitude hmF2 and the electron density NmF2, as well as the electron density above, derived by the model are lower than those of the IRI model. The F3/C model reproduces observations of F3/C well at 410-km altitude while the IRI model overestimates them. The overestimation of the IRI model becomes large with decrease of EUV flux. It is found that the topside vertical scale height of the F3/C model shows high values not only magnetic dip equator but also middle latitude. The results differ significantly from those of IRI, but agree with those observed by topside sounders, Alouette and ISIS satellites.

Ionospheric total electron content observed during the 24 October 1995 solar eclipse

Abstract During the solar eclipse of October 24, 1995, the effect of an eclipse on the total electron content (TEC) of the ionosphere can be investigated by using measurement of the Global Positioning System (GPS). The TEC derived from five GPS ground-based receivers have been used to observe ionospheric variations over the geomagnetic equatorial, equatorial anomaly, and mid-latitude regions. The deviations in the TECs on the eclipse day from those on reference days show that during the eclipse days the ionosphere experienced some changes. Four features of the TEC deviations, pre-ascension (PA), major depression (MD), sunset ascension (SA), and secondary depression (SD) have been observed. Possible mechanisms explaining in the four features are investigated and discussed.

Derivation and error analysis of echo phase parameters for the dynasonde

Ionospherically reflected echoes are received with a four-element multiplexed interferometer array and two phase-matched receivers in implementations of the National Oceanic and Atmospheric Administration dynasonde. From these data, six phase parameters (Φ0, Φx, Φy, Φt, Φp, and Φƒ) are obtained and used to derive echo location, Doppler velocity, wave polarization, and virtual range. Since 2 π aliasing is an inherent feature of interferometric spaced antenna phase measurements, the phase parameters cannot be derived directly from the measured phase values using the method of least squares. In this work, we introduce a general procedure for the derivation of these parameters that (1) employs a “zero-freedom” technique to derive initial estimates of the phase parameters, (2) derives shifted values of the measured phases from the six estimates, and (3) uses the method of least squares in conjunction with the shifted phases to improve the phase parameter estimates. This procedure minimizes the phase ambiguity inherent in interferometric phase measurements and derives phase parameters that approach the ideal least squares result. Furthermore, the ionospheric echo “quality” is quantified by two error parameters, defined asŝ, the least squares of the measured phase errors, and EP, the RMS phase error. It is also shown that the value of ŝ relative to the square of the standard deviation of measured phase is equal to the number of degrees of freedom in the phase parameters. Applying these techniques to data acquired with the Utah State University dynasonde, we derive a standard deviation of <2° in the measured echo phase.