Wei-Hsiung Tsai

Seasonal variations of the ionospheric total electron content in Asian equatorial anomaly regions

The ionospheric total electron contents (TEC) in both northern and southern equatorial anomaly regions are examined by using the Global Positioning System (GPS) in Asian area. The TEC contour charts obtained at YMSM (25.2°N, 121.6°E; 14.0°N geomagnetic) and DGAR (7.3°S, 72.4°E; 16.2°S geomagnetic) stations in 1997, solar minimum, are investigated. It is found that the ionospheric crests manifest remarkable seasonal variations. The TEC values on both northern and southern equatorial anomaly crests yield their maximum values during the vernal and autumnal months, but the winter anomaly does not appear in the southern region. Results show that both crests are fully developed around midday in winter, postnoon in equinoxes and late afternoon in summer, and the two crests move significantly equatorward in winter but slightly poleward in summer and autumn. These phenomena can be fully explained by a combined theory of the transequatorial netural wind, the subsolar point, and the auroral equatorward wind.

A study of tomographically reconstructed ionospheric images during a solar eclipse

The low-latitude ionospheric tomography network (LITN) consists of a chain of six Naval Navigation Satellite System (NNSS) receiving stations established along 121°E longitude from a geographic latitude of 14.6°N to 31°N. It is specifically designed to observe large-scale ionospheric variations over the equatorial anomaly region by using tomographic imaging techniques. Recently, the network LITN was applied to observations of the October 24, 1995, solar eclipse. Two-dimensional images of ionospheric electron density during the eclipse period were reconstructed. These images and the corresponding results from a nearby ionosonde were compared with those for a reference day. It is shown that during the eclipse day the ionosphere experienced some large-scale changes. In particular, four episodes of electron density enhancement or depression have been identified. (1) The maximum enhancement occurred before the maximum phase of the solar eclipse at approximately 7°–10°N geomagnetic latitude at the 275–300 km ionospheric height. (2) The second enhancement appeared roughly 3 1/2 hours after the maximum obscuration at 15°–22°N geomagnetic latitude and 300–325 km ionospheric height. (3) The largest electron density depression occurred roughly 2 hours after the maximum obscuration at approximately 9°–15°N geomagnetic latitude and on both the bottom and topside ionosphere. (4) The second depression occurred about 4 hours after the maximum obscuration at approximately 5.5°N geomagnetic latitude and mainly on the topside ionosphere. More detailed study suggests that the two enhancements have their origins in the ionospheric day-to-day variations, the first depression is related to the combined photochemical and the equatorial fountain effects, and the second depression may have its origin in geomagnetic coupling between conjugate ionospheres. These observations are interpreted within the framework of ionospheric dynamics in the equatorial anomaly region.

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.

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.

The low-latitude ionospheric tomography network (LITN)—initial results

Abstract The Low-latitude Ionospheric Tomography Network (LITN), a chain of six stations located along the 121 °E meridian receiving signals from the Transit NNSS satellites, carries out tomographic investigations of the ionosphere in this equatorial anomaly region. The technical aspects of the network are introduced. Because of the steep latitudinal gradients of the ionosphere in this region, a special procedure has been developed to facilitate the reconstruction process. Model simulations are carried out to help formulate the best reconstruction algorithm. Initial results from the first set of data from the full network are presented, and the diurnal behavior of the anomaly discussed. Comparisons of the reconstructed electron density profiles with those derived from ionograms and the reconstructed vertical TECs with those observed appear to indicate that the LITN can be used to provide a two-dimensional image of the ionosphere in the equatorial region.

IRI model application in low latitude ionospheric tomography

Abstract We report the progress in developing a Low-latitude Ionospheric Tomography Network (LITN) along the 121°E meridian(Figure 1). LITN consists of an array of NNSS receiving stations covering the latitude range from Manila to Shanghai, and is specifically designed to enable large scale tomographic imaging over the equatorial anomaly region. Recently, we successfully applied computerized tomographic technique to obtain two-dimensional(2D) images of ionospheric electron density using measured TEC data from the network. The reconstruction is based on multiplicative algebraic reconstruction technique using the IRI model as an initial guess. To verify the quality of the reconstruction image, electron density profiles are reproduced from the image and compared with the profiles measured by ionosonde. It will be shown that IRI models can be used to facilitate such reconstruction.

A hybrid simulation of contact discontinuity

Contact discontinuities in a collisionless plasma are studied by hybrid simulations, in which ions are treated as particles and electrons are considered as a fluid. It is demonstrated that contact discontinuity with a stable density ramp can exist in cases with a finite electron temperature. An electron pressure gradient is present across the contact discontinuity, leading to the presence of a parallel electric field and hence field-aligned potential increase ({Delta}{Phi}{parallel}) in the transition region. By reflecting ions at the discontinuity, this parallel electric potential peak reduces the interpenetration between hot and cold ions and maintains a stable density ramp across the contact discontinuity. The ratio of the field-aligned electric potential energy to ion thermal energy, e{Delta}{Phi}{parallel}/k{Tau}{sub i}, is found to be an increasing function of {Tau}e/{Tau}i, where {Tau}e and {Tau}i are respectively the electron and in temperature. 11 refs., 3 figs.

A study on the cosmic electron density profile

Abstract This paper shows a simulation process of retrieving ionospheric electron density from the Global Positioning System/Meteorology (GPS/MET) to examine the accuracy of the retrieval procedure for the COSMIC project 3 during solar minimum and maximum periods. Results show that the error of the maximum electron density ( NmF 2 ) is under 25%, and the error is greater in the solar maximum than in the solar minimum period. The mean error of the height of NmF 2 (i.e. hmF 2 ) is less than about 13 km and seems to have no correlation with solar activity.