C.H. Liu

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.

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.

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.

New observational techniques for studying the dynamics of the middle atmosphere using the Chung Li VHF radar

Abstract Taking advantage of the newly developed volume scattering model for MST radar and the unique features of the Chung Li VHF radar, several novel observational techniques have been developed and implemented. Techniques such as oblique spaced antenna (OSA), the frequency domain analysis of spaced antenna data, the full spectrum analysis (FSA) and the multifrequency frequency domain interferometer (FDI) will be discussed and experimental results will be presented. Potential applications of the new techniques to study the dynamics of the middle atmosphere will be discussed.

Anatomy of plasma structures in an equatorial spread F event

This paper investigates the small scale plasma structures observed by ROCSAT-1 in the equatorial F region through the newly developed Hilbert-Huang transform (HHT) method in the time (space) domain under the frozen-in approximation. The new method allows us to decompose the non-stationary, nonlinear data into a finite number of intrinsic scale modes. In this report the structures of vertical ion velocity and horizontal density gradient inside a plasma bubble are analyzed mode by mode anatomically without making the usual linearization assumption. We found that the intrinsic modes for velocity and density gradient of the selected event have identical wave form for structures with scales between 300 m and 50 m. This implies that the vertical velocity fluctuations induced from the electric field follows the exact Boltzmann relation in the limited regime of scale length between 300 m and 50 m. A spectral break at 50 m is clearly seen in the velocity HHT spectrum. The spectral form of velocity differs greatly from that of density gradient at scale lengths shorter than 50 m.

Numerical simulations of the saturated gravity wave spectra in the atmosphere

Abstract A two dimensional numerical model is used to compute the saturation of small scale gravity waves in the region near the critical level. The vertical wave number spectrum of horizontal velocity fluctuations in the unstable region (USR) where shear instability develops is found to be governed by wave-shear interaction and follows a theoretical saturation spectrum ~ ω b 2 /2 m 3 . Wave-shear interaction is also found to be responsible for the observed fact that the variance of vertical velocity fluctuations is significantly lower than the level predicted by linear gravity wave theory. On the other hand, the corresponding spectrum in the stable region (SR) following a much shallower spectrum ~m −2 is found to result from the combined effects of wave-wave interactions and eddy diffusion. The key step in our simulation is the separate parameterization of horizontal and vertical eddy diffusion coefficients instead of a constant molecular viscosity coefficient.

Measurement of vertical phase and group velocities of atmospheric gravity waves by VHF radar

Abstract A systematic method of deriving from MST radar data the group velocity and phase velocity of the atmospheric wave along the radar beam direction is proposed and verified by a series of numerical simulations. We apply the method to two data sets measured by Chung-Li radar under different background wind conditions. It is found that the vertical group velocity and phase velocity are mostly in the opposite direction when the background wind is weak. The energy source of downgoing wave packets was evidently related to the instability in the upper height range (10.5–11.7 km) where strong wind shear existed. When the background wind and wind shear are stronger, the vertical group and phase velocities may propagate in the same direction. We also found from numerical simulation and data analysis that the wave packet of gravity waves following power law spectrum are short-lived. A by-product of the group velocity measurement is that the horizontal wavelength may also be deduced from a vertical radar beam measurement from the dispersion relation if it is valid.

A study of velocity fluctuation spectra in the troposphere and lower stratosphere using MU radar

Abstract We present an analysis of the vertical wave number and frequency spectra of atmospheric motions in the height ranges between 5 and 25 km observed using the Shigaraki, Japan, MU radar during a 4-day period in January 1988. The vertical wave number spectrum of the horizontal velocity fluctuation is found to saturate at large wave numbers satisfying power law ~ N 2 2K 3 z while departing from this −3 power law at small wave numbers. Frequency spectra of the oblique radial velocity fluctuations can be fitted by a Garrett-Munk gravity wave model spectrum. However, the vertical velocity fluctuation cannot be fitted simultaneously. The observed spectra are too steep and their energy levels are too low compared with the results from model prediction. Also, the vertical profiles of the energy densities of the horizontal velocity fluctuations are found to be positively correlated to the background wind velocity profile. These characteristics of the observed spectra are satisfactorily explained by dynamic instability and wave-wave interactions in the regions below the critical layer through nonlinear numerical simulations. The correlation between the background wind and the horizontal velocity fluctuations is shown to result from wave-shear interaction.

E region observations over Chung‐Li during the SEEK Campaign

During the Sporadic E Experiment over Kyushu (SEEK) campaign E-region field aligned irregularities (FAI) were observed with the Chung-Li VHF Radar (operated on 52 MHz) in Taiwan between August 15-22, 1996. The characteristics of the quasi-periodical echoes from the E-region about 80 km north of the Chung-Li radar are studied. During the same period ionogram records were taken with the Digisonde located at the Chung-Li radar site. Strong sporadic-E layers were detected with the Digisonde when the FAI, observed with the VHF radar, were also very intense. The variations of sporadic E-layer critical frequency and of the field-aligned irregularities were studied. The latter were estimated to be located at altitudes 95-115 km occurring in layers of 5-15 km thickness, moving downwards at a rate of about 2.5 km/h. There seemed to be a change over of the FAI echoes from lower to upper E-region every night approximately at 23 hour LT. The distinction between post-sunrise and post-sunset FAI echoes observed earlier by MU radar was not clearly seen in Chung-Li. A preliminary examination of the morphology of the fine structures within the layers indicates quasi-periodic features with 5-10 minutes period. Many of these periodic fine structures were observed at the Chung-Li VHF radar to have opposite slopes than those detected earlier with the MU radar in Japan, which is located 1500 km north of the Chung-Li VHF radar.

Doppler velocities obtained by the EISCAT VHF radar and the dynasonde during the PMSE95 campaign

Abstract In this paper, we examine the Doppler velocities simultaneously derived from the EISCAT VHF radar and the dynasonde, under the PMSE conditions. Spectral and filtering analyses are applied to study the velocities in detail. It is found that the high- and the low-frequency motions derived from the two radars correlate differently, high for low-frequency and low for high-frequency component. The agreement/discrepancy between the two velocities are interpreted in terms of radar scattering/operation principles and characteristics of acoustic gravity waves.