Chien-Ya Wang

Interferometry observations of three‐dimensional spatial structures of sporadic E irregularities using the Chung‐Li VHF radar

By using the interferometry technique implemented at the Chung-Li VHF radar, the striated echoes with quasi-periodic characteristics in the range-time-intensity plot generated from the electron density irregularities associated with sporadic E layer are investigated. It is shown that the Es irregularities above 110 km drifting mostly westward along a stationary path of a few kilometers width are responsible for the striated echoes. Considering the field-aligned property of the Es irregularities and the geometry of the echoing region over the Chung-Li radar site, it indicates that this stationary path is the cross section of a tilted layer which has a sharp electron density gradient in the direction across the layer parallel to the magnetic field line in the E region and orients geographically 72°NW. The observations also demonstrate that the echoing regions of the Es irregularities over the Chung-Li radar station are confined on the right side of a tilted thin plane with the thickness of a few kilometers at the elevation angle of 52° in the radar viewing region. These characteristics can be explained by using the radar backscatter from field-aligned targets in the field-perpendicular direction. The behavior of the sporadic E layer in the equatorial anomalous region is also investigated and discussed, and a descending sporadic E layer modulated by the gravity waves is observed. The descent rate of the layer is about 3.6 m/s, considerably larger than that reported by other investigators. The primary gravity wave modulating the sporadic E layer has a period of 12–15 min and propagates upward in phase with a vertical wavelength of about 50 km. Moreover, a positive correlation between the peak intensity of radar returns from Es irregularities below 110 km and the vertical shear of their horizontal drift velocity is seen. This feature, combined with the positive correlation between radar backscatter and the Doppler spectral width, strongly suggests that the crucial role the neutral wind plays in the excitation of the Es irregularities below 110 km cannot be ignored.

Morphology of sporadic E layer retrieved from COSMIC GPS radio occultation measurements: Wind shear theory examination

On the basis of the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC)-measured fluctuations in the signal-to-noise ratio and excess phase of the GPS signal piercing through ionospheric sporadic E (Es) layers, the general morphologies of these layers are presented for the period from July 2006 to May 2011. It is found that the latitudinal variation in the Es layer occurrence is substantially geomagnetically controlled, most frequent in the summer hemisphere within the geomagnetic latitude region between 10° and 70° and very rare in the geomagnetic equatorial zone. Model simulations show that the summer maximum (winter minimum) in the Es layer occurrence is very likely attributed to the convergence of the Fe+ concentration flux driven by the neutral wind. In addition to seasonal and spatial distributions, the height-time variations in the Es layer occurrence in the midlatitude (>30°) region in summer and spring are primarily dominated by the semidiurnal tides, which start to appear at local time around 6 and 18-h in the height range 110-120-km and gradually descend at a rate of about 0.9-1.6-km/h. In the low-latitude (<30°) region, the diurnal tide dominates. The Horizontal Wind Model (HWM07) indicates that the height-time distribution of Es layers at middle latitude (30°-60°) is highly coincident with the zonal neutral wind shear. However, Es layer occurrences in low-latitude and equatorial regions do not correlate well with the zonal wind shear. Key Points Examination of Es layer summer maximum phenomenon Global distribution of COSMIC-retrieved Es layer Es layer formation and wind shear mechanism.

Interferometry investigations of VHF backscatter from plasma irregularity patches in the nighttime E region using the Chung‐Li radar

Backscatter spectra with extremely narrow spectral width of only 2 to 7 m/s and small mean Doppler velocity of 10 to 30 m/s associated with plasma irregularity patches in nighttime sporadic E layers have been obtained with the Chung-Li VHF radar. The radar interferometer shows these echoes to be highly aspect sensitive with striated structures in the horizontal and azimuthal planes. By considering the spatial distributions of the Es echoes and the geometry of the effective radar beam, we propose a schematic model in which multiple thin and tilted Es layers with steep electron density gradients are drifting across the illuminated region. The results show that the layers appear to be tilted toward east at angles of 3° to 14°, and their horizontal drift velocities (obtained by tracking the patches with the interferometer phase) imply primarily eastward motions at speeds of 7.5 to 56.2 m/s.

Coordinated observations of F region 3 m field-aligned plasma irregularities associated with medium-scale traveling ionospheric disturbances

Three meter field-aligned irregularities (3 m FAIs) associated with medium-scale traveling ionospheric disturbances (MSTIDs) that occurred on 5 February 2008 were observed by using the Chung-Li 52 MHz coherent scatter radar. Interferometry measurements show that the plasma structures responsible for the 3 m FAI echoes are in a clumpy shape with a horizontal dimension of about 10–78 km in a height range of 220–300 km. In order to investigate the dynamic behaviors of the plasma irregularities at different scales in the bottomside of F region, the VHF radar echo structures from the 3 m FAIs combined with the 630 nm airglow images provided by the Yonaguni all-sky imager are compared and analyzed. The results show that the radar echoes were located at the west edge of the depletion zones of the 630 nm airglow image of the MSTIDs. The bulk echo structures of the 3 m FAIs drifted eastward at a mean trace velocity of about 30 m/s that is in general agreement with the zonal trace velocity of the MSTIDs shown in the 630 nm airglow images. These results suggest that the observed F region 3 m FAIs for the present case can be regarded as the targets that are frozen in the local region of the MSTIDs. In addition, the radar-observed 3 m FAI echo intensity and spectral width bear high correlations to the percentage variations of the 630 nm emission intensity. These results seem to suggest that through the nonlinear turbulence cascade process, the MSTID-associated 3 m FAIs are very likely generated from the kilometer-scale plasma irregularities with large amplitude excited by the gradient drift instability.

Meteor radar wind over Chung‐Li (24.9°N, 121°E), Taiwan, for the period 10–25 November 2012 which includes Leonid meteor shower: Comparison with empirical model and satellite measurements

The neutral winds in the mesosphere and lower thermosphere (MLT) region are measured by a newly installed meteor trail detection system (or meteor radar) at Chung-Li, Taiwan, for the period 10–25 November 2012, which includes the Leonid meteor shower period. In this study, we use the 3 m field-aligned plasma irregularities in the sporadic E (Es) region in combination with the International Geomagnetic Reference Field model to calibrate the system phase biases such that the true positions of the meteor trails can be correctly determined with interferometry technique. The horizontal wind velocities estimated from the radial velocities of the meteor trails and their locations by using a least squares method show that the diurnal tide dominates the variation of the MLT neutral wind with time over Chung-Li, which is in good agreement with the horizontal wind model (HWM07) prediction. However, harmonic analysis reveals that the amplitudes of the mean wind, diurnal, and semidiurnal tides of the radar-measured winds in height range 82–100 km are systematically larger than those of the model-predicted winds by up to a factor of 3. A comparison shows that the overall pattern of the height-local time distribution of the composite radar-measured meteor wind is, in general, consistent with that of the TIMED Doppler Interferometer-observed wind, which is dominated by a diurnal oscillation with downward phase progression at a rate of about 1.3 km/h. The occurrences of the Es layers retrieved from fluctuations of the amplitude and excess phase of the GPS signal received by the FORMOSAT-3/COSMIC satellites during the GPS radio occultation (RO) process are compared with the shear zones of the radar-measured meteor wind and HWM07 wind. The result shows that almost all of the RO-retrieved Es layers occur within the wind shear zones that favor the Es layer formation based on the wind shear theory, suggesting that the primary physical process responsible for the Es layer events retrieved from the scintillations of the GPS RO signal is very likely the plasma convergence effect of the neutral wind shear.

Cross Spectral Analysis of CODAR-SeaSonde Echoes from Sea Surface and Ionosphere at Taiwan

It is well known that the primary targets responsible for first-order sea echoes observed by a High-Frequency (HF) radar are the advancing and receding ocean waves with the wavelengths at Bragg scales. However, in light of the fact that the ionospheric sporadic E (Es) and F layers may be present in the viewing range of the HF radar for ocean wave detection, the radar returns reflected from the F and Es layers may significantly contaminate the ocean wave power spectrum. The characteristics of the first-order sea echoes and ionospheric interferences measured by the CODAR-SeaSonde in Taiwan area are analyzed and presented in this article. The coherences and phases of the normalized cross spectra of the sea and ionospheric echoes between different pairs of the receiving channels are calculated, respectively. One of the striking features presented in this report is that the ionospheric echo heights scaled from the ionogram observed by the Chung-Li ionosonde are about 30 km lower than those observed by the DATAN CODAR-SeaSonde. It is also found that the coherences of the sea echoes are generally smaller than those of the ionospheric echoes by about 15% on average, and the phase fluctuations (standard deviations) of the sea echoes are substantially larger than those of the ionospheric layer reflection echoes. In addition, statistics show that the sum of the mean phases of the ionospheric echoes between the three receiving channel pairs is approximately zero, while it is not for the sea echoes. These results seem to suggest that the use of the discrepancies in the characteristics of the coherences and phases between the sea and ionospheric echoes may provide a potential means to be helpful to distinguish the sea and ionospheric echoes in the CODAR-SeaSonde observed cross power spectrum.

Sea surface observations using a VHF radar with multireceiver beamforming technique

A VHF pulse radar was used to observe the sea surface in the northern Taiwan Strait. Radar frequency is 52 MHz, and a range cell was 300 m. Four vertical dipole antennas, arranged linearly along the coastline with 3 meter separation between adjacent antennas, were used to collect the radar returns. The self-adaptive Capon method was employed in this study to determine the direction of arrival (DOA) of the sea echoes. Analysis shows that pronounced semidiurnal tidal modes were presented in the temporal variations of observed DOA, echo power, and Doppler velocity.