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.

[1] With interferometry measurements made with Chung-Li 52 MHz VHF radar, characteristics of layer-type and clump-type plasma structures of 3-m field-aligned irregularities are investigated for the first time. Long-term statistics show that the thickness and the zonal extent of the layer-type plasma structures are in the ranges of approximately 0.1-4 km and 5-28 km, respectively. However, the vertical and zonal extents of the clumptype plasma structure are comparable with each other, ranging from 2 to 12 km. These features clearly imply that the cross sections of the layer-type and clump-type plasma structures intercepted by a fan-like effective radar beam are, respectively, in highly elongated and nearly circular shapes. The mean height of the clump-type plasma structures is slightly higher than that of the layer-type plasma structures by about 5 km. From interferometry-measured echo distributions about the perpendicularity to the local magnetic field line, it is found that the mean aspect angle of the 3-m field-aligned irregularities for the layer-type plasma structures is about 0.05°smaller than that for the clump-type plasma structures. In addition, the height dependence of the aspect angle of the layer-type plasma structures is different from that of the clump-type plasma structures. The former tends to increase with height, while the later decreases with altitude. The zonal trace velocities of the layer-type and clump-type plasma structures estimated from their temporal displacements in the horizontal plane are, respectively, in ranges of −150-100 m/s and −180-80 m/s, in which negative (positive) values indicate eastward (westward) drift. The characteristics of the Doppler spectra of the echoes from the layer-type and clump-type plasma structures are also analyzed and the relations between the spectral parameters and the aspect angles are also discussed in this article.

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.

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 (E s ) 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 E s 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 E s layers occur within the wind shear zones that favor the E s layer formation based on the wind shear theory, suggesting that the primary physical process responsible for the E s layer events retrieved from the scintillations of the GPS RO signal is very likely the plasma convergence effect of the neutral wind shear.