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] In this paper, presented for the first time the three-dimensional global morphology and seasonal variations of scintillation index (S4 index) measured from the signal-to-noise ratio (SNR) intensity fluctuations of L1 channel of GPS radio occultation (RO) signals using FORMOSAT-3/COSMIC (in short, F3/C) satellites for a low solar activity year 2008. The S4 index, which confined around AE30 magnetic latitudes, is found to start around post-sunset hours (1900 MLT, magnetic local time) and often persists till post-midnight hours (0300 MLT) between 150 and 350 km altitudes during equinox and northern winter seasons while no activity is observed during southern winter season. However, high latitudes are characterized with no scintillation activity beyond 150 km during any season, which implying that in the solar minimum period the drives of instabilities in the auroral, cusp and polar cap regions, namely the gradient drift and velocity shear, are absent. The S4 index at F region altitudes during magnetically quiet times is more intense and extends to higher latitudes than that observed during disturbed time consistent with earlier studies. The equatorial S4 index appears below the peak of F2 layer (hmF2) during most of the seasons although the associated intensities and the time of maximum occurrences are relatively higher and earlier during vernal equinox followed by autumn equinox. This equinoctial asymmetry could be primarily attributed to the asymmetries in eastward drift velocities, thermospheric meridional winds and plasma densities. Further, the global maps of S4 index at E region altitudes (between 75 and 125 km) show strong seasonal variations with highest activity during northern and southern summer solstice in the middle latitudes while it appears on both sides of magnetic equator with less or no activity at and around the equator during equinox seasons. The absence of S4 index along the equator can be understood in terms of the vanishing vertical component of the magnetic field lines that can inhibit the vertical movement and layered deposition of ionized particles of thin irregular electron density layers such as Es-layers. Keeping in view the importance of these valuable database, we would like to emphasize that the F3/C GPS RO technique can be used to study the ionospheric irregularities at GHz frequency globally directly from the high-rate L1 data, which reiterating its importance as a powerful tool to explore the terrestrial ionosphere on a global scale.

Abstract. 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 kilometer's 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 72øNW. 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 ø 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.

A new technique, using beam broadening effects, has been developed to measure the aspect sensitivity of atmospheric clear air VHF radar echoes. It uses the relatively broad antenna beam of the vertical-pointing antenna of the new Chung-Li stratospheric-tropospheric radar (25øN, 120•'E). The aspect sensitivity measurement using this method is straightforward and free from convolution effects introduced by the finite width of the antenna beam pattern. The observed results agree very well with other measurements. The authors propose a turbulent layer model to explain the aspect sensitivity of the echoes. In this model, anisotropic turbulence is confined to a very thin (few meters) region at the boundary of a turbulent layer. This region is responsible for the aspect sensitivity of the echoes obtained from the vertical direction. The isotropic echoes obtained from the oblique beam arise from the isotropic turbulence embedded in the center of the layer, with 30-300 m in vertical extent. We show in an appendix that the magnitude of the partial reflection coefficient is much more sensitive to the shape, length scale, and smoothness, than to the slope of the refractive index profile. Therefore the functional shape of the refractive index profile is very important for estimating the reflection coefficient. Large errors can be made when assuming, for simplicity, nonphysical profiles. For partial reflecting mechanisms to be important, steplike discontinuities, confined within length scales of the order of a meter, would be required. Papernumber 88RS03859. 0048-6604/89/88 R S-03859508.00 not so clearly understood. It has been established that tropospheric, stratospheric, and mesospheric VHF radar echoes are aspect sensitive, i.e., the echo power depends on antenna beam pointing angle, and are much larger when the antenna is directed to the vertical. This aspect sensitivity was first reported independently by Gaye and Green [1978] and by Rb'tt•ter and Liu [1978] for the troposphere and stratosphere and by Fukao et al. [1979] for the mesosphere. Other observations have followed [Rb'ttqer and Vincent, 1978; R6ttqer et al., 1981; Sato et al., 1985; Tsuda et al., 1986; Waterman et al., 1985] including quantitative measurements of its angular dependence. Aspect sensitivities as high as 20 dB have been reported [Tsuda et al., 1986] at 50 MHz, 10 dB being more common [Riittqer et al., 1981; Hockinq et al., 1986], falling at a rate of approximately 1 dB per degree. There is no consensus on the mechanism responsible for the enhancement of the signals when one points vertically or close to vertical. Several have been proposed, including anisotropic turbulence 113 114 WOODMAN AND CHU: ASPECT SENSITIVITY OF VHF BACKSCATTERTABLE 1. Characteristics ofChung-Li VHF Radar Characteristic

(2010), Reply to comment by Lei et al. on "A new aspect of ionospheric E region electron density morphology," J. Geophys. Res., 115, A07314, doi:10.1029 [1] Since the FORMOSAT-3/COSMIC satellites were launched in April 2006, ionospheric electron density profiles have been retrieved from the excess phase of the GPS signal by using the radio occultation technique and can be accessed from the Web site http://www.cosmic.ucar.edu/. On the basis of these electron density profiles and the use of data quality control criteria developed by Yang et al. [2009], Chu et al.[2009] investigated E region electron density morphology and showed that the general properties of the COSMICretrieved E region electron density are in good agreement with the predictions of the Chapman layer theory that was developed in accordance with photochemical process and controlled by solar zenith angle. Nevertheless, Chu et al.[2009] found existences of salient enhancements in the noontime E region electron density not only at the geomagnetic equator but also in the geomagnetic latitude regions ±15°−35°, which cannot be explained by the Chapman layer theory. In addition, they also provided compelling evidence to show the presence of longitudinal wave number 3 and 4 structures of the equatorial electron density in a height range of 100-200 km, which is in excellent agreement with longitudinal structures of equatorial electrojet intensity derived from equatorial magnetic field data obtained by the Ørsted, CHAMP, and SAC-C satellites during the years 1999-2006.[2] On the basis of the simulation result obtained by Yue et al. [2010], Lei et al. [2010] question the validity of the E region electron density retrieved by the GPS radio occultation technique. They argue that because of the presence of the ionospheric electron density gradient in the horizontal direction that violates the spherical symmetry assumption of the Abel transform for inverting ionospheric electron density profile from calibrated total electron content along the GPS raypath, the COSMIC-measured E region electron density enhancements in midlatitude regions were caused by the retrieval error of the GPS radio occultation process. From Figure 1 of Lei et al. [2010] were true and able to be representative of the general GPS occultation-retrieved results, the morphologies of the COSMIC-measured E region electron density should be in accord with those of the simulation results. Namely, the E region electron density retrieved by COSMIC satellites should be much greater (smaller) than true measurement made by the ground-based ionosonde in geomagnetic latitude regions ±30°-50°(±10°-30°). In order to validate the simulation-retrieved results, we compare peak values of E layer electron density N m E between COSMIC retrieval and global ionosonde measurement in the different latitudinal regions for July 2006 to July 2009. The COSMIC data were selected for comparison if the separation between COSMIC occultation point and ionosonde station is 10 min in time and 2.5°in space. As shown in Figure 1, ...