Yen-Hsyang Chu

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.

First measurements of neutral wind and turbulence in the mesosphere and lower thermosphere over Taiwan with a chemical release experiment

[1] Neutral winds and turbulence structure in the mesosphere and lower thermosphere were measured with a sounding rocket chemical release experiment carried out in Taiwan. Trimethyl aluminum (TMA) was used as a tracer of the drift of the background atmosphere. The results show that the measured neutral wind had a maximum speed of 154 m/s at a height around 105 km. The wind vectors were found to rotate clockwise with height in the altitude range from about 98 to 121 km. On the basis of the hodograph analysis of the measured neutral wind vectors, an upward propagating inertio-gravity wave with intrinsic period of 11.2 hours and vertical wavelength of 19.5 km was believed to be primarily responsible for the height variations of the neutral wind and the Richardson number. Large vertical wind shears were observed near 100 km and 106 km. Turbulent structures were observed in much of the trail below 110 km, but there were enhanced structures in the altitude range between the two large shears, i.e., at and near the altitude of the maximum wind speed. Comparing the positions of the turbulent features with expected atmospheric stability zones induced by an upward propagating gravity wave indicates that the turbulence structures were primarily located within the wave-induced convectively unstable zone. The structures are therefore interpreted as counterrotating vortices within the convective instability zone of a breaking gravity wave. Moreover, the relation between the horizontal separations l (about 1.8 km) of the turbulent structures and the vertical extent h (about 0.7 and 1.5 km) of the wind shear zones with Richardson numbers less than 0.25 did not conform to the predicted l/h ratio of a factor of approximately 8 predicted by simple linear Kelvin-Helmholtz instability theory for the primary billow structures associated with the instability. These results suggest that the observed structures were not the primary billows but were more likely associated with a secondary instability, such as the counterrotating vortices that develop later in the evolution of the instability. In general, the observations reported here support the interpretation that the turbulence evident in the trail was very likely generated in the convectively unstable zone induced by the inertio-gravity wave propagating through the region with large temperature gradients � 32 K/km produced by wave breaking processes.

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.

The severe depletion of turbulent echo power in precipitation observed using the Chung-Li VHF Doppler radar

Earlier investigations of atmospheric precipitation made with a VHF radar demonstrate that below the level of melting (i.e., 0°C isotherm) the VHF echo intensities from precipitation particles are generally much weaker than those from turbulent refractivity fluctuations by about 20–30 dB. However, in this article, using Chung-Li VHF radar data, the precipitation echoes in the height range from about 2.5 to 4 km, which is below the melting level during the season of spring over the Taiwan area, may far exceed the turbulent returns in intensity by about 15 dB, contradicting earlier observations. A comparison of the echo power profiles of the precipitation with those of refractivity fluctuations reveals that the weak backscatter from refractivity can be attributed to abnormally severe depletion of turbulent echo power in a convective cloud with fairly intense upward air motion and moderate precipitation. Furthermore, the correlation between hydrometeor terminal velocity and precipitation echo power is positive, while the air motion and turbulent echo power are anticorrelated. On the basis of these observations a plausible mechanism is proposed to explain the abnormal depletion of refractivity echo power.

First observations of type-1 sporadic E Irregularities in the equatorial anomaly region using the Chung-Li VHF radar

In this paper we present the characteristics of mid-latitude irregularities first observed with the 52 MHz Chung-Li radar. Intense type-1 echoes with Doppler velocities of about 270 m/s (slightly smaller than the nominal ion-acoustic wave speed of 360 m/s) in the height range from 103.1 km to 104.5 km lasted for about 2 minutes. The range-time variations of sequential power spectra indicate that the patch of type-1 irregularities moved toward the radar at a rate of about 42 m/s. The zonal drift motions of the type-1 and type-2 irregularities obtained from an interferometry technique showed that the irregularities drifted eastward with an average speed of 84 m/s.

The investigations of the atmospheric precipitations by using Chung-Li VHF radar

In this paper the results of precipitation observed by Chung-Li VHF radar during the passage of typhoon Susan through the Taiwan area on June 2, 1988, are presented. We find that VHF radar can not only observe the falling speed of the rain drops but can also detect that of the ice particles. The observational results show that the former is about 6–8 m/s, while the latter is about 1 m/s, consistent with those observed by the conventional Doppler meteorological radar. The echo power due to precipitation is generally far smaller than that due to atmospheric refractivity fluctuations. However, it is enhanced strikingly at the height of melting layer (about 5 km in Taiwan area), and its value can be greater than that due to refractivity fluctuations by about 5 dB. Moreover, it also shows that the feature of bright band only occurs in the echo power profile of precipitation, and it cannot be observed in that of atmospheric refractivity. The echo power profiles of the vertical and oblique beams (with zenith angle 17°) are also examined. They indicate that there is no aspect sensitivity for the echoes scattered from the atmospheric refractivity fluctuations during the precipitating environment. However, for the echo power of precipitation the feature of aspect sensitivity does exist above the melting layer and disappears below this altitude. The correlation between the horizontal wind speed and the Doppler spectral width of precipitation is found to be fairly high, with a correlation coefficient of 0.86. This suggests that the beam-broadening effect due to the advection of horizontal wind may be one of the important factors contributed to the precipitation spectral width and has to be taken into consideration when the precipitation spectra are analyzed.

A study of the frequency coherence of stratospheric and tropospheric radar echoes made with Chung‐Li VHF radar

In the past few years the technique of Frequency Domain Interferometry(FDI) has been developed on VHF radar. By using this technique, the characteristics of a very thin atmospheric layer structure, which is embedded in the radar volume and can not be resolved by the conventional VHF radar with only one operational frequency, can be determined through the calculation of the coherence and the phase from two echo signals with slightly different operational frequencies. In this article the results of a frequency domain interferometry experiment carried out on Chung-Li radar are presented. The use of four discrete frequencies and three different frequency separations distinguishes this study from previous investigations. It is shown experimentally that the signal-to-noise power ratio indeed affects significantly the magnitude of the observed coherence. For the first time the dependence of coherence on frequency separation is also investigated. The results show that coherence decreases with frequency spacing with the roll-off rate ranging from −0.2/MHz to −1.2/MHz, depending on the atmospheric structure. After best fitting the theoretical model to the observed coherence taken from a height range with no atmospheric layer existing, the effective range resolution can be found. Employing this information, the thicknesses of two prominent and lasting layers locating at height 7.2 km and 7.8 km are estimated through the least square fitting process, and found to be 114 and 186 meters, respectively.

System phase calibration of VHF spaced antennas using the echoes of aircraft and incorporating the frequency domain interferometry technique

Using multiple-receiver arrays, the techniques of spaced antennas and spatial domaininterferometry have been widely utilized in mesosphere-stratosphere-troposphere (MST)VHF and meteor radars. As these techniques are applied, the phase imbalance between thereceivingchannelswillcauseabiasedresultifitisnotconsideredintheuseofphaseanglesof the radar returns received in different receiving channels. In view of this, many methodshave been employed for the phase calibration of receiving channels. In this paper, wepropose one more method/procedure for the calibration, in which the commercial airplaneflying routinely is used as a radar target. We measure the zenith and azimuth angles of theairplane with a video camera and estimate the range and height of the airplane fromobservation using the frequency domain interferometry (FDI) technique. Consequently, thetrack of the airplane in the vicinity of the radar site can be determined. The phasedifference between the two radar echoes received by a pair of receiving channels is thenpredicted in accordance with the track of the airplane. Comparing the predicted phasedifference with that observed, the phase imbalance between a pair of receiving channelscould be obtained. Tens of cases show consistent results and so verify the reliability of themethod. Also shown is the drift of system phase imbalance from season to season, whichmight be caused by the temperature dependence of antenna system components.

Reply to comment by Lei et al. on A new aspect of ionospheric E region electron density morphology

[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 COSMIC‐ retrieved 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 Orsted, 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] (identical to Figure 2 of Yue et al. [2010]), the simulation‐retrieved E region electron densities around 100 km in equinox season are approximately 50– 100% and 150–200% greater than the “true” model values at the geomagnetic equator and in geomagnetic latitude regions ±30°−50°, respectively. In addition, the former are about 150–200% smaller than the latter in geomagnetic latitude regions ±10°−30°. If the simulation‐retrieved results obtained by Yue 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 NmE 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. Figure 1 presents an example of latitudinal variation in histograms of percent errors of NmE between COSMIC retrieval and ionosonde measurement for spring (March–May) 2007–2009, in which the percent error (PE) is defined by