Three‐dimensional spatial structures of midlatitude type 1 Es irregularities

2005

Ann. Geophys.

Abstract. Beam broadening effects on the Doppler radar spectrum induced by along- and cross-radar-beam winds are theoretically investigated in this article. Analytical expression of beam broadening spectrum for a vertical beam subject to a constant vertical wind in the absence of horizontal wind is derived first. We find that the resultant beam broadening spectral shape is in an exponential form, rather than a Gaussian shape. The peak of the exponential spectrum corresponds to the true vertical wind velocity, and the spectral width σ is a function of vertical wind velocity w and the half-power beam width θ1/2 of the radar beam, in accordance with the expression σ=0.09wθ1/22. The beam broadening spectrum for a vertical radar beam subject to both vertical and horizontal winds is numerically studied, and the result shows that the spectral shape is distorted from the Gaussian form. The estimated vertical wind velocity of the distorted spectrum is very close to the true vertical wind velocity with a difference of less than 0.6% for a typical case. However, its spectral width is substantially greater than that of the conventional Gaussian spectrum broadened by horizontal wind only. The difference of the spectral widths Δσ (in unit of percentage) in the two, as a function of the half-power beam width and the ratio of vertical to horizontal wind velocities, can be well approximated by a parabolic equation. With this equation and the conventional Gaussian beam broadening spectral width caused by horizontal wind, the true width of the distorted beam broadening spectrum induced by vertical and horizontal winds can be inferred without complicated numerical computation. Through a transformation of wind velocity from vertical and horizontal winds into along- and cross-radar-beam winds defined on the cross section of an oblique radar beam, the results obtained from the case of vertical radar beam can be directly applied to the case of the oblique beam. We show that in some special cases the beam broadening spectral widths of an oblique beam subject to both vertical and horizontal winds will be exceedingly narrower than those estimated by the existing formula, irrespective of the presence of strong horizontal wind velocity.

Section: Discussionsupporting

confidence: 82%

Section: Discussionmentioning

confidence: 99%

Effects of along- and cross-radar-beam winds on Doppler radar spectrum

2005

Ann. Geophys.

“…Note from the vertical planes shown in Figure 4 that the echo patterns of the lower plasma structure at height about 101 km generally distributed along the line with zenith angle of about 38.5°. This feature reveals that the configuration of the lower plasma structure assembled by the field‐aligned irregularities with exceedingly small aspect angle (about 0.1°) was very likely in a patch‐like shape, rather than a horizontal thin layer [ Chu and Wang , 2002a]. Examining Figure 4 in more detail indicates that the solidness of the upper thin layer and lower patch‐like irregularity were changed quasi‐periodically.…”

Section: Observationsmentioning

confidence: 99%

“…It is well known that the use of multiple baseline interferometry can resolve the spatial structure of the field‐aligned irregularities in the expected echoing region by positioning the true height and angular position of the irregularities. Mounting evidences show that the 3‐m E s irregularities may gather into a clump with a finite extent in horizontal and vertical directions [ Yamamoto et al , 1994; Chu and Wang , 1997; Wang and Chu , 2001; Hysell et al , 2002a] or assemble in a layer structure with very thin thickness in vertical and wide extent in horizon [ Chu and Wang , 2002a, 2002b]. Experimental results obtained by using the Chung‐Li VHF radar implemented with interferometry show that the plasma structure of the E s irregularities in the expected echoing region may locate at more than 10° in a zonal direction away from the radar beam axis, and the zonal drift velocity of the structure can be as large as 100 m/s or more [ Chu and Wang , 1999; Wang and Chu , 2001].…”

Section: Introductionmentioning

confidence: 99%

Spatial distribution of random velocity fluctuations of 3‐m field‐aligned irregularities in ionospheric sporadic E layer

Wang

2007

J. Geophys. Res.

Self Cite

[1] In this article we attempt to estimate the spatial distribution of the random velocity fluctuation of 3-m field-aligned irregularities (FAI) in sporadic E (E s ) region, which were responsible for the quasi-periodic (QP) echoes in range-time-intensity (RTI) plot. As previously shown by Chu and Wang (2003), beam broadening effect may be significant in spectral width measurements. However, using interferometry technique this effect could be removed by estimating the zonal drift velocity and the interferometry-resolved zonal extent of the plasma irregularities. By applying this correction to 1 hour of data we found that the ratio of the beam broadening spectral width to the observed spectral width vary between 5 and 40% with a mean value of 14.2% and a standard deviation of 6.3%. After removing the beam-broadening contribution from observed Doppler spectral width, the mean velocity fluctuations of the 3-m plasma irregularities at a height of 105 km are in the range of 5 to 12 m/s. In addition, the spatial distribution of the FAI velocity fluctuation is nearly uniform in the echoing region over a span of 8°( corresponding to a horizontal extent of about 20 km) in geomagnetic zonal direction. The plausible mechanism responsible for the uniform distribution of the random velocity fluctuation of the 3-m FAI in the nighttime E s region is discussed.

“…Irrespective of large inclination angle of geomagnetic field line and weak dc electric field intensity, a number of observational evidences in support of the generation and existence of type 1 irregularities in midlatitude sporadic E (Es) region have been provided recently by various HF and VHF radars [ Schlegel and Haldoupis , 1994; Haldoupis et al , 1996b; Huang and Chu , 1998; Hysell and Burcham , 2000; Chu and Wang , 2002]. In general, the occurrence of midlatitude type 1 irregularies is very rare compared with those in equatorial and auroral regions.…”

Section: Introductionmentioning

confidence: 99%

Plasma structures of 3‐meter type 1 and type 2 irregularities in nighttime midlatitude sporadic E region

Wang

2002

J. Geophys. Res.

Self Cite

[1] Although the plausible mechanisms involved in the generation of midlatitude type 1 sporadic E (Es) irregularities have been suggested, rare observational evidence is provided to validate the proposed plasma structure associated with the midlatitude type 1 Es irregularities. In this article, the type 1 echoes observed by the Chung-Li VHF radar located in the equatorial anomalous region are interferometrically analyzed and the corresponding plasma structure of the type 1 irregularities is reconstructed. We find that the plasma structure has sharp lateral and top and bottom boundaries with thickness of about 1-2 km and horizontal extent of about 3-5 km in E-W direction. Its dimension in N-S direction cannot be resolved by using interferometry technique because of considerably narrow width of expected echoing region in elevation. The observed Doppler velocity of the type 1 echoes can be as low as 220 m/s, substantially smaller than nominal ion acoustic wave speed (about 360 m/s) in Es region. The spatial structure of the concurrent type 2 irregularities is also reconstructed. The result strongly suggests that it be a well-defined thin layer with thickness of 1-2 km and horizontal extent of 9-17 km in E-W direction, very different from that of the type 1 irregularities. The whole structure of type 1 irregularities moves bodily toward east at speed of about 31 m/s, and no vertical displacement of the structure is observed. Although the movement of the layer structure of the type 2 irregularities in E-W direction is indistinct, it descends remarkably at a rate of 10.3 m/s. These features imply that for the present case the factors governing the dynamic behavior of the type 1 and type 2 irregularities are different and independent, irrespective of the fact that the clump of the type 1 irregularities separates from that of concurrent type 2 irregularities only by about 4 km in distance.