On the basis of interferometer technique implemented at a VHF coherent scatter radar, a method of measuring tilted angle and true horizontal extent of a thin layer structure containing 3-m field-aligned irregularities (FAIs) in ionospheric sporadic E (E-s) region is proposed in this article. The essential part of this method is to measure the length and the orientation of the striation-like echo pattern resolved by interferometry technique, which is the result of the interception of the layered structure by a fan-like effective antenna beam pattern defined by the product of the physical antenna beam and the aspect sensitivity of the backscatter from E-s FAIs. From the projections of the striated echo pattern on mutually orthogonal planes, the zenith and azimuth angles of the unit vector normal to the layer can be estimated in accordance with a set of line equations describing the length and the orientation of the echo pattern. It is found that the E-s thin layer for the present case is primarily tilted in the northeast-southwest direction with a maximum zenith angle of 11 degrees. In addition, quasi-periodic variations in layer height and tilted angle with a period of approximately 3-5 min were observed, in which the peak-to-peak amplitudes of the layer height variation were in a range of 0.3-1.2 km. These features provide solid evidences that the thin E-s layer was modulated by a quasi-periodic oscillation not only in the layer height, but also in the tilt of the layer surface. Interferometer measurement reveals that the layer is in an anisotropic shape with major axis aligned in the northwest-southeast direction making an angle of approximately 45 degrees with respect to the west direction. The width (or minor axis length) of the thin layer varied with time in a range 8.8-13 km, and the axial ratio of the anisotropy is no less than 1.73. Although the azimuth alignment of the anisotropic thin layer is in harmony with that of the E-s layer instability proposed by Cosgrove and Tsunoda (2002), the measured Doppler velocity, drift direction of the FAIs, and interferometer-resolved horizontal extent of the layer do not favor the theoretical predictions. In light of the fact that the oscillating period is larger than Brunt-Vaisala period in E-s region and the amplitude of the layer modulation is consistent with that of a propagating gravity wave, it suggests that the oscillation was very likely associated with a short-period gravity wave with period of 3-5 min propagating in the northeast-southwest direction.
