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 E(s) 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 E(s) 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 degrees NW. The observations also demonstrate that the echoing regions of the E(s) 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 degrees 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 E(s) 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 E(s) irregularities below 110 km cannot be ignored.