Statistical investigations of layer‐type and clump‐type plasma structures of 3‐m field-aligned irregularities in nighttime sporadic E region made with Chung‐Li VHF radar

摘要

With interferometry measurements made with Chung‐Li 52 MHz VHF radar, characteristics of layer‐type and clump‐type plasma structures of 3‐m field‐aligned irregularities are investigated for the first time. Long‐term statistics show that the thickness and the zonal extent of the layer‐type plasma structures are in the ranges of approximately 0.1–4 km and 5–28 km, respectively. However, the vertical and zonal extents of the clumptype plasma structure are comparable with each other, ranging from 2 to 12 km. These features clearly imply that the cross sections of the layer‐type and clump‐type plasma structures intercepted by a fan‐like effective radar beam are, respectively, in highly elongated and nearly circular shapes. The mean height of the clump‐type plasma structures is slightly higher than that of the layer‐type plasma structures by about 5 km. From interferometry‐measured echo distributions about the perpendicularity to the local magnetic field line, it is found that the mean aspect angle of the 3‐m field‐aligned irregularities for the layer‐type plasma structures is about 0.05° smaller than that for the clump‐type plasma structures. In addition, the height dependence of the aspect angle of the layer‐type plasma structures is different from that of the clump‐type plasma structures. The former tends to increase with height, while the later decreases with altitude. The zonal trace velocities of the layer‐type and clump‐type plasma structures estimated from their temporal displacements in the horizontal plane are, respectively, in ranges of ?150–100 m/s and ?180–80 m/s, in which negative (positive) values indicate eastward (westward) drift. The characteristics of the Doppler spectra of the echoes from the layer‐type and clump‐type plasma structures are also analyzed and the relations between the spectral parameters and the aspect angles are also discussed in this article.