Plasma structures responsible for sporadic E region quasi-periodic echoes

We used the interferometry technique to reconstruct the spatial structures of 3-m field-aligned irregularities (FAI) in the nighttime sporadic E (Es) layer responsible for quasi-periodic (QP) echoes. We found that the scattering regions that give rise to the QP striations with positive range rates occurred in patchy patterns with zonal and vertical extents of approximately 5-20 and 3-8 km, respectively. These echo patterns appeared quasi-periodically with periods of 2-6 min and moved horizontally from southeast to northwest at an approximately constant height. One of the important findings of this study is that the zonal trace velocities of the kilometer-scale plasma structures were approximately the same as the zonal drift velocities of the 3-m FAI. The former were deduced from the temporal displacement of the echo patterns on the horizontal plane, and the latter were estimated from the observed Doppler velocities of the 3-m FAI combined with the angular positions resolved by the interferometry technique. This feature strongly implies that the small-scale FAI were "frozen" within the large-scale plasma structures responsible for the QP echoes. On the basis of this frozen-in property, the meridional component of the drift velocity of the plasma structures can be obtained from that of the 3-m FAI, irrespective of the limitations of the extraordinarily narrow width of the expected echoing region in the north-south direction. Our results show that the maximum drift velocity of the plasma structures is as large as approximately 135 m/s in the direction from southeast to northwest. On the basis of the long dwelling time (greater than 6 min) and small horizontal dimension (average of approximately 10 km) of the QP scattering regions, we speculate that the configuration of the large-scale plasma structures responsible for the QP echoes is likely to be a horizontally elongate shape whose major axes are primarily aligned in the meridional direction with respect to geomagnetic north.