On the nature of 150-km radar echoes over the magnetic dip equator

We present new experimental findings that provide insight into the nature o f 150-km echoes. A finding, using data obtained with a 49.92-MHz radar on P ohnpei (6.96 degrees N, 158.19 degrees E, 0.5 degrees magnetic dip angle), is an unusual east-west asymmetry in which the altitude profile of echo occ urrence depends on viewing direction. The other is the narrowness of Dopple r spectral widths associated with these echoes. When considered with other known properties of 150-km echoes, a scenario for 150-km echoes emerges in which 3-m-scale field-aligned irregularities (FAI) are imbedded in tilted, sheetlike structures in plasma density. These meter-scale FAI are envisione d to have narrow Doppler spectral widths if excited directly by a linear pl asma instability, and they would consist of a narrow angular spectrum of pl ane plasma waves if the instability is weak. The latter could produce the o bserved east-west viewing asymmetry.