Range Effect in the Spectral Width of E-Region HF Coherent Echoes

SuperDARN/CUTLASS Finland HF radar observations of E-region coherent echoes are presented for a two-hour event on 12 February 1999. The Burg spectrum method is applied to study microstructure of the spectra. It is discovered that many spectra with enhanced width are two-peaked with a systematic pattern in their occurrence; namely, the double-peak echoes are typically observed at intermediate or far range, farther than the power maximum of the E-region echo band in the range profile. It is argued that double-peak echoes occur because the radar can access, at certain ranges, both the bottom and the top parts of the electrojet layer where the irregularity velocities are quite different. Ray tracing analysis with measured EISCAT density profiles confirm the possibility of such scattering layers. We also show that the velocity of both peaks was below the EISCAT observed ExB convection velocity which is consistent with the idea that the velocity of E-region decameter irregularities is depressed due to collisions.