Observations of double-peaked E region coherent spectra with the CUTLASS Finland HF radar

Cooperative UK Twin Located Auroral Sounding System Finland HF radar observations of E region coherent spectra are presented for several events. The Burg spectrum method is applied to study microstructure of the spectra. It is discovered that many spectra are two-peaked, and quite often there is a systematic pattern in their occurrence; the double-peak echoes are typically observed at intermediate and far ranges of the E region echo band and at ranges farther than the power maximum in the range profile. The typical separation between the peaks is about 150 m/s and hardly changes with the azimuth of observations. Velocities of two components in a double-peak spectrum are typically larger and smaller than the velocity of the unresolved spectrum and single-peaked velocity of echoes at shorter ranges. It is hypothesized that the two components of the echoes occur because of signal reception from the top and bottom of the electrojet layer.