COHERENT ECHOES DURING EISCAT UHF COMMON PROGRAMS

Coherent echoes, whatever their origin, appear as narrow spectra super imposed on the normal incoherent scatter spectra recorded by the EISCA T UHF and VHF radars. Their narrow width, higher than normal amplitude and the induced spectral asymmetry in the overall spectrum enable the m to be easily found by a simple computer algorithm which has routinel y been used to pick out such spectra from Common-Programme (CP) data, usually in real time. The UHF CP-1 data have been searched back to 198 6 for such echoes. Most of the echoes found in this way are caused by noise spikes, but radio interference, satellite echoes, and, of most i nterest, enhanced ion acoustic echoes caused by plasma processes are a ll found. We present a summary of the findings of this search, with sp ecial emphasis on the CP-1 data set. Examples of remote site enhanced ion acoustic shoulders are presented, as well as some unexplained echo es from Tromso.