HF radar probing of the lower magnetosphere

The first observations of ion acoustic waves at altitudes of about 4000 km in the auroral zone were carried out using the powerful SURA HF radar in 1991. Here we report additional observations made with the Russian SURA radar and the Ukrainian UTR 2 receiving system in December 1992. Although echoes similar to the earlier observations were detected, we performed a number of diagnostics and have concluded that no unambiguous magnetospheric echoes were seen at SURA during the 1992 campaign. (There is some indication that the more sensitive UTR 2 system received magnetospheric echoes.) Some aspects of the coding scheme used in the experiments that might lead to spectral artifacts are discussed. A new antenna under construction-at SURA will allow transmission at twice the frequency used to date and should permit more definitive observations in the future.