A CLIMATOLOGY OF QUIET DISTURBED IONOSPHERIC CONDITIONS DERIVED FROM 22 YEARS OF WESTERBORK INTERFEROMETER OBSERVATIONS/

Knowledge of the quiet and disturbed conditions in the propagation med ium is essential for quality control of transatmospheric radio signals . This holds equally for the troposphere and the ionosphere. This pape r describes a climatology of ionospheric irregularities obtained from observations of celestial radio sources by radio interferometry, i.e. by the Westerbork Synthesis Radio Telescope (WSRT) in The Netherlands. This instrument is located at geomagnetic mid-latitude. All WSRT cali brator observations in the 22-year period 26 June 1970-31 December 199 1 have been checked for manifestations of ionospheric effects. Althoug h seasonal effects are clear, the occurrence and 'strength' of ionosph eric irregularities show no dependence on solar activity. Assuming tha t the frequency of occurrence of ionospheric disturbances in spring an d autumn are similar, it is found that 'ionospheric' winter starts on day 348 +/- 3 and all seasons last for 3 months. Medium-scale travelli ng ionospheric disturbances (TIDs) occur most frequently during the da ytime in winter periods. The occurrence of non-periodic irregularities is, however, nor a function of lime in the day. The daily variation i n the amplitude and frequency of the occurrence of the TIDs suggests t hat the solar terminator and Joule heating near the electrojets do not contribute substantially to their generation. Generation of gravity w aves may be caused by winds and tides in the lower thermosphere-mesosp here. This has to be investigated further. On the basis of the availab le data, a 'disturbance measure', indicating to what extent the ionosp here is 'quiet', is proposed. The output of this project may be of imm ediate use for different ionospheric investigations, such as ionospher ic modelling and the study of excitation mechanisms for ionospheric ir regularities. (C) 1996 Elsevier Science Ltd