POLAR LACUNA ON IONOGRAMS .1. BRIEF MORPHOLOGY

The causes of polar F lacuna or complete disappearance of F layer trac es on Polar Cap ionograms are re-examined using data selected from 30 years' records at DUMONT d'URVILLE (South magnetic pole, invariant lat itude L = -80.5). We recall the debate about the nature and definition of polar lacuna. We evaluate the constraints of the respective ionoso nde equipments, and re-establish the reality of F2 lacuna by some high -resolution ionogram f-plots. We review the diurnal and seasonal chara cteristics of hourly lacuna occurrence for the three types:F1, F2 and ''F3'' lacuna (for F1 and F2 lacuna together). From 60-s series at DUM ONT d'URVILLE we comment on the distinctive life-times and on precurso r phenomena for the three types. This evidence suggested (Rodger 1989) a common origin for E and F layer lacunae.