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by ground backscatter carried out at Torrecliiaruccia (S. Marinella, Rome)
since 1957, as to tlie occurrence of tlie propagation via sporadic-ZJ ionization,
Tlie seasonal and diurnal occurrence, as well as tlie azimutlial distribution
of tlio Es eelioes are diseussed.
Tliere was a good correlation between tlie time occurrence of Es eelioes
and the occurrence of Es ionization, the top frequency of which exceeds a
criticai value related to the frequency of the sounding radio waves. To
account for a lack of correlation between the azimutlial distribution of the
eelioes and tlie azimutlial distribution of foEs, it is suggested that the latter
one depends even 011 an azimutlial variation in the vertical radiation diagrain
of the antenna, due to some characteristics of the ground.
A statistics of Es "clouds" was performed; about 60% of the observed
clouds were " moving clouds ", their main movement occurring towards
South-West; the mean dimension of these clouds was about 250 km.
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