Sporadic E tidal variabilities and characteristics observed with the Cyprus Digisonde

In this study, ionogram observations made with the Cyprus digisonde (35 degrees N; 33 degrees E) are analyzed by applying an ionosonde height-time-intensity (HTI) methodology. The aim is to study dominant periodicities and diurnal patterns in occurrence and altitude transport of sporadic E (Es) and intermediate descending layers (IDL), which are impacted upon by solar thermospheric tides via the windshear layer formation mechanism in the E and lower F region ionosphere. The results show the diurnal occurrence and altitude descent of sporadic E to be dominated by a semidiurnal tide-like periodicity, which prevails, with some differences, in all seasons. It is characterized by a daytime layer starting near sunrise at similar to 125 km, followed by a nighttime layer appearing first in late afternoon at similar to 130 km; both layers descend in altitude with speeds between about 2 and 3 km/h, therefore reaching the 100 km level in similar to 10-12 h. Also, a terdiurnal tide-like periodicity is present in daily Es occurrence and altitude descent but only in summer solstice. In addition, the data show fast-descending layers to originate at F region altitudes near sunrise and sunset, which are subject to a semidiurnal periodicity. Although these layers are rarely seen by the digisonde below about 180 km for most times and never during the night, the data hint that they connect with sporadic E layers below, therefore, both IDLs and Es form an integral tidal ion layer system in the lower ionosphere. An exceptional result is the detection during daytime of strong intermediate descending layers in March equinox, starting at F region and descending downwards with speeds of similar to 10 km/h. Although an effort is made to interpret this equinoctial IDL signature, more work is needed for its understanding. (C) 2014 Elsevier Ltd. All rights reserved.