A new methodology of ionosonde height–time–intensity (HTI) analysis is introduced which allows the investigation of sporadic E layer (Es) vertical motion and variability. This technique, which is useful in measuring descent rates and tidal periodicities of Es, is applied on ionogram recordings made during a summer period from solstice to equinox on the island of Milos (36.71N; 24.51E). On the average, the ionogram HTI analysis revealed a pronounced semidiurnal periodicity in layer descent and occurrence. It is characterized by a daytime layer starting at 120km near 06 h local time (LT) and moving downward to altitudes below 100km by about 18 h LT when a nighttime layer appears above at_125 km. The latter moves also downward but at higher descent rates (1.6–2.2 km/h) than the daytime layer (0.8–1.5 km/h). The nighttime Es is weaker in terms of critical sporadic E frequencies (foEs), has a shorter duration, and tends to occur less during times close to solstice. Here, a diurnal periodicity in Es becomes dominant. The HTI plots often show the daytime and nighttime Es connecting with weak traces in the upper E region which occur with a semidiurnal, and at times terdiurnal, periodicity. These, which are identified as upper E region descending intermediate layers (DIL), play an important role in initiating and reinforcing the sporadic E layers below 120–125 km. The observations are interpreted by considering the downward propagation of wind shear convergent nodes that associate with the S2,3 semidiurnal tide in the upper E region and the S1,1 diurnal tide in the lower E region

Abstract. Sporadic E layers (E s ) follow regular daily patterns in variability and altitude descent, which are determined primarily by the vertical tidal wind shears in the lower thermosphere. In the present study a large set of sporadic E layer incoherent scatter radar (ISR) measurements are analyzed. These were made at Arecibo (Geog. Lat. ∼18 • N; Magnetic Dip ∼50 • ) over many years with ISR runs lasting from several hours to several days, covering evenly all seasons. A new methodology is applied, in which both weak and strong layers are clearly traced by using the vertical electron density gradient as a function of altitude and time. Taking a time base equal to the 24-h local day, statistics were obtained on the seasonal behavior of the diurnal and semidiurnal tidal variability and altitude descent patterns of sporadic E at Arecibo. The diurnal tide, most likely the S(1,1) tide with a vertical wavelength around 25 km, controls fully the formation and descent of the metallic E s layers at low altitudes below 110 km. At higher altitudes, there are two prevailing layers formed presumably by vertical wind shears associated mainly with semidiurnal tides. These include: 1) a daytime layer starting at ∼130 km around midday and descending down to 105 km by local midnight, and 2) a less frequent and weaker nighttime layer which starts prior to midnight at ∼130 km, descending downwards at somewhat faster rate to reach 110 km by sunrise. The diurnal and semidiurnal-like pattern prevails, with some differences, in all seasons. The differences in occurrence, strength and descending speeds between the daytime and nighttime upper layers are not well understood from the present data alone and require further study.