Seasonal variability and descent of mid-latitude sporadic E layers at Arecibo

Abstract

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.

Coordinated sporadic E layer observations made with Chung-Li 30MHz radar, ionosonde and FORMOSAT-3/COSMIC satellites

“…It is worth pointing out that, because of the limitation of the vertical resolution of the COSMIC-retrieved ionospheric electron density profile (around 1 km) using piercing technique (Schreiner et al, 1999), fine structures of the Es layer cannot be shown from the COSMIC measurement. Nevertheless, it has been shown that the radio occultation technique is valid in the retrieval of ionospheric Es layer (Arras et al, 2009;Christakis et al, 2009;Yang et al, 2009). As shown, pronounced electron density layers with peaks centered at heights 102.5, 102 and 106 km and with the thicknesses of about 10, 7 and 7 km, respectively, were observed.…”

Sporadic E layer at mid-latitudes: average properties and influence of atmospheric tides

“…The corresponding estimated wavelength is (42 ± 3) km. It is worth noting that the unloading altitude is in this case lower than those characterizing the previous cases, and this is correct due to the missing action performed by the semidiurnal tides, which are characterized by an unloading altitude higher than that of the diurnal tide (Haldoupis et al, 2006;Christakis et al, 2009). In fact, the semidiurnal tides influence the Es layer dynamics at altitudes higher than 110 km, while the diurnal tide influences the Es layer dynamics at altitude lower than 110 km and, in the absence of other dynamical inputs, can drag the Es layer down to altitudes lower than 90 km.…”

“…Hence, the Es layer descent patterns are approximately represented by the linear fits represented in Fig. 6a, the negative angular coefficient of which represents the Es descent velocity that, according to the wind-shear theory, is equivalent to the phase velocity of the convergence node characterizing the neutral wind profile associated with the tides (Christakis et al, 2009). The first trace appears at about 04:00 UT, at an altitude of 126 km, and descends with a medium velocity of (−6.4 ± 0.5) km h −1 to an altitude of 105 km at 08:00 UT; the second trace appears at about 10:00 UT, at an altitude of 110 km, and descends with a medium velocity of (−1.3 ± 0.1) km h −1 to an altitude of 100 km at 17:00 UT; the third trace appears at about 18:00 UT, at an altitude of 120 km, and descends with a medium velocity of (−3.6 ± 0.4) km h −1 to an altitude of 103 km at 21:00 UT.…”

Semidiurnal tidal signature in sporadic E occurrence rates derived from GPS radio occultation measurements at higher midlatitudes

“…As known from theory and ground-based observations (Haldoupis et al, 2006;Christakis et al, 2009) the E s layer descent follows the phase velocity of the SDT convergent node at altitudes well above about 100 km. Below, the descent velocity slows down due to enhanced ion-neutral collisions.…”