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The COSMIC radio occultation observations are used to investigate the wavenumber‐4 (WN4) structure of the Sporadic E (Es) layer during 2007–2018. The WN4 pattern is observed in the Es occurrence rate over the middle‐ and low‐latitudes. This pattern is stronger at the low‐latitudes than at the middle‐latitudes. It shows the peak intensity in latitude bands between 5° and 15° dip latitudes in both hemispheres. The WN4 structure occurs predominantly during daytime. It appears above 110 km in the morning, then descends slowly down to about 105 km, and finally disappears after sunset. The WN4 structure moves eastward with a speed of approximately 90°/day. It is prominent in boreal autumn and summer, and disappears in boreal winter. The dominant role of the DE3 tide for the formation of the WN4 structure is supported by similar features between the Es occurrence rate and the DE3 tide of the zonal wind shear, such as the phase, eastward speed, seasonal variation, symmetrical distribution, and strong activity at the low‐latitudes. Key Points: The wavenumber‐4 (WN4) structure is observed in the sporadic E layer over the middle‐ and low‐latitudesThe diurnal, seasonal, and altitudinal variations of the WN4 pattern are analyzedThe diurnal eastward non‐migrating tide with zonal wavenumber‐3 plays the dominant role in the formation of the WN4 structure [ABSTRACT FROM AUTHOR]

To study the influence of solar and geomagnetic activity on Es layers over low, mid and high latitude stations, hourly values of parameters of Es layer during four solar cycles (1970–2010) over different latitude stations are considered. It has been suggested that there is a relationship between solar and geomagnetic activity with Es layer, and the correlations are different over low, mid and high latitude stations. The reasons of different behavior of Es frequency parameters to solar and geomagnetic activity are also studied. [ABSTRACT FROM AUTHOR]

In this paper different spectral analyses are employed to investigate the tidal and planetary wave periodicities imprinted in the following two main characteristics of the sporadic E (Es) layer: the top frequency (ftEs) and the lowest virtual height (h′Es). The study is based on ionograms recorded during the summertime of 2013, and precisely in June, July, August and September, by the Advanced Ionospheric Sounder by Istituto Nazionale di Geofisica e Vulcanologia (AIS-INGV) ionosondes installed at Rome (41.8°N, 12.5°E) and Gibilmanna (37.9°N, 14.0°E), Italy. It was confirmed that the diurnal and semidiurnal atmospheric tides play a fundamental role in the formation of the mid-latitude Es layers, acting through their vertical wind-shear forcing of the long-living metallic ions in the lower thermosphere, and at the same time it was found that the planetary atmospheric waves might affect the Es layers acting through their horizontal wind-shear forcing with periods close to the normal Rossby modes, that is 2, 5, 10 and 16 days. The wavelet analysis shows also that the ftEs and h′Es tidal oscillations undergo a strong amplitude modulation with periods of several days and with important differences between the two parameters. This amplitude modulation, characterizing markedly the first thirty days of the ftEs spectrogram, suggests that Es layers are affected indirectly by planetary waves through their nonlinear interaction with the atmospheric tides at lower altitudes. This study wants to be a continuation of the Haldoupis et al. (2004) work in order to verify their results for the foEs characteristic and on the other hand to extend the study also to the h′Es characteristic not yet shown so far. Anyhow, the study confirms that ionosonde data, especially those registered in summertime, represent a powerful tool for studying tidal and planetary waves properties and their climatology in the mesosphere–low-thermosphere region. [ABSTRACT FROM AUTHOR]

Sporadic sodium layers (SSLs) were studied with sodium (Na) lidar at Beijing (40.5°N, 116.0°E) and Haikou (19.5°N, 109.1°E) in China. The altitude distribution, strength factors, and occurrence time of SSL and sporadic E (E) layer events were statistically analyzed at both observing sites. SSL occurrence had a maximum near 22:00 local time, and E layer occurrence had a maximum before midnight. The altitude distributions of SSL and E layer events were better correlated at Haikou than at Beijing. All the average values of height and strength factor for SSL and E layer events, as well as the average of the maximum frequency that can be reflected by the E layer (fE), at Beijing were higher than those at Haikou. A better correlation between SSL and E layer events was also found at Haikou, and the formation of SSLs was considered to likely depend on the seasonal variability of chemical and dynamical processes. [ABSTRACT FROM AUTHOR]

Abstract: Sporadic E layers are formed dominantly by wind shear mechanism, but their formation and dynamics are driven also by ionospheric electric fields. Investigation of low latitude sporadic E layers under quiet conditions shows that Es layer formation during post sunset hours can be disrupted or enhanced depending upon the vertical structure of the vertical electric field arising from sunset electrodynamic processes. During magnetic storms the formation and disruption of these layers are also strongly controlled by vertical Hall electric field induced by the zonal magnetospheric electric fields that penetrates to equatorial/low latitude ionosphere. Observational results on storm time Es layer responses in the Brazilian and Indian-Asian longitudes are compared. An under-shielding prompt penetration electric field (PPEF) of westward polarity that dominate the night side ionosphere, or an over-shielding electric field also of westward polarity in the evening sector can cause formation of sporadic E layers near 100km, while an eastward polarity electric field, (under-shielding/over-shielding depending upon local time) can lead to disruption of an Es layer in progress. Ionization convergence/divergence leading to the Es layer formation/disruption is driven by a vertical Hall electric field, induced by the primary zonal PPEF, in the presence of storm associated enhanced ratio of field line integrated Hall to Pedersen conductivity (∑ H )/(∑ P ). A downward polarity of the Hall electric field leads to Es layer formation, while an upward polarity causes the Es layer disruption. An enhancement in the ∑ H /∑ P ratio can result from E layer conductivity enhancement due to energetic particle precipitation peculiar to the longitude of the South Atlantic Magnetic Anomaly (SAMA) and/or from a drastic reduction in integrated Pederson conductivity in the form of reduced foF2 that is observed in all longitudes. [Copyright &y& Elsevier]

This paper provides a comprehensive update on sporadic E layers that is placed in the context of atmosphere-ionosphere coupling, exemplified here by the fundamental windshear theory processes that govern sporadic E layer formation and variability. Some basics of windshear theory are provided first, followed by a summary of key experimental results, their interpretation and physical understanding. The emphasis is placed on the wind shear control of the diurnal and sub-diurnal variability and altitude descent of sporadic E layers and the key role behind these properties of the diurnal and semidiurnal tides. Furthermore, the paper summarizes recent observations that establish a role also for the planetary waves in sporadic E layer occurrence and long-term variability. The possible mechanisms behind this interaction are examined and evidence is presented which shows that planetary waves affect sporadic E layers indirectly though the amplitude modulation of tides at lower altitudes in the MLT region. Only a brief mention is made about gravity wave effects on sporadic E, which apparently exist but cannot be as crucial in layer forming as thought in the past. There is now enough evidence to suggest that mid- and low-latitude sporadic E is not as 'sporadic' as the name implies but a regularly occurring ionospheric phenomenon. This may suggest that the sporadic E layer physics can be incorporated in large-scale atmosphere-ionosphere coupling models. [ABSTRACT FROM AUTHOR]

Abstract: A technique of modelling the one-hop radio wave propagation at middle latitudes in the presence of sporadic E-Layer is presented. The technique is focused on the performance of the long-term forecast of the maximum usable frequency range and on the increase of the radio communication reliability. Examples of calculation for medium-distance paths are shown. [Copyright &y& Elsevier]

The observations of daytime ionospheric scintillation are attributed to E-region irregularities at high and equatorial latitudes. In this paper, VHF amplitude scintillations recorded during the daytime period from 1991 to 1999 at low latitude station Varanasi (geomag. lat. = 14° 55' N, long. = 154° E) are analyzed to study the behaviour of sporadic-E irregularities during the active solar and magnetic periods. The daytime digital scintillation data have been analyzed to study some important parameters of scintillation producing sporadic-E irregularities like auto-correlation function, power spectral densities, signal de-correlation time etc. We report the behaviour of these parameters under weak and strong scintillation conditions. The results are also discussed in the light of recent works. [ABSTRACT FROM AUTHOR]