On the governing dynamics of the VHF radar echoes from the mesosphere and collision‐dominated lower E region over Gadanki (13.5°N, 79.2°E)

Abstract

In the present communication, the role of neutral dynamical processes in governing the morphology and seasonal variations of daytime VHF radar echoes from the mesosphere and the collision‐dominated lower E region is discussed. While the seasonal variations of mesospheric echo occurrence show semiannual variation with their maxima in the equinoxes, the occurrence of E region echoes shows strong annual variation peaking in the summer with two secondary maxima identical to semiannual variation of the mesospheric echoes. The occurrence rates of the E region echoes, however, display year‐to‐year variability. Semiannual variations in the occurrence of low‐latitude mesospheric echoes are closely linked with similar variations in the occurrences of temperature inversion, large wind shear, tides, and gravity wave activities in the same height region. In the case of E region echoes, the summer maximum and equinoctial secondary maxima are found to be linked with similar variations in wind and wind shear, Es activity, tidal, and gravity wave activities. Further, the summer maximum is found to be strongly correlated with that of meteor flux and nonmigrating diurnal tide. This finding is significant given the fact that such wind field in the presence of metallic ions could form both Es and field‐aligned irregularities. The present results show that tides and gravity waves are the most important dynamical factors governing the morphology and seasonal variations of the mesospheric as well as lower E region echoes observed over Gadanki. These results are discussed in the light of current understanding on the dynamical coupling of the mesosphere and lower E region through tides and gravity waves.

On the Type‐A and Type‐B 150‐km Radar Echoes

“…Given that the metallic ions could reduce the incoherent scatter spectral width by about 20% (Tepley & Mathews, 1985), the enhanced echoes with narrow spectral width could be associated with such effect on the NEIS. Further, the fact that the meteor flux maximizes in the local summer in the Indian sector (Selvaraj et al, 2017) and they are likely to be uplifted by the prevailing daytime eastward electric field, it is quite reasonable to consider the presence of metallic ions in the 150‐km altitude region. Also the meteor ablation rate being higher during the solar minimum than in solar maximum (e.g., Campbell‐Brown, 2019) stimulates to consider the role of metallic ions to understand the reason for more occurrence of Type‐B echoes during local summer and low solar activity (Patra & Pavan Chaitanya, 2016).…”

Seasonal Variations of Low‐Latitude Migrating and Nonmigrating Diurnal and Semidiurnal Tides in TIMED‐SABER Temperature and Their Relationship With Source Variations

“…However, Gurubaran and Rajaram () observed maximum activity of diurnal tide in June in addition to the semiannual variation and noted its biennial variation. Selvaraj et al () noted shorter vertical wavelength of diurnal tide in mesospheric winds over Tirunelveli (8.7°N, 77.8°E) and attributed it to the nonmigrating tide. The present study shows large enhancement of DE4 during June, when DW1 is minimum.…”

Multiple Ionospheric Descending Layers Over Arecibo

“…It has been well reported that the convergence of the ions forms the Es through the wind shear driven by the tides and waves (Krall et al, 2020;Axford, 1963;Whitehead, 1961;Mathews, 1998;Haldoupis, 2012;Selvaraj et al, 2017). Based on the descending speed of the Es, USD, and USN layers, dominant control by the semidiurnal and diurnal tides is identified (Christakis et al, 2009).…”

“…Layering phenomena have been observed in the lower ionosphere for many decades (Christakis et al, 2009;Mathews, 1998;Whitehead, 1989;Mathews & Bekeny, 1979;Shen et al, 1976;Axford, 1963;Whitehead, 1961;McNicol & Gipps, 1951;Haldoupis, 2012;Earle et al, 2000;Fujitaka & Tohmatsu, 1971, 1973 globally, and in particular at Arecibo using the powerful 430 MHz incoherent scatter radar. Early power profile observations showed the remarkable complexity of so-called sporadic layers, and the development of the wind shear theory, as applied to metallic ions, has also proceeded over several decades (Axford, 1963;Whitehead, 1961;Mathews, 1998;Haldoupis, 2012;Selvaraj et al, 2017). The studies broadened to include somewhat higher altitudes, encompassing so-called intermediate descending layers (IDLs).…”