E region radar echoes from low‐latitude field‐aligned irregularities due to gravity waves and tides: A case study using radar, lidar, and radiosonde observations and simulations

Abstract

On the night of 5 February 2011, the height-time plot of echo from E region field-aligned irregularities (FAI) received by the Indian mesosphere-stratosphere-troposphere radar at Gadanki (13.5°N, 79.2°E) shows a slow-descending layered structures separated by nearly 8 km in the height region 80-100 km and vertically elongated structures with quasiperiodic variations at higher heights. The stratospheric thermal structure also shows similar slowly descending structures with nearly 8 km separation. Hodograph of 7-9 km band-pass filtered radiosonde zonal and meridional winds over Gadanki form clearly an ellipse, and the dominant period obtained from the ratio of major to minor axes of the ellipse is nearly 24 h indicating the presence of a nonmigrating diurnal tide with vertical wavelength of 8 km. The two-dimensional spectrum of temperature perturbations, which are assumed to be due to gravity waves, shows dominant periods near 1 h and 1.5 h. The spectrum of FAI echo also shows similar dominant periods. Altitude-time cross section of vertical ion velocity is computed using winds composed of observed gravity wave and tidal parameters. As the diurnal tide over Tirunelveli (8.7°N, 77.8°E) shows weaker amplitudes above 90 km, the removal of tidal contribution clearly reveals the observed FAI echo structure of vertically slanting plasma blob structures at higher heights and slowly descending continuous structures separated by 8 km at lower heights. The present study clearly demonstrates the role of tides and gravity waves in the formation of descending echo structures and quasiperiodic echoes respectively.

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)

“…It is interesting to note that the vertical wavelength of diurnal tide is ~7 km in June and July when compared to 10–25 km in other months. We surmise that the diurnal tide with large amplitude and short vertical wavelength in June is nonmigrating consistent with the results reported by Sridharan and Sandhya []. The short vertical wavelength suggests low downward phase progression, and hence, the tidal forcing can be effective for long period in converging ions forming E s layer.…”