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\begin{document}

\title{Multiple Ionospheric Descending Layers Over Arecibo}



\author[1]{Selvaraj Dharmalingam}%
\author[2]{Mani Sivakandan}%
\author[3]{Michael P. Sulzer}%
\affil[1]{Arecibo Observatory / UCF}%
\affil[2]{Leibniz Institute of Atmospheric Physics at the University of Rostock}%
\affil[3]{Arecibo Observatory}%


\vspace{-1em}



  
  \date{June 14, 2024}


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\begin{abstract}
Observations using Arecibo Observatory's highly sensitive Incoherent
Scattering Radar (AO-ISR) show ionospheric descending layers from as
high as \$\textbackslash{}sim\$400 km, much higher than earlier studies,
with continuity down to 90 km. The AO-ISR was operated to observe the
ion-line and plasma-line with coded-long-pulse for high temporal and
spatial resolution of 35/10 seconds and 300 m, respectively, during
01-06 February 2019. We found multiple layering structures descending
from 400 km to 90 km in all these six days. These layers are
traditionally called intermediate descending layers (IDLs)
(\$\textgreater{}\$130 km and below F-peak), upper semi-diurnal daytime
\$\textbackslash{}\&\$ nighttime layers (110 km-130 km), and lower
diurnal layers(\$\textless{}\$110 km). We have denoted the new daytime
descending layers above the hmF2 as top-side descending layers (TDLs).
All these layers are collectively named ion descending layers (IonDLs)
since all of them are connected with some discontinuity at the F1-peak
(i.e., 170 km), except for the daytime lower-diurnal layer. The most
pronounced IonDLs occur in the twilight times. IonDLs mainly occur in
shear zones of the vertical ion drifts and are favored by downward ion
drifts, and their descent speeds increase with increasing altitude. The
estimated phase velocities of the waves in the F-region are comparable
with the descending speed of the IonDLs. Furthermore, IonDLs/IDLs occur
with and without spread-F events but intensified spread-F events raise
their beginning altitude. The TDLs and IDLs are driven by gravity waves
with time periods of 1.5-4 hours.%
\end{abstract}%



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