  High altitude platforms (HAPs)-aided terrestrial-aerial communication
technology based on free-space optical (FSO) and Terahertz (THz) feeder links
has been attracting notable interest recently due to its great potential in
reaching a higher data rate and connectivity. Nonetheless, the presence of
harsh vertical propagation environments and potential aerial eavesdroppers are
two of the main challenges limiting the reliability and security of such a
technology. In this work, a secrecy-enhancing scheme for HAP-aided
ground-aerial communication is proposed. The considered network consists of
HAP-assisted communication between a ground station and a legitimate user under
the threat of an aerial and ground eavesdropper. Thus, the proposed scheme
leverages (i) HAP diversity by exploiting the presence of multiple flying HAPs
and (ii) the use of a hybrid FSO/THz transmission scheme to offer better
resilience against eavesdropping attacks. An analytical secrecy outage
probability (SOP) expression is derived for the scheme in consideration.
Results manifest the notable gain in security of the proposed scheme with
respect to both (i) the single-HAP and (ii) THz feeder-based benchmark ones,
where the proposed scheme's SOP is decreased by four orders of magnitude using
$4$ HAPs with respect to the first benchmark scheme, while a $5$-dB secrecy
gain is manifested with respect to the second benchmark one.
