  Driven by B5G and 6G technologies, multi-network fusion is an indispensable
tendency for future communications. In this paper, we focus on and analyze the
\emph{security performance} (SP) of the \emph{satellite-terrestrial downlink
transmission} (STDT). Here, the STDT is composed of a satellite network and a
vehicular network with a legitimate mobile receiver and an mobile eavesdropper
distributing. To theoretically analyze the SP of this system from the
perspective of mobile terminals better, the random geometry theory is adopted,
which assumes that both terrestrial vehicles are distributed stochastically in
one beam of the satellite. Furthermore, based on this theory, the closed-form
analytical expressions for two crucial and specific indicators in the STDT are
derived, respectively, the secrecy outage probability and the ergodic secrecy
capacity. Additionally, several related variables restricting the SP of the
STDT are discussed, and specific schemes are presented to enhance the SP. Then,
the asymptotic property is investigated in the high signal-to-noise ratio
scenario, and accurate and asymptotic closed-form expressions are given.
Finally, simulation results show that, under the precondition of guaranteeing
the reliability of the STDT, the asymptotic solutions outperform the
corresponding accurate results significantly in the effectiveness.
