  Free-space optics (FSO)-based satellite communication systems have recently
received considerable attention due to their enhanced capacity compared to
their radio frequency (RF) counterparts. This paper analyzes the performance of
physical layer security of space-to-ground intensity modulation/direct
detection FSO satellite links under the effect of atmospheric loss,
misalignment, cloud attenuation, and atmospheric turbulence-induced fading.
Specifically, a wiretap channel consisting of a legitimate transmitter Alice
(i.e., the satellite), a legitimate user Bob, and an eavesdropper Eve over
turbulence channels modeled by the Fisher-Snedecor $\mathcal{F}$ distribution
is considered. The secrecy performance in terms of the average secrecy
capacity, secrecy outage probability, and strictly positive secrecy capacity
are derived in closed-form. Simulation results reveal significant impacts of
satellite altitude, zenith angle, and turbulence strength on the secrecy
performance.
