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.