###### Abstract
[28] evaluated the outage probability performance of an asymmetric FSO/RF of multiusers satellite-UAV-terrestrial system, and the authors considered DF relay UAV node for transmission and RF link exploit transmit beamforming scheme. However, in all these previous research works on a mixed FSO/RF, multiusers integrated satellite-terrestrial systems are DF-based relay node and the secrecy performance of cooperative satellite terrestrial relaying networks was not considered. Recently, some research studies have examined hybrid satellite-terrestrial systems in the context of physical layer security. The secrecy performance of a hybrid satellite relay network with multiple external eavesdroppers was studied [29] with the assumption that multiple eavesdroppers receive signals from the terrestrial networks. In Reference [30], the secrecy outage performance of a land mobile satellite system was evaluated, where both the legitimate user and eavesdropper utilize the diversity technique with multiple antennas. Moreover, the physical layer secrecy performance of a hybrid satellite and terrestrial free space optical cooperative network was presented in Reference [31], where the eavesdropper was assumed to intercept the satellite signal. In addition, the secrecy performance of RF eavesdropping for a HAP aided satellite system was investigated in Reference [32]. However, the security performance of all aforementioned studies was not based on multiple legitimate users, and DF relay was considered for the system. Motivated by this, the performance of the secrecy performance of a mixed FSO/RF integrated satellite-high altitude platform (HAP) relaying networks for terrestrial multiusers with the existence of an eavesdropper. The HAP relay node is considered to utilize AF relaying with different detection schemes for FSO signal detection. The closed-form expression for the system equivalent end-to-end cumulative distribution function is derived. Thus, to quantify the system performance, the exact closed-form expressions of the system connection outage probability, secrecy outage probability, and strictly positive secrecy capacity are obtained under the different HAP detection schemes. To gain better insight, the asymptotic expression for the system SOP is derived. The main contributions of this work are summarized as follows:  
1. The exact closed-form expression for the system equivalent end-to-end cumulative distribution function (CDF) is derived by exploiting the [PERSON]'s G and [PERSON]'s H functions. To the best of authors' knowledge, the derived CDF is novel as a mixed Gamma-Gamma and shadowed-Rician structure under AF relaying protocol is not found in the existing literature. 2. The analytical closed-form expressions of the system performance in terms of connection outage probability (COP), secrecy outage probability (SOP), and strictly positive secrecy capacity (SPSC) are obtained. 3. The asymptotic expression for the system secrecy outage probability is derived to obtain physical insight. 4. Relative to Reference [26], where multiple users are considered, the system performance was not based on physical layer secrecy. In addition, the RF link was subjected to Nakagami-m distributions. In this paper, the system performance is based on physical layer secrecy, and the RF links are subjected to Rician fading distributions. * Relative to Reference [32], where the HAP relay aided node employed DF relaying protocol, and the system only considered a single terrestrial user. The HAP in this paper is an AF-based relaying protocol, and the multiple legitimate users are considered at the ground station. The rest of the paper is organized as follows. The system and channel models are provided in Section 2. In Section 3, the exact closed-form expression for the system equivalent end-to-end cumulative distribution function is presented We derive the analytical expressions of the COP, SOP, and SPSC subject to different detection schemes in Section 4. Numerical results and discussions are depicted in Section 5.