## 1 Introduction  
With the development of deep space exploration, improving the communication performance of deep space links has become a challenging topic. Free space optical (FSO) is a viable solution thanks to its advantages of high data transmission and large capacity [1, 2]. However, the communication link will face great challenges during superior solar conjunction when the Sun is located between the Earth and the probe [3]. At this time, due to the random changes in the refractive index of the coronal turbulence, the FSO link encounters the irregular solar wind plasma from the Sun, resulting in dramatic signal fluctuations. This phenomenon is called solar scintillation [4]. The essence of solar scintillation is wave scattering, which is directly related to signal amplitude fluctuations, phase fluctuations and angle of arrival fluctuations. The impact of coronal turbulence on the Earth-detector link has been reported in the astronomy and communication fields [5, 6], and will lead to the performance of the communication links degrading severely, and may even cause the interruption. [PERSON] et al. derived the closed-form amplitude fluctuation expression of optical waves propagating in non-Kolmogorov solar wind turbulence during superior solar conjunction, for the first time, based on the Rytov approximation, and briefly discussed the bit error rate (BER) of the FSO system under the log-normal (LN) channel [7]. Then, a further study uses M-ary PPM modulation to improve the BER and average channel capacity in the deep space FSO communication under the LN channel [8]. So far, most of the research in deep space optical communications have focused on FSO communication systems. However, deep space communications are affected by solar activities, and irregular solar wind over long distances will cause signal distortion and fading. In addition, the atmosphere around the earth will also produce signal attenuation. Therefore, further enhancement in the performance of FSO system is required. In near-Earth communication, the combination of FSO technology and RF technology has become a complementary communication technology to alleviate the interference of atmospheric turbulence [9]. As a dual-hop relay system with asymmetric channels, the hybrid RF/FSO relay system combines the advantages of RF systems and FSO systems. Nowadays, there have been numerous studies on hybrid RF/FSO systems [9; 10; 11]. The authors in [10] discussed an RF/FSO system, obeying the Nakagami-m/Gamma-Gamma distribution, derived the end-to-end probability density function (PDF) expression, and simulated the BER and channel capacity performance. Based on the Rayleigh/Gamma-Gamma model proposed by [9], the outage probability, the average BER and channel capacity of the system are comprehensively studied [11]. This model is robust in the simulation of turbulence at different intensities and has been recognized by many researchers. The theoretical study of the hybrid RF/FSO system is well established and it has been proven to perform better than the FSO system in various scenarios. In [12; 13], the theory of 2-hop downlink in deep space communications is discussed, and high link availability is demonstrated. Some papers successively suggested that the hybrid system can be applied to deep space communication [1; 2; 3; 14; 15; 16; 17]. In [14; 15], it was the first time that an RF-Optical method was proposed to improve the performance of deep space communications. A reasonable and promising scheme of the FSO/RF system was introduced in [2], in which the optical signal was first transmitted from the deep space detector to the geostationary orbit satellite through FSO, and then to the receiver on the Earth via RF. After that, [PERSON] proposed that a hybrid RF/FSO system could be used to mitigate the signal attenuation caused by solar scintillation [3]. Recently, considering the channel state information (CSI), the outage probability of the hybrid RF/FSO deep space-terrestrial system based on the Rician/Gamma-Gamma distribution was studied [16].