  Providing continuous bandwidth over several tens of GHz, the Terahertz (THz)
band (0.1-10 THz) supports space-air-ground integrated network (SAGIN) in 6G
and beyond wireless networks. However, it is still mystery how THz waves
interact with the channel medium in SAGIN. In this paper, a universal
space-air-ground attenuation model is proposed for THz waves, which
incorporates the attenuation effects induced by particles including condensed
particles, molecules, and free electrons. The proposed model is developed from
the insight into the attenuation effects, namely, the physical picture that
attenuation is the result of collision between photons that are the essence of
THz waves and particles in the environment. Based on the attenuation model, the
propagation loss of THz waves in the atmosphere and the outer space are
numerically assessed. The results indicate that the attenuation effects except
free space loss are all negligible at the altitude higher than 50 km while they
need to be considered in the atmosphere lower than 50 km. Furthermore, the
capacities of THz SAGIN are evaluated in space-ground, space-sea, ground-sea,
and sea-sea scenarios, respectively.
