Schematic diagram of a spherical particle hovering over the surface. The particle of radius r_a hovers over the planar regolith at the height of H(≫ r_a). The solar irradiation (the long red arrow) is originated from the direction . A tiny surface patch with an area of dA reflects the solar radiation and emits the thermal radiation to the particle (the dashed and squiggly arrows, respectively, to the directionof ) with the emission angle of e′. In our model, e″ = e′ holds because of the planar surface assumption. The isothermal regolith region (gray dashed) is assumed to be a circle with a radius of r₀(≫ H ≫ r_a). No radiation outside this region is considered because the patch right below the particle already fills most of the field of view (r₀ ≫ H).