tokens: refer to φ w = 6 V, hν = 10.29 V (for λ ν = 121.57 nm), T d ≈ 205 K (for latitude θ = 70°), and non-thermal photoelectron distribution with A = 0.1. The parameter q is observed to increase with φ o . It is also noticed that the effect of dust charge fluctuation on the wave dispersion is pronounced for small z o values; for instance, q > 1 for z o = 30 and φ o > 0.2 (Fig. 5a ) and for φ o = 1.0 and z o 104 (Fig. 5b ). The positive plasma potential φ o infers enhanced population density of the photoelectrons in the vicinity compared to the case φ o = 0; this may induce additional shielding to the floating dust particles and effectively reduces the coupling between charged dust particles. It may result in a decrease in the phase speed and frequency of the propagating DA wave. This nature has been depicted in Figs 5(c) and (d) . The calculations corresponding to non-zero φ o are of significance to the lunar exospheric plasma, which exhibits a photoelectron sheath with a positive potential and varies with the altitude. Another interesting point evident from the wave dispersion (Figs 2 – 5 ) is the occurrence of a cut off with z o , Δ, and φ o for which ω r takes imaginary values, and it refers to a damped wave solution for the excited DA modes. Figure 5. (a) and (b) ( top panel ): Parameter q A = [(Λ ch /Ω ch ) n de ] A and (c) and (d) ( bottom panel ): wave dispersion ( ω r / k ) in long-wavelength regime ( kλ e 1) for the non-thermal distribution of photoelectrons. Left-hand and right-hand columns refer to parameters’ variation with φ o and z o , respectively. The calculations refer to the parameters φ w = 6 V, hν = 10.29 eV, T d ≈ 205 K, and A = 0.1. The black (solid), red (dash), green (dot), blue (dash–dotted), magenta (dash–dot–dotted), and brown (short dash) in the left -hand and right-hand columns correspond to the magnitude of the varying parameter z o = 30, 50, 100, 150, 200, and 250, and φ o = 0.1, 0.5, 0.8, 1.0, and 1.2, respectively. As understood from the discussion, the significant coupling between floating dust and electrons within the photoelectron sheath is a favourable condition for generating such low-frequency DA modes on the sunlit locations, and these physical conditions strongly depend on dust charge z o and constituent sheath plasma parameters (i.e. density and potential). It is of interest to comment on the influence of the solar wind on the excitation of these DA modes. The solar wind is an essential constituent configuring the electrostatic environment over Moon. Due to larger photoemission current (∼5 μA m −2 ), the surface charging over the sunlit location and sheath population (∼500 cm −3 ) is dominated by emitted photoelectrons, and the solar wind plasma (density ∼5 cm −3 and current ∼10 nA m −2 ) essentially helps in attaining dynamic equilibrium of the constituent charging currents. In the wave generation processes, the relative motion of solar wind to the photoelectron sheath plasma contributes in exciting
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