The photoelectron sheath and floating fine positively charged dust particles constitute two-component dusty plasma in the sunlit lunar regolith's vicinity. By including the charge fluctuation into photoelectron-dust dynamics, the lunar exospheric plasma is proposed to support the propagation of long-wavelength dust acoustic (DA) modes. Using the standard approach based on the dynamical equations for continuity, momentum, plasma potential, and dust charging along with Fowler's treatment of photoemission and non-Maxwellian nature of the sheath photoelectrons, the wave dispersion is derived. The dust charge variation modifies the usual DA wave dispersion and excites the ultralow frequency modes that propagate with sufficiently low phase speed. Such ultralow frequency modes are predicted as pronounced for smaller values of dust charge and sheath potential. The DA wave dispersion is also depicted as sensitive to the photoelectrons' energy distribution within the sheath. The quantitative estimates suggest that the nominal exospheric plasma may exhibit DA waves propagating with frequencies of the order of unity.

We report a comprehensive study by the UV spectrograph LAMP (Lyman-Alpha Mapping Project) onboard the Lunar Reconnaissance Orbiter to map the spatial distribution and temporal evolution of helium atoms in the lunar exosphere, via spectroscopy of the He I emission line at 58.4 nm. Comparisons with several Monte Carlo models show that lunar exospheric helium is fully thermalized with the surface (accommodation coefficient of 1.0). LAMP-derived helium source rates are compared to the flux of solar wind alpha particles measured in situ by the ARTEMIS twin spacecraft. Our observations confirm that these alpha particles (He++) are the main source of lunar exospheric helium, representing 79 per cent of the total source rate, with the remaining 21 per cent presumed to be outgassing from the lunar interior. The endogenic source rate we derive, (1.49 +/- 0.08) x 10(6) cm(-2) s(-1), is consistent with previous measurements but is now better constrained. LAMP-constrained exospheric surface densities present a dawn/dusk ratio of similar to 1.8, within the value measured by the Apollo 17 surface mass spectrometer LACE (Lunar Atmosphere Composition Experiment). Finally, observations of lunar helium during three Earth's magnetotail crossings, when the Moon is shielded from the solar wind, confirm previous observations of an exponential decay of helium with a time constant of 4.5 d

An open question of the electrostatic charge development on the lunar surface in the electron-rich region within the permanently shadowed craters (PSCs) is addressed. We propose that the fine dust grains on the crater surface may act as efficient field emission centres generating electrons via quantum field tunnelling. This return current may be sufficient to establish a steady-state dynamical equilibrium for the surface-plasma system. This leads to the crater surface attaining a finite electric potential. Our analysis illustrates that the PSC having similar to 100 nm dust, covering 1 per cent of the surface area within the electron-rich region, may acquire a negative potential of few hundred volts in the steady-state condition.

Context. Recent measurements by IBEX and detailed modeling have changed our understanding of the flow of the interstellar medium through the solar system. In particular, a time dependence of the direction of the interstellar medium flow has been proposed, and a new population of helium atoms, called the warm breeze, has been discovered. Aims. We aim to constrain the structure of the interstellar medium close to the downwind focusing cone using the sensitive LAMP FUV/EUV imaging spectrograph onboard the Lunar Reconnaissance Orbiter. Methods. We measured the brightness of the emission line from interstellar helium atoms resonantly scattering solar photons at 58.4 nm (HeI) and compare it to our modified cold model of interstellar HeI sky brightness as a function of ecliptic latitude and longitude. Additionally, we compared LAMP observations to a model with time-dependent inflow direction and a model of the brightness of the warm breeze, to see if they can be distinguished by LAMP. Results. We find that the LAMP observations agree within error bars to our modified cold model, which in turn is consistent with the latest interstellar helium flow parameters found with IBEX. Our model can therefore be applied to other UV spectroscopic observations of the interstellar helium. However, LAMP observations cannot distinguish between our model and a model with a different inflow direction, since the latter has negligible effect on the 2D brightness of the interstellar HeI emission line. For the same reason, LAMP could not detect the effect of the warm breeze. We note a discrepancy between solar irradiances measured by TIMED/SEE and those measured by SDO/EVE. We recommend using values from SDO/EVE. Finally, we derive a value of LAMP sensitivity at the EUV wavelength (58.4 nm) of 0.485 +/- 0.014 Hz/Rayleigh. Conclusions. These measurements pave the way to observations of the interstellar wind from lunar orbit.