Recent studies have highlighted the crucial role of abiotic processes, such as photodegradation and microclimatic fluctuation, in accelerating dryland litter decomposition. In grasslands, substantial amounts of dead plant material persist upright above the soil surface after senescence, experiencing distinct microclimatic conditions compared to surface litter. However, our understanding of how ultraviolet (UV) exposure and microclimatic conditions influence their decomposition is limited. To address this knowledge gap, we conducted a field experiment manipulating UV radiation for both soil surface litter and standing litter and monitored their microclimatic conditions in a semi-arid grassland. Our findings indicate that UV exposure enhanced the decomposition of soil surface litter by alleviating the constraint of lignin on litter decomposition, while having no significant influence on standing litter. Although the mean levels of thermal-hydric conditions were lower, more intense fluctuation of temperature and air humidity was detected in standing litter. These higher-level microclimatic fluctuations facilitated the release of dissolved organic carbon, potentially increasing the availability of labile substrates to microbes. Meanwhile, standing litter released more photo-sensitive phenols, leading to decreased sensitivity to UV exposure. Consequently, while UV exposure initially increased standing litter decomposition during the early stage, its influence eventually diminished. These findings underscore the critical yet differing roles of microclimatic conditions and UV exposure in the decomposition of standing and surface litter. Relying solely on knowledges derived from surface litter decomposition and microclimate conditions may not accurately capture the patterns of grassland litter degradation. The limited precipitation in drylands is widely believed to restrict litter decomposition. However, the observed litter decomposition rates in these regions often exceed model predictions based on climatic conditions. In arid and semi-arid steppes, the sparse vegetation cover results in intense ultraviolet (UV) radiation reaching soil surface, thereby triggering photodegradation of the recalcitrant compounds in litter. Additionally, substantial dead plant materials persist standing for months to years, and undergoes more intense warm-cold and wet-dry cycles. However, there have been limited studies assessing the intricate interplay between UV radiation and microclimate fluctuations on litter decomposition in drylands. In this study, we found that UV radiation facilitated the degradation of recalcitrant compounds and enhanced litter degradability of soil surface litter. More intense temperature and humidity fluctuations increased the concentration of dissolved organic carbon in standing litter. This increase could provide more labile substrate to decomposers, consequently accelerating litter decomposition. Moreover, microclimatic fluctuation enhances the loss of photo-sensitive compounds in litter, reducing litter's sensitivity to UV exposure. Our study highlighted the significant role played by the interaction between climatic conditions and UV radiation in driving litter decomposition in water-limited steppes, contributing to a better understanding of carbon turnover in dryland ecosystem. The degradation of soil surface litter was mainly driven by warmer and wetter microclimate and ultraviolet (UV) exposure Greater microclimatic fluctuation in standing litter enhanced the release of dissolved labile carbon, thereby accelerating its decomposition Greater microclimatic fluctuation increased the loss of photo-sensitive compound and weakened sensitivity of standing litter to UV radiation

The local topography of the crater makes permanently shadowed craters (PSCs) over Moon electrically complex. The plasma environment in PSCs is generally characterized by diffused solar wind (SW) plasma. Its dynamics splits the crater into two distinct plasma regions, viz., electron rich region (ERR) and quasineutral region, which essentially describes the electric potential distribution on the crater's surface. Herein, we discuss the electrostatic surface charging of PSCs and illustrate that the fine particles overlying the crater surface significantly contribute to establishing a finite electric potential on the crater surface. We depict that these fine particles act as efficient field emission centers generating electrons via quantum field tunneling and suffice in countering the diffused SW charging flux, establishing steady state charging equilibrium over the crater's surface.

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.

Large spatial extent of biomass burning occurs in northeast region of India during annual dry season for shifting cultivation purposes. Characterization of optical properties of resultant biomass burning aerosols is important for the study of atmospheric radiative process and for remote sensing of both Surface and atmospheric properties in these regions. In the present study, physical and optical properties of biomass burning aerosols in Arunachal Pradesh, North Eastern Region of India have been studied for the first time using ground based measurements using a MICROTOPS-II sunphotometer, an Aethalometer, a quartz crystal microbalance impactor (QCM), SO2 analyser, and an UV meter. Aerosol size distribution suggested dominance of accumulation mode particle loading during burning days compared to normal days. The slope of data points between simultaneous measurements of AOD (500 nm) and UVery suggested that every 0.1 increase in aerosol optical depth (AOD) causes 0.1 minimal erythermal dose (MED h(-1)) reduction during normal day and reduction of 0.36 MED h(-1) in ground reaching UVery during biomass burning periods. Diurnal variations of black carbon aerosol (BC) concentrations increased by a factor of similar to 2 during morning and evening hours compared to afternoon hours during biomass burning period. Daily average black carbon aerosol loading and SO2 concentrations were found to be high during burning day compared to background values. The proportion of BC to total aerosol mass concentration was observed to be similar to 5% during normal days and similar to 14% during burning days. The changes in black carbon mass concentration values have implications for estimating radiative forcing due to aerosols over the region. (C) 2008 Elsevier B.V. All rights reserved.