Non-linear solar EUV-driven sodium release from the lunar surface: a contrast to the linear PSD model

ABSTRACT The correlation between solar Extreme Ultra-Violet (EUV) radiation above 8.8 eV and the release of sodium from the lunar surface via photon-stimulated desorption (PSD) is investigated. We use simultaneous measurements of EUV photon flux and Na optical spectral line flux ($F_{Na}$) from the lunar exosphere. Data were acquired with the high-resolution (R$\sim$72 000) Echelle Spectrograph on the 2.34-m Vainu Bappu Telescope during the lunar first quarter (2024 January–March), observing Na I D2 and D1 flux at altitudes below $\sim$590 km from the surface. Simultaneous EUV and FUV measurements were acquired from the GOES-R Series Extreme Ultraviolet Sensor (EUVS), while NUV data were obtained from the Total and Spectral Solar Irradiance Sensor-1 (TSIS-1) aboard the ISS. We correlated $F_{Na}$ with EUV photon flux from EUVS across six bands spanning 256–1405 Å (48.5–8.8 eV) and NUV (2000–4000 Å) from TSIS-1. A non-linear rise in lunar exospheric sodium with increasing EUV and FUV fluxes was observed, contrasting with previous linear PSD models. The EUV radiation above 10 eV drives sodium release, with 256-304 Å wavelengths as dominant contributors. Additionally, the NUV flux and $F_{Na}$ are positively correlated, indicating the role of sodium release. The zenith column density averages $3.3\times 10^{9}$ atoms cm$^{-2}$, with Characteristic temperatures averaging at $\sim$6700 K and scale heights of $\sim$1500 km. Elevated temperatures and sodium densities during solar activity suggest enhanced Na release during flares. These results emphasize the need for a revised PSD model above 8.8 eV and improved constraints on the PSD cross-section.