Presentation Information
[PPS02-P03]Developing numerical model of photoelectrons and its application to the dayside lunar surface potential variations
*Masahisa Kato1, Yuki Harada2, Yoshifumi Saito3, Masaki N. Nishino3, Futoshi Takahashi4, Hisayoshi Shimizu5, Shaosui Xu6, Andrew R. Poppe6, Jasper S. Halekas7 (1.Kyoto University, 2.Nagoya University, 3.Japan Aerospace Exploration Agency, 4.Kyushu University, 5.The University of Tokyo, 6.University of California, Berkeley, 7.University of Iowa)
Keywords:
Lunar surface potential
Since the Moon does not possess a dense atmosphere, its surface directly interacts with ambient charged particles. On the dayside, photoelectrons are emitted due to solar irradiation, and the electrostatic potential of the lunar surface adjusts to balance electric currents carried by incoming and outgoing charged particles. To investigate the temporal and spatial variability of the dayside lunar surface potential, we develop a numerical model of the energy spectra of photoelectrons emitted from the surface. By comparing the model results with in situ observations around the Moon, we estimate the acceleration and deceleration of emitted electrons caused by the potential difference between the lunar surface and orbiting spacecraft. We present two applications of the model to lunar observations. First, we compare the model with Kaguya observations to examine the spatial distribution of the surface potential in response to variations in the crustal magnetic field strength. Electrons tend to be more decelerated above magnetic anomalies, which is interpreted as the effect of an upward electric field formed by the decoupling of electrons and ions. This behavior corresponds to the positive surface potential on crustal magnetized regions. Second, we apply the model to ARTEMIS observations to investigate the variability of the lunar surface potential as a function of solar activity. We estimate the dayside lunar surface potential based on the calculated photoelectron energy spectra and the characteristic downward electron fluxes to the lunar surface. We find that the lunar surface potential can exceed +100 V in the terrestrial magnetotail lobes when an extreme solar flare occurs. Large dayside lunar surface potentials can lead to drastic spatial variations in surface potential, driven by surface topography, which acts as barriers for charged particles and photons and lead to inhomogeneity of surface potentials. This method can be applied to the evaluation of charged particle and electrostatic environments around airless bodies.