Radio occultation measurement of the electron density near the lunar surface using a subsatellite on the SELENE mission

Abstract

[1] The electron density distribution in the vicinity of the lunar surface was explored with the radio occultation technique using a subsatellite on the SELENE mission. Although the measurements suffer from contamination by the terrestrial ionosphere and interplanetary plasma, an analysis of more than 300 measurements provides adequate statistics and reveals a general trend. The result suggests that a dense ionosphere covering the whole sunlit side, as suggested by the radio occultation measurements on the Soviet Luna 19 and 22 missions, does not exist. However, weak signatures of electron density enhancement with densities on the order of 100 cm À3 are observed below 30 km altitude at solar zenith angles less than 60 . The statistically averaged density reaches a peak at around 15 km altitude and decreases gradually at higher altitudes and toward the surface. Although the suggested electron layer is thinner and less extended horizontally than that reported by Luna 19 and 22, the existence of such an ionized layer is still difficult to explain by conventional ionosphere generation mechanisms. An alternative source of electrons may be required.Citation: Imamura, T., et al. (2012), Radio occultation measurement of the electron density near the lunar surface using a subsatellite on the SELENE mission,

On the origin of the ionosphere at the Moon using results from Chandrayaan‐1 S band radio occultation experiment and a photochemical model

“…A linear fit was then applied to the first 2 min of Doppler residual data prior to occultation, and the fit was extended for last 1 min of the observations. It is worth mentioning that similar electron density profiles were obtained using Selene RO measurements [Imamura et al, 2012] as well. Only if the phase changes during the occulted period exceeded one sigma of the fluctuation during first 2 min, we assumed that the extra phase change in the signal was due to occulted lunar ionosphere.…”

“…This experiment presented an evidence for the existence of a thin ionosphere around Moon with a density of the order of 10 2 cm −3 . Radio occultation of the Moon's ionosphere by SMART 1 has also shown the existence of ionosphere with columnar density of the order of 10 13 el/cm 2 [Pluchino et al, 2008] which was further confirmed by the Selene RO measurements [Imamura et al, 2012]. The estimated columnar density of the lunar ionosphere was in the range of 3-5 × 10 14 el/cm 2 .…”

“…The model calculations suggest that in the absence of transport, inert ions, namely Ar + , Ne + , and He + , dominate lunar ionosphere (density ∼5 × 10 4 cm −3 ). Radio occultation of the Moon's ionosphere by SMART 1 has also shown the existence of ionosphere with columnar density of the order of 10 13 el/cm 2 [Pluchino et al, 2008] which was further confirmed by the Selene RO measurements [Imamura et al, 2012].The ionosphere at Moon is believed to be highly variable [Vyshlov, 1976] with several factors controlling its plasma density. Assuming the Moon's exosphere to have CO 2 , H 2 O, O, OH, H 2 , CH 4 , and CO molecules in addition to the inert gases, the model calculations suggest that the lunar ionosphere is dominated by molecular ions, namely H 2 O + , CO + 2 , and H 3 O + , with near-surface density ∼250 cm −3 .…”

Dual‐spacecraft radio occultation measurement of the electron density near the lunar surface by the SELENE mission

“…The nominal observation mode of the SELENE radio occultation experiment, which used a single spacecraft, covered a considerably wider SZA range. The results obtained with this observation mode suggest a weak electron density enhancement with densities on the order of 100 cm −3 below 30 km altitude at solar zenith angles less than 60° [ Imamura et al , 2012]. Although the classical ionosphere generation mechanism does not seem to work near the lunar surface, photoelectron emission from exospheric dust particles is promising.…”

Measuring the Earth's Synchrotron Emission From Radiation Belts With a Lunar Near Side Radio Array

“…A more recent experiment with LRS (Lunar Radio Science) on Kaguya‐SELENE by the Japanese space agency has found evidence of transient enhancements in surface electron density around 250/cm but only within a solar zenith angle of 60°. They used radio occultation experiments with multiple spacecraft to probe the lower lunar atmosphere (Imamura et al, ). SELENE did not find a large persistent enhancement like Luna over the whole dayside.…”