Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p51d..04i&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #P51D-04
Other
5729 Ionospheres (2459), 6250 Moon (1221)
Scientific paper
Lunar ionosphere is generally thought to be as thin as 1 cm-3; the process that will prevent the accumulation of newly produced ions near the lunar surface is the impingement of the solar wind magnetic field on the lunar surface, which induces an electric field that sweeps away ions. In harmony with this prediction, most of the radio occultation experiments performed with radio stars failed to detect the lunar ionosphere. Radio occultation experiments conducted with the Soviet Luna 19 and 22 spacecraft, on the other hand, detected large electron densities near the dayside lunar surface. Vyshlov (1974) obtained peak electron densities of 500--1000 cm-3 at heights of 5--10 km, with a gradual decrease at higher altitudes with a scale height of 10--30 km. The measured densities are difficult to explain theoretically, and thus the generation mechanism of the lunar ionosphere is a major issue, with even the validity of the previous observations still under debate. If a thick lunar ionosphere exists, possible mechanisms to maintain the ionized layer are the effect of the remnant magnetic field which stands off the solar wind magnetic field, certain processes that enhance the neutral gas concentration, or charged dust grains that are lifted up by the near-surface electric field. The electron density profiles above the lunar surface are being observed by radio occultation during the SELENE (KAGUYA) mission using sub-satellites. The systematic measurements will establish the morphology of the lunar ionosphere and reveal its dependence on various conditions, thereby providing clues to the generation mechanism. The S-band (2.2GHz) and X-band (8.5GHz) signals transmitted by the Vstar sub- satellite is received at the Usuda Deep Space Center in Japan. The most serious error source is the temporal variation in the terrestrial ionosphere during measurements. In the region where the contribution of the lunar ionosphere is virtually absent, i.e. at altitudes above ~100 km, a gradual variation caused by the terrestrial ionosphere is observed. This variation is extrapolated into the near-moon portion and subtracted from the observed one, thereby eliminating the influence of the terrestrial ionosphere to some extent. In addition to this method, we also use the Rstar sub- satellite, which transmits coherent two signals in S-band, to measure the terrestrial ionosphere during the lunar occultation of Vstar; the subtraction of the Rstar's measurement from the Vstar's measurement gives the lunar ionosphere. The opportunities of the latter method are rather limited, however. More than 100 measurements using Vstar and more than 10 measurements using Rstar and Vstar have been conducted during the first half of the mission. Although the error due to the fluctuation of the terrestrial ionosphere is rather significant, there seems to be a tendency that the electron density increases on the morning side of the moon.
Ando Hiroki
Futaana Yoshifumi
Hanada Hideo
Imamura Takashi
Iwata Takahiro
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