1 .2 Previous Radio Occultation Measurements Of The Martian Ionosphere And The Lunar Ionosphere

The previous radio occultation measurements at Mars are listed in Table 1. In the missions using a single frequency, successful observations of the ionosphere are limited to ingress occultations in 2-way mode (Mariner 4, 6, 7, 9). The Viking mission, which used two frequencies in 1-way mode, successfully observed the ionosphere in both ingress and egress occultations.

table 1 Radio occulation measurements at Mars

The mechanism for maintaining the nightside ionosphere of Mars is thought to be sim-ilar to that of Venus. The prime sources of the nightside ionosphere of Venus are thought to be the transport of atomic ions from the dayside [Knudsen et al., 1980] and the electron precipitation to the nightside atmosphere [Knudsen et al., 1985]. The transport from the dayside almost disappears at low solar activity while the electron precipitation exists also at low solar activity [Knudsen, 1992].

By analogy with Venus, we expect the existence of the transport from the dayside to the nightside and the electron precipitation to the nightside atmosphere also on Mars. In situ measurements of the nightward transport have not been done, but the electron pre-cipitation in the vicinity of Mars was detected by HARP instrument onboard the Phobos 2 spacecraft [Verigin et al., 1991; Shutte et al., 1995]. The models of the nightside ionos-phere of Mars were developed assuming the existences of the electron precipitation [ Verigin et al., 1991; Haider et al., 1992] and the transport from the dayside [FOX et al., 1993]. The measurement of the nightside ionosphere by the Nozomi radio occultation, together with other in situ plasma measurements, will reveal the source of the nightside plasma.

The detection of the lunar ionosphere by a radio occultation technique was reported in the Luna 22 mission [ Vyshlov, 1976]. They showed the peak electron concentrations of 500-1000 cm^-3 at altitudes of 5-10 km above the sunlit lunar surfaces.

Photoionization theories cannot explain the thick ionosphere reported by the Luna 22 mission. Assuming that the ionosphere is composed of the photoelectrons from the mate-rial of the lunar sunlit surface, the electron layer would exist only within several tens of meters above the surface [Reasoner et al., 1972]. Such a photoelectron layer was observed by the Apollo missions [Stern, 1999]. Assuming that the ionosphere is composed of the photoelectrons supplied by the ionization of neutral gases, the electron layer would be removed by solar wind because the time constant of ionization is much longer than that of the pick-up process [Bauer, 1996]. The measurement of the lunar ionosphere by the Selene radio occultation will improve our knowledge on the source of plasma near the moon.