2.2 Martian Ionosphere

For the calculation of the phase shift by the Martian ionosphere, the dayside and nightside electron density profiles observed by the Viking mission (see Section 1.2) are adopted (Figure 3a). The adopted velocities of the spacecraft are the maximum and the minimum velocities in the model orbit of Nozomi (Figure 3b).

Fig.3.Calculation condition for the Martian ionosphere.
(a)Electron density profile and solid curve indicates the nightside profile[Lindal et al.,1979].
(b)Planned orbit of Nozomi.

The phase shifts caused by the radio occultation of the Martian dayside and nightside ionos-pheres are shown in Figures 4 and 5, respectively. The phase shifts due to the fluctuation of the terrestrial ionosphere along the ray path are also shown for comparison. The density of the Martian dayside ionosphere is large enough to be detected. On the other hand, the Martian nightside ionosphere is difficult to be detected, since the fluctuation of the terrestrial ionos-phere is as large as the signature of the Martian nightside ionosphere.

Fig.4.Phase shifts caused by the radio occultation of the Martian dayside ionosphere.Dashed curve and dotdashed curve indicate the phase shifts when the velocity of the spavecraft is maximum(3.27 km s^-1)and minimum(0.24 km s^-1),respectively.Solid curve indicates the phase shift due to the fluctuation of the terrestrial ionosphere calculated from GPS data in a typical case(Section 3).
Fig.5.Phase shift caused by the radio occultation of the Martian nightside ionosphere.Dashed curve and dot-dash curve indicate the phase shift when the velocity of tha spacecraft is maximum(3.27 km s^-1) and minimum (0.24 km s^-1),respectively.Solide curve and dotted curve indicate the phase shift due to the fluctuation of the terrestrial ionosphere for disturbed and quiet conditions,respectively,that are calculated from GPS data (Section 3).