Home The Institute of Space and Aeronautical Science Report A Feasibility Study for Observing Small Lunar and Martian Ionospheres by Radio Occultation Technique

3.2 Results

As we obtained similar results from the above two methods, we show only the which are obtained by the standard deviation method.

Before the analysis, we check whether there is any deviation of the distribution of the measurements in the field of view from the receiver. Figure 8 shows the distribution of the measurements every 6 hours. The measurements at low elevations are removed to avoid the effect of the fluctuation far from UDSC. It is concluded that the directions of the measurements above the elevation of 30° are not localized in specific areas.





Fig.8.Distribution of the observations by GPS stellites in the field of view from the receiver between(a) 0900 and 1500 LT,(b) 1500 and 2100 LT,(c) 2100 and 0300 LT, and (d) 0300 and 0900 LT.The brightness ndicates the number of samplings in each area.Solid curves represent the orbits of the GPS satellites.

The dependence of the activity of the fluctuation on Kp index is shown in Figure9. The relation between the activity of the fluctuation and Kp index is not clear. However,we can not exclude the possibility of the dependence of the fluctuation activity on Kp index because the number of samples in high Kp period is not enough. In the present study, we use the data obtained under the condition that Kp index is in the range between 0 and 2 minus.



Fig.9.Dependence of the typical fluctuation amplitude of the terrestrial ionosphere on Kp index in
(a) July 1999 and (b)December 1999.Standard deviation of the samples is indicated by error bars.

Figure 10 shows the dependence of the fluctuation amplitude of the terrestrial ionos-phere on local time. The samplings are nearly uniformly distributed over local time. From June to August, the peak amplitude occurs between 2200 and 2300 LT. From September to October, any clear peak can not be seen. From November to December, there is a peak between 1200 and 1300 LT. The local time of the peak activity of the fluc-tuation changes with season.



Fig.10.Dependence of the fluctuation activity of the terrestrialionosphere on local time in (a) May 1999.(b) Jun 1999,(c) July 1999,(d) August1999,(e) September 1999,(f) October 1999,(g) November 1999,(h) December 1999. Standard deviation o the samples is indicated by error bars.

The possibility of the detection of the Martian nightside ionosphere and the lunar ionosphere is investigated for the summer and winter cases using the above results. We regard the fluctuation of the terrestrial ionosphere as a noise. Then, the signal-to-noise ratio R is defined as
(5)



where I planet is the maximum total electron content along the ray path in the planetary ionosphere (see Section 2) and I planet is the fluctuation of the total electron content of the terrestrial ionosphere. The percentage detection possibility P at each local time is defined as
(6)


where Ndetectable is the number of samples whose R value is more than 2, 5, and 10, and Nall is the total number of samples at each local time.

Figures 11 and 12 are the results for the Martian nightside ionosphere and the lunar ionosphere, respectively. In both cases, December is more suitable for the measurement than July because of the smaller fluctuation of the terrestrial ionosphere in December. Daytime is suitable for the measurement in July, while nighttime is suitable for the mea-surement in December.



Fig.11.Possibility of the detection of the Martian nightside inosphere in (a) July 1999 and (b) December 1999. Solid curve, dashed curve, and dot-dashed curve indicate the possibilities when the signal-to-noise ratio is required to be larger than 2, 5, and 10, respectively.



Fig.12.Possibility of the detection of the lunar inosphere in (a) July 1999 and (b) December 1999. Solid curve and dashed curve indicate the possibilities when the signal-to-noise ratio is required to be larger than 2 and 5, respectively.

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