1.3 The Effect Of The Terrestrial Ionosphere On The Radio Occultation Measurement

Radio occultation measurements have several error sources:

1. Fluctuation of the plasrna density in the terrestrial ionosphere.

2. Fluctuation of the plasma density in the interplanetary space.

3. Fluctuation of the frequency of the oscillator.

4. Recording error at the ground station.

5. Deviation of the atmospheric structure from spherical symmetry.

The effect of the interplanetary plasma on the radio occultation measurements can be negligible if the ray path is far from the sun, because the power of the fluctuations in the time scale of the radio occultation measurement is small [ Woo et al., 1979]. Here we exam-ine the first error source, the density fluctuation in the terrestrial ionosphere. The densi-ty fluctuation in the terrestrial ionosphere causes the fluctuation of the amplitude and the phase of the received signal. Since the phase shift is the primary source of informa-tion in our experiment, we focus on the phase fluctuation caused by the disturbances in the terrestrial ionosphere. It will be shown in Section 2 that the phase fluctuation caused by the terrestrial ionosphere is comparable to those by the Martian nightside ionosphere and the lunar ionosphere.

It should be noted that only short term fluctuations become serious error sources, since long term fluctuation can be removed by extrapolating the data before occultation. An example of the elimination of the long term variation in a previous work is shown in Figure 2 [Mizuno, 1991]. The extrapolation of the phase shift before the occultation was used to remove the long term variation. The time scale of the extrapolation was several minutes in this case.

Fig.2.Phase shift by the Neptune ionosphere measured in the Voyager 2 mission.(a)Phase shift before the substraction of a long term variation.(b)Phase shift after the subtraction. The figures are taken from Mizuno.

1. Maximum occurrence and magnitude of the phase scintillation are observed in the nighttime [Aarons et al., 1996; Doherty et al., 1994].

2. The phase scintillation takes place in high Kp index period rather than in low Kp index period [Aarons et al., 1996; Aarons 1997; Doherty et al., 1994].

3. In the mid-latitude, the magnitude of the short term ^*1(1 minute) phase scintillation is smaller than that of the long term (15 minutes) scintillation [Doherty et al., 1994].

We can obtain the TEC information over Japan from the GPS network of the Geographical Survey Institute (GSI) in Japan, GPS Earth Observation Network (GEONET). The wave structure of the mid-latitude Traveling lonospheric Disturbances (TID) in the nighttime was observed by GEONET [Saito et al., 1998].

The goals of this study are:

1. To compare the phase shifts by the Martian nightside ionosphere and the lunar ionosphere with the phase shift by the short term fluctuation of the terrestrial ionosphere.

2. To clarify the magnitude and the local-time dependence of the fluctuation of the terrestrial ionosphere over UDSC by analyzing the TEC data observed by the GPS network.

3. To develop a method to estimate the fluctuation of the terrestrial ionosphere along the ray path of the radio occultation by using the TEC data observed by the GPS network.

^*1The time scale of the radio occullation measuurement is about several minutes. The "short term" means a short time scale compared with the time scale of the radio occullation measurement.