4.1 Method

There are several ways to estimate the fluctuation of the TEC of the terrestrial ionos-phere by using the GPS network. In general, it is difficult to find a GPS satellite-ground receiver pair whose ray path is close to the ray path of the radio occultation measure-ment of the planetary atmosphere. Therefore, we use the TEC data measured by many GPS satellite-ground receiver pairs to estimate the fluctuation of the terrestrial ionos-phere along the ray path of the radio occultation.

We assume that the fluctuation of the terrestrial ionosphere is localized on the 300 km altitude surface, which is the altitude of the F-region peak according to the IRI90 model [Saito et al., 1998]. Then, the TEC fluctuation along the ray path of each GPS satellite-receiver pair is considered to have its origin at the intersection where the ray path cross-es the 300 km surface. The raw TEC data by the GPS network are the slant columns of the TEC along the ray paths from GPS satellites to ground receivers. To convert the slant column to the vertical column at the intersection, the observed TEC is multiplied by the ratio of the thickness of the ionosphere to the length of the ray path within the ionosphere (Figure 13), on the assumption that plasma in the ionosphere exists between 250 km and 450 km altitudes [Saito et al.. 1998]. The TEC value at the intersection of the ray path of the radio occultation is estimated by averaging the above TEC values inside a circle with the center at the intersection (Figure 14). The averaged value is converted to the slant TEC along the ray path of the radio occultation.

Fig.13.Conversion of the slant column to the vertical colum.


Fig.14.Mehod to estimate the fluctuation of the terrestrial ionosphere by using the GPS network.
The TEC data inside a circle (solid symbols) are used for the estimation.