3 PROPERTIES THE FLUCTUATION OF THE TERRESTRIAL INOSPHERE

3.1 Method

The dependences of the fluctuation of the terrestrial ionosphere on local time and sea-son are investigated by using the TEC data observed by the GPS network. We can obtain the terrestrial TEC along the ray path of GPS satellite-ground receiver pair as the raw data.

An offset peculiar to each GPS satellite-ground receiver pair is included in the raw TEC data. Since we are interested in the short term component of the fluctuation in the terrestrial ionosphere, the trend component due to the variation of the path length in the terrestrial ionosphere associated with the movement of the GPS satellite and long term variations is removed by subtracting the running average. By removing the trend com-ponent, the offset peculiar to the GPS satellite-ground receiver pair is also removed.

The typical amplitude of the short term fluctuation is evaluated by two methods. One method is to calculate the standard deviation of the fluctuation of the TEC every one hour after removing a long term variation which is obtained by the running average of 10 minutes. The other method is to calculate the power spectrum P(f)of the fluctuation of the TEC and obtain the typical amplitude A as the root of the power:

(4)

where f₁ and f₂ are the maximum and the minimum frequencies, respectively, to be considered.

The sampling interval of the TEC data is 30 seconds. The measurement error of the raw TEC value is less than 10¹⁴ m^-2 [Hernandez-Pajares et al., 1998]. The property of the observations used in the analysis is listed in Table 3.

Table 3 Theproperty of the observations.