Optimum mean ionospheric height in total electron content observations

Abstract

In order to obtain an accurate TEC (Total Electron Content) from the differential Doppler frequency measurements of two coherent signals transmitted from NNSS (Navy Navigation Satellite System) satellites received at a pair of ground stations, it is necessary to solve an integral constant which depends greatly on the assumed mean ionospheric height h_s. We propose here a novel method which enables us to determine h_s in a reasonable way as follows. By examining the rms sum of the calculated difference of TEC_v\u27s ("composite difference" of the vertically corrected TEC) from six pairs of the four stations, Kokubunji, Sendai, Ebetsu, and Wakkanai, located in the northern part of Japan in the North-South direction, with various assumed h_s values, we select the combinations of neighboring nearest stations, Kokubunji-Sendai and Ebetsu-Wakkanai only, for which we obtain suitable h_s\u27s with which the composite difference takes a single minimum for each pair of the stations. Then, we determine the latitudinal dependence of h_s by least-squares fit to a straight line for these locally deduced values. The mean ionospheric height h_s as a function of latitude thus derived should be optimum. In practice, we examined the data set during the period from November 11 to 18,and from 21 to 25,1994,throughout which magnetic activity remained low with K_p index in the range of 0_+～3_+, and we found 13 events to apply the present method effectively.After showing a possible signature of medium-scale TID (Traveling Ionospheric Disturbance) observed at the time of K_p=4_ on May 4,1994,we give some comments on the observations of the medium-scale TID in the polar region