Determination of electron content at a low-latitude station with off-zenith transverse point

Abstract

The importance of the choice of mean ionospheric height (hM) in the determination of latitude variation of total electron content (TEC) from satellite transmissions at a low-latitude station is investigated. By considering the concept of magneto-ionic mode coupling in the vicinity of the transverse region, the Faraday rotation as a function of latitude for a frequency of 40 MHz has been theoretically computed for Calcutta (12°N geomagnetic) by using typical electron concentration profiles representing different ionospheric conditions. This result was used to determine the criterion for the proper choice of hM consistent with least error in the determination of TEC over the entire latitude range of interest. It is found that the correct choice corresponds to the height of the transverse point at that latitude (TP) for which the Faraday rotation is a minimum. An incorrect choice of this height leads to substantial error in the computed electron content at latitudes to the north of TP, the error being as large as 3% per 10 km deviation from the best choice of hM. A method of deriving the correct choice of hM from actual Faraday rotation records is indicated.