A differential‐Doppler study of traveling ionospheric disturbances from Millstone Hill

From January 1971 to March 1973, five satellites of the Navy Navigation Series (Transit) in polar orbits at 1000 km altitude were tracked from Millstone Hill to secure differential‐Doppler measurements of N‐S gradients in the ionosphere. In all, over 2000 passes were observed, encompassing all local times and seasons. All of the differential‐Doppler records have been scanned to secure statistical information on the incidence, wavelength, amplitude, and location of the traveling ionospheric disturbances detected. Most of the TID's had wavelengths in the range of 150–350 km and were seen to the south of the station, suggesting that they were traveling from north to south. The incidence peaked at 60% of all passes at 0800 local time and dropped to less than 5% for a few hours before sunrise. The seasonal variation is less clear‐cut, but the incidence seems to be increased in winter and the equinoxes and reduced in summer. This may be simply a consequence of the dependence of the detectability on total content. While the character of the fluctuations and a north‐to‐south motion would be consistent with a source in the auroral ionosphere, there seems to be no increase in incidence during magnetically disturbed times. Moreover, in summer the waves are seen equally to the north and the south of the station. It seems possible that there may be many sources for these waves distributed in latitude and that the observed locations are controlled by the filtering effects of F region winds.