Validity of three-station methods of determining ionospheric motions

Abstract

Ionospheric ‘drift’ data are routinely analysed in a fashion that yields a ‘drift velocity and a ‘characteristic velocity’, but the physical significance of these velocities is not yet clear. A fairly straightforward interpretation is available if the data refer to ionization irregularities generated by wind-borne turbulence, but no corresponding interpretation has been established to meet the possibility that they refer instead to irregularities generated by propagating atmospheric waves. The present paper examines that possibility in a preliminary way. The result is elementary in the case of a single dominant atmospheric wave: the ‘drift velocity’ accurately represents the horizontal trace velocity of the wave. When two further waves are incorporated, however, with azimuths symmetrically disposed about that of the first wave, it is found that the analysed velocities bear no sensible relationship to the velocities of the waves that went into their making. To the extent that the three-wave model may be representative of the broad wave spectra that can be anticipated at ionospheric heights in practice, and to the extent that drift data relate to such spectra, this result sheds doubt on the physical relevance of the data-reduction techniques that are currently employed, and of the ‘velocities’ they produce. A corresponding uncertainty in the study of radio-star scintillations and of ‘travelling ionospheric disturbances’ is also revealed. The analysis gives some hints as to steps that might prove beneficial in the future processing of ‘drift’ data. More importantly, it serves to emphasize the objectives of identifying conditions in which present techniques are physically irrelevant, and of providing new techniques to deal with the problems that arise when a broad atmospheric wave spectrum is operative.