Seasonal variations of electron densities below 100 km at mid-latitudes—I Differential absorption measurements

Abstract

The differential absorption experiment (D.A.E.) for measuring electron densities in the lower ionosphere, using partial reflections, is examined. A model of the scattering process which invokes sharply bounded horizontally stratified irregularities is found to be more satisfactory than one based on volume scattering. The ratio of calculated reflection coefficients for the two magneto-ionic components, R x R o , is found to be insensitive to the geometry of reflecting boundaries and the fractional changes in electron concentration, ΔN N , provided the refractive index n tends to unity; but is affected by the propagation angle with respect to the magnetic field. The nature of irregularities in the lower ionosphere is discussed, and it is concluded that perturbations in collision frequency, Δv v , are not significant for the calculation of R x R o . Models of collision frequency profile, v( h) for three seasons are derived from meteorological data. Differences in probable collision frequencies between northern and southern hemispheres are examined. The sensitivity of the method, to changes in assumed v( h), is then discussed. The capability of the D.A.E. to produce electron density profiles of good height resolution and accuracy is evaluated. Errors involved in the assumption of v( h) and the measurement of the amplitude ratio of the received magneto-ionic components, R x R o , are discussed.