Electron Density Distribution at Noon in the Lower Ionosphere

ABSTRACT

The new pulse interaction technique introduced by Fejer (1955) has been employed at Armidale (lat. 30°32'S; log. 151°38'E) to investigate the lower ionosphere. High sensitivity of measurement of the impressed modulation on the wanted echo, at least 0.005 per cent, was achieved by the use of a powerful transmitter of about 200 kW. peak-power and high-gain aerial systems for each of the disturbing, the wanted transmitters and the receiver. The receiving system consisted of a high-gain narrow band amplifier and a phase-sensitive detector. The ordinary mode of propagation was used for both the disturbing and the wanted echoes.

The Fejer curves of percentage modulation impressed on the wanted echo as a function of the time delay between the wanted echo and the disturbing ground pulse were obtained near noon during January 1963 (local summer). Some of these curves were analysed using the wave-interaction theory originally developed by Bailey (1937, 1938) and the standard magnetoionic theory. An electron density profile from the level of onset of modulation to the level of reflection of the wanted echo in the E layer has been derived. It is found that the ionization below the E layer is stratified into two separate layers.

The model of two D layers, consisting of the D_α and the D_β, was originally postulated by Bain and Bracewell (1952) to explain their VLF observations. More recently Nicolet and Aikin (1960) and Moler (1960) found causes for two layers on theoretical grounds. The present observations confirm the earlier predictions.

It is proposed that the D_α (upper) and the D_β (lower) should be called the D and the C layers following the original Appleton nomenclature.