HF and VHF absorption techniques in radio wave probing of the ionosphere

Abstract

A critical examination of the techniques of measurement of ionospheric absorption by HF and VHF radio waves and the scientific significance of these measurements is presented. The techniques discussed include the use of (1) pulsed transmission with reflection from one of the ionospheric layers (A1 technique), (2) cosmic radio noise (A2 technique), (3) CW waves from commercial transmitters (A3 technique) and (4)f_min. For each technique the method of analysis, along with indication of its limitations and possible areas of improvement, is outlined. Discussions on the scientific relevance of the technique include: (a) the aeronomic implications of morphological analysis, such as those giving the dependence of absorption with frequency, with solar zenith angle and with solar activity, (b) the Winter anomaly in midlatitudes and the anomalous nature of equatorial absorption, (c) the ionospheric implications of flare effects in absorption, (d) recent attempts to develop electron density profiles from single or multifrequency absorption measurements and comparison of selected profiles with those obtained by other techniques at similar latitudes and times and (e) estimates of collision frequencies at E- and F-region heights and some recent attempts to obtain, information on electron temperature.