Relationship Between Riometer Absorption and Medium‐ and High‐Frequency Auroral Absorption: Vertical Incidence

Abstract

Some known factors influencing the accuracy of D‐ and E‐region ionospheric radio absorption estimates at auroral latitudes, determined from Riometer (Relative Ionospheric Opacity Meter) measurements of incident cosmic noise, are briefly reviewed. Then, measured and modeled height distributions of electron concentration at auroral latitudes, corresponding to various levels of auroral radio‐wave absorption determined by riometers, are analyzed. It is first confirmed that riometer absorption can be equated to vertical incidence absorption on the upward or downward legs. Then the frequency dependence of absorption is derived for the Medium Frequency (MF) and High Frequency (HF) ranges at vertical incidence. In both cases, absorption values at different frequencies are calculated using the phase‐integral method, incorporating realistic height distributions of effective electron‐neutral particle collision frequencies. The results indicate that the frequency dependence deviates from that observed at low and middle latitudes, particularly when absorption levels are high. It is proposed that the found changes to the frequency dependence for auroral latitudes may be used for practical estimations of absorption on MF and HF propagation predictions at vertical incidence, based on the widely available riometer observations. In a companion paper, formulae are proposed to determine oblique‐incidence absorption in MF‐HF propagation predictions using vertical‐incidence estimates.