Radio-quiet ionosonde observations for monitoring absorption in the HF band

Abstract

This work presents a technique to monitor ionospheric absorption in the HF radio band using ionosondes. For this purpose, the ionosondes were run in a radio-silent mode, recording not reflections of their own emitted signals but oblique signals from transmitters of opportunity. We present first the quiet-time data, showing what the typical diurnal variations in the spectrograms are. We then demonstrate that enhancements of ionospheric absorption resulting from either solar X-rays or particle precipitation events can be detected in these data. We show several examples of such signatures in the spectrograms, in time series integrated over a wide band of frequencies, and in small frequency band time series. The aim of this paper is to introduce this type of observations and demonstrate the principle of its use for absorption monitoring.