Measuring ionospheric absorption based on received ionosonde amplitudes using sounding frequencies above 10 MHz – preliminary results

The ionosphere consists of various ionised layers in Earth’s upper atmosphere, located roughly between 60 and 1000 km. Space weather events like solar flares cause enhanced absorption of radio waves in the ionosphere most notably in the lowest part of it, the D-region (ca. 60–100 km altitude range) which can weaken radio signals and can pose difficulties to radio communication at certain frequencies. There exist several methods to qualitatively and/or quantitatively assess the absorption in the layers of Earth’s ionosphere. In the present study, one such method is focused on, namely, the so-called ionosounding technique in which an instrument called the ionosonde actively emits radio pulses towards the ionosphere over a selected frequency sweep, typically between 1.5 and 12 MHz, and the passive antenna system of the same instrument receives the reflected echoes. Based on the received amplitudes of the echoes, the D-region absorption in the ionosphere can be quantified.

State-of-the-art DPS-4D ionosondes are installed in Dourbes, Belgium and Sopron, Hungary, respectively, with the two stations being part of the same international network of ionosondes (GIRO network). There have been various research collaborations between the two groups in the past (e.g., participating in the same European project called T-FORS). In a paper by Buzás et al., 2023, we investigated the impact of solar flares on the absorption of radio waves emitted by ionosondes. As a continuation of this study, we looked for other ways to quantify the ionospheric absorption and to compare our results with other methods. One such method is to utilize the upper, higher-frequency part of the spectrum (practically 10–30 MHz) of the ionosonde where usually there are no reflections from the emitted electromagnetic pulses. Basically the instrument “listens” to the background noise (either of terrestrial or extraterrestrial origin) received by the antenna system at these frequencies. In this mode of measurement, it is possible to extract information on the ionospheric absorption. 

Here, we aim to show our preliminary results concerning the possibility of the utilization of the above-10 MHz-part of the ionosonde spectrum. To this end, we analysed ionosonde amplitude data recorded at Dourbes and Sopron stations both during quiet periods and periods with M- and X-class solar flare events in 2024. The seasonal and diurnal variation of some selected frequency bands are discussed, as well as the ionospheric response at different frequencies during the flare events. 

References:

Buzás, A., Kouba, D., Mielich, J., Burešová, D., Mošna, Z., Koucká Knížová, P., & Barta, V. (2023). Investigating the effect of large solar flares on the ionosphere based on novel Digisonde data comparing three different methods. Frontiers in Astronomy and Space Sciences, 10, 1201625.