Ionospheric absorption variation during the Mother Day Superstorm in May 2024 as observed by different types of ionosonde data

Space weather events such as solar flares and energetic particle events cause enhanced absorption of radio waves in the lower ionosphere, posing difficulties to radio communication at certain frequencies. Increases in ionospheric absorption are due to enhancement of the ionisation in the D region which can be related to the following sources: (1) increases in hard X-rays during solar flares, which affects the day-lit side of the earth especially at lower latitudes, (2) impacts by high-energy solar protons, which can reach the D region in the polar cap were the field lines of the geomagnetic field are open , and (3) precipitation of electrons due to recombination events in the magnetotail, which can produce D-region ionisation in the auroral oval region.

Determination of the changes in ionospheric absorption is possible using ionosounding techniques in which the ionosonde actively emits radio pulses towards the ionosphere over a selected frequency sweep (typically between 1.5 and 14 MHz), and the passive antenna system of the same instrument receives the reflected echoes. The absorption can be defined by the minimum frequency reflected by the ionosphere what can be recorded on the ionograms (fmin parameter, Barta el a. 2019). It can also be quantified based on the received amplitudes of the echoes (Buzás et al. 2023). An alternative approach to analyze the signal-to-noise ratio  of radio waves recorded on ionograms during solar events (de Paula et al. 2022). Another method to determine the absorption variation is to use the instrument in "listening mode" and analyze the background noise observed in the HF band (practically in 10–30 MHz range) during solar events.

The main purpose of the current study is to investigate the ionospheric absorption changes over Europe during the Mother’s Day Superstorm, determined from different type of data recorded by ionosondes at midlatitudes. A detailed analysis of the probable sources of the absorption changes —solar flare effects, polar cap and/or auroral absorption— will be discussed. Furthermore, we will compare the advantages and disadvantages of the different methods based on the results.

 

References:

Barta, V., Sátori, G., Berényi, K. A., Kis, Á., & Williams, E. (2019). Effects of solar flares on the ionosphere as shown by the dynamics of ionograms recorded in Europe and South Africa. Ann. Geophys. 37, 747-761. https://doi.org/10.5194/angeo-37-747-2019.

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. Front. Astron. Space Sci., 10:1201625. https://doi.org/10.3389/fspas.2023.1201625.

de Paula, V., Segarra, A., Altadill, D., Curto, J. J., & Blanch, E. (2022). Detection of solar flares from the analysis of signal-to-noise ratio recorded by Digisonde at mid-latitudes. Remote Sens., 14, 1898. https://doi.org/10.3390/rs14081898.