Investigating the variation of the ionospheric absorption during large solar flares based on modern Digisonde data

As a result of the enhanced X-ray and EUV fluxes following large solar flares, the electron density of the ionospheric layers increases. Furthermore, it causes higher absorption or even partial or total fade-out of the emitted radio waves which can be measured with ionosondes and Digisondes by studying the amplitude of the reflected electromagnetic waves [1,2]. In the present study, the ionospheric response to large solar flares has been investigated using the ionosonde data measured at the Průhonice (Czech Republic, 49.98° N, 14.55° E) and San Vito (Italy, 40.6° N, 17.8° E) stations in September 2017, the most active solar period of Solar Cycle 24. A novel method [3] to calculate and investigate the absorption of radio waves propagating in the ionosphere is used to determine the absorption during large solar flare events (M and X class). Subsequently, the absorption data are compared with the indicators derived from the fmin method (fmin, the minimum frequency is considered as a qualitative proxy for the “nondeviative” radio wave absorption occurring in the D-layer). Total and partial radio fade-out and increased values (with 2–5 MHz) of the fmin parameter were experienced during and after the intense solar flares (> M3). Furthermore, the signal-to-noise ratio (SNR) measured by the Digisondes was used as well to quantify and characterize the fade-out events and the ionospheric absorption. The combination of these three methods may prove to be an efficient approach to monitor the ionospheric response to solar flares. [1] Sripathi, S., Balachandran, N., Veenadhari, B., Singh, R., and Emperumal, K.: Response of the equatorial and low-latitude ionosphere to an intense X-class solar flare (X7/2B) as observed on 09 August 2011, J. Geophys. Res.-Space, 118, 2648–2659, 2013. [2] Barta, V., Sátori, G., Berényi, K. A., Kis, Á., and Williams, E. (2019). Effects of solar flares on the ionosphere as shown by the dynamics of ionograms recorded in Europe and South Africa. Annales Geophysicae, Vol. 37, No. 4, pp. 747-761 [3] Sales, G. S., 2009, HF absorption measurements using routine digisonde data, Conference material, XII. International Digisonde Forum, University of Massachusetts