Solar Flares in the 25th Cycle of Activity: Effect on Ionospheric Disturbance and GNSS Signal Strength

The paper presents the results of a study of the effect of solar flares and geomagnetic storms that occurred in the first half of the 25th solar activity cycle on ionospheric disturbance. Special attention is paid to the study of the signal-to-noise ratio for various signal components on Global Navigation Satellite System (GNSS) receivers. Ionospheric responses were recorded in the form of an increase in the median values of the Rate of Total Electron Content Index (ROTI) for the study region during the action of X- and M-class solar flares to 0.2 TECU/min with a duration of up to 30 min. The ionosphere’s response to a powerful geomagnetic storm on May 10–11, 2024, was also recorded, with an increase in the median ROTI to 0.24 TECU/min. The ionospheric disturbances lasted throughout the action of the magnetic storm. The study of changes in the median values of the signal-to-noise ratio across the study region revealed decreases during individual solar flares with both sharp and smooth gradients. The flares that caused a decrease in the signal-to-noise ratio were accompanied by radio emission at the operating frequencies of GNSS. The reduction values reached 3–5 dBHz for most groups of GNSS signal components, with the exception of the component in the L1 frequency subband. When examining individual vectors for the line of sight (LOS) between a GNSS satellite and receiver, signals with data transmission delays of up to 40 min were detected. There were also sharp decreases in the signal-to-noise ratio by 7 dBHz or more.