摘要：On the 28 October 2021, a total of thirteen solar flares occurred. Of these, there were two M- and one X-class solar flares while the rest fell within the C-class category. The first and second M-class flares peaked at 0740 UT and 1028 UT, respectively, while the X1.0 class reached the peak at 1535 UT. While it is known that strong solar flares cause total absorption of frequencies in the high frequency (HF) range, the F2 layer was consistently present on 28 October 2021 as shown by the critical frequency of the F2 layer (foF2). However, using South African ionosonde data over Hermanus (34.4°S, 19.2°E) and Grahamstown (33.3°S, 26.5°E), we show that the lower frequencies in the E and F1 layers were the most affected by the M-class solar flares. Following the peak of the M2.2 solar flare at 1028 UT, an enhancement reaching about 12% over the background in total electron content (TEC) is observed on 28 October 2021 especially in the Southern hemispheric part of the African sector within the latitude range of 20°–40°S. In response, mid latitude ionosonde data showed that there were no echoes from the E and F1 layers for about 2 hr during ≈1025–1225 UT, restricting complex modes of HF communication such as double hops (1F1E) utilizing a combination of more than one ionospheric layer.
