Absence of high frequency echoes from ionosondes during the 23–25 April 2023 geomagnetic storm; what happened?

We report an unusual event on absence of high frequency (HF) echoes in ionosonde observations from the ionospheric F2 region during the geomagnetic storm of 23-25 April 2023. This event was observed in both southern and northern hemispheres over two stations, Grahamstown (33.3 degrees S, 26.5 degrees E), South Africa and Pruhonice (50.0 degrees N, 14.6 degrees E), Czech Republic. Significant O/N2 depletion over the stations was observed by TIMED/GUVI, indicating a strong negative ionospheric storm. This is unique since absence of echoes in ionosonde measurements is usually due to strong radio absorption in the ionosphere associated with solar flares. However, there was no flare activity during the periods of "absent" F2 HF echoes. On the other hand, the ionosonde detected echoes from E-layer. TIEGCM simulation reproduced TIMED/GUVI O/N2 depletion and showed that NmE was larger than NmF2 on dayside over Pruhonice. TIMED/GUVI O/N2 also showed a clear spatial gradient in the O/N2 depleted regions, suggesting F-region ionosphere was tilted. By estimating the critical frequency of the F2 layer using GNSS observations, we have shown that it wasn't possible for the ionospheric electron density to reach depletion levels prohibiting reflection of HF echoes from ionosondes. We suggest that this phenomena may have been caused by either (a) maximum electron density of E layer exceeding that of F2 layer and/or (b) ionospheric tilting which made the signals to be reflected far away from the ionosonde locations.