Abstract:
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°S, 26.5°E), South Africa and
Pruhonice (50.0°N, 14.6°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 signalsto be reflected far away from the ionosonde
locations.
