Automated detection of low-altitude isolated mesospheric radar echoes using YOLOv8: evidence for an ionospheric “C layer” phenomenon below 60 km altitude?

The Earth's ionosphere is created by incident solar radiation and extends from approximately 60 to 800 km altitude. Within the ionosphere distinct regions are formed based on the number density of the dominant chemical species and their ionisation by the incident solar ultraviolet radiation and X-rays. The generally accepted lowermost ionospheric region is called D region and extends during daytime from, for example, 90 km downwards to approximately 60 km. Previous studies suggested the presence of an ionospheric C layer around 60 km mostly for observations at very low frequencies. In recent years, rather faint echoes from below the typically altitude-continuous D region have been observed during the sunlit period using a monostatic 3.17 MHz ground-based radar system at polar latitudes. The aim of this study is to distinguish radar echoes from altitudes near 60 km from regular D region echoes and attribute them to a possible C layer source. To our knowledge, this study might be the first systematic investigation of such a phenomenon through monostatic radar observations. Following an initial manual inspection of the raw data and the corresponding radar image spectra, an automated deep learning approach was developed and employed to detect these isolated low-altitude echoes. We used the pattern recognition tool YOLO (You Only Look Once) to gain statistical information on the occurrence of these radar echoes over four years of radar measurements, which covered conditions ranging from minimum to maximum solar activity. The median altitude of these radar echoes is found to be 58.5 km (σ=3.9 km), a thickness of ≈ 6 km per layer, and where the majority of detections exhibit a relatively narrow power spectrum. Substantial annual variability was found for these parameters and the occurrence rate, essentially separating them into summer and winter. The reduced occurrence rates during the solar maximum year 2024 and with that also a reduced intensity of galactic cosmic rays (GCR) suggest their role as an ionisation source for the observed layers.