Automated Detection of Low-altitude Isolated Mesospheric Radar Echoes Using YOLOv8: Evidence for a C-Layer Phenomenon near 60 km Altitude?

Abstract. The Earth's ionosphere is created by the incident solar radiation and extends from approximately 60 km to 800 km altitude. Within the ionosphere distinct regions are formed based on the number density of the dominant chemical species and their ionization by the incident solar ultraviolet radiation and X rays. The lowermost ionospheric region is called D region and expands during daytime downwards to approximately 60km. In the recent years, rather faint echoes from below the typically continuous D region have been observed during the sunlit period using a 3.17 MHz ground based radar system at polar latitudes. To our knowledge, this study might be the first evidence of such a phenomenon through consistent radar observations. Following an initial manual inspection of the raw data and the corresponding radar image spectra, an automated deep learning approach was 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 preferred altitude of these radar echoes is found to be near 58 km with typically little variability, and where the majority of detections show a rather narrow radar 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 suggest the role of galactic cosmic rays as an ionisation source.