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

Aug 27, 2026

Uploaded by Aurelia

The study investigates isolated low-altitude mesospheric radar echoes near 60 km that have been reported beyond the typically altitude-continuous D-region. The work aims to separate these echoes from regular D-region returns and assess whether they indicate an ionospheric “C layer” source below 60 km. After a manual review of radar data and spectra, an automated deep-learning method was developed using YOLOv8. Over four years of monostatic 3.17 MHz polar-latitude measurements, the median echo altitude is 58.5 km with layer thickness near 6 km, showing strong seasonal variability and reduced occurrence during solar maximum 2024.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages17

Reading Time43 min

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Introduction

Background on ionospheric layers and monitoring

D region formation and measurement challenges

Motivation for low-altitude echo detection

Atmos. Chem. Phys., 26, 9571–9587, 2026 [https://doi.org/10.5194/acp-26-9571-2026](https://doi.org/10.5194/acp-26-9571-2026)[ ](https://doi.org/10.5194/acp-26-9571-2026)© Author(s) 2026 . This work is distributed under the Creative Commons Attribution 4 .0 License. Automated detection of low-altitude isolated mesospheric radar echoes using YOLOv8: evidence for an ionospheric “C layer”phenomenon below 60 km altitude? Yadu Krishnan Krishnakumar 1,3 , Toralf Renkwitz2,3 , and Andreas Ahrens3 1Escuela Técnica Superior de Ingeniería y Sistemas de Telecomunicación (ETSIST), Universidad Politécnica de Madrid, Campus Sur, Calle Nikola Tesla s/n, 28031 Madrid, Spain 2Leibniz-Institute of Atmospheric Physics at the University of Rostock, Kühlungsborn, Germany 3Faculty of Engineering, Hochschule Wismar, University of Applied Sciences: Technology, Business and Design, 23966 Wismar, Germany Correspondence: Yadu Krishnan Krishnakumar ([yadu.krishnakumar@alumnos.upm.es](yadu.krishnakumar@alumnos.upm.es)) Received: 23 February 2026 – Discussion started: 6 March 2026 Revised: 16 May 2026 – Accepted: 15 June 2026 – Published: 8 July 2026 Abstract. 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 (􀀛 D 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. Published by Copernicus Publications on behalf of the European Geosciences Union. Research article 9572 Y. K. Krishnakumar et al. : Automated detection of LIME 1 Introduction With the presence of incident solar radiation, mainly ultraviolet (UV, Lyman-alpha) and X-rays, the ionosphere is formed as atoms and molecules in the Earth's upper atmosphere are ionised. With the ionisation of mostly nitrogen, oxygen as well as nitric oxide, free electrons are created that accumulate in certain ionospheric regions (see e.g. Hargreaves, 1992 ; Schunk and Nagy, 2009) . The layers within the regions vary in intensity and peak heights with diurnal, annual and (multi-) decadal periodicities. The general understanding is, that the layers stretch from t

FAQs

What problem does the study address regarding ionospheric radar observations?

It distinguishes isolated radar echoes near 60 km from regular D-region echoes to evaluate whether they originate from an ionospheric “C layer” phenomenon below 60 km.

How is the automated detection approach implemented?

An automated deep-learning method was developed after manual inspection, using YOLO (You Only Look Once) to detect isolated low-altitude echoes and quantify their occurrence statistics.

What observational results are reported for the detected echoes?

The median altitude is found to be 58.5 km with a layer thickness of about 6 km per layer, with most detections showing relatively narrow power spectra and pronounced seasonal variability.

Aug 27, 2026

Uploaded by Aurelia