Characterization of polar mesospheric VHF radar echoes during solar minimum winter 2019/2020. Part I: Ionisation

• Published in: Journal of atmospheric and solar-terrestrial physics 2021-09, Vol.221, p.105684, Article 105684
• Main Authors: Renkwitz, Toralf, Latteck, Ralph, Strelnikova, Irina, Johnsen, Magnar G., Chau, Jorge L.
• Format: Article
• Language: English
• Subjects: D-region electron density; Geomagnetic activity; Middle atmosphere trends; Partial reflection radar; Polar mesosphere winter echoes; VHF radar observations
• ISSN: 1364-6826; 1879-1824
• DOI: 10.1016/j.jastp.2021.105684

The exceptionally solar and geomagnetic quiet winter of 2019–2020 provides a very useful scenario to study polar mesospheric radar echoes. Such a condition limits the impact of ionisation caused by geomagnetic storms and particle precipitation and allows to investigate the role of moderate electron densities as well as neutral dynamics in formation and transport of the structures causing these echoes. For this purpose we used the continuous operation of the sensitive VHF Middle Atmosphere Alomar Radar System (MAARSY) in conjunction with the Saura partial reflection radar throughout the winter as key instruments to monitor the Mesosphere. Eight months of radar measurements are analyzed in respect to occurrence, echo power, spectral widths and their relation to practically common volume measurements of electron densities. VHF polar mesospheric echoes observed during 2019/2020 were generally more faint and occurred less frequent than in previous year. The very low occurrence rates of 6.5% at most for the months November to February as well as the complete absence of echoes below 56 km throughout the season are very remarkable. Highest occurrences of echoes with spectral widths mostly below 3 ms−1 were seen for electron densities of 3 ⋅ 108 m−3, while on average Ne ~ 5.5⋅ 108 m−3 were observed for the presence of VHF echoes, representing a necessary prerequisite to observe such VHF radar echoes. VHF radar echoes occurring at altitudes below 75 km furthermore seem to show preferences to distinct electron number density windows. •Polar mesospheric VHF radar echoes are investigated for the exceptional solar minimum winter 2019/2020.•Simultaneous operation of VHF and partial reflection radars in quasi‐common volume.•Weaker VHF radar echoes and partially very low occurrence rates were observed.•Highest PMWE occurrence was found for electron density of 3e8 / m^3.•PMWE show distinct preference on certain electron density for individual altitudes.