Investigation of PMSE layers during solar maximum and solar minimum

Abstract. Polar Mesospheric Summer Echoes (PMSE) are a phenomenon that are measured in the upper atmosphere during the summer months and can occur in several layers. In this study, we aimed to investigate the relationship between PMSE layers ranging from 80 to 90 km altitude, and the solar cycle. We used 230 hours of observations from the EISCAT VHF radar located near Tromsø, Norway, and applied a previously developed classification model to identify PMSE layers. The observations were taken during the solar maximum of the solar cycle with the years 2013, 2014 and 2015, and during the solar minimum of the solar cycle with the years 2019 and 2020. Our analysis focused on parameters such as the altitude, thickness, and echo power in the PMSE layers, as well as the number of layers present. Our results indicate that the average altitude of PMSE, the echo power in the PMSE and the thickness of the layers is on average higher during solar maximum than during solar minimum. In the considered observations, the electron density at 92 km altitude and the echo power in the PMSE are positively correlated with the thickness of the layers. In addition, we found that higher electron densities at ionospheric altitudes might be necessary to observe multi-layered PMSEs. Furthermore, we observed that the thickness decreases as the number of multi-layers increase. Based on comparisons with previous studies, we hypothesized that the thickness of PMSE layers may be related to the vertical wavelength of gravity waves, with larger wavelengths potentially resulting in thicker layers. Also, an interesting parallel is seen between the thickness of Noctilucent Clouds (NLC) multi layers and PMSE multi layers, where both NLC and PMSE have a similar distribution of layers greater than 1 km in thickness.