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. Polar mesosphere summer echoes (PMSEs) are strong radar echoesobserved in the polar mesopause during the local summer. Observations of layeredPMSEs carried out by the European Incoherent Scatter Scientific Associationvery-high-frequency (EISCAT VHF) radar during 2004–2015 in the latest solarcycle are used to study the variations of the PMSE occurrence ratio (OR).Different seasonal behavior of PMSEs is found by analyzing the seasonalvariation of PMSE mono-, double-, and tri-layer OR. A method was used tocalculate the PMSE mono, double-, and tri-layer OR under a different electrondensity threshold. In addition, a method to analyze the correlation ofthe layered PMSE OR with the solar 10.7 cm flux index (F10.7) and geomagnetic Kindex is proposed. Based on it, the correlation of the layered PMSE OR withsolar and geomagnetic activities is not expected to be affected bydiscontinuous PMSEs. It is found that PMSE mono-, double-, and tri-layer ORs arepositively correlated with the K index. The correlation of the PMSE mono- anddouble-layer OR with F10.7 is weak, whereas the PMSE tri-layer OR showsa negative correlation with F10.7.

Characteristics of the layered polar mesosphere summer echoes occurrence ratio observed by EISCAT VHF 224 MHz radar