Abstract
Abstract. Polar mesosphere summer echoes (PMSEs) are strong radar echoes observed in the polar mesopause during the local summer. Observations of layered PMSEs carried out by the European Incoherent Scatter Scientific Association very-high-frequency (EISCAT VHF) radar during 2004–2015 in the latest solar cycle are used to study the variations of the PMSE occurrence ratio (OR). Different seasonal behavior of PMSEs is found by analyzing the seasonal variation of PMSE mono-, double-, and tri-layer OR. A method was used to calculate the PMSE mono, double-, and tri-layer OR under a different electron density threshold. In addition, a method to analyze the correlation of the layered PMSE OR with the solar 10.7 cm flux index (F10.7) and geomagnetic K index is proposed. Based on it, the correlation of the layered PMSE OR with solar and geomagnetic activities is not expected to be affected by discontinuous PMSEs. It is found that PMSE mono-, double-, and tri-layer ORs are positively correlated with the K index. The correlation of the PMSE mono- and double-layer OR with F10.7 is weak, whereas the PMSE tri-layer OR shows a negative correlation with F10.7.
Highlights
The ionosphere is an important part of the near-Earth space environment, and the mesosphere is the coldest region in the Earth’s atmosphere
We have presented a new calculation method for calculating the layered Polar mesosphere summer echoes (PMSEs) occurrence ratio, which is different from the method given in Sect. 4.2, so that the layered PMSE OR is relatively accurate
We found that the variation trends of the layered PMSE ORs with different thresholds are largely consistent
Summary
The ionosphere is an important part of the near-Earth space environment, and the mesosphere is the coldest region in the Earth’s atmosphere. Polar mesosphere summer echoes (PMSEs) are strong echoes detected by radars from mediumfrequency (MF) to ultra-high-frequency (UHF) bands in the polar summer mesopause, and PMSEs have been considered to be possible indicators of global climate change (Thomas and Olivero, 2001). Radar waves in the very-high-frequency (VHF) band are backscattered due to the irregularities of electron density with spatial scales of about half the wavelength of the radar. This has been confirmed by Blix et al (2003) from simultaneous rocket and radar observations. One remarkable feature of all PMSEs is the fact that the radar echoes often occur in the form of two or more distinct layers which can persist for periods of up to several hours. The layering mechanism leading to these multiple structures has only been poorly understood in spite of some previous attempts involving gravity waves, the general thermal structure, and Kelvin–Helmholtz instabilities (Röttger, 1994; Klostermeyer, 1997; Hill et al, 1999; Hoffmann, 2005)
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