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

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.

Can VHF radars at polar latitudes measure mean vertical winds in the presence of PMSE?

Mean vertical velocity measurements obtained from radars at polar latitudes using polar mesosphere summer echoes (PMSEs) as an inert tracer have been considered to be non-representative of the mean vertical winds over the last couple of decades. We used PMSEs observed with the Middle Atmosphere Alomar Radar System (MAARSY) over Andøya, Norway (69.30∘ N, 16.04∘ E), during summers of 2016 and 2017 to derive mean vertical winds in the upper mesosphere. The 3-D vector wind components (zonal, meridional and vertical) are based on a Doppler beam swinging experiment using five beam directions (one vertical and four oblique). The 3-D wind components are computed using a recently developed wind retrieval technique. The method includes full non-linear error propagation, spatial and temporal regularisation, and beam pointing corrections and angular pointing uncertainties. Measurement uncertainties are used as weights to obtain seasonal weighted averages and characterise seasonal mean vertical velocities. Weighted average values of vertical velocities reveal a weak upward behaviour at altitudes ∼84–87 km after eliminating the influence of the speed of falling ice. At the same time, a sharp decrease (increase) in the mean vertical velocities at the lower (upper) edges of the summer mean altitude profile, which are attributed to the sampling issues of the PMSE due to disappearance of the target corresponding to the certain regions of motions and temperatures, prevails. Thus the mean vertical velocities can be biased downwards at the lower edge, and the mean vertical velocities can be biased upwards at the upper edge, while at the main central region the obtained mean vertical velocities are consistent with expected upward values of mean vertical winds after considering ice particle sedimentation.

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

Polar Mesosphere Summer Echoes (PMSE) are strong radar echoes observed in polar mesopause during local summer. Measurements of layered PMSE observed by the European Incoherent Scatter Scientific Association Very high frequency (EISCAT VHF) radar from 2004 to 2015 in the latest solar cycle, can be used to study the variations of PMSE occurrence ratio (OR). The seasonal variation of PMSE mono-, double- and tri-layer occurrence ratio was analyzed, and there is different seasonal behavior. A method was given to calculate the PMSE mono-, double- and tri-layer occurrence ratio under different electron density threshold conditions. In addition, the correlation between PMSE layered occurrence ratios and solar 10.7 cm flux index (F10.7), and the correlation between PMSE layered occurrence ratios and geomagnetic K index were analyzed respectively in this study. It can be obtained that PMSE mono-, double- and tri-layer OR are positively correlated with the K index. The correlation coefficient between PMSE mono- and double-layer OR and F10.7 is weak, and the PMSE tri-layer OR has a negative correlation with F10.7.