Polar mesosphere summer echoes (PMSE): Review of observations and current understanding

[摘要] Polar mesosphere summer echoes (PMSE) are verystrong radar echoes primarily studied in the VHFwavelength range from altitudes close to thepolar summer mesopause. Radar waves are scatteredat irregularities in the radar refractive indexwhich at mesopause altitudes is effectively determinedby the electron number density. For efficientscatter, the electron number density must revealstructures at the radar half wavelength (Braggcondition for monostatic radars; ~3 m fortypical VHF radars). The question how such smallscale electron number density structures arecreated in the mesopause region has been alongstanding open scientific question for almost30 years. This paper reviews experimental andtheoretical milestones on the way to an advancedunderstanding of PMSE. Based on new experimentalresults from in situ observations with soundingrockets, ground based observations with radarsand lidars, numerical simulations withmicrophysical models of the life cycle ofmesospheric aerosol particles, and theoreticalconsiderations regarding the diffusivity ofelectrons in the ice loaded complex plasma of themesopause region, a consistent explanation forthe generation of these radar echoes has beendeveloped. The main idea is that mesosphericneutral air turbulence in combination with asignificantly reduced electron diffusivity due tothe presence of heavy charged ice aerosolparticles (radii ~5–50 nm)leads to thecreation of structures at spatial scalessignificantly smaller than the inner scale of theneutral gas turbulent velocity field itself.Importantly, owing to their very low diffusivity,the plasma structures acquire a very longlifetime, i.e., 10 min to hours in the presenceof particles with radii between 10 and 50 nm.This leads to a temporal decoupling of activeneutral air turbulence and the existence of smallscale plasma structures and PMSE and thus readilyexplains observations proving the absence ofneutral air turbulence at PMSE altitudes. Withthis explanation at hand, it becomes clear thatPMSE are a suitable tool to permanently monitorthe thermal and dynamical structure of themesopause region allowing insights into importantatmospheric key parameters like neutraltemperatures, winds, gravity wave parameters,turbulence, solar cycle effects, and long termchanges.

[学科分类] 大气科学