. Polar mesosphere summer echoes (PMSE) are very strong radar echoes primarily studied in the VHF wavelength range from altitudes close to the polar summer mesopause. Radar waves are scattered at irregularities in the radar refractive index which at mesopause altitudes is effectively determined by the electron number density. For efficient scatter, the electron number density must reveal structures at the radar half wavelength (Bragg condition for monostatic radars; ~3 m for typical VHF radars). The question how such small scale electron number density structures are created in the mesopause region has been a longstanding open scientific question for almost 30 years. This paper reviews experimental and theoretical milestones on the way to an advanced understanding of PMSE. Based on new experimental results from in situ observations with sounding rockets, ground based observations with radars and lidars, numerical simulations with microphysical models of the life cycle of mesospheric aerosol particles, and theoretical considerations regarding the diffusivity of electrons in the ice loaded complex plasma of the mesopause region, a consistent explanation for the generation of these radar echoes has been developed. The main idea is that mesospheric neutral air turbulence in combination with a significantly reduced electron diffusivity due to the presence of heavy charged ice aerosol particles (radii ~5–50 nm) leads to the creation of structures at spatial scales significantly smaller than the inner scale of the neutral gas turbulent velocity field itself. Importantly, owing to their very low diffusivity, the plasma structures acquire a very long lifetime, i.e., 10 min to hours in the presence of particles with radii between 10 and 50 nm. This leads to a temporal decoupling of active neutral air turbulence and the existence of small scale plasma structures and PMSE and thus readily explains observations proving the absence of neutral air turbulence at PMSE altitudes. With this explanation at hand, it becomes clear that PMSE are a suitable tool to permanently monitor the thermal and dynamical structure of the mesopause region allowing insights into important atmospheric key parameters like neutral temperatures, winds, gravity wave parameters, turbulence, solar cycle effects, and long term changes.

/pdf/polar-mesosphere-summer-echoes-pmse-review-of-observations-3mmz8vo62v.pdf

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

Modelling of particle charging in the polar summer mesosphere: Part 1—General results

The relationship between N(e) and mesospheric aerosols near the mesopause is studied. The full distribution of charges on mesospheric aerosols is calculated, including dust and ice particles with radii ranging from 1 to 400 nm. The N(e) and ion density N(i) are obtained and ionization height profiles are calculated. The effects of dust and ice particles on N(e) and N(i) are studied for a wide range of assumed conditions. The results indicate that aerosol concentrations associated with visible polar mesospheric clouds are unlikely to cause a severe N(e) depletion. The pronounced 'bite-out' of N(e) at about 87 km in the summertime may be caused by a large concentration of small ice particles in a narrow cold layer near the mesosphere. Net negative charge on mesospheric aerosols may severely inihibit coagulation, so that mesospheric dust would not grow significantly. A higher supersaturation with respect to water vapor would be needed for heterogeneous nucleation of ice crystals.

Charging of mesospheric particles - Implications for electron density and particle coagulation

We report the results from simultaneous radar and rocket measurements of a PMSE event where for the first time the rocket measured dust and plasma within the radar beam We find very clear correspondence between the measured dust charge density profile and the radar backscatter profile as a function of height We find that even very small amounts of charged dust is associated with an appreciable PMSE radar backscatter Although we find it likely that the dust layer corresponds fully with the PMSE layer there is a possibility that the upper part of the PMSE layer may be influenced by ion clusters which are too small to be detected by the rocket dust probe

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000GL012420

First common volume observations of layered plasma structures and polar mesospheric summer echoes by rocket and radar

A composite sounding rocket payload was launched from Andoya Rocket Range (69°17′N) on August 8, 1971, 12.11 GMT; it reached a peak altitude of 99.1 km. The main purpose of the launch was to study the ion composition around the cold summer mesopause and its relation to ambient temperature, production, and loss rates. Ions were detected by the mass spectrometer from a height of 70.7 km on the ascent to a height of 69.8 km on the descent, water-cluster ions H+(H2O)n dominating the composition below 84 km. Cluster ions of a higher order of hydration appeared with increasing height until a rapid cutoff occurred in the region from 84.5 to 86.0 km. Simultaneously, metal ions Mg+ and Fe+ appeared, and NO+ and O2+ emerged as dominating species. The largest cluster ion found during ascent was (109 ±1)amu, probably H+(H2O)6, detected in one spectrum at 84.5 km. During descent the largest cluster detected was (91 ±1) amu, probably H+(H2O)5, found in three spectra between 85.9 and 82.2 km. In the cluster-ion region the concentration of O2+ exceeded that of NO+, whereas NO+ exceeded O2+ above the region, the crossover taking place near the water-cluster cutoff.

Positive-ion composition measurement in the upper mesosphere and lower thermosphere at a high latitude during summer

Rocket measurements showing removal of electrons above the mesopause in summer at high latitude

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Rocket measurements showing removal of electrons above the mesopause in summer at high latitude