The thermal and dynamical state of the atmosphere during polar mesosphere winter echoes

Abstract

Abstract. In January 2005, a total of 18 rockets were launched from the Andøya Rocket Range in Northern Norway (69 • N) into strong VHF radar echoes called "Polar Mesosphere Winter Echoes" (PMWE). The echoes were observed in the lower and middle mesosphere during large solar proton fluxes. In general, PMWE occur much more seldom compared to their summer counterparts PMSE (typical occurrence rates at 69 • N are 1-3% vs. 80%, respectively). Our in-situ measurements by falling sphere, chaff, and instrumented payloads provide detailed information about the thermal and dynamical state of the atmosphere and therefore allow an unprecedented study of the background atmosphere during PMWE. There are a number of independent observations indicating that neutral air turbulence has caused PMWE. Ion density fluctuations show a turbulence spectrum within PMWE and no fluctuations outside. Temperature lapse rates close to the adiabatic gradient are observed in the vicinity of PMWE indicating persistent turbulent mixing. The spectral broadening of radar echoes is consistent with turbulent velocity fluctuations. Turbulence also explains the mean occurrence height of PMWE (∼68-75 km): viscosity increases rapidly with altitude and destroys any small scale fluctuations in the upper mesosphere, whereas electron densities are usually too low in the lower mesosphere to cause significant backscatter. The seasonal variation of echoes in the lower mesosphere is in agreement with a turbulence climatology derived from earlier sounding rocket flights. We have performed model calculations to study the radar backscatter from plasma fluctuations caused by neutral air turbulence. We find that volume reflectivities observed during PMWE are in quantitative agreement with theory. Apart from turbulence the most crucial requirementCorrespondence to: F.-J. Lübken (luebken@iap-kborn.de) for PMWE is a sufficiently large number of electrons, for example produced by solar proton events. We have studied the sensitivity of the radar echo strength on various parameters, most important electron number density and turbulence intensity. Our observational and theoretical considerations do not provide any evidence that charged aerosol particles are needed to explain PMWE, in contrast to the summer echoes which owe their existence to charged ice particles.

Statistical characteristics of PMWE observations by the EISCAT VHF radar

“…Later investigations confirmed this early proposal (e.g. Lübken et al, 2006Lübken et al, , 2007Rapp et al, 2011;Morris et al, 2011). However, measurements with lidar and radar (Kirkwood et al, 2002;Stebel et al, 2004;Belova et al, 2005;Kero et al, 2008;La Hoz and Havnes, 2008) have triggered the hypothesis that small charged aerosol particles (probably of meteoric origin) play a similar role as in the case of PMSE (e.g.…”

“…In addition, Lübken et al (2006) presented calculations (their Fig. 6) showing that volume reflectivities at the EISCAT VHF radar wavelength can be up to 10 −13 m −1 if charged particles with 4 nm radii are assumed.…”

“…Collis et al, 1992;Lübken et al, 2007;Kirkwood et al, 2006b;Brattli et al, 2006). Note also that Lübken et al (2006) reported smaller spectral widths (their Fig. 12) measured with the ALWIN VHF radar.…”

“…However, as opposed to PMSE, the spectral width is increased inside the PMWE layer (e.g. Collis et al, 1992;Lübken et al, 2006).…”

Estimate of size distribution of charged MSPs measured in situ in winter during the WADIS-2 sounding rocket campaign

“…Moving with the background flow and bound to neutral air turbulence, charged MSPs are also suggested to be potentially involved in the formation of so-called polar mesospheric winter echoes (PMWEs) (e.g., Kavanagh et al, 2006;Lübken et al, 2007;Kero et al, 2008;La Hoz and Havnes, 2008;Havnes and Kassa, 2009;Havnes et al, 2011;Strelnikova and Rapp, 2013;Stebel et al, 2004;Belova et al, 2008). There are also theories explaining the formation of PMWEs by neutral turbulence and an increased electron density (e.g., Lübken et al, 2006). Moreover, even a connection to infrasound has been suggested (Kirkwood et al, 2006).…”