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 are 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 absolute magnitude of 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 requirement for PMWE is a sufficiently large number of electrons, for example produced by solar protons. 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.

Isolated lower mesospheric echoes seen by medium frequency radar at 70° N, 19° E

“…The selection process thus effectively removes all data that normally would yield useful wind and turbulence values in the upper mesosphere and lower thermosphere, and leaves noise and dominant anomalous echoes. The effect of applying the selection criteria to the data of the 20 January 2005 is somewhat spectacular, not least when comparing with the PMWE reported by Lübken et al (2006). See also Seppälä et al (2006) for a further description of this period.…”

“…The object of this study is to map the occurrences of such echoes facilitating comparisons with other observations. For example, the similarity and simultaneity of the echo structure for the 20 January 2005 with VHF radar results presented by Lübken et al (2006) are particularly striking. In presenting a number of such echo events since 2001 selected from the MF radar dataset (which spans 1997 to present), we find that virtually all echo occurrences coincide with enhanced solar proton fluxes suggesting that substantial ionisation of the mesosphere is a necessary condition.…”

“…The presence of such strong echoes below 70 km combined with absence of usable signal above is an unusual condition occurring on only a few days each year. This current study was instigated when we noted that the January 2005 PMWE event reported by Lübken et al (2006) at VHF coincided exactly with such a low-altitude strong reflection at MF. Apart from determining reflected power, the Tromsø MF radar is able to measure both wind speed (from the motion of the reflected radio wave diffraction pattern on the ground) (Meek, 1980) and rough estimates of turbulent energy dissipation rates (from echo fading times) (Hall et al, 1998).…”

“…It should come as no surprise therefore that phenomena described as Polar Mesospheric Winter Echoes (PMWE) (e.g. Kirkwood et al, 2002;Belova et al, 2005;Lübken et al, 2006;Zeller et al, 2006) and perennial equatorial mesospheric echoes (e.g. R. Woodman and J. Chau, private communication) should exist.…”

“…R. Woodman and J. Chau, private communication) should exist. While the reader might be forgiven for asking why we refer to PMSE and PMWE etc., since anomalous echoes seem to appear all year round and at non-polar latitudes, perusal of Lübken et al (2006) and references therein will quickly reveal that different mechanisms are visualized depending on height, latitude, season and method of observation.…”