Neutral air turbulence and temperatures in the vicinity of polar mesosphere summer echoes

Abstract

[1] A total of 8 sounding rocket flights with measurements of neutral air turbulence in the upper mesosphere have been performed in the past 10 years with simultaneous and nearly co-located radar measurements of polar mesosphere summer echoes (PMSE). These measurements took place close to the rocket ranges in northern Norway (Andøya Rocket Range, 69°N) and in northern Sweden (Esrange, 68°N). A detailed comparison demonstrates that there is no apparent correlation between PMSE and neutral air turbulence and that in fact turbulence is absent in the majority of all PMSE events (no turbulence in 7 out of 10 PMSE layers). This suggests that neutral turbulence and other mechanisms affecting the neutral atmosphere at very small spatial scales play a minor role in creating PMSE, contrary to the speculations published in the literature. The main mechanism for creating PMSE remains unidentified. A comparison of PMSE with simultaneous temperature profiles derived from falling sphere and ionization gauge measurements shows that PMSE are practically always present at altitudes where the temperature is low enough for water ice particles to exist. This supports the general understanding that PMSE are closely related to charged water ice particles. On the other hand, the measurements also demonstrate that low enough temperatures are not sufficient for PMSE to exist. Temperature lapse rates were deduced from the high-altitude-resolution ionization gauge measurements. Within the PMSE layers the temperature lapse rate is typically +1-2 K/km with a rather large variability of ±5-10 K/km. Adiabatic lapse rates have never been found within a PMSE layer, which suggests that turbulence cannot have been active for a substantial period. This again supports the idea that neutral air turbulence plays a minor role in creating PMSE. Probably the only common physical reason for PMSE and turbulence is the background temperature profile, which supports the creation of ice particles (since temperatures are very low) and which provokes the breaking of gravity waves and creation of turbulence since the temperature gradient changes at the mesopause.

Modelling the impact of noctilucent cloud formation on atomic oxygen and other minor constituents of the summer mesosphere

“…Lübken found that significant turbulence during this period only occurred between 78 and 97 km, peaking around 90 km. Later rocket measurements confirmed these results [Lübken et al, 2002]. Figure 2 shows that this is in contrast to the predictions of 2-D models, which predict a much broader region of turbulence extending from 70 km to above 110 km.…”

Simultaneous observations of a Mesospheric Inversion Layer and turbulence during the ECOMA-2010 rocket campaign

“…However, numerical simulations showed that only vigorous turbulence revealing heating rates of ∼ 10 K h −1 is capable of producing temperature inversions (Liu et al, 2000). Such strong turbulence, however, has rarely been observed (see, e.g., Lübken, 1997;Lübken et al, 2002) in contrast to the temperature inversion layers, which are common phenomena.…”

Small scale density variations of electrons and charged particles in the vicinity of polar mesosphere summer echoes

“…Unfortunately, these observations can only be conducted when the CONE sensor is in ram position during the downleg part of the rocket trajectory (whereas all the observations presented here were obtained on the upleg part of the rocket trajectory). As described in Lübken et al (2002), the CONE sensor identified turbulent activity at altitudes between 87.5 and 90.5 km and between 92.5 and 93.5 km on the downleg part of the trajectory. If we would assume that the turbulence is homogeneous over the distance between upleg and downleg of the rocket trajectory (horizontal distance ∼50 km), we could classify the spectrum shown in Fig.…”

“…Ulwick et al, 1988). However, in recent years two experimental facts provided strong evidence against some parts of this theory: first, in situ measurements of neutral air turbulence in PMSE showed that neutral air turbulence regularly existed only in the upper altitude range with PMSE (Lübken et al, 2002). Secondly, in situ measurements of plasma parameters of the PMSE environment like number densities of electrons, positive ions and charged particles showed that in the majority of cases the charge residing on the particles is significantly less than the free electron number density (Havnes et al, 2001;Blix et al, 2003) contrary to the predictions of the Cho et al (1992)-theory. Recently, Rapp and Lübken (2003) proposed a possible solution for these two dilemmas: they reconsidered the question of electron diffusion in the vicinity of charged aerosol particles and found that in principle each perturbation of the negatively (positively) charged aerosol number density should also be reflected in anticorrelated (correlated) perturbations of the electron number density, independently of the actual ratio between charged particle number density and electron number density.…”

“…First ideas that the electron number density structures are directly related to neutral air turbulence were soon discarded since it turned out that under typical conditions in the mesopause region a length scale of 3 m is far in the viscous subrange, where any structures are efficiently destroyed by molecular diffusion (e.g. Lübken et al, 2002). The first breaktrough in the understanding of PMSE was achieved by Kelley et al (1987) who proposed that the electrons at the polar summer mesopause are low diffusivity tracers due to the presence of large and heavy positive ions such as water cluster ions.…”