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Abstract. 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 explaCorrespondence to: M. Rapp (rapp@iap-kborn.de) nation 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.

International audienceIn recent times it has become increasingly clear thatreleases of trace gases from human activity have a potentialfor causing change in the upper atmosphere. However,our knowledge of systematic changes and trends inthe temperature of the mesosphere and lower thermosphereis relatively limited compared to the Earths loweratmosphere, and not much effort has been made to synthesizethese results so far. In this article, a comprehensivereview of long-term trends in the temperature of the regionfrom 50 to 100 km is made on the basis of the availableup-to-date understanding of measurements and model calculations.An objective evaluation of the available datasets is attempted, and important uncertainly factors arediscussed. Some natural variability factors, which arelikely to play a role in modulating temperature trends,are also briefly touched upon. There are a growing numberof experimental results centered on, or consistent with,zero temperature trend in the mesopause region (80–100km). The most reliable data sets show no significant trendbut an uncertainty of at least 2 K/decade. On the otherhand, a majority of studies indicate negative trends inthe lower and middle mesosphere with an amplitude ofa few degrees (2–3 K) per decade. In tropical latitudesthe cooling trend increases in the upper mesosphere.The most recent general circulation models indicateincreased cooling closer to both poles in the middlemesosphere and a decrease in cooling toward the summerpole in the upper mesosphere. Quantitatively, thesimulated cooling trend in the middle mesosphere producedonly by CO₂ increase is usually below the observedlevel. However, including other greenhouse gasesand taking into account a “thermal shrinking” of theupper atmosphere result in a cooling of a few degreesper decade. This is close to the lower limit of the observednonzero trends. In the mesopause region, recentmodel simulations produce trends, usually below 1 K/decade,that appear to be consistent with most observationsin this regio

[1] Gravity waves (GWs) are a ubiquitious dynamical feature of the polar summer mesopause region. During three summer campaigns, in 1991, 1993 and 1994, we launched seven sounding rockets from the north Norwegian island Andøya. Each of these payloads carried an ionization gauge capable of measuring the total atmospheric density at a high spatial resolution. From these measurements, temperature profiles were determined for altitudes between 70 and 110 km, with an altitude resolution of 200 m. The temperature profiles reveal significant rms variations that are as large as 6 K at 80 km, 10 K at 85 km, and even 20 K at 95 km. During three out of the seven launches a bright noctilucent cloud (NLC) was simultaneously detected by our ground-based lidar and by rocket-borne in situ experiments. During these flights, the NLC is located close to a local temperature minimum below the mesopause. We then estimated gravity wave parameters from accompanying falling sphere and chaff wind observations and found signatures that the wave periods during the NLC cases were on the order of 7-9 hours, with corresponding horizontal wavelengths of 600-1000 km. Motivated by these observations, we used a microphysical model of NLC generation and growth to study the interaction between GWs and NLC. Based on recently measured and modeled temperatures and water vapor mixing ratios, and our gravity wave parameter estimates, we find that the NLC layer indeed follows the motion of the cold phase of the wave by means of a complex interplay between ice crystal sedimentation, transport by the vertical wind, and simultaneous growth. It turns out that the history of individual particles significantly influences the observed properties of NLC. Furthermore, we find that GWs with periods longer than 6.5 hours amplify NLC while waves with shorter periods tend to destroy NLC. In addition, we can only find a correlation between local temperature minima and the location of the NLC provided that the wave periods are longer than $6 hours, which is consistent with our wave parameter estimates.

[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.

[1] Triggered by recent experimental evidence showing that some parts of the Cho et al. [1992] theory describing electron diffusion in the vicinity of charged aerosol particles cannot be correct, we reconsider the process of electron diffusion under the conditions of the polar summer mesopause region. The key idea is that perturbations in the distribution of charged aerosol particles created for example by neutral air turbulence almost immediately lead to (anticorrelated) perturbations in the electron number density due to simple charge neutrality and zero net current arguments. We obtain analytical solutions of the coupled diffusion equations for electrons, charged aerosol particles, and positive ions subject to the initial condition of anticorrelated perturbations in the charged aerosol and electron distribution. The main signatures of these solutions are in line with available in situ evidence of small-scale plasma structures in the vicinity of polar mesosphere summer echoes (PMSE), i.e., electron perturbations are anticorrelated to both perturbations in the distributions of negatively charged aerosol particles and positive ions. The lifetime of these perturbations is proportional to the square of the aerosol particle radius such that the presence of particles with radii larger than $10 nm allows for the existence of electron number density perturbations up to several hours after the initial creation mechanism has stopped. These results are almost independent of the ratio between the aerosol charge number density and the number density of free electrons. These electron perturbations potentially give rise to a radar reflectivity comparable to values observed with 50 MHz VHF radars. Our model results can readily explain why in situ measurements of neutral air turbulence have repeatedly shown active turbulence only in the upper part of the PMSE layer whereas turbulence was basically absent in the lower part. Furthermore, our model concept qualitatively yields the correct altitude profile of the mean PMSE occurrence frequency based on the measured altitude profile of the turbulence occurrence frequency.

We present the results of temperature soundings performed on almost 180 days since 1980 in the 50to 120-km altitude range at Andenes, Norway (69øN latitude). Most of the temperature profiles were obtained by ground-based lidar; the others were derived from in situ density measurements. We present (1) monthly mean temperatures for 9 months of the year, including the mesopause temperature and altitude, and examine (2) seasonal effects, (3) interannual variability, (4) systematic differences to CIRA 1986, and (5) longer-term effects related to the solar activity cycle. The main results are as follows: the mesopause is high (98 km) and warm (192 K) in a long period from October to March and low (88 km) and cold (129 K) in June and July. The transition between these two states in August is much faster than commonly anticipated. An intercomparison of our monthly mean temperature profiles with CIRA 1986 shows significant deviations. Above 80 km we find positive correlations between temperature and solar activity with regression coefficients in the order of 0.15 K/SFU. 1.altitudes below 75-80 km and therefore do not cover the mesopause region. During the last 10 years we have obtained at 69øN latitude (Andenes, Norway) temperature profiles on approximately 180 days in this part of the atmosphere. Our measurements cover the altitude range from 50 to 120 km, though with different statistics. The purpose of this paper is to summarize (1) the results of our measurements and to examine (2) seasonal effects, (3) interannual variability, (4) systematic differences to existing reference atmospheres, and (5) longer-term effects related to solar cycle. MEASUREMENT TECHNIQUESTemperature profiles were measured directly by a sodium lidar and were deduced from density measurements performed by passive falling spheres, ionization gauges, and mass spectrometers. Details of the instruments and of the data reduction are presented elsewhere (see below). Here we will only summarize the most important characteristics of the experiments.The sodium lidar allows to measure temperature profiles in the altitude region 80 to 110 km from the measurement of the 20,841 20,842 LOBKEN AND VON ZAHN: THERMAL STRUCTURE OF THE POLAR MESOPAUSE REGION Doppler broadening of the laser-excited sodium D 2 resonance line of free sodium atoms. For integration periods of 10 min the altitude resolution is 1 km between 80 and 100 km and 2 km above. To acquire temperature data, the instrument needs clear sky and darkness. With this technique, winter conditions were investigated by Neuber et al. [1988] and late summer conditions by Kurzawa and yon . Here we employ recent theoretical results of yon der Gathen [1991] to correct in retrospect some of our measured temperatures for small, higher order effects caused by the finite natural linewidth of the Na hyperfine structure absorption lines and Na saturation effects. The total corrections are small (typically -4 K) and depend on instrumental and atmospheric parameters, such as the laser beam divergence, the energy of t...

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