F.-J. Lübken

Thermal structure of the mesopause region at polar latitudes

We present the results of temperature soundings performed on almost 180 days since 1980 in the 50-to 120-km altitude range at Andenes, Norway (69°N 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.

Nearly zero temperature trend in the polar summer mesosphere

A series of in-situ temperature measurements performed by falling spheres at high latitudes in the last ∼10 years is compared with historical temperature data collected by the rocket grenade technique in the mid 1960s. Concentrating on summer (here from mid May to mid August) and on altitudes between 50 and 85 km, where natural variability and instrumental uncertainties are small, the observed trend is very small, if present at all. Taking into account all data from 50 to 85 km the mean trend is −0.024 K/y with a statistical error of ±0.014 K/y, i.e., practically zero. The mean deviation of the grenade temperatures from the falling sphere data is 0.8±6.4 K and the differences are randomly distributed. We have also analyzed various subsets of the entire data set, e.g., in a restricted altitude range where lidars have shown large trends at mid latitudes, namely from 55 to 75 km. The main result remains unchanged: The deviation between ‘old’ and ‘new’ data is practically zero and the temperature trend in the high latitude summer mesosphere is certainly much smaller (if any at all) compared to that observed at mid latitudes.

Experiments revealing small impact of turbulence on the energy budget of the mesosphere and lower thermosphere

We have measured a total of 17 in situ profiles of small-scale density fluctuations (typical resolution: meters) in the lower thermosphere and upper mesosphere, which are used to derive turbulent parameters, such as the turbulent energy dissipation rate e, the turbulent diffusion coefficient K, and the mean turbulent velocity wturb. The accuracy of the absolute numbers is unprecedented thanks to the very high spatial resolution and a recently improved data analysis procedure. Concentrating on the 12 flights which were performed during winter conditions at high latitudes (69°N), we find mean energy dissipation rates of 1–2 mW kg−1 in the lower mesosphere (<75 km) and 10–20 mW kg−1 in the upper mesosphere and lower thermosphere (<100 km). The corresponding heating rates are approximately 0.1 and 1 K d−l, respectively. These values are at least 1 order of magnitude smaller than most of the previous measurements and are also significantly smaller than typical values assumed in models. Our observations suggest that the heating effect of turbulence is negligible compared to the most prevailing terms of the heat budget. It can be shown by theoretical considerations involving the turbulent energy budget equation that cooling by turbulent heat conduction is also negligible if e is small.

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

Abstract Rocketborne measurements of electron and positive ion number densities in the vicinity of noctilucent clouds and/or polar mesosphere summer echoes frequently give evidence of pronounced disturbances relative to a smooth background profile. A model is applied to study the disturbances in terms of diffusional charging of aerosol particles with special regard to the background plasma conditions, i.e. the electron/ion production rate and the coefficient of dissociative recombination which depends on the positive ion composition. It is demonstrated that the disturbances in electron and positive ion profiles are a complex function of the aerosol radius, the aerosol number density, the production rate, and the recombination coefficient. However, if the latter two are known, the model allows the determination of aerosol radii and number densities from the measurement of positive ion and electron number densities. Furthermore, we show that nearly all combinations of electron biteouts, positive ion biteouts, and positive ion enhancements can exist depending on the properties of the aerosol particles and the background plasma conditions. Concerning electrons the presence of aerosol particles will always lead to a depletion. The magnitude of this depletion increases with increasing aerosol size and number density. Concerning positive ions both depletions and enhancements can occur. While small electron/ion production rates favour deep depletions in both electrons and positive ions, enhancements in positive ions are most likely created if the recombination coefficient is large implying large positive cluster ions. However, enhancements of electrons cannot be created by the variation of the above mentioned parameters. Thus observations of electron enhancements are a strong indication of a charging mechanism different from the diffusional charging discussed in this paper, e.g. photo emission. We have applied the charging model to in situ observations of electrons and positive ions in the vicinity of noctilucent clouds and polar mesosphere summer echoes. These results are presented in a companion paper by Lubken and Rapp (J. Atmos. Sol. Terr. Phys. (2001), 63, 771–780).

First in situ temperature measurements at the Antarctic summer mesopause

The Arctic summer mesopause at ∼88 km is the coldest-known place (∼130 K) in the terrestrial atmosphere and is ∼60 K colder in summer than winter. Indirect evidence has suggested that the summer mesopause temperatures in the Antarctic are a few Kelvin warmer than in the Arctic. However, reliable measurements have not been available at southern high latitudes to verify this. We report the very first in situ temperature observations in the summer mesosphere from Antarctica based on rocket-borne falling spheres launched from Rothera (68°S, 68°W). The first of 24 successful launches, on 5 January 1998, showed a mesopause temperature of 129 K at 87 km, surprisingly close to northern hemisphere (NH) mean summer values. During January the mesospheric temperatures are similar to the northern summer, but the difference increases to several Kelvin in February.

Neutral air turbulence in the upper atmosphere observed during the Energy Budget Campaign

A number of different experimental techniques employed in the campaign provided measurements on the fine scale structure of the upper atmosphere, from which information about turbulent intensity, eddy transport and eddy dissipation rates may be extracted. The turbulent state of the mesosphere was shown to be highly variable and significant differences were found between observations obtained during the four salvoes launched during different degrees of geomagnetic disturbance.

In situ measurements of turbulent energy dissipation rates and eddy diffusion coefficients during MAP/WINE

Abstract In situ measurements of turbulent energy dissipation rates 6 and eddy diffusion coefficients in the 65–120 km altitude range using mass spectrometers, positive ion probes and foil clouds are presented. The mass spectrometer and the ion probe data were both analyzed with two independent theoretical approaches to derive e. In general, the derived quantities agree with the values discussed in the literature, although noticeable differences occur in certain rocket flights. An intercomparison of the spectral power densities derived from the observed neutral gas and positive ion density fluctuations supports the assumption that under certain circumstances positive ions can be used as passive tracers for neutral gas number density fluctuations. The observed e profiles exhibit a local minimum around 80 km altitude.

First in‐situ observations of neutral and plasma density fluctuations within a PMSE layer

The NLC-91 rocket and radar campaign provided the first opportunity for high resolution neutral and plasma turbulence measurements with simultaneous observations of PMSE (Polar Mesospheric Summer Echoes). During the flight of the TURBO payload on August 1, 1991, CUPRI and EISCAT observed double PMSE layers located at 86 and 88 km altitude, respectively. Strong neutral density fluctuations were observed in the upper layer but not in the lower layer. The fluctuation spectra of the ions and neutrals within the upper layer are consistent with standard turbulence theories. However, we show that there is no neutral turbulence present in the lower layer and that something else must have been operating here to create the plasma fluctuations and hence the radar echoes. Although the in situ measurements of the electron density fluctuations are much stronger in the lower layer, the higher absolute electron density of the upper layer more than compensated for the weaker fluctuations yielding comparable radar echo powers.

Electron temperature control of PMSE

Recently an artificially induced modulation of polar mesosphere summer echos (PMSE) was observed when the electrons at PMSE altitudes were heated by a ground based heating facility [Chilson et al., 2000]. The PMSE disappeared within a few seconds when the heater was switched on and reappeared within a few seconds when the heater was switched off again. We explain these observations employing a model of electron diffusion in the environment of positive ions and negatively charged aerosols which takes into account enhanced electron temperatures. If the electron temperature equals the neutral gas temperature, electron diffusion is efficiently inhibited due to the multipolar electric field between the aerosols, positive ions, and electrons. If the electron temperature is enhanced, however, the electron diffusivity is increased which compensates the effect of charged aerosols such that spatial structures in the electron gas at scales as small as the radar half wavelength are efficiently destroyed by diffusion. Furthermore, an enhanced electron temperature increases the aerosol charge which supports the decay of electron disturbance through plasma interference effects. The heating experiment has demonstrated that the reduction of electron diffusivity by charged aerosol particles is the underlying physical mechanism for the existence of PMSE.

Microphysical and turbulent measurements of the Schmidt number in the vicinity of polar mesosphere summer echoes

During the ECHO campaign in 1994 neutral and electron density fluctuations were measured together with charged aerosols on the same sounding rocket launched close to a VHF radar detecting polar mesosphere summer echoes (PMSE). For the first time this combination of measurements allows for an independent test of the microphysical and the turbulence interpretations of the Schmidt number (Sc). The Schmidt number characterizes the reduction of the electron diffusivity by charged aerosols, which leads to an enhancement of the electron density fluctuations at small spatial scales. In one of the flights charged aerosols were observed at ∼83–89km together with correlated depletions in electron density (‘biteouts’). We have applied a model of aerosol charging to the measured plasma profiles and determined a mean aerosol radius of ∼8nm and a mean aerosol charge of 1e-. In the microphysical description of electron diffusion these parameters correspond to Sc∼420. Spectral analysis of the electron density fluctuations showed enhancements of spectral densities at small scales suggesting likewise a Schmidt number much larger than unity. Using an energy dissipation rate of 67mW/kg as derived from neutral air turbulence measurements on the same rocket we get from the electron spectra Sc=385 which is in excellent agreement with the microphysical result. Apart from this turbulent layer we observe no significant disturbances in neutral air number densities below ∼87km which confirms earlier indications that processes must exist to create PMSEs which are not directly coupled to neutral air turbulence.