Summer Mesopause Research Articles

Influences of ice particles on the ion chemistry of the polar summer mesosphere

In the polar summer mesosphere, charge is distributed over a wide range of constituents closely connected to phenomena like noctilucent clouds (NLCs) and polar mesosphere summer echoes (PMSEs). In this paper, we study how the presence of ice particles influences mesospheric ion chemistry, and how this may feed back on the particle population. To this end, we present an ion‐chemical model that for the first time features close coupling with cluster growth and ice particle charging. Starting out from molecular ion reactions, the H+(H2O)n proton hydrate chain is described using the Thomson model and Natanson's recombination scheme. Under most mesospheric conditions, electron capture by particles is expected to enhance the lifetimes and concentrations of positive ions and clusters. This has important consequences for the total charge density and mobility in the environment of particle layers. Extending the proton hydrate chain to large cluster sizes, we also quantify the efficiency of ionic nucleation of mesospheric ice particles. While ionic nucleation is not feasible as a major mesospheric nucleation process, it can become efficient given moderate atmospheric variations as induced by gravity waves. This leads to a scenario of rapid generation of populations with many small particles in local temperature minima. We show that electron capture to existing particles can significantly enhance the ionic nucleation of new particles. In summary, there are many potential connections between ion chemistry and layered phenomena in the mesosphere that should be included in comprehensive models of NLC/PMSE. Unfortunately, uncertainties in ionic reaction rates are a persistent problem and in great need of laboratory measurements representative for cold summer mesopause conditions.

Hemispheric differences in the temperature of the summertime stratosphere and mesosphere

We study two processes which may govern interhemispheric differences in the temperature of the summertime middle atmosphere. The first is the direct radiative effect arising from the eccentricity of Earth's orbit. The second factor is the difference in gravity‐wave filtering due to the hemispheric asymmetries in the summertime mean winds of the troposphere and lower stratosphere. Using two different gravity‐wave drag parameterizations and a zonal wind climatology, we find greater gravity wave induced acceleration of the zonal flow in the southern summer lower stratosphere, which leads to weaker gravity wave drag in the southern upper mesosphere. Using a two‐dimensional chemical‐dynamical model, we evaluate the temperature changes caused by these drag differences and compare them with those caused by direct radiative asymmetry. The radiative asymmetry peaks in the upper stratosphere but is nonnegligible between 20 and 80 km. The dynamical asymmetry has a primary contribution in the upper mesosphere and a secondary contribution in the lower stratosphere. Overall, our results support the idea that poleward of 30° the southern middle atmosphere is warmer than the north by 3–8K between 20 and 85 km. As a result, our model suggests that the relative humidity of the northern summer mesopause region is greater than in the south. This implies that mesospheric clouds (PMCs and NLCs) should be more frequent and more extensive in the north than in the south.

Resonant nonlinear interactions between atmospheric waves in the polar summer mesopause region

Data obtained from the mobile SOUSY VHF radar at Andoya/Norway in summer 1987 have been used to study the nonlinear interactions between planetary waves, tides and gravity waves in the polar mesosphere, and the instability of background atmosphere above the mesopause. It is observed that 35-h planetary wave, diurnal, semidiurnal and terdiurnal tides are the prominent perturbations in the Lomb-Scargle spectra of the zonal wind component. By inspecting the frequency combinations, several triads are identified. By bispectral analysis it is shown that most bispectral peaks stand for quadratic coupling between tidal harmonics or between tide and planetary or gravity wave, and the height dependence of bispectral peaks reflects the variation of wave-wave interactions. Above the mesopause, the occurrence heights of the maximum L-S power spectral peaks corresponding to the prominent wave components ten to increase with their frequencies. This may result from the process in which two low frequency waves interact to generate a high frequency wave. Intensities of the planetary wave and tides increase gradually, arrive at their maxima, and then decay quickly in turn with increasing height. This kind of scene correlates with a “chain” of wave-wave resonant interactions that shifts with height from lower frequency segment to higher frequency segment. By instability analysis, it is observed that above the mesopause, theRichardson number becomes smaller and smaller with height, implying that the turbulent motion grows stronger and stronger and accordingly the background atmosphere more and more instable. It is suggested that the wave-wave sum resonant interaction and the wave dissipation due to instability are two dominant dynamical processes that occur in the mesopause region. The former invokes the energy transfer from lower frequency waves to higher frequency waves. The latter results in the heating of the atmosphere and accelerating of the background flow.

Icy particles in the summer mesopause region: Three‐dimensional modeling of their environment and two‐dimensional modeling of their transport

In summertime, layers of icy particles form in the high‐latitude mesopause region, causing the phenomena of noctilucent clouds and polar mesosphere summer echoes. We study the formation of these layers by means of a 3‐D general circulation model of the middle atmosphere (COMMA/IAP) including new modules for the mesospheric chemistry and for the microphysics of icy particles. We initialize the mesopause region with an ensemble of two million particles and investigate their time‐dependent transport in a 2‐D Lagrangian way. We use the model to study in considerable detail ice particle formation by heterogeneous nucleation on smoke particles, freeze‐drying of the mesopause region by formation of ice particles, and the influences of tidal waves and particle eddy diffusion on ice particle and noctilucent cloud (NLC) layer behavior. We present model results on the requirements for adequate size of smoke particles to act as condensation nuclei in the mesopause region (directly after initialization > 0.7 nm radius; after 8 hours > 2 nm), the types of ice particle trajectories, the timescale for growth of particles (6 to 8 hours to radii > 20 nm) and sublimation of ice particles (duration of sublimation phase typically 6 hours), the typical altitude range of maximum NLC brightness (82.5–83.0 km), the size distributions of ice particles (closer to Gaussian than lognormal in shape), the importance of the vertical component of the tidal winds on NLC layer behavior, and the dominance of semidiurnal over diurnal tidal effects. Furthermore, the model allows us to make predictions on a number of NLC features that are either currently under study or may become observable in the near future, such as properties of NLC at high arctic latitudes, and the development of layers with strongly enhanced water vapor mixing ratios near the lower border of the NLCs.

Small‐scale temperature variations in the vicinity of NLC: Experimental and model results

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.

Response of the mesosphere to human‐induced perturbations and solar variability calculated by a 2‐D model

We have used the improved NCAR interactive 2‐D model (SOCRATES) to investigate the chemical and thermal response of the mesosphere to composition changes from the preindustrial era (∼1850) to the present, to doubling the CO2 concentration, and to the 11‐year solar flux variability. The calculations show that all regions in the model mesosphere have cooled relative to the preindustrial times. The mesopause region has cooled by ∼5 K and the winter pole by up to 9 K near 60 km. Ozone mixing ratio has decreased by about 5% in the lower mesosphere and by about 30% near the summer mesopause region (caused by a dramatic increase in [OH]). Doubling the CO2 abundance cools the whole mesosphere by about 4–16 K and has a complicated effect on O3, which exhibits an alternating increase/decrease behavior from the lower mesosphere up to the mesopause region. Similar results are obtained, in both magnitude and structure, for the O3 response to a decrease in solar UV flux. Similarities are also found in the response of T, OH, and H to these two perturbations. These results lead to the conclusion that the long‐term increase in the well‐mixed greenhouse gases, in particular CO2, alters the thermal structure and chemical composition of the mesosphere significantly and that these anthropogenic effects are of the same magnitude as the effects associated with the 11‐year solar cycle. Thus, the difference in the timescales involved suggests that the anthropogenic signal over periods of typically 10 years is smaller than the signal generated by the 11‐year solar variability. Finally, analysis of the results from a simulation of the combined perturbations (2 × CO2 + 11‐year solar variability) shows that, for the most part, the solar variability does not interact with increasing CO2 and vice versa; that is, the two effects are additive.

Cluster ions and ice particle nucleation: Positive feedback at the summer mesospause

Large proton hydrate clusters H+(H2O)n can act as condensation nuclei for ice particles near the polar summer mesopause. However, laboratory data pose strong requirements on temperature and humidity for this nucleation mechanism to be feasible. Growth to sufficient cluster sizes is governed by competition between clustering reactions and recombination with free electrons. Since the abundance of free electrons can be strongly reduced by capture to ice particles, the presence of ice particles enhances the growth of cluster ions. This leads to a positive feedback mechanism with particle nucleation being favored in regions where particles already exist. Since this mechanism can amplify irregularities in the ionospheric particle and charge distribution, it is of potential interest for the understanding of polar mesosphere summer echoes (PMSE) and other layered phenomena. We demonstrate these ideas using a dusty ion‐chemical model of the D‐region.

D region electron number density limits for the existence of polar mesosphere summer echoes

We investigate the dependence of polar mesosphere summer echoes (PMSE) and mesosphere summer echoes (MSE) on the background electron number density. Both a lower and upper limit are quantified below and above which PMSE cannot exist. The result is that PMSE occur for a very wide range of electron number densities between ∼300–500/cm3 and ∼105/cm3. A comparison of the diurnal variation of MSE observed at Kühlungsborn (54°N) with current model estimates of the electron number density at midlatitudes shows that at least ∼300–500 electrons /cm3 are necessary for PMSE to exist. This lower limit is consistent with all available electron number density measurements obtained from sounding rockets in the vicinity of PMSE. It is shown that the existence of a lower electron number density limit can be understood in terms of the standard theory of the scattering of VHF waves in the D region. We have then analyzed PMSE observations during the major solar proton event on 14 July 2000. We have estimated the electron number density at PMSE altitudes based on proton and electron flux measurements obtained with detectors on board the GOES‐8 and ACE spacecrafts in combination with an ion‐chemical model. Comparing the electron number densities at 87 km altitude with the average PMSE signal to noise observations (SNR) we find a significant negative correlation between SNR and the electron number densities for densities on the order of ∼105/cm3. We propose that this negative correlation is due to a limited amount of aerosol particles: current PMSE theories assume that electron irregularities in the VHF band can only exist if more than ∼50% of the free electrons are bound to aerosol particles which thus reduce the electron diffusivity due to ambipolar forces. If, however, the electron number density increases significantly above the aerosol number density, this condition can no longer be fulfilled. On the other hand, the fact that PMSE is present up to electron number densities of ∼105/cm3 raises several important questions on our current understanding of aerosol particles around the polar summer mesopause and their role in the creation of PMSE: either many more aerosol particles exist than has been anticipated so far (with corresponding implications for the nucleation of these particles) or our current understanding of the role of aerosol particles in the creation of PMSE is not complete.

Studies for the Odin sub-millimetre radiometer: III. Performance simulations

Odin is a small, low-cost satellite with a combined astronomical and aeronomical mission. The mission is divided on an equal basis between astronomy and aeronomy. The aeronomy objectives can be divided into four main subjects: stratospheric ozone chemistry, mesospheric ozone chemistry, the summer mesopause region, and the coupling between atmospheric regions. The primary instrument on Odin is the millimetre and sub-millimetre radiometer (SMR), which is used both for astronomy and aeronomy. It is the first satellite to use sub-millimetre frequencies for limb-sounding mode. Odin is also equipped with an optical spectrometer (OSIRIS). This paper is the third of a three-part series and describes the choices of observing modes and the expected performance of the SMR instrument for the aeronomy mission. The relevant frequencies are identified and the exact selection of lines is made. This is followed by a detailed simulation study to determine the achievable altitude coverage together with the corresponding vertical resolution for each retrievable species. An indication of the expected uncertainties is also given, showing, for example, a high-sensitivity to mesospheric water vapour and stratospheric chlorine monoxide. However, a complete analysis of observation uncertainties must await launch and the completion of the validation programme. PACS Nos.: 42.68A, 07.07D, 07.57K

Mean characteristics of the spectrum of horizontal velocity in the polar summer mesosphere and lower thermosphere observed by foil chaff

Thirty-three foil chaff rockets were flown at high latitudes during the summers of 1987, 1988, and 1991.Vertical profiles of horizontal wind were obtained at high resolution (25 m) in the 67– 103 km region of the mesosphere and lower thermosphere. This resolution (at that range of altitude) is unique and provides sensitivity to more than two decades of vertical wavenumbers. This study utilizes the unique data set to present observed spectra and their mean characteristics. Average vertical wavenumber spectra indicate that there is excellent agreement with the saturated gravity wave spectral model developed by Dewan and Good (J. Geophys. Res. 91(D3) (1986) 2742) and Smith et al. (J. Atmos. Sci. 44(10) (1987) 1404), which has a slope of −3 and a spectral amplitude proportional to the buoyancy frequency squared. This excellent agreement provides further support for the saturated spectrum theory in a mean sense and suggests that saturation processes are present in the horizontal flow and act to produce turbulence. We can therefore expect that turbulence is enhanced near the polar summer mesopause. Dominant vertical wavelength obtained from the average spectra is estimated to be 12.8 km .

Scattering phase functions and particle sizes in noctilucent clouds

The MIDAS‐DROPPS campaign conducted in Norway in 1999 provided a comprehensive study of the high‐latitude summer mesopause region with rocket‐borne and ground‐based instrumentation. Optical photometers were flown on two rocket payloads through substantially different mesospheric conditions. On both flights, distinct noctilucent cloud (NLC) layers were detected. We present the first analysis of NLC scattering phase functions observed from sounding rockets. Applying Mie calculations, the angular dependence of the scattering is used to extract information about particle sizes. The first flight featured a weak NLC with small particles (r ≤ 20 nm) located below the core of a strong polar mesosphere radar echo (PMSE). The second flight took place in the absence of any detectable PMSE and probed a bright NLC optically dominated by particles in the size range 40–50 nm.

Measurements of water vapour in the mesosphere with the spectral absorption line imager (SALI)

Water vapour concentration is one of the most important, yet one of the least known quantities of the mesosphere. Knowledge of water vapour concentration is the key to understanding many mesospheric processes, including the one that is primary focus of our investigation, Polar Mesospheric Clouds (PMC). The processes of formation and occurrence parameters of PMC constitute an interesting problem in their own right, but recent evidence had been provided which suggests that PMC are a critical indicator of climate change. In this context the feasibility of a low cost method of water vapour measurements using an instrument carried aloft by a sounding rocket has been examined and some of the results discussed. It is proposed to measure the strength of the 936nm water absorption line in a solar occultation configuration employing a CCD detector. This leads to the design of a small, low cost and low-mass instrument, which can be flown on a small rocket, of the type of the Orbital Sciences Corporation Viper 5. Alternatively the instrument can be flown as a “passenger” on larger rocket carrying other experiments. In either case flight costs are relatively low. Some performance simulations are presented showing that the instrument we have designed will be sufficiently sensitive to measure water vapor in concentrations that are expected at the summer mesopause, about 85 km height. Sufficient payload design work was carried out showing that the structural, thermal and electrical requirements for a flight on the Viper 5 rocket can be met and thus making the experiment feasible for a flight.

Electrical structure of noctilucent cloud and polar mesosphere summer echo regions

A number of recent coordinated rocket and ground-based investigations have concentrated on the cold summer mesopause region where the occurrence of noctilucent clouds (NLCs) and polar mesosphere summer echoes (PMSEs) provides excellent examples of layering processes observable over both relatively small and large altitude scales. This paper provides an overview of our recently flown instrumentation and the resulting measurements of mesospheric electrical structure during observed NLC/PMSE conditions. We begin with a brief description of our different probe techniques used to characterize this region's electrical structure, specifically, its polar components of conductivity, the mobilities and concentrations of different positively charged species, and the relative concentration of electron density, including its large- and small-scale vertical structure. Evidence for an electron “bite-out” at NLC/PMSE altitudes exists in our measurements of negative electrical conductivity and the Langmuir probe current data for collected electrons. A very challenging problem has been the detection of charged particles or aerosols, which we have addressed by using different probe geometries and charge collection schemes. Blunt probes operating under ram-flow conditions have measured current fluctuations suggestive of impacting aerosols/dust in both negatively and positively charged forms. Also, swept-voltage Gerdien condensers with stabilized flow geometries have identified very small mobility values that are believed to indicate the presence of sub-visible positive charged particles. The number densities of these species are particularly pronounced in the NLC/PMSE regions.

Capture rates of electrons and positive ions by mesospheric aerosol particles

Aerosol particles exist in the vicinity of the extremely cold polar summer mesopause and can be visually observed in the form of noctilucent clouds ( Jesse (1885) Met. Zeit. 2, 311–312). Being embedded into the plasma of the D-region, these particles become charged by means of plasma diffusion and lead to huge VHF radar echoes known as Polar Mesospheric Summer Echoes ( Ecklund and Balsley (1981) J. Geophys. Res. 86, 7775–7780). The diffusional charging of submicron particles has been treated in detail by Natanson (1960, Sov. Phys. Tech. Phys. 5, 538–551) and with some modifications by Fuchs (1963, Geophys. Pura. Appl. 56, 185–193). Previous model investigations of the charging of mesospheric aerosol particles (e.g., Rapp and Lübken (1999) Earth Planets Space 51, 799–807) applied analytical approximations to the rigorous Natanson theory which Natanson (1960) had derived for the case that the mean free path of the ions, λ, is much larger than the radius of the particles, r A. This conditions is satisfied for particles around the mesopause where λ∼1 cm and r A ⩽100 nm ( Thomas and Mc Kay (1985) Planet. Space Sci. 33 1209–1244). The aim of this note is to investigate the applicability of Natanson's analytical approximations for models of mesospheric aerosol particles.

Lidar observations of seasonal variation of diurnal mean temperature in the mesopause region over Fort Collins, Colorado (41°N, 105°W)

Two campaigns from February 1997 to January 1999 have resulted in 18 sets of continuous temperature measurements in the mesopause region over Fort Collins, Colorado (41°N, 105°W), each covering a complete diurnal cycle, reasonably well distributed throughout the year. It is shown that the winter‐summer transitions, based on diurnal means, are abrupt and clear, further strengthening the concept of two‐level mesopause, and demonstrate the robustness of this picture. Similar to data taken at Urbana, Illinois, our data at Fort Collins also demonstrate the need to observe over a complete diurnal cycle to deduce the uncontaminated mean temperature structure and seasonal variations in the mesopause region. The difference between annual diurnal, nighttime, and daytime means may be ascribed to a temperature wave propagating with a downward phase speed of ∼0.8 km/h. Our data show that the nighttime annual mean is colder than the diurnal annual mean by no more than 2 K below 88 km and warmer by no more than 3 K above 88 km where the three means are nearly the same. As in the Urbana data, we also observed the altitude of a secondary temperature minimum in the summer to be ∼96 km, which without strong dynamical cooling in the summer would have been the summer mesopause altitude.

Observations of the breakdown of an atmospheric gravity wave near the cold summer mesopause at 54N

Recently, it was shown from a single set of airglow/lidar observations in Urbana, Illinois (40N) that some small‐scale wave‐like structure seen in OH airglow images can be associated with the breakdown, via a convective instability, of an atmospheric gravity wave. A second set of simultaneous airglow/lidar observations, showing wave breakdown, has been obtained over Kühlungsborn, Germany (54N) during a period when noctilucent clouds (NLCs) were also observed. This showed that the wave breakdown process can occur under the same cold, low altitude summer mesopause conditions that support the occurrence of NLCs. This new set of data shows that the first observations were not a unique event, and give credence to the suggestion that some of the small scale structure seen in NLC displays may be due to wave breakdown.

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.

Sensitivity of mesospheric mean flow, planetary waves, and tides to strength of gravity wave drag

A global circulation model which extends from the surface to 125 km is used to study how the strength of gravity wave drag affects the dynamics of the mesosphere. The strength of gravity wave drag has a strong influence on the zonal mean state of the mesosphere, in particular the magnitude and variability of the summer mesopause temperature and the shear on the top of the mesospheric jets. This change in the zonal mean state strongly affects the susceptibility of the mesosphere to baroclinic and/or barotropic instability and hence the formation of the 2‐day wave. The 2‐day wave, in turn, interacts nonlinearly with the diurnal tide, producing secondary waves and a reduction in amplitude of the diurnal tide. Previous studies with quasi‐linear mechanistic tidal models have captured some semiannual variation in tidal amplitude through direct interactions between tides and gravity waves. Our fully nonlinear global circulation model results support an alternative explanation in terms of interactions between planetary waves and tides.

First observations of polar mesosphere summer echoes in Antarctica

A 25‐kW peak power 50‐MHz radar was installed at the Peruvian base on King George Island, Antarctica (62°S), in early 1993. A search for polar mesospheric summer echoes (PMSEs) was made during late January and early February of the first year of operation with negative results. These results have been reported in the literature [Balsley et al., 1993; 1995]. We report here results obtained during the austral summer of the second year (1994) of operation. Observations during the second year were begun earlier, i.e., closer to the austral summer solstice. PMSEs were observed during this period, albeit the echoes were much weaker than what one would expect based on earlier Poker Flat radar results at a comparable latitude (65°N) in the Northern Hemisphere. A large and measurable asymmetry in PMSE strength in the two hemispheres therefore exists. We explain this asymmetry by postulating a difference in summer mesopause temperatures between the two hemispheres of ∼7.5 K. This difference has been estimated using an empirical relationship between the variations of the Poker Flat PMSE power as a function of temperature given by the mass spectrometer incoherent scatter extended (MSISE‐90) model.

Comparison between observed and computed polar mesosphere summer echoes

A recently developed model for simulating VHF radar signals from the polar summer mesopause region predicts a predominant effect of the temperature on the generation of these so‐called polar mesosphere summer echoes (PMSE). This effect is due to the strongly nonlinear influence of the temperature on the saturation water vapor pressure over ice determining the concentration of ice particles which in turn affects the concentration of free electrons by electron scavenging. In particular, the height and the temperature of the mesopause as well as height regions with enhanced temperature gradients should significantly control the PMSE layer structure whereas other relevant parameters such as the meteoric dust size distribution, the water vapor mixing ratio, and the energy dissipation rate essentially affect the overall echo intensity. In the present study, temperature profiles and radar signal‐to‐noise ratios observed simultaneously in summer 1987 over Andøya (northern Norway) are used for comparing simulated and measured PMSE under the condition of low cosmic noise absorption. In most cases the model reproduces the observed number of PMSE layers as well as the layer heights and widths with accuracies of 1 km, and the deviations between observed and computed maximum signal‐to‐noise ratios do not exceed 10 dB. Since the temperature profiles and the signal‐to‐noise ratios have been measured at distances of about 40 km, at least part of the discrepancies can be attributed to the horizontal nonuniformity of the mesopause region.