Mesospheric Layer Research Articles

Uptake of Fe, Na and K atoms on low-temperature ice: implications for metal atom scavenging in the vicinity of polar mesospheric clouds

Ice clouds form in the mesosphere between 80 and 90 km, at high latitudes during summer when the temperature falls below 150 K. There is strong evidence that the water-ice particles in these clouds scavenge metal atoms that are produced in the mesosphere by meteoric ablation. In the present study the uptake of Fe, Na and K on an ice film was studied in a fast flow tube over a temperature range of 80-150 K, covering the temperatures over which ice clouds form in the upper mesosphere. The uptake was found to be highly efficient and mostly in the diffusion-limited regime, requiring accurate measurements of the diffusion coefficients of the metal atoms in He: DFeHe = 366 (+/- 17) (T/296 K)(1.85 +/- 0.07), DNaHe 286 (+/- 13) (T/296 K)(1.68 +/- 0.04) and DKHe = 247 (+/- 15) (T/296 K)(1.69 +/- 0.07) Torr cm2 s(-1). Measured values of the diffusion coefficients in N2 are 112 (+/- 4), 125 (+/- 4) and 88 (+/- 4) Torr cm2 s(-1) at 293 K for Fe, Na and K, respectively. The uptake of Na and K was observed to be extremely efficient from 80-150 K, with lower limits of gamma Na > 0.09 and gamma K > 0.05, although it is likely that gamma is much closer to unity. The uptake of Fe on cubic ice is close to unity efficiency above 135 K, but gamma Fe decreases to only 3 x 10(-3) at 80 K. Uptake of Fe on amorphous ice films is much more efficient than on cubic ice films below 130 K. These results are interpreted using quantum calculations of the metal atoms adsorbed onto a 12-H2O model ice surface. Finally, it is shown that the uptake of Fe, Na and K on low-temperature ice is sufficiently fast to explain the substantial depletions in the mesospheric metal layers that are observed in the presence of mesospheric ice clouds.

Challenges in the limb retrieval of noctilucent cloud properties from Odin/OSIRIS

Limb-viewing satellite instruments have become a major tool for the observation of noctilucent clouds (NLC), providing distinct advantages in the study of these thin mesospheric layers. The geographical coverage and the continuity of the data provide exclusive information on global distribution, seasonal variation, and planetary wave patterns. In combination with spectroscopic measurements, this provides the possibility to infer global information about NLC particle sizes. However, retrieval procedures for satellite-borne NLC observations are complex. Major challenges are related to the limb geometry, the discrimination between cloud and background, and albedo effects. This paper, describes the analysis of NLC observations from the OSIRIS instrument onboard the Odin satellite. Retrieval challenges and solutions are discussed with special emphasis on the spectral analysis of NLC particle properties. We show that OSIRIS can provide comprehensive information about individual clouds. However, intercomparisons between different satellites or ground-based instruments are generally limited to studies on a climatological basis.

Global measurement of the mesospheric sodium layer by the star occultation instrument GOMOS

We present the first global measurement of the sodium mesospheric layer obtained from the processing of about 100 000 star occultations by the GOMOS instrument onboard the ENVISAT satellite. The retrieval method is developed on the basis of a simple DOAS retrieval applied to averaged transmittances. The vertical inversion of the sodium slant path optical thickness is performed by using a modified Gaussian extinction profile. A global climatology is derived by using monthly bins of 20 degrees in latitude. The high variability of the sodium layer is confirmed on a global scale as well as the presence of an important modulation in the annual cycle. Also, we present some evidence for the existence of a diurnal cycle characterized by an increase of the sodium concentration in daylight.

Potential distribution around sounding rockets in mesospheric layers with charged aerosol particles

It is shown that large potential variations can exist inside the wake of sounding rockets moving with supersonic velocity through the summertime mesopause region. The potential distribution is driven by charge separation induced by the shock wave of the rocket. The low conductivity conditions due to the attachment of free electrons to aerosol particles allow large electric fields, of the order of V/m, to be maintained. The numerical results from a simple model are consistent with recent electric field measurements carried out by the DROPPS‐1 sounding rocket.

The mesospheric metal layer topside: a possible connection to meteoroids

Abstract. In the past, many studies have been carried out to demonstrate the influence of meteoroids on the atmospheric metal layer, observed roughly in the altitude range 80–105 km. Even with the capability of present day resonance lidars to measure metal densities within single meteor trails, it has been difficult to prove any influence of meteors on the average metal layer. In contrast to approaches taken earlier, we discuss here the seasonal characteristics of potassium, calcium, calcium ion, iron and sodium above 110 km altitude where the average nocturnal densities are so low that the existence of a baseline level of metal atoms and ions is often overlooked. By comparing simultaneous and common-volume observations of different metal layers at one location, we demonstrate that despite their different seasonal characteristics at lower altitudes remarkably similar seasonal characteristics are observed at higher altitudes. In addition, a qualitative agreement is also found for potassium at different latitudes. A comparison of metal densities at 113 km altitude with known meteor showers indicates a strong influence of shower meteoroids on the topside of the metal layers. Simultaneous observations of K along with Ca, Fe and/or Na permit the calculation of abundance ratios, which at 113 km altitude are quite similar to values measured in single meteor trails by ground based lidars. Furthermore, the increase in densities throughout summer is strong evidence for the influence of sporadic meteoroids on the high metal layers. This increase correlates well with the seasonal variation of sporadic micrometeor input independent of meteor showers. Given these evidences, we contend that there is a direct influence of ablating meteoroids on the topside of the mesospheric metal layer.

A time-resolved model of the mesospheric Na layer: constraints on the meteor input function

Abstract. A time-resolved model of the Na layer in the mesosphere/lower thermosphere region is described, where the continuity equations for the major sodium species Na, Na+ and NaHCO3 are solved explicity, and the other short-lived species are treated in steady-state. It is shown that the diurnal variation of the Na layer can only be modelled satisfactorily if sodium species are permanently removed below about 85 km, both through the dimerization of NaHCO3 and the uptake of sodium species on meteoric smoke particles that are assumed to have formed from the recondensation of vaporized meteoroids. When the sensitivity of the Na layer to the meteoroid input function is considered, an inconsistent picture emerges. The ratio of the column abundance of Na+ to Na is shown to increase strongly with the average meteoroid velocity, because the Na is injected at higher altitudes. Comparison with a limited set of Na+ measurements indicates that the average meteoroid velocity is probably less than about 25 km s-1, in agreement with velocity estimates from conventional meteor radars, and considerably slower than recent observations made by wide aperture incoherent scatter radars. The Na column abundance is shown to be very sensitive to the meteoroid mass input rate, and to the rate of vertical transport by eddy diffusion. Although the magnitude of the eddy diffusion coefficient in the 80–90 km region is uncertain, there is a consensus between recent models using parameterisations of gravity wave momentum deposition that the average value is less than 3×105 cm2 s-1. This requires that the global meteoric mass input rate is less than about 20 td-1, which is closest to estimates from incoherent scatter radar observations. Finally, the diurnal variation in the meteoroid input rate only slight perturbs the Na layer, because the residence time of Na in the layer is several days, and diurnal effects are effectively averaged out.

Seasonal variation of mesospheric iron layers at Arecibo: First results from low‐latitudes

We present the annual variation of the distribution of neutral iron of the low‐latitude mesopause region measured at the Arecibo Observatory using resonance fluorescence lidar. A comparison was made between our low‐latitude measurements and similar mid‐and high‐latitude observations of Fe column abundance, centroid height, and RMS width of the layer. We find that the annual variation of the height and width of the layer are essentially anti‐correlated with previous measurements, while the column abundance lags its midlatitude counterpart by roughly three months. We suggest that the latitudinal differences in Fe layer properties result from variations in the distribution of mesospheric ozone and O2, which in turn affect the iron chemistry at the two latitudes.

Gravity waves in Fabry–Perot measurements made at Mt. John (44°S,170°E), New Zealand

Observations of winds in mesospheric airglow layers have been made at Mt. John (44°S,170°E), New Zealand for some years. We present a modelling study of airglow emissions which shows that the properties of wind detection based on airglow emission means that high-frequency gravity waves are effectively filtered from the wind spectrum observed. This filtering means that any waves with periods of the order of hours should be detectable in the record (as they will not be hidden in the noise of the higher-frequency waves ubiquitous at these heights). One example of such a wave is shown. As part of the analysis, we show that because the airglow layers differ in width, some waves might be observed in only one airglow layer, even when present in both.

Structure of the mesospheric Na layer at 40°N latitude: Seasonal and diurnal variations

Lidar observations obtained throughout the diurnal and annual cycles at Urbana, Illinois (40°N, 88°W), are used to examine the structure of mesospheric Na between 76 and 108 km. The Na layer is strongly influenced by seasonal and diurnal variations in solar illumination and by seasonal variations of mesospheric temperatures. Although there is little evidence of direct tidal perturbations in Na density, 24 hour oscillations dominate the diurnal variations. Na abundance and densities above 90 km are controlled by photoionization and charge exchange reactions during the day and recombination at night. The abundance is maximum at sunrise, and the peak‐to‐peak diurnal variation averages more than 30%. Below 85 km, density variations are controlled by photochemistry which interrupts the conversion of Na to the bicarbonate reservoir, and by warmer temperatures which enhance the liberation of Na from the reservoir. In this region the peak‐to‐peak diurnal Na variations can exceed 200%. The combined effects of photoionization above 90 km and photochemistry below 85 km induce a strong 24 hour oscillation in the layer centroid height. The peak‐to‐peak variation exceeds 1 km. The lowest centroid height is at local noon when Na densities below 85 km are maximum and photoionization above 90 km is strongest. The seasonal variations in Na densities and abundance are influenced primarily by changes in mesopause region temperatures which are coldest during midsummer. Below 95 km, reactions leading to the bicarbonate sink and the liberation of Na from this reservoir dominate Na chemistry. These reactions and their temperature dependencies are responsible for the large annual variation in Na abundance (2.3 × 109 to 5.3 × 109 cm−2). The measured correlation between Na abundance and the layer weighted temperature is +0.89. The ratio of the abundance to temperature variations is 1.9 × 108 cm−2K−1. The seasonal Na density and temperature perturbations are well‐correlated (+0.6 to +0.9) below 98 km and negatively correlated above (−0.2 to −0.5).

Mesospheric Na layer at extreme high latitudes in summer

Summertime observations of the mesospheric Na layer at high latitudes are reported from the 1993 Airborne Noctilucent Cloud (ANLC‐93) campaign in the Canadian Arctic and at the Amundsen‐Scott South Pole Station. Measurements at the South Pole reveal a layer that has a smaller column abundance and is significantly higher and thinner than at midlatitudes. Using a model that was essentially optimized to wintertime conditions at high northern latitudes, the South Pole layer can be modeled satisfactorily if the rate coefficient for the reaction between sodium bicarbonate and atomic hydrogen is set to k(NaHCO3 + H → Na + H2O + CO2) = 1.1 × 10−11 exp (−910/T) cm3 molecule−1 s−1. In particular, the model is able to reproduce the small scale height of about 2 km observed on the underside of the layer. It is then shown that this steep gradient in the atomic Na mixing ratio can be sustained against vertical eddy diffusion because of the sufficiently rapid chemical cycling between Na its major reservoir, NaHCO3. This justifies the assumption in the model that the vertical transport of Na species can be treated in terms of a single continuity equation describing total sodium. The observations from the campaigns in both hemispheres show that the Na abundance has a temperature dependence of about 2 × 108 cm−2 K−1 at temperatures below 170 K, in good accord with the model. About 40% of this dependence appears to be caused by the activation energies of the reactions that partition sodium between atomic Na and NaHCO3, and the remainder by the temperature dependence of the odd‐oxygen/odd‐hydrogen chemistry in the upper mesosphere.

VHF radar observations of turbulent structures in the polar mesopause region

Abstract. The mobile SOUSY VHF Radar was operated in the summer of 1987 during the MAC/SINE campaign in northern Norway to study the polar mesosphere summer echoes (PMSE). Measurements of the spectral width indicate that two types of structures occur. In general mesospheric layers are bifurcated exhibiting a narrow spectral width and a well-defined aspect sensitivity. However, for about 10% of the observation time cells of enhanced turbulence characterized by extremely broad spectral widths appear predominantly in the upper sublayer above 86 km. Identification and separation of beam and shear broadening allows a determination of the turbulence-induced component of the spectral width. This case study reveals that during several events these cloud-like structures of enhanced turbulence move with an apparent velocity of several tens of meters per second which is almost identical with the phase trace velocity of simultaneously observed waves. Since, at that time, the Richardson number was less than a quarter, it was concluded that these turbulent cells were generated by a Kelvin-Helmholtz mechanism. The horizontal extent of these structures was calculated to be less than 40 km. A general relation between spectral width and echo power was not detected. The turbulent component of the spectral width was used to calculate typical values of the energy dissipation rate at times where narrow spectral width dominates and during periods of enhanced turbulence. In addition, the outer scale of the inertial subrange (buoyancy scale) was estimated. For the first time the occurrence and motion of this type of structures of enhanced spectral width is analyzed and discussed in detail.

VHF radar observations of turbulent structures in the polar mesopause region

The mobile SOUSY VHF Radar was operated in the summer of 1987 during the MAC/SINE campaign in northern Norway to study the polar mesosphere summer echoes (PMSE). Measurements of the spectral width indicate that two types of structures occur. In general mesospheric layers are bifurcated exhibiting a narrow spectral width and a well-defined aspect sensitivity. However, for about 10% of the observation time cells of enhanced turbulence characterized by extremely broad spectral widths appear predominantly in the upper sublayer above 86 km. Identification and separation of beam and shear broadening allows a determination of the turbulence-induced component of the spectral width. This case study reveals that during several events these cloud-like structures of enhanced turbulence move with an apparent velocity of several tens of meters per second which is almost identical with the phase trace velocity of simultaneously observed waves. Since, at that time, the Richardson number was less than a quarter, it was concluded that these turbulent cells were generated by a Kelvin-Helmholtz mechanism. The horizontal extent of these structures was calculated to be less than 40 km. A general relation between spectral width and echo power was not detected. The turbulent component of the spectral width was used to calculate typical values of the energy dissipation rate at times where narrow spectral width dominates and during periods of enhanced turbulence. In addition, the outer scale of the inertial subrange (buoyancy scale) was estimated. For the first time the occurrence and motion of this type of structures of enhanced spectral width is analyzed and discussed in detail.

Simulation of lidar measurements of gravity waves in the mesosphere

Lidar measurements of mesospheric gravity wave characteristics are simulated using a simple numerical model. Simple theory is used to superimpose gravity wave fluctuations upon typical background densities and sodium abundances in the height range 80–100 km, which we then assume are perfectly observed by a Rayleigh and Na lidar, respectively. These lidar “data” are then analyzed for their gravity wave content using the standard reduction and analysis methods which are routinely applied to these data sets. Our goal is to assess whether this analysis of lidar data faithfully recovers the underlying wave field. In both quasi‐monochromatic and spectral wave studies, the height range over which Na number density can be measured and the shape of the background Na profile impose a limit on the gravity wave information which can be extracted from Na lidar data. The limitations due to background layer shape do not exist for Rayleigh lidar measurements. The simulations reveal that quasi‐monochromatic gravity waves with vertical wavelengths larger than approximately 10 km may not be reliably retrieved from Na lidar data. It is also shown that there are limitations on the accurate extraction of spectral parameters from Na lidar data due to the limited vertical extent of the mesospheric Na layer.

ALOHA‐93 measurements of intrinsic AGW characteristics using airborne airglow imager and groundbased Na wind/temperature lidar

Monochromatic Acoustic Gravity Waves (AGWs) with periods < 1 hour are a prevalent feature in the mesospheric airglow layers. These waves are important dynamically and energetically to the region where their temporal and spatial morphology are not well established. The purpose of this study is establish the intrinsic AGW characteristics over an extended region (as flown by the NCAR Electra aircraft) and to present the data in terms of the predicted spectral domain defined by the Brunt‐Vaisala frequency and the diffusive filtering limit proposed by Gardner [1994]. On October 21, 1993, observations were made from the NCAR Electra aircraft during a 6 hour flight in a large triangle N and W of Maui, for a integral distance of ∼3000 km. The entire area observed [∼1 M km²] had a monochromatic AGW propagating toward the NW and the western half had a SW propagating wave superimposed. These waves were also observed with the Michelson interferometer on the aircraft and an airglow imager at the Haleakala location during this time. Intrinsic phase velocities were computed where the Na Wind/Temperature (W/T) lidar at Haleakala provided a measure of the mean wind to compensate phase velocities observed with the imager. The data were tabulated and plotted in an AGW spectral reference frame and compared to cutoff conditions predicted by diffusive filtering theory.

Lidar observations of a lot of sporadic sodium layers in mid‐latitude

We have routinely observed mesospheric Na layers since November 1991 with a Na ground‐based lidar above Tokyo Metropolitan University (TMU) at Hachioji, Tokyo (35.6°N, 139.4°E) in mid‐latitude. In the past, the sporadic Na layers have been observed commonly at low‐and high‐latitude lidar sites [Kwon et al., 1988; Batista et al., 1989; von Zahn et al., 1987] but rarely observed at mid‐latitude sites [Senft et al., 1989]. Contrary to expectations, we could observe more than 100 events of the sporadic Na layers for two years from November 1991 until October 1993 in this mid‐latitude lidar site, and the large sporadic Na layers had been observed especially during spring and summer. Most of these events were accompanied by sporadic E layers and the most enhancement of the sporadic E layers preceded that of the sporadic Na layers by 15 to 30 minutes.

Gravity wave activity in the stratosphere and mesosphere at the South Pole

A Rayleigh/resonant lidar was installed at the Amundsen-Scott South Pole in December 1989 and made observations of the mesosphere and stratosphere through October 1990. The wintertime lidar observations yielded a high resolution data set of sodium (Na) density profiles in the mesosphere and polar stratospheric cloud (PSC) backscatter ratio profiles in the stratosphere. These observations illustrate a variety of wave phenomena present in the Antarctic middle atmosphere. The observations of the mesospheric Na layer were used to characterize gravity wave activity in the upper mesosphere between 80 and 105 km. Strong wave activity was observed throughout the winter and exhibited remarkable similarity with observations from lower latitude sites. The mean density variance at the South Pole was (6%) 2, which is similar to that observed at a variety of lower latitude sites. Strong coherent oscillations were observed in the bottomside density contours of the of the Na layer with periods near the local inertial period. Fabry-Perot spectrometer measurements at South Pole confirm the presence of similar oscillations in the OH winds and temperatures and suggest that the oscillations are associated with planetary-scale waves. Stratospheric backscatter ratio profiles show downward motions similar to the phase progressions of upwardly-propagating gravity waves. Both backscatter ratio profiles and the radiosonde horizontal wind profiles show 1–5 km vertical structures with periods near the local inertial period. With no tropospheric convection during the polar night, and little orographic forcing over the relatively featureless Antarctic plateau, these observations suggest that geostrophic adjustment of the Antarctic stratospheric jet may be a major source of these gravity waves.

Gravity waves in the upper mesosphere over Antarctica: Lidar observations at the South Pole and Syowa

Lidar observations of the mesospheric Na layer were made at the south pole(90°S) and Syowa (69°S) during the winters of 1990 and 1985, respectively. These observations are used to characterize the gravity wave activity in the upper mesosphere at both sites. Strong wave activity is observed throughout the winter at both the south pole and Syowa and shows remarkable similarity with observations from several midlatitude and low‐latitude sites. The quasi‐monochromatic gravity waves exhibit the same general relationships between their wavelengths, observed periods, and amplitudes as observed at lower latitudes. The average growth length of these waves is approximately 26 km, indicating that the wave field at both Antarctic sites is strongly influenced by dissipation and saturation processes. The spectra and variances of the density perturbations associated with quasi‐random wave field at the south pole are reported. The vertical wavenumber and temporal frequency spectra follow power‐law shapes. The mean index of the vertical wavenumber spectrum is −2.4, and the mean characteristic wavelength is 14 km. The mean index of the temporal frequency spectrum is −1.7. The mean density variance at the south pole is (5.7%)2 and is similar in magnitude to that observed at a variety of lower‐latitude sites. With no tropospheric convection during the polar night and little orographic forcing over the relatively featureless Antarctic plateau, these observations suggest that nonlinear processes, rather than the source characteristics, primarily determine the characteristics of the gravity wave field in the upper mesosphere. These observations show two other distinct features. The mean Na layer over Antarctica is significantly lower and broader (centroid height ≈ 90 km and rms width ≈ 4.8 km) than at lower latitudes, reflecting the stronger downwelling and warmer winter temperatures in the mesopause region at high latitudes. Strong coherent oscillations were observed in the bottomside density contours of the Na layer with periods close to the inertial period. These oscillations were also observed in OH airglow measurements and appear to be associate with planetary scale waves.

Structure and seasonal variability of the nighttime mesospheric Fe layer at midlatitudes

Lidar measurements of mesospheric Fe were conducted for 325 h during 75 nights at Urbana, Ill. (40°N, 88°W), in fall 1989 and from spring 1991 through summer 1992. The Fe layer abundance and root‐mean‐square (RMS) width have strong annual variations, with minima in summer. The abundance varied from 3.5 × 109 to 25 × 109 cm−2 with a mean of 10.6 × 109 cm−2, and the RMS width varied from 2.3 to 5.3 km with a mean of 3.4 km. The centroid height of the Fe layer has a strong semiannual variation, with minima at the solstices. The centroid varies from 86.0 to 90.3 km and has a mean of 88.1 km. Sporadic Fe (Fes) layers were present about 27% of the total observation time. The Fe measurements are compared with the extensive Na layer observations obtained during the past decade at Urbana and with common volume observations made simultaneously on 24 nights with a Na temperature lidar. The mean Fe column abundance is approximately twice the mean Na column abundance. The Fe layer centroid height is also on average nearly 4 km lower and the RMS width is approximately 24% narrower than the corresponding Na layer parameters. A chemical model of the mesospheric Fe layer is described and compared to various experimental results. The reaction of Fe with O3 to form FeO on the bottom side of the layer and the subsequent reaction of FeO with CO2 to form FeCO3 appear to be the dominant chemical sinks for Fe. The temperature dependency of the latter reaction may explain the annual variation in the column abundance. The lidar observations and the chemical model calculations suggest that the expected cooling of the mesopause region by approximately 10 K due to the doubling of CO2 and other greenhouse gases during the next century may reduce the mean Fe abundance by as much as 45% and the mean Na abundance by 55%.

Simultaneous observations of sporadic E, Na, Fe, and Ca+ layers at Urbana, Illinois: Three case studies

We describe three sporadic layering events observed in the mesospheric metal layers above Urbana, Illinois, using resonance fluorescence lidars and an MF radar. On June 6–7, 1991, a sporadic Na layer and a prominent temperature inversion layer were observed near 90 km. The formation of this low‐altitude event appears to involve the release of Na from a chemical reservoir or from the surfaces of dust and smoke particles, perhaps facilitated by the high temperatures in the inversion layer. On December 10, 1991, sporadic Fe and Ca+ layers formed near 94 and 95 km, respectively, and on December 17–18, 1991, sporadic E, Fe, Na, and Ca+ layers formed simultaneously near 100 km. The characteristics of these December events suggest that sporadic Fe and high‐altitude sporadic Na layers are formed by the neutralization of Fe+ and Na+ in the associated sporadic E layers.

Evidence for substantial seasonal variations in the structure of the mesospheric Fe layer

Lidar measurements of mesospheric Fe were conducted at Urbana, IL (40°N, 88°W) during the fall of 1989 and the spring through fall of 1991. The Fe layer abundance, centroid height and rms width varied substantially during the observation period with minimum values for all three parameters in summer. The abundance varied from 3.3×109 ‐ 25×109 cm−2, the centroid height from 86.7 – 90.4 km and the rms width from 2.1 – 3.9 km. Sporadic Fe (Fes) layers were also frequently observed. The Fe measurements are compared with the extensive Na layer observations obtained during the past decade at Urbana. The mean Fe column abundance is more than twice the average Na column abundance. The Fe layer is also on average approximately 3.6 km lower and 32 % narrower than the Na layer. The seasonal behavior of the Fe layer is discussed in light of our preliminary temperature measurements of the mesopause region as well as a simple chemical model.