Chester S. Gardner

Analytical models for the responses of the mesospheric OH* and Na layers to atmospheric gravity waves

Analytic models are developed to describe gravity wave induced perturbations in the high ν OH* Meinel Band emissions and in atomic Na density. The results are used to predict the fluctuations in OH* intensity and rotational temperature, Na abundance, and the centroid heights of the OH* and Na layers. The OH* model depends critically on the assumed form for the atomic oxygen profile. In this study the O profile is modeled as a Chapman layer, which is in excellent agreement with MSIS-90. We also neglect the wave-induced redistribution of O3 because the chemical lifetime of ozone in the mesopause region is short compared to most gravity wave periods. Under these conditions the OH* response is ΔV/Vu ≃ −3[1 - (z - zOH)hOH + (z − zOH)2/σ12]Δρ/ρu, where ΔV/Vu are the relative emission rate fluctuations, Δρ/ρu are the relative atmospheric density fluctuations, zOH ≃ 89 km is the layer centroid height, hOH ≃ 3.6 km, and σ1 ≃ 8.0 km. By using these results, we show that cancellation of the induced perturbations in emission intensity and rotational temperature is significant for short vertical wavelengths. The amplitude attenuation in both parameters is proportional to exp(−m2σ2OH/2), where m = 2π/λz and σOH ≃ 4.4 km is the rms thickness of the OH* layer. For example, at λz = 15 km, the predicted rotational temperature perturbation is only 20% of the atmospheric temperature perturbation. Because the most sensitive instruments are only capable of accuracies approaching ±2 K, there are few reported observations of waves with λz ≤ 15 km. The cancellation effects are not as limiting in OH intensity observations because the relative intensity perturbations are larger than the relative temperature perturbations, and intensities can be measured more accurately than temperature, especially with broadband instruments. Fluctuations in the emission rate are largest on the bottomside of the OH* layer, ∼ 3.75 km below the layer peak (∼89 km), where the effects due to the redistribution of atomic oxygen dominate. Fluctuations in rotational temperature are largest near the peak of the OH layer, where the volume emission rate is largest. The ∼3.75 km separation between the maxima of the intensity and rotational temperature perturbations is largely responsible for the phase differences observed in the fluctuations of these parameters. Rotational temperature and Krassovskys ratio are found to be very sensitive to the form of the background temperature profile. Wave-induced OH* layer centroid height fluctuations coupled with the mean lapse rate of the background temperature profile can contribute significantly to the observed rotational temperature fluctuations, especially for the shorter wavelength waves λz ≤ 15 km. The OH* intensity fluctuations are relatively insensitive to the temperature profile as well as variations in atomic oxygen density and therefore appear to be excellent tracers of gravity wave dynamics. OH temperature observations are best suited for studying long-period waves, including tides, with λz ≥ 15 km.

Wave breaking signatures in OH airglow and sodium densities and temperatures: 1. Airglow imaging, Na lidar, and MF radar observations

The Collaborative Observations Regarding the Nightglow (CORN) campaign took place at the Urbana Atmospheric Observatory during September 1992. The instrumentation included, among others, the Aerospace Corporation narrowband nightglow CCD camera, which observes the OH Meinel (6–2) band (hereafter designated OH) and the O2 atmospheric (0–1) band (hereafter designated O2) nightglow emissions; the University of Illinois Na density/temperature lidar; and the University of Illinois MF radar. Here we report on observations of small-scale (below 10-km horizontal wavelength) structures in the OH airglow images obtained with the CCD camera. These small-scale structures were aligned perpendicular to the motion of 30- to 50-km horizontal wavelength waves, which had observed periods of about 10–20 min. The small-scale structures were present for about 20 min and appear to be associated with an overturned or breaking atmospheric gravity wave as observed by the lidar. The breaking wave had a horizontal wavelength of between 500 and 1500 km, a vertical wavelength of about 6 km, and an observed period of between 4 and 6 hours. The motion of this larger-scale wave was in the same direction as the ≈30- to 50-km waves. While such small-scale structures have been observed before, and have been previously described as ripple-type wave structures [Taylor and Hapgood, 1990], these observations are the first which can associate their occurrence with independent evidence of wave breaking. The characteristics of the observed small-scale structures are similar to the vortices generated during wave breakdown in three dimensions in simulations described in Part 2 of this study [Fritts et al., this issue]. The results of this study support the idea that ripple type wave structures we observe are these vortices generated by convective instabilities rather than structures generated by dynamical instabilities.

Ranging performance of satellite laser altimeters

Topographic mapping of the Earth, Moon, and planets can be accomplished with high resolution and accuracy using satellite laser altimeters. These systems employ nanosecond laser pulses and microradian beam divergences to achieve submetre vertical range resolution from orbital altitudes of several hundred kilometres. In this paper the authors develop detailed expressions for the range and pulsewidth measurement accuracies and use the results to evaluate the ranging performances of several satellite laser altimeters currently under development by NASA for launch during the next decade. The analysis includes the effects of the target surface characteristics, spacecraft pointing jitter, and waveform digitizer characteristics. The results show that ranging accuracy is critically dependent on the pointing accuracy and stability of the altimeter, especially over high-relief terrain where surface slopes are large. At typical orbital altitudes of several hundred kilometres, single-shot accuracies of a few centimetres can be achieved only when the pointing jitter is on the order of 10 mu rad or less. >

Mesospheric Na layer at 40°N: Modeling and observations

A complete monthly record of the annual variation of Na and temperature in the upper mesosphere has been obtained from 3 years of nighttime lidar observations at two midlatitude sites, Urbana-Champaign, Illinois (40°N), and Fort Collins, Colorado (41°N). The Na density exhibits a strong annual variation at all heights between 81 and 107 km, with the column abundance of the layer peaking in early winter and then decreasing by nearly a factor of 4 to a midsummer minimum. There are also significant semiannual components to the variations in the centroid height and thickness of the layer. The nighttime temperature profile between 81 and 105 km exhibits a high winter mesopause at about 101 km and a summer mesopause at about 85 km. During spring and autumn, the mesopause oscillates apparently randomly between these states. A seasonal model of the Na layer was then constructed incorporating recent laboratory studies of the pertinent neutral and ionic reactions of the metal. The background atmospheric composition was provided from three off-line models, as well as from UARS/Microwave Limb Sounder satellite measurements of H 2 O. With a small number of permitted adjustable parameters, the model is able to reproduce many observed features of the Na layer remarkably well, including the monthly variation in column abundance and layer shape. The biggest discrepancy is during midsummer, when the modeled layer is displaced 2-3 km above that observed, although a factor contributing to this is that the lidar observing period during summer was relatively short and the effect of the diurnal tide could have been incompletely sampled. Both the observations and the model show that Na density and temperature are highly correlated below 96 km (correlation coefficient equal to 0.8-0.95), mostly as a result of the influence of odd oxygen/hydrogen chemistry on the partitioning of sodium between atomic Na and its principal reservoir species, NaHCO 3 . Above 96 km, a weak negative correlation (-0.2) is explained by the dominance of ion-molecule chemistry. Finally, it was shown that if the eddy diffusion coefficient in the middle mesosphere is significantly smaller or if the global meteoric influx is much larger than the values used in the present model, then processes for permanently removing gas-phase Na species in the mesosphere, such as polymerisation and deposition onto dust particles, will need to be included.

A review of the mesosphere inversion layer phenomenon

An active topic of current research in aeronomy is the study of the dynamics of the mesosphere and lower thermosphere (MLT) from 60 to 130 km, especially in regard to the influences that govern variability. The physical processes of this region are diverse and complex with strong coupling between the MLT and the adjacent atmospheric regions brought about largely by the propagation and dissipation of atmospheric gravity waves (GWs) from sources above and below. The measurements of MLT winds and temperatures required for such studies represent daunting technical challenges. At low and midlatitudes the mesosphere inversion layer (MIL) phenomenon, a ∼10 km wide region of enhanced temperatures (ΔT ∼ 15–50K), is observed with great regularity in both the upper mesosphere (60–70 km) and the mesopause (90–100 km). Observations are largely based upon Rayleigh and Na temperature lidar systems but coherent radar observations have shown that the MIL phenomenon is linked to layers of turbulence occurring in both the topside and the bottomside regions. GW activity is believed to play an important role in the development of a linkage between the MIL and the tidal structure through GW coupling that results in an amplification of the tidal thermal structure. This linkage is readily evident for the upper MIL but is seen only occasionally for the lower MIL. Further study of MIL properties should emphasize continual 24 hour temperature observations, especially for the lower MIL region, to confirm the linkage of the development of the MIL to the MLT tidal structure.

Lidar, radar and airglow observations of a prominent sporadic Na/sporadic E layer event at Arecibo during AIDA-89

Abstract Sporadic Na (Na,) layer events were frequently identified during 160 h of lidar observations at Arecibo in January, March and April 1989. Most were accompanied by sporadic E ( E .) layers. The most spectacular Na s E s , event occurred on the night of 30–31 March when both the Na and electron abundances between 90 and 100 km increased by approximately 700% during a period of 2.25 h starting at 2100 LST. The maximum Na density was almost 42,000 cm 3 . The vertical and temporal structure of the Na and electron densities were remarkably similar during the event. The ratio of the average Na enhancement to the electron density varied from a maximum of 3.5 Na atoms/electron at 98 km to about 0.5 Na atoms/ electron below 94 km. Between 93 and 97 km the electron enhancement preceded the Na enhancement by 15–30 min. Above 97 km and below 93 km the Na and electron density variations were in phase. The data suggest that the E s , layer triggered the release of Na from a reservoir, but the E s layer was not the source of the major Na s layer. Two minor Na s layers were observed between 101 and 107 km after midnight LST which were also accompanied by intense s layers and enhancements of the O( 1 S) emission intensities. The abundances of these high altitude Na s layers were less than 1% of the electron abundances. These Na, layers appear to be caused by the conversion of Na in the E s layer to Na through a set of clustering reactions involving N 2 CO 2 and H 2 O.

Performance analysis of adaptive-optics systems using laser guide stars and sensors

Many current wave-front-reconstruction systems use localized phase-slope measurements to estimate wave fronts distorted by atmospheric turbulence. Analytical expressions giving the performance of this class of adaptive-optics system are derived. Performance measures include the mean-square residual phase error across the aperture, the optical transfer function, the point-spread function, and the Strehl ratio. Numerical examples show that the mean-square residual error and the Strehl ratio are sensitive to variations of the photon noise in the wave-front sensor and to variations in the sensor spacing and the actuator spacing. The Strehl ratio degrades rapidly as the diameters of the individual slope sensors are made larger than the Fried seeing-cell diameter r0 and when the sensor signal levels fall below 100 counts per slope measurement. On the other hand, the resolution of the optical system is relatively unaffected by moderate changes in the photon noise or the densities of sensors and actuators. The diameter of the individual slope sensors can be as much as 1.5 times r0 without significant degradation in angular resolution. These performance measures are particularly important in the design of adaptive telescopes used for imaging in astronomy. For adaptive telescopes using laser guide stars, these measures can be used to determine the key design parameters for the laser.

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%.

Lidar observations of the meteoric deposition of mesospheric metals.

The mesospheric sodium and iron layers at an altitude between about 80 and 110 kilometers are routinely monitored by atmospheric physicists using resonance fluorescence lidar techniques because these constituents are excellent tracers of mesopause chemistry and dynamics. The mesospheric metals are the products of meteoric ablation. Existing ablation profiles are model calculations based in part on radar observations of the ionized background atmosphere left in the wake of high-speed (> 20 kilometers per second) meteoroids. Thin trails of neutral metal atoms, ablated from individual meteoroids, are occasionally observed with high-power lidars. The vertical distribution of 101 sodium and 5 iron meteor trails observed during the past 4 years at Urbana, Illinois; Arecibo, Puerto Rico; and near Hawaii is approximately Gaussian in shape with a centroid height of 89.0 (� 0.3) kilometers and a root-mean-square width of 3.3 (� 0.2) kilometers. This directly measured ablation profile is nearly the same as the mean iron layer profile but is considerably different from existing models and the distribution of ionized meteor trails observed by radars. A lower limit on the influx to the mesopause region from the lidar meteors is approximately 1.6 x 103 sodium and 2.7 x 104 iron atoms per second per square centimeter, which corresponds to an annual flux of meteoric debris into the mesosphere of about 2.0 (�0.6) gigagrams. Because the lidars can detect only the ablation trails left by the larger meteors, the observations suggest that the actual meteoric influx may be larger than the more recently reported values, which range between 16 and 78 gigagrams per year.

Lidar observations of the temperature profile between 25 and 103 km: Evidence of strong tidal perturbation

Simultaneous observations of the middle atmosphere and lower thermosphere by Na Wind/Temperature (W/T) and Rayleigh/Raman lidar and other ground-based measurements were conducted the night of October 21, during the ALOHA-93 campaign. Rayleigh/Raman lidar measurements provide density from 25 to 90 km and temperature from 25 to 85 km. From the same location, Na W/T lidar measurements were also obtained between ∼83 and 103 km. The combined data provides continuous temperature from 25 to 103 km. Strong perturbations which may be associated with the diurnal tide are observed in the temperature and wind profiles.