Hydrodynamic and hydromagnetic stability

[1] Since 1958, over 400 chemical tracer measurements have been made of the wind profiles in the upper mesosphere and lower thermosphere. The measurements cover a wide range of latitudes, longitudes, seasons, and local times. The results from the analysis of the data set show that a wind maximum in the altitude range between 100 and 110 km is a consistent feature of the observations at midlatitudes and low latitudes. The wind speed associated with the maximum exceeds 100 m s−1 in over 60% of the observations. Large shears are found, especially on the bottomside of the wind maximum, and the observed shears are often large enough to be either unstable or approaching instability. The wind maximum and shear features are characteristics of the overall data set and are not tied to particular observation sites, particular seasons, or special conditions.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2001JA000218

Winds and shears in the mesosphere and lower thermosphere: Results from four decades of chemical release wind measurements

[1] For over 30 years it has been recognized that atmospheric gravity waves (AGWs) in the 80–110 km region significantly perturb the basic atmospheric state, perhaps causing instability regions and subsequent turbulence. It has also been recognized for nearly as long that AGWs cause strong fluctuations in the airglow emissions that originate in this altitude region. Airglow images have been obtained since 1973, and they have shown structures that have mainly been attributed to the passage of AGWs through this region as predicted theoretically. The AGWs have been assumed to originate largely in the troposphere because of either convective activity or the flow of air over large mountain ranges. However, intensive analysis of the properties of a class of small-scale features currently known as ripples [Taylor et al., 1997; Nakamura et al., 1999] suggests that these features are not AGWs but are rather instability features generated in situ. The basis for this hypothesis is examined in this review, and it is concluded that while there is support for the instability hypothesis as the origin of ripple features, at present the exact nature of the instabilities causing these features is not known.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2003RG000131

Instability layers and airglow imaging

Since February 1985, the 2-MHz narrow beam radar near Adelaide, Australia, has been used to measure the short-term root-mean-square fluctuating velocities of radio wave scatterers in the altitude range 80–100 km. Data have been recorded about every 2 to 3 hours. This is in addition to the routine measurement of winds in the region between 60 and 100 km altitude. Under certain assumptions, these fluctuating velocities can then be converted to turbulent energy dissipation rates and eddy diffusion coefficients. These measurements are presented, and their interpretation as turbulence is discussed. The possibility of contamination from high frequency, short wavelength gravity waves is considered and a correction procedure to allow for such contamination is developed. It is concluded that the corrected measurements represent a good indication of turbulence activity. Between 84 and 92 km, the turbulence intensities were approximately constant with increasing height, but between 80 and 84 km turbulence was relatively weak. Seasonal variations are discussed, and important interannual differences between 1985 and 1986 are noted.

Two years of continuous measurements of turbulence parameters in the upper mesosphere and lower thermosphere made with a 2‐MHz radar

Mass densities derived from drag data are compared with densities determined by mass spectrometers, EUV extinction measurements, and observations of the growth of chemiluminescent clouds for an altitude of 150 km. Drag densities fall very close to the Cira 1965 mean atmosphere and are 10 to 20% higher than the average for all rocket measurements discussed. Mass spectrometric and EUV extinction measurements agree quite well for n(N2) and n(O2) but differ significantly for n(O). In order to derive an improved set of parameters of thermospheric density and composition, it is proposed that the Cira 1965 mean density be lowered by 10%, that the measured n(N2) and n(O2) number densities be maintained, and that n(O) be chosen such that the number densities become consistent with the mass density p. This results in the following reference set for 150-km altitude: ρ = 1.96 × 10−12 g cm−3; n(N2) = 2.6 × 1010 cm−3; n(O) = 2.3 × 1010 cm−3; n(O2) = 2.5 × 109 cm−3; n(Ar) = 5 × 107 cm−3; mean molecular weight M = 22.9 g mole−1.

K.H. Lloyd

Papers

Turbulence, billows and gravity waves in a high shear region of the upper atmosphere

Absorption of a grazing ray calculated by ray tracing

Calculation of the effective height of the Earth's shadow

Atmospheric structure in the lower thermosphere

Reply to comments by D. Layzer and J.F. Bedinger on: Turbulence, billows and gravity waves in a high shear region of the upper atmosphere