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.
A sodium lidar at Syowa station (69°S, 39°E), Antarctica, measured temperature profiles from 80 to 110 km between March and September during 2000 and 2001. Monthly averaged temperature profiles in the mesopause region in wintertime (June, July and August) are nearly 20 K lower than those in the northern hemisphere sites. The measured winter mesopause altitudes (∼99 km) are in good agreement with those measured at Fort Collins (41°N) and Andøya (69°N) in winter. The mesopause temperatures (∼175 K) in winter months however are about 20 K lower than those observed from a northern hemisphere conjugate site, Andøya. The lower winter mesopause temperatures measured at Syowa station, which are consistent with southern hemisphere mesopause temperatures measured by a shipborne lidar, suggest the existence of a hemispheric difference. Possible causes for this difference are discussed.
The influences of the joints adjacent to the human tibia, the surrounding soft tissue and the fibula on the vibrational parameters of the tibia (that is natural frequency, mode shape and damping ratio) are in need of clarification. Using five cadaveric human legs, the resonant frequency and damping ratio were measured with an accelerometer placed on the medial tibial condyle while the medial malleolus was impacted by an impulse hammer. The legs were subsequently dissected and the measurements were made in six conditions. The mode shape was obtained by use of modal analysis. Hammer impacts were given at 15 points along the three surfaces of the tibia, while an accelerometer was attached to the surface of the medial malleolus. Resonant frequencies ranged from 275 to 405 Hz in the intact specimens and successively increased with removal of the skin, muscles and foot, and then decreased with the removal of the femur and the fibula. Damping ratios successively decreased. The transitional pattern of natural frequencies was similar to that of the resonant frequencies. The mode shape represented the first bending mode and it corresponded to the condition of the tibia in which the proximal and distal ends of the bone were free.
We report the first substantive inter‐site comparison of high southern latitude mesopause region winter temperatures. Davis (69°S, 78°E) hydroxyl rotational nightly‐mean temperatures are significantly correlated with Syowa (69°S, 39°E) sodium lidar nightly‐mean temperatures at 87 km made in 2000 (correlation coefficient of 0.68) and 2001 (0.51) despite a site separation of ∼1500 km. The Davis winter average temperature in 2001, 203 K, differs from the Syowa value of 201 K only by the uncertainty in the measurement. A more substantial 7 K separates the year 2000 winter averages, Davis again being warmer. The temperature difference between the sites is attributed to either variations in the hydroxyl layer or short‐term cooling events in the mesopause region above Syowa that are not detected above Davis. Syowa winter temperatures at 87 km are ∼7 K cooler than for an equivalent latitude northern hemisphere site.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.