[1] Abstract: Earth's near-surface layer, its lithosphere, is broken into quasi-rigid plates that form the upper thermal boundary layer for mantle convection. Since the discovery of plate tectonics, it has been widely conjectured but only recently demonstrated that this peculiar style of convection may be facilitated by an upper mantle low viscosity zone (LVZ) over which the plates glide easily. The LVZ, or ''asthenosphere,'' concept dates from 19th century investigations of isostatic support of mountain belts and is supported by modern evidence for a seismic low velocity zone and by studies of postglacial rebound and dynamic compensation of the Earth's gravity field. Here we show in both two-dimensional (2-D) Cartesian and 3-D spherical Earth models that combining a pronounced LVZ and a plastic yield stress to allow localized weakening of the cold thermal boundary layer results in a distinctly plate tectonic style of convection, with $30% toroidal surface motion for the 3-D case. Recycling of water into the upper mantle at subduction zones is a plausible cause of Earth's LVZ, whereas Venus is dry and lacks both an LVZ and plate tectonics.

Abstract. Vertical heat flux and cooling rate profiles of the mesopause region resulting from dynamic processes were computed using wind and temperature measurements made

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Abstract. Sixty-five hours of Na lidar observations of vertical and horizontal winds, temperature, and Na density were obtained during eight different nights in 1994 and 1995 at the Starfire Optical Range, New Mexico, using a 3.5 rn diameter telescope. The highresolution data are used to study the spectra of gravity wave perturbations in the mesopause region. Wave activity was strong during the observations. The average variances of temperature, relative atmospheric density, horizontal wind, and vertical wind were 80 K 2, 28 (%)2, 1100 m2/s 2, and 4.3 m2/s 2, respectively. The temperature, relative density, and horizontal wind spectra are generally consistent with the large body of published measurements and with the predictions of gravity wave theory. The observed temporal frequency (c0 o) and vertical wave number (m) spectra of vertical winds are both very shallow. The indices of the co o spectra vary between-0.59_+0.13 and-1.2_+0.09, and the mean value is -0.76. The indices of the rn spectra vary between-0.83_+0.04 and -1.48+_0.03, and the mean value is -1.1. In contrast, the indices of the horizontal wind m spectra vary between-2.8+_0.10 and-3.2+0.13 with a mean of-3.0. These large differences imply that the underlying intrinsic spectra are not separable. However, the observed vertical wind rn spectra are not consistent with the nonseparable theories which predict index values near + 1. By using mathematical and numerical models, we show that the observed rn spectra are distorted by Doppler and critical layer effects associated with the height-varying mean wind field. This distortion is greatest at high values of rn and leads to observed vertical wind rn spectra which are steeper than the underlying intrinsic spectra. Although the intrinsic spectra are definitely shallower (i.e., indices more positive) than the observations, it is not possible to determine if the measurements are entirely consistent with any of the nonseparable wave dissipation theories.

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