James F. Vedder

Irreversible transport in the stratosphere by internal waves of short vertical wavelength

The U-2 aircraft was instrumented and flown in the stratosphere during the Stratosphere-Troposphere Exchange Projects experiments of April 1984 to provide a set of simultaneous measurements by fast responding sensors that would aid in the identification of the modes of cross-jet transport. The measurements confirm the preexperimental deductions that transport is dominated by waves, not by large-scale circulations. Monotonic gradients of trace constituents normal to the jet axis, with upper stratospheric tracers increasing poleward and tropospheric tracers increasing equatorward, are augmented by large-scale confluence as the jet intensifies during cyclogenesis. These gradients are rotated, intensified, and significantly increased in area as their mixing ratio surfaces are folded by the differential transport of a very low frequency, transverse wave. The quasi-horizontal transport produces a laminar structure with stable layers rich in upper stratospheric tracers alternating vertically with less stable layers rich in tropospheric tracers. The transport proceeds toward irreversibility as higher frequency, shear-gravity waves extend the folding to smaller horizontal scales. It becomes irreversible when these short waves actually fold the isentropic surfaces and small-scale mixing develops. The progression to higher wave numbers is a discrete, not a continuous, cascade with major gaps in the observed horizontal wavelengths. The wave modes are identified by matching the observed amplitudes and phases against those obtained by linear perturbation theory. Prior to mixing, the wave-generated perturbations maintain the correlations produced by advecting the larger-scale mean gradients; thus the high resolution measurements support the linear turbulence closure assumption.

Lunar microcraters: Implications for the micrometeoroid complex

Abstract The contributions of lunar microcrater studies to understand the overall micrometeoroid environment are summarized and compared to satellite data. In comparison with small-scale laboratory studies, most lunar crater morphologies are compatible only with impact velocities > 3·5 km/sec and projectile densities between 1–8 g/cm 3 ; a mean value is most likely 2–4 g/cm 3 . The particles arenon-porous and fairly equi-dimensional; needles, platelets, rods, whiskers and other highly asymmetric particle shapes can be excluded. Data on projectile chemistry is sparse and non-diagnostic at present. The crater diameters are converted into projectile masses via small scale laboratory impact experiments. Accordingly, the observed span of crater pit diameters (0·1 μm–1 cm) corresponds to a particle mass range of ≈ 10 −15 –10 −3 g. This large, dynamic detection range is a unique feature of the lunar rock detector. Absolute crater densities on different rocks vary from “production” to “equilibrium” conditions. After normalization of such densities, relative microcrater size frequencies are obtained to deduce a mass frequency distribution for particles 10 −15 –10 −3 g. There is evidence that this distribution is bimodal. A radiation pressure cutoff at 10 −12 g particle mass does not exist. The micrometeoroid flux obtained from lunar rocks is compatible with satellite data. There is indication that the micrometeoroid flux may have been lower in the past. Some speculative astronomical consequences concerning the origin of micrometeoroids are discussed.

On the age of stratospheric air and ozone depletion potentials in polar regions

Observations of the nearly inert, man-made chlorofluorocarbon CFC-115 obtained during January 1989 are used to infer the age of air in the lower stratosphere. These observations together with estimated release rates suggest an average age of high-latitude air at pressure altitudes near 17-21 km of about 3 to 5 years. This information is used together with direct measurements of HCFC-22, HCFC-142b, CH{sub 3}Br, H-1301, H-1211, and H-2402 to examine the fractional dissociation of these species within the Arctic polar lower stratosphere compared to that of CFC-11 and hence to estimate their local ozone depletion potentials in this region. It is shown that these HCFCs are much less efficiently dissociated within the stratosphere than CFC-11, lowering their ozone depletion potentials to only about 30-40% of their chlorine loading potentials. In contrast, the observations of CH{sub 3}Br and the Halons considered here confirm that they are rapidly dissociated within the stratosphere, with important implications for their ozone depletion potentials. 20 refs., 4 figs., 3 tabs.

In Situ Northern Mid-Latitude Observations of ClO, O3, and BrO in the Wintertime Lower Stratosphere

In order to test photochemical theories linking chlorofluorocarbon derivatives to ozone(O3) depletion at high latitudes in the springtime, several related atmospheric species, including O3, chlorine monoxide(ClO), and bromine monoxide (BrO) were measured in the lower stratosphere with instruments mounted on the NASA ER-2 aircraft on 13 February 1988. The flight path from Moffett Field, California (37�N, 121�W), to Great Slave Lake, Canada (61�N, 115�W), extended to the center of the polar jet associated with but outside of the Arctic vortex, in which the abundance of O3 was twice its mid-latitude value, whereas BrO levels were 5 parts per trillion by volume (pptv) between 18 and 21 kilometers, and 2.4 pptv below that altitude. The ClO mixing ratio was as much as 65 pptv at 60�N latitude at an altitude of 20 kilometers, and was enhanced over mid-latitude values by a factor of 3 to 5 at altitudes above 18 kilometers and by as much as a factor of 40 at altitudes below 17 kilometers. Levels of ClO and O3 were highly correlated on all measured distance scales, and both showed an abrupt change in character at 54�N latitude. The enhancement of ClO abundance north of 54�N was most likely caused by low nitrogen dioxide levels in the flight path.

Microcraters formed in glass by low density projectiles

Abstract Microcraters were produced in soda-lime glass by the impact of low density projectiles of polystyrene (ρ = 1.06 g/cm 3 ) with masses between 0.7 and 62 picograms and velocities between 2 and 14 km/s. The morphology of the craters depends on the velocity and angle of incidence of the projectiles. For normal incidence at 3 km/s, the projectile leaves a dent; and between 3.5 and at 4.6 km/s, a continuous lip is formed. For velocities greater than 5.2 km/s at normal incidence, an extensive spallation zone surrounds the central pit; the ratio of the central pit diameter to the projectile diameter ( D C /d ) increases from 1.25 at 5.2 km/s to 1.75 at 14 km/s; and D C /d is independent of projectile mass for constant velocity. The transitions in morphology of the craters formed by polystyrene spheres occur at higher velocities than they do for more dense projectiles. For oblique impact, the craters are elongated and shallow with the spallation threshold occurring at higher velocity. For normal incidence, the total displaced mass of the target material per unit of projectile kinetic energy increases slowly with the kinetic energy of the projectile according to the relation: M e =230 E 1.1 picogram.

Microcraters in glass and minerals

Abstract The morphology of craters formed in glass, oligoclase, and olivine by micron-size projectiles with velocities up to 14 km/sec changes with the velocity and angle of incidence of the projectile. An energy-dispersive X-ray analysis of the crater provides information about the projectile composition in some cases. These results are an aid in the interpretation of microcraters observed on lunar samples.

Accretionary Processes in the Early Solar System: An Experimental Approach

Micrometer-size silicate flakes do not accrete during impacts in the velocity range 1.5 to 9.5 kilometers per second. Conventional accretionary theories for silicate bodies are applicable only to particles whose orbits are similar. Metal-silicate fractionation in the solar system may have been affected by differences in the accretionary behavior of the metal and silicate particles.

Microparticle accelerator of unique design

A microparticle accelator of unique design, which produces high-velocity, micrometer-sized projectiles of any cohesive material, is described. In the source, an electrodynamic levitator, single particles are charged by ion bombardment in high vacuum. The vertical accelerator has four drift tubes, each initially at a high negative voltage. After injection of the projectile, each tube is grounded in turn at a time determined by the voltage and charge/mass ratio to give four acceleration stages with a total voltage equivalent to about 1.7 MV. The delay times may be set manually or controlled automatically by the particles charge/mass ratio measured in the source by the operator just before ejection. At the entrance to the accelerator, the particle generates a signal that initiates the timing sequence. In the target chamber, detectors record the passage of the particle and provide information on charge, velocity, and position. Trajectories usually pass within a 1-mm-radius circle 1 m below the fourth drift tube. Velocities between 0.5 and 15 km/s have been attained with projectiles of various materials and shapes for cratering studies and calibration of micrometeoroid detectors. About 20 projectiles per day can be accelerated.

Microcraters formed in hot glass by hypervelocity projectiles

Microcraters were formed in heated soda-lime glass by the normal incidence of spheres of plastic or fused silica with diameters between 0.8 and 4.5μm and velocities between 2.5 and 10 km s−1. The morphology of the craters in targets at temperatures up to 800°C is little different from those formed in unheated glass. Spallation still occurs to the same extent and above the same velocity threshold, but the spalls sag and sharp edges become dull in a few seconds at temperatures above the softening point. There is a small increase in the flow of glass from the central pit into a narrow lip at the higher temperatures, but this lip is often removed by spallation, especially at the higher velocities of impact. There is no evidence of a splashed lip with strings of melt overlying the spalled area. The results in conjunction with other evidence suggest that most lunar craters of micrometer size with a smooth central pit, splashed lip, and a spallation zone are the result of primary impacts.