William J. McNeil

Differential Ablation of Cosmic Dust and Implications for the Relative Abundances of Atmospheric Metals

Although it is generally accepted that extraterrestrial material is the source of metals in the upper atmosphere, the relative abundances of mesospheric metal atoms and ions present us with a conundrum. Lidar observations have consistently shown that the abundances of neutral metals in the atmosphere and the abundances of these metals in the meteoric material that falls to Earth are significantly disproportionate. The column density of neutral sodium is perhaps 2 orders of magnitude larger than that of calcium, while the abundances in meteorites are approximately equal. By contrast, ion mass spectroscopy has shown that the abundances of the meteoric ions match reasonably well those in the meteorites. We present here a model that attempts to address these discrepancies. At the heart of the model is the concept of differential ablation, which suggests that more volatile metals sublimate earlier in the descent of a cosmic dust particle than do the less volatile components. We model three different meteoric metals: sodium, magnesium, and calcium. Results suggest that sodium ablates to a greater extent than does calcium and that it ablates at a substantially higher altitude. Deposition at lower altitudes leads to more rapid conversion of the atomic calcium into complexes through three-body reactions. Thus the depletion of calcium arises from both a decrease in deposition and an increase in the rate of removal of that which is deposited. We examine the behavior of the model in several respects, comparing predicted results with measurements and finding reasonable agreement. We argue that the success of this model indicates that differential ablation is a key factor in the determination of the relative abundances of meteoric metals in the mesosphere.

A statistical model of auroral ion precipitation: 2. Functional representation of the average patterns

In the study of Hardy et al. (1989) the average pattern of auroral ion precipitation was determined as a function of corrected geomagnetic latitude, magnetic local time (MLT), and Kp using data from the SSJ/4 detectors on the satellites of the Defense Meteorological Satellite Program (DMSP). One product of this study was a set of hemispheric maps of the integral number flux and integral energy flux of the precipitating ions for each of seven levels of activity as defined by Kp. In order to facilitate the use of these maps in magnetospheric, ionospheric, and thermospheric models, they have been fit to a functional form. To accomplish this, the latitudinal variation in the integral energy and number flux in each of 48 half-hour MLT bins of the original statistical study, for each Kp level, was fit to a general form of the Epstein function. This functional form was chosen since it fits well the asymmetric shape of the latitudinal profile of the integral energy and number flux. The Epstein function fits to the latitudinal profiles require six variables in each MLT zone. To further reduce the number of variables needed to specify a map, the MLT variation of each of the six variables was expanded as a sixth-order Fourier series with 13 coefficients. The resulting fit reproduces well the original statistical data. The distribution of differences between the original data and the fit normalized to the original data shows agreement that is generally better than 25% for both the integral energy and number flux.

A model for meteoric magnesium in the ionosphere

We present a model of neutral and ionized magnesium in the equatorial ionosphere at altitudes of 50–350 km. The model was developed in an attempt to understand recent spectroscopic measurements made from the space shuttle. The model incorporates ablation of cosmic dust particles, ion and neutral chemistry, diffusion, and vertical drift due to the equatorial electric field. We find that in the absence of electric field drift, there is little or no variation in Mg and Mg+ density profiles above 120 km. With the electric field included, the high-altitude behavior of the Mg+ ion changes dramatically. At dawn, almost all the ion density is confined to the 90- to 125-km altitude range, while at dusk there is significant ion density as high as 350 km. A radiance model is developed to allow for direct comparison with measurements. The model incorporates shielding of incident solar UV by ozone and variable local time of observation and tangential height of instrument line of sight. Comparisons between model and measurement are encouraging.

Comprehensive model for the atmospheric sodium layer

We present a steady state model of the midlatitude nighttime sodium layer which is comprehensive in the sense that it includes deposition of the metal, diffusion, and kinetics as well as the seasonal variations in the atmosphere. The model is used to compute seasonal variations in the layer parameters and the sodium nightglow. The behavior of the model reproduces well the observed behavior of the layer. We find that the main contributor to the seasonal variation is the temperature dependence of the reaction recycling atomic sodium from NaHCO 3 . Seasonal variations in the atmospheric minor species including ozone are secondary effects at best. The relative contributions of all factors are compared.

Impact of a major meteor storm on Earth's ionosphere: A modeling study

A comprehensive model of the effect of a major meteor storm on Earths ionosphere is presented. The model includes meteor stream mass distributions based on visual magnitude observations, a differential ablation model of major meteoric metals, Fe and Mg, and state-of-the-art modeling of the chemistry and transport of meteoric metal atoms and ions subsequent to deposition. Particular attention is paid to the possibility of direct ionic deposition of metallic species. The model is validated by calculating the effect of annual meteor showers on the background metal atom and ion abundances. A metallic ion density increase of up to 1 order of magnitude is observed, in agreement with in situ measurements during showers. The model is exercised for a hypothetical Leonid meteor storm of the magnitude reported in 1966. The model predicts the formation of a layer of metal ions in the ionospheric E region that reaches peak densities of around 1 × 105 cm−3, corresponding to a 2 order of magnitude increase of the quiescent nighttime E region density. Although sporadic E layers reaching or exceeding this density are relatively common, the effect is different in that it persists on the order of days and would be observed over nearly one-half the globe. The model predictions are consistent with the available 1966 Leonid storm data. In particular, the observation of enhanced, predawn sporadic E activity points to efficient collisional ionization of meteoric metals, as assumed in the model.

The 1996 Leonid shower as studied with a potassium lidar: Observations and inferred meteoroid sizes

We report on the observation and analysis of meteor trails that are detected by ground-based lidar tuned to the D 1 fine structure line of K. The lidar is located at Kuhlungsborn, Germany. The echo profiles are analyzed with a temporal resolution of about 1 s and altitude resolution of 200 m. Identification of meteor trails in the large archive of raw data is performed with help of an automated computer search code. During the peak of the Leonid meteor shower on the morning of November 17, 1996, we observed seven meteor trails between 0245 and 0445 UT. Their mean altitude was 89.0 km. The duration of observation of individual trails ranges from 3 s to ∼30 min. We model the probability of observing a meteor trail by ground-based lidar as a function of both altitude distribution and duration of the trails. These distributions depend on the mass distribution, entry velocity, and entry angle of the meteoroids, on the altitude-dependent chemical and dynamical lifetimes of the released K atom, and on the absolute detection sensitivity of our lidar experiment. From the modeling, we derive the statistical likelihood of detection of trails from meteoroids of a particular size. These bracket quite well the observed trails. The model also gives estimates of the probable size of the meteoroids based on characteristics of individual trails.

Hazards of Hypervelocity Impacts on Spacecraft

Hypervelocity impacts by space particles, such as meteoroids and debris, pose hazards to spacecraft. The limits of velocity of meteoroid and debris are derived. Characteristic properties of hypervelocity impacts are momentum transfer, penetration, plasma production, localization, and suddenness. Using McDonnell’ s empirical formulas derived from laboratory experiments, impact penetrations and plasma production rates in the space environment are calculated. When the critical temperature theorem for Maxwellian space plasmas is used, the energy of the plasma generated is shown to be too low to induce any signie cant spacecraft charging. The plasma generated, however, can induce a transient, sustained or avalanche discharge between differentially charged surfaces. The dischargecurrent depends notonly on theplasma density generated but also on theneutral gas released on impact. A scenario of impact induced hazard following days of passage of a high-energy plasma cloud, such as a coronal mass ejection cloud, is discussed. Some mitigation methods are discussed. Finally, we discuss whether electrons can be accelerated to high energies in a meteor trail.

The Role of Plasma Processes in the Space Shuttle Environment

Abstract Sources of enhanced ionization in the Space Shuttle environment are considered, with particular emphasis on the plausibility of a plasma discharge resulting from the Critical Ionization Velocity (CIV) mechanism. These sources are then compared with ion loss processes. It is found that within the shuttle environment the loss of ions is faster than ion production from CIV, leading to the conclusion that the observed enhancement in ionization arises from sources other than plasma interactions. It is suggested that the seemingly anomalous ionization levels can be attributed to firings of thrusters of the attitude control system. In addition, the connection between the shuttle glow phenomenon and the enhanced ionization is considered and found to be circumstantial and not causal, thereby strengthening the argument that the primary source of the shuttle glow is chemical interactions of atmospheric gases with surfaces or with surface-adsorbed gases.

Models of thermospheric sodium, calcium and magnesium at the magnetic equator

Abstract In attempting to interpret recent observations of metals in the lower thermosphere, we have developed several comprehensive models. These models explicitly include the deposition of the metal atoms from ablation of cosmic dust, time-dependant chemical reactions, and transport both by diffusion and by electric fields. The modeling presented here focuses on the depletion of the mesospheric column density of calcium relative to sodium neutrals, which is also observed in the thermospheric measurements. In an effort to obtain the measured abundances, we examine the consequences of differential ablation of the metal oxides comprising cosmic dust. In the ablation model, sodium is assumed to come off first while the less volatile calcium and magnesium remain in liquid form to be ablated later at somewhat lower altitudes. Model results show a strong dependance on the initial velocity of the cosmic dust particle, with an “average” cosmic dust velocity reproducing the measurements quite well.

Mg+ and other metallic emissions observed in the thermosphere

Limb observations of UV dayglow emissions from 80 to 300 km tangent heights were made in December 1992 using the GLO instrument, which flew on STS-53 as a Hitchhiker-G experiment. STS-53 was at 330 km altitude and had an orbit inclination of 57 degree(s). The orbit placed the shuttle near the terminator for the entire mission, resulting in a unique set of observations. The GLO instrument consisted of 12 imagers and 9 spectrographs on an Az/El gimbal system. The data was obtained over 6 days of the mission. Emissions from Mg+ and Ca+ were observed, as were emissions from the neutral metallic species Mg and Na. The ultimate source of the metals is ablation of meteors; however, the spatial distribution of the emissions is controlled by upper mesospheric and thermospheric winds and, in the case of the ions, by the electromagnetic fields of the ionosphere. The observed Mg+ emission was the brightest of the metal emissions, and was observed near the poles and around the geomagnetic equator near sunset. The polar emissions were short-lived and intense, indicative of auroral activity. The equatorial emissions were more continuous, with several luminous patches propagating poleward over the period of several orbits. The instrumentation is described, as are spatial and temporal variations of the metal emissions with emphasis on the metal ions. These observations are compared to previous observations of thermospheric metallic species.