Abstract

Abstract. Most of the extraterrestrial dust entering the Earth's atmosphere ablates to produce metal vapours, which have significant effects on the aeronomy of the upper mesosphere and lower thermosphere. A new Chemical Ablation Model (CAMOD) is described which treats the physics and chemistry of ablation, by including the following processes: sputtering by inelastic collisions with air molecules before the meteoroid melts; evaporation of atoms and oxides from the molten particle; diffusion-controlled migration of the volatile constituents (Na and K) through the molten particle; and impact ionization of the ablated fragments by hyperthermal collisions with air molecules. Evaporation is based on thermodynamic equilibrium in the molten meteoroid (treated as a melt of metal oxides), and between the particle and surrounding vapour phase. The loss rate of each element is then determined assuming Langmuir evaporation. CAMOD successfully predicts the meteor head echo appearance heights, observed from incoherent scatter radars, over a wide range of meteoroid velocities. The model also confirms that differential ablation explains common-volume lidar observations of K, Ca and Ca+ in fresh meteor trails. CAMOD is then used to calculate the injection rates into the atmosphere of a variety of elements as a function of altitude, integrated over the meteoroid mass and velocity distributions. The most abundant elements (Fe, Mg and Si) have peak injection rates around 85 km, with Na and K about 8 km higher. The more refractory element Ca ablates around 82 km with a Na:Ca ratio of 4:1, which does therefore not explain the depletion of atomic Ca to Na, by more than 2 orders of magnitude, in the upper mesosphere. Diffusion of the most volatile elements (Na and K) does not appear to be rate-limiting except in the fastest meteoroids. Non-thermal sputtering causes ~35% mass loss from the fastest (~60–70 km s−1) and smallest (10−17–10−13 g) meteoroids, but makes a minor contribution to the overall ablation rate.

Highlights

  • There are two principal sources of meteoroids in the Earth’s atmosphere (Ceplecha et al, 1998; Williams, 2002)

  • In the last few years, high-powered large aperture (HPLA) radars have reported direct observations of the meteor head echo

  • In this paper we describe a new Chemical Ablation Model (CAMOD), which contains the following processes: sputtering by inelastic collisions with air molecules before the meteoroid melts; evaporation of atoms and oxides from the molten particle; diffusion-controlled migration of the volatile constituents (Na and K) through the molten particle; and impact ionization of the ablated fragments by hyperthermal collisions with air molecules

Read more

Summary

Introduction

There are two principal sources of meteoroids in the Earth’s atmosphere (Ceplecha et al, 1998; Williams, 2002). In the last few years, high-powered large aperture (HPLA) radars have reported direct observations of the meteor head echo (i.e. the ball of plasma around the ablating particle as it descends through the atmosphere) This enables measurements of the direction of the line-of-sight velocity, deceleration and meteoroid mass to be made A consensus around 20–40 t d−1 has emerged in the last 5 years, but this remains highly uncertain and is in any case a crude average that does not take account of seasonal and latitudinal variations (Janches et al, 2006; Fentzke and Janches, 2008) Because of their very high entry velocities, meteoroids undergo rapid frictional heating by collision with air molecules, and their constituent minerals subsequently vaporize. We will explore the curious finding that multiple metal resonance lidar observations of the same section of an individual meteor trail rarely observe more than one metal (von Zahn et al, 1999, 2002)

Thermal ablation
Non-thermal mass loss
Ionization of the ablated fragments
Diffusion-controlled ablation of alkali metals
Vapour – molten meteoroid thermodynamic equilibrium
Integration and the initial distribution of the meteoroids
Results and discussion
Heat transport in the meteoric particle
Melting of the meteoroid
Diffusion-controlled ablation of Na and K
Elemental ablation profiles in the atmosphere
Differential ablation of Ca: sensitivity to model parameters
Conclusions
Full Text
Paper version not known

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call