A chemical model of meteoric ablation

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. DiffusionCorrespondence to: J. M. C. Plane (j.m.c.plane@leeds.ac.uk) of the most volatile elements (Na and K) does not appear to be rate-limiting except in the fastest meteoroids. Nonthermal 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.

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“…The majority of the incoming particles are too small to 89 generate visually detectable meteoroids (particles need to be larger than 1 mm radius), but 90 ionization of the ablated atoms by hyperthermal collisions with atmospheric molecules 91 forms electrons which enable detection by radar, either through specular reflection from 92 the resulting electron trail or by incoherent scatter from the ball of electrons around the 93 meteoroid termed the head echo (e.g. (Janches, et al, 2009) Vondrak, et al, 2008). An unablated particle will then sediment through the 99 atmosphere under gravit E 100 general atmospheric circulation and eventually reach the surface, where it is termed a 101 micrometeorite (MM).…”

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“…Finally, only for the least sensitive MU radar results and utilizing an extreme revision of the widely used ionization probability, the high velocity meteors become the dominant population in the distribution even though they are the minority of the incoming flux. It is important to note that the only atmospheric parameter affecting the ablation is the atmospheric density profile experienced by the incoming meteoroid as shown in Vondrak et al (2008). While the ambient plasma density may have an impact on the formation of meteor trails, in particular the nonspecular ones , it does not affect the head echo.…”

“…This would be consistent with the MU and PFISR radar detections. Na and K are particularly of interest because, as volatiles, they are the elements that will ablate more easily off the meteoroid's body according to differential ablation (Vondrak et al 2008;Janches et al 2009), and the most likely candidates to produce enough electrons when particles are small and slow to make the meteors visible by a radar (see Paper I).…”

“…Originally, Vondrak et al (2008) utilized the function derived by Jones (1997). In Paper I, we have found, however, that using this ip b , which is universally utilized in radar meteor research Janches et al 2009;Nesvorný et al 2011a;Weryk & Brown 2013, among some assorted type of investigations), ZoDy over-predicts the Arecibo observed rates by 10 to 20 times and the peak of the slow portion of the line of sight velocity distribution by 2 to 3 orders of magnitude.…”

“…Our approach utilized the Chemical Ablation Model (CABMOD) developed by Vondrak et al (2008) to calculate the electron production profiles as a function of altitude of a given particle, and the radar antenna Gain pattern, G, to estimate the meteor signal-to-noise ratio (S/N) as a function of the physical location of the particle's path through the radar beam. This approach results in the calculation of the probability that a particle would be detected by the radar as a function of its dynamical parameters, based on the specification of what portion of the illuminated radar volume in the Earth's mesosphere, where ablation occurs and meteors are produced, will have the sensitivity required to detect the given particle.…”