Radar Detectability Studies of Slow and Small Zodiacal Dust Cloud Particles. III. The Role of Sodium and the Head Echo Size on the Probability of Detection

Abstract

We present a path forward on a long-standing issue concerning the flux of small and slow meteoroids, which are believed to be the dominant portion of the incoming meteoric mass flux into the Earth's atmosphere. Such a flux, which is predicted by dynamical dust models of the Zodiacal Cloud, is not evident in ground-based radar observations. For decades this was attributed to the fact that the radars used for meteor observations lack the sensitivity to detect this population, due to the small amount of ionization produced by slow-velocity meteors. Such a hypothesis has been challenged by the introduction of meteor head echo (HE) observations with High Power and Large Aperture radars, in particular the Arecibo 430 MHz radar. Janches et al. developed a probabilistic approach to estimate the detectability of meteors by these radars and initially showed that, with the current knowledge of ablation and ionization, such particles should dominate the detected rates by one to two orders of magnitude compared to the actual observations. In this paper, we include results in our model from recently published laboratory measurements, which showed that (1) the ablation of Na is less intense covering a wider altitude range; and (2) the ionization probability, b ip , for Na atoms in the air is up to two orders of magnitude smaller for low speeds than originally believed. By applying these results and using a somewhat smaller size of the HE radar target we offer a solution that reconciles these observations with model predictions.

Meteoric Metal Chemistry in the Martian Atmosphere

“…Thus, it appears that at least 50% of the magnesium needs to be injected as Mg + . This implies an unexpectedly high probability of ionization for hyperthermal collisions of Mg with CO 2 , compared to what is expected for Mg colliding with O 2 and N 2 (Janches et al, 2017). In the case of Fe, 50% ionization occurs for collisions with CO 2 at speeds in excess of~35 km s À1 (Thomas et al, 2016), which is relatively high for cosmic dust particles entering the Martian atmosphere.…”

The Impact of Comet Siding Spring's Meteors on the Martian Atmosphere and Ionosphere

“…One effect of the relatively high entry velocity of the Siding Spring dust is that a significant fraction of the evaporating metal atoms would have ionized immediately through hyperthermal collisions with CO 2 molecules. Using the ionization efficiencies recently reported by Janches et al (2017),~60% of the Mg atoms, 60% of the Fe, and 90% of the Na would have ionized. This is in contrast to the persistent layer, which is mostly ionized through charge exchange with O 2 + (Molina-Cuberos et al, 2003).…”

Meteoroids at the Moon: Orbital Properties, Surface Vaporization, and Impact Ejecta Production

“…It is important to note that we use these values because they are the latest published estimates of these quantities. However, they remain a debated quantity as currently they overpredict measurements by sensitive radars, in particular those from JFCs meteoroids (Janches et al, ). Nevertheless, the results presented here can be scaled if revised estimates of the population mixing ratios are reported in the future.…”