Coordinated Optical and Radar measurements of Low Velocity Meteors
Abstract
To better estimate which luminous e fficiency ( τ) value is compatible with contemporary values of the ionization coe fficient (β), we report a series of simultaneous optical and specular echo radar measurements of low speed (v < 20 km/s) meteors. We focus on the low speed population as secondary ionization is not relevant and the initial trail radii are small, minimizing model assumptions required to estimate electron line density. By using the large decrease in expected ionization coe fficient at such low speeds, we attempt to better define the likely ratio of photon to electron production. This provides an estimate of the probable luminous e fficiency, given that recent lab measurements of ionization e fficiency agree with established theory (Jones, 1997; DeLuca et al., 2018) suggesting β is more constrained than τ .
Optical measurements were performed with two pairs of autonomously operated electron-multiplied charge coupled device cameras (EMCCDs) co-located with the multi-frequency Canadian Meteor Orbit Radar (CMOR) (Brown et al., 2008). Using the timing and geometry of individual meteors measured by both the radar and multi-station EMCCD systems, the portion of the optical lightcurve corresponding to each specular radar echo is measured and the received echo power used to estimate an electron line density. A total of 1249 simultaneous EMCCD and radar meteors were identified from observations between 2017 – 2019 with 55 having in atmosphere speeds below 20 km/s. A subset of 36 events were analyzed in detail, with 29 having speed < 20 km/s. These meteors had G-band magnitudes at the specular radar point between +4 and +7.7, with an average radiant power of 5W (assuming a 945 W power for a zero magnitude meteor). These correspond to a typical magnitude of +6. Following the procedure in Weryk & Brown (2013b), the ratio of electron line density (q) to radiant power (I) provides a direct estimate of the ionization coefficient (β) to luminous efficiency (τ) ratio for each event. We find that β / τ strongly correlates with radiant power. All our simultaneous meteors had asteroidal-like orbits and six were found to be probable iron meteoroids, representing 20% of our slow <20 km/s sample. Luminous efficiency values averaged 0.6% at low speed, ranging from < 0.1% to almost 30%. No trend of luminous efficiency with speed was apparent, though a weak correlation between higher values of τ and radiant power may be present.
1. Introduction
The measurement of meteoroid mass using either optical or radar observations of meteors requires knowledge of the amount of ablation energy partitioned into photon or electron production. The associated luminous efficiency (τ) and ionization coefficient (β) are quantities which have historically been measured in the laboratory (eg. Slattery & Friichtenicht (1967); Friichtenicht et al. (1968)) or estimated from meteor measurements. While the range in luminous efficiency estimates is very large (Subasinghe et al., 2017), recent laboratory measurements of the ionization efficiency (DeLuca et al., 2018; Thomas et al., 2016) are in comparatively better agreement with the theoretical estimates from Jones (1997). Moreover, both measurements and theory suggest a rapid drop in ionization efficiency for speeds below 20 km/s.
Characterizing and understanding the meteoroid population encountering the Earth at low velocities (v < 20 km/s) has become increasingly important in recent years. In the last decade, new dynamical meteoroid models (eg. (Nesvorný et al., 2010; Yang & Ishiguro, 2015)) have predicted a large population of small, slow meteoroids originating from Jupiter-family comets which should dominate the mass influx to the Earth (Carrillo-Sánchez et al., 2016), in contrast to predictions from some earlier models (Liou et al., 1995). These new models have produced estimates for the speed distribution as a function of meteoroid mass for such models (Carrillo-Sánchez et al., 2016), providing testable predictions. Similarly, recent studies (eg. Borovicka et al. (2005); ˇ Campbell-Brown (2015)) have suggested that a significant fraction of low speed meteoroids may be iron in composition, with the fraction appearing to increase with decreasing mass (Capek et al., 2019).
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