Coordinated Optical and Radar Measurements of Low Velocity Meteors

To better estimate which luminous efficiency ($\tau$) value is compatible with contemporary values of the ionization coefficient ($\beta$), we report a series of simultaneous optical and specular echo radar measurements of low speed meteors. A total of 1249 simultaneous EMCCD and radar meteors were identified. A subset of 36 events were analyzed in detail, with 29 having speed less than 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) and masses of 1 - 10 mg. These correspond to a typical magnitude of +6. We find that $\beta$ / ${\tau}$ strongly correlates with radiant power. All our simultaneous meteors had asteroidal-like orbits and six were found to be probable iron meteoroids, representing 20 percent of our slow velocity sample. Luminous efficiency values averaged 0.6% at low speed, ranging from less than 0.1% to almost 30%. No trend of luminous efficiency with speed was apparent, though a weak correlation between higher values of luminous efficiency and radiant power may be present.