Direct determination of the micrometeoric mass flux into the upper atmosphere

The advent of radar micrometeor observations at Arecibo Observatory (AO) has enabled direct estimates of the meteoric mass flux into the upper atmosphere. These observations yield on average ∼3200 events per day in the 300 m diameter Arecibo beam. Doppler velocity estimates are found for approximately 50% of all events and of these, approximately 55% (26.5% of the total) also yield measurable (linear) decelerations. Assuming spherical particles of canonical density 3 gm/cc, the meteoric masses obtained range from a few micrograms to a small fraction of a nanogram. This approach yields an average mass of 0.31 microgram/particle for the 26.5% of all particles that manifest observable deceleration. The 45% with velocities, but not decelerations, correspond to particle masses larger than a few micrograms. However if we assume that all observed particles average 0.31 micrograms each, we find a mass flux of about 1.4× 10_-5 kg/km₂-day over the whole Earth. Detailed annual whole-Earth mass flux per decade of particle mass is calculated and compared with those of Ceplecha et al. [1]. Our results fall below those of Ceplecha et al. for observed mass fluxes however inclusion of those particles for which we cannot explicitly determine mass yield similar fluxes. Many of the particles we observe show evidence of catastrophically disintegrating in the meteor zone. We thus suggest that the majority of micrometeoroid mass is deposited in the 80-115 km altitude region where ionospheric and atmospheric manifestations such as sporadic E and neutral atomic metal layers are well documented. We further suggest that the "background" diurnal micrometeor mass flux is sufficient to dominate the average lower atmosphere mass influx from the annual meteor showers.