Meteoroid sputtering, high-altitude radar and optical meteors, and sources for lower-thermospheric metals

Observations of radar and optical meteors at altitudes above the ablation-defined meteor zone point to a “new” sputtering source of high-altitude metals while also yielding clues to the radio science of radar meteors. That high-altitude radar meteors (HARMs) are visible at altitudes of up ~180 km using the 50 MHz Jicamarca Radio Observatory (JRO) implies that a significant fraction of the directly-sputtered meteoroid material is ionized as the collisional mean free path (MFP) at these altitudes is many tens of meters. Also, that high-altitude klB FAI (Field-Aligned Irregularity) trail-echoes are observed in association with HARM head-echoes offers insight into the meteor-associated plasma processes as well as the radar scattering mechanism. In particular, HARMs reveal the onset, as the meteoroid enters the upper atmosphere, of radar scattering which must be from the minimum, radar detectible, electron production. The maximum Radar backscattering Cross-Section (RCS) from N electrons, σ_BS = 4πν_e²N², is the result of totally coherent scattering from N, equally illuminated and closely spaced, electrons where r_e, is the classical electron radius. An estimate for N is given. Overall, this approach yields a direct, physics-based path linking the RCS of observed head- and trail-echoes to the meteoroid sputtering process and to the ion atmospheric “capture” process whereby the head-echo “plasma” is embedded into the atmosphere. In addition, recent lidar observations of atomic Fe, Na, and K at altitudes well above the traditional meteor zone violate the usual assumptions regarding thermospheric metals and form an instructive complication to the above “simple meteoroid” scenario as many meteoroids appear to be fragmenting at high altitudes. This “cold fragmentation” likely points to complex “dirty-ice” and “dust-ball” meteoroids comprised of small, dense grains weakly bound together by more volatile substances. These meteoroids may disperse into an extended “coma” that presents a more complex and larger surface area atmospheric-interaction region. We briefly discuss sputtering, the impact energies needed for onset of sputtering, sputtering yield, and the observational evidence available for interpretation of meteoroid sputtering as a source of aeronomically interesting metals above the classical meteor zone. We further suggest that a thermospheric “reservoir” of nanometer-sized dust play an important aeronomic role.