Meteor Trails in the Lower Thermosphere: What Do Large Radars Really Detect?
Abstract:Tens of millions of detectable meteors ablate in the Earth's upper atmosphere each second, creating turbulent plasma trails that persist for up to minutes as they dissipate into the background ionosphere. These trails produce easily detected radar signals with a wealth of information. This talk will present a detailed analysis of two aspects of meteor physics: (1) the early evolution of meteors as they first ionize and create radar reflections called head echoes, and (2) the later evolution as meteor plasmas develop turbulence and create radar signals called non-specular or range spread meteor trail echoes.
Head echoes form when sublimated material from a meteoroid initially collides with atmospheric molecules and ionize. Kinetic theory shows that this plasma develops over a length-scale close to the ion mean-free-path but with a highly non-Maxwellian velocity distribution. We have developed an analytical model that improves the quantitative interpretation of head echo radar measurements and ionization efficiency (called the Beta parameter). This will help us calculate meteoroid and atmosphere parameters from radar head-echo observations.
Non-specular meteor trail echoes develop when meteor plasmas become turbulent allowing the reflection of radar signals. We will analyze this system using 3D simulations of a dense column of meteor plasma embedded in a background ionosphere/thermosphere. While the meteor diffuses across the Earth's geomagnetic field B0, large electric fields develop because of the interplay between highly mobile but magnetized electrons and the heavier but collisionally demagnetized ions. These fields point mostly perpendicular to B0 and change slowly in the direction of B0. These simulations show that the electric field causes a substantial restructuring of the ionospheric plasma outside the trail but connected to it via B0. They also demonstrate the diffusive expansion of the trail and the development of waves both within and outside the trail. These massively parallel particle-in-cell (PIC) simulations reveal the nature of the highly anisotropic diffusion of meteors with some interesting surprises.