Quantifying the Effect of Meteoroid Ablation Rate on Meteor Plasma Formation using 3D Particle-in-Cell Simulation

Meteor plasma head echoes are frequently observed by high-power large-aperture (HPLA) radar when radio waves scatter from the meteor head plasma, which is the plasma surrounding the meteoroid as it enters Earth's atmosphere. This plasma is generated when thermally ablated neutral meteoric particles undergo high-energy ionizing collisions with oncoming atmospheric particles. To meaningfully deduce meteoroid properties from meteor head echo observations, a plasma density distribution must be assumed. Three-dimensional electrostatic particle-in-cell (PIC) simulations can predict the density distribution surrounding an ablating meteoroid. Previous results demonstrate that the ion density of a meteor head predicted via the massively parallel EPPIC (Electrostatic Parallel PIC) software matches an analytical model derived via gas dynamics, and that the inclusion of the background magnetic field can cause electron density variations of up to 70% within a 10-centimeter radius of the meteoroid. Unfortunately, prior simulation results assume a neutral particle ablation rate that is unrealistically small, at around 10^-12 kg/s. Since HPLA radar instruments frequently observe meteors in the range of 10^-8 to 10^-9 kg that fully ablate within fractions of a second, one would expect the ablation rate to be much larger. Increasing the ablation rate in a plasma simulation is difficult, since doing so increases the plasma density and therefore decreases the smallest Debye length that must be resolved. In this work, the ablation rate is increased from 10^-12 kg/s to 10^-11 kg/s and then 10^-10 kg/s, as high as we could achieve without prohibitively increasing grid resolution. As one might expect, the plasma density in the near-meteoroid region increases proportionally to the ablation rate. When Earth's magnetic field is not included in the simulation, the shape of the meteor plasma is qualitatively similar to prior results with the lower ablation rate. The increased ablation rate appears to reduce the influence of the magnetic field on the plasma shape within the near-meteoroid region, since in comparing the cases with and without Earth's magnetic field, electron density varies by less than 10% within a 10-centimeter radius of the meteoroid, unlike the 70% variation that appears in cases with lower ablation rate. However, the increased ablation rate results in increased variation between the meteor plasma shape farther from the meteoroid as transition to the trail region begins. Therefore, it is clear that the meteoroid mass loss rate is an important factor in determining how radio waves will scatter and generate head echo signatures in radar data.