Radio wave modification of plasma irregularities associated with mesospheric dust clouds

Scales, W. and Chen, C.

Virginia Tech, Blacksburg, Virginia, USA

Polar mesospheric summer echoes PMSEs are radar echoes, typically in the 50 MHz to 1.3 GHz range, produced by scattering from electron irregularities in the earth’s summer polar mesosphere. The electron irregularities believed to produce PMSEs result from electron charging on subvisible dust that exists in the mesosphere. This dust consists of ice particles created due to the cold temperatures in this region of the atmosphere. Experimental observations have shown that PMSEs may be modulated by radio wave heating the irregularity source region with a ground-based ionospheric heating facility. Early experiments showed that the PMSE strength may be initially suppressed after the turn-on of the radio wave heating. It was also recently predicted and verified that the PMSE strength may be enhanced above the undisturbed level for a period of time after the turn-off of the radio wave heating. This has been termed the ‘PMSE overshoot effect’. An understanding of the temporal behavior of PMSE strength during radio wave heating shows great promise as a diagnostic of the mesospheric dust layer. Early theoretical modeling work considered what is thought to be the two primary physical processes causing the PMSE modulation, electron diffusion and dust charging, separately. The successful prediction of the overshoot effect incorporated Boltzmann electrons and ions as well as dust charging effects. However, incorporation of Boltzmann electrons neglects finite diffusion time effects. Since the relative timescales for dust charging and ambipolar diffusion may be comparable depending on the mesospheric conditions and irregularity scale-size, important temporal behavior is not incorporated in recent models. This work describes a new model that incorporates both finite diffusion time effects as well as dust charging. The model utilizes fluid ions described by continuity and momentum equations, electrons whose behavior is determined from quasi-neutrality, and the charged dust grains are described by the a Particle-In-Cell PIC method. The model has been used to investigate temporal behavior of electron irregularities during electron temperature enhancement associated with radio wave heating over a range of dust and plasma parameters. The model predicts that the temporal behavior of the irregularities during radio wave heating depends on the ratio of the electron-ion ambipolar diffusion time to the dust particle charging time. Due to the dependence of this ratio on irregularity scale-size, these results have important implications for observations of PMSE modification at different observing radar frequencies. Therefore, new possibilities may exist for diagnosing the dust layer with radio wave modification.