Role of grain size and particle velocity distribution in secondary electron emission in space plasmas

TLDR

Cosmic dust grains, always immersed in plasma, acquire charge largely through secondary electron emission, and the grain size has a decisive but underappreciated influence on the equilibrium potential. The authors performed a systematic study of conducting and insulating spherical grains of various sizes in both Maxwellian and generalized Lorentzian plasmas to quantify secondary emission effects. They found that secondary emission yield rises sharply as grain size decreases, reaching very high values for grains comparable to the electron penetration depth, is larger for insulators, produces more positive equilibrium potentials in plasmas with broad non‑Maxwellian tails, can cause opposite charges on different sized grains, and may significantly affect accretion in polydisperse dusty plasmas.

Abstract

By virtue of being generally immersed in a plasma environment, cosmic dust is necessarily electrically charged. The fact that secondary emission plays an important role in determining the equilibrium grain potential has long been recognized, but the fact that the grain size plays a crucial role in this equilibrium potential, when secondary emission is important, has not been widely appreciated. Using both conducting and insulating spherical grains of various sizes and also both Maxwellian and generalized Lorentzian plasmas (which are believed to represent certain space plasmas), we have made a detailed study of this problem. In general, we find that the secondary emission yield δ increases with decreasing size and becomes very large for grains whose dimensions are comparable to the primary electron penetration depth, such as in the case of the very small grains observed at comet Halley and inferred in the interstellar medium. Moreover, we observe that δ is larger for insulators and equilibrium potentials are generally more positive when the plasma has a broad non‐Maxwellian tail. Interestingly, we find that for thermal energies that are expected in several cosmic regions, grains of different sizes can have opposite charge, the smaller ones being positive while the larger ones are negative. This may have important consequences for grain accretion in polydisperse dusty space plasmas.