On the internal physical conditions in dust probes: transport, heating and evaporation of fragmented dust particles

Abstract

We study the conditions within, and dynamics of fragmented mesospheric dust particles inside, the Faraday-cup type dust probe MUDD using numerical simulations with a dedicated model. The transport of singly charged fragments from impacting NLC particles on the main grid in MUDD, have been calculated on the basis of supplementary models of the neutral gas conditions and electric field structure within the probe. The theoretical model includes the effects of drag from neutral molecules, electric forces, as well as heating of -- and evaporation from -- the fragments. The model equations have been improved to be valid for nanoscale particles with a broad range of intrinsic properties, in the molecular flow regime. We find that the size range for unambiguous detection of pure MSP fragments of mass density rho_s=3000 kgm^(-3), is limited to fragments of radii between 1.5 nm and 2.1 nm with a 0.3 nm resolution; i.e. for the two existing detection modes of retarding potentials 10 V and 20 V. In the zero potential reference mode, fragments with radii smaller than 0.8 nm are stopped completely by neutrals. Fragments of pure ice content are found to evaporate rapidly, and will not contribute significantly to the measured currents at the bottom plate. Ice particles which contribute to the currents have to be larger than 3 nm, which renders the common assumption that ice particles smaller than 3 nm in radius must stick to probe surfaces [Tomsic, 2001; Havnes and Næsheim, 2007] redundant. From the study of alternative potential modes in MUDD, it is found that is is possible to improve the detectable size distribution of MUDD significantly by using lower retarding or accelerating potentials than the modes which already exist. Results from the E-field modeling suggest that the production of secondary charges have been somewhat underestimated due to very strong field anomalies near the edges of the probe. We also find plausibly large uncertainty factors from the investigations of initial fragment velocity, dynamic shape and heating of fragments during collisions with G2.