Abstract
Abstract A recent work aided by Rosetta in situ measurements set constraints on the dust-to-gas mass emission ratio and the size distribution of dust escaping the nucleus of comet 67P/Churyumov–Gerasimenko near perihelion. Here we use this information along with other observables/parameters as input into an analytical model aimed at estimating the number density of electrons attached to dust particles near the position of Rosetta. These theoretical estimates are compared to in situ measurements of the degree of ionization. The comparison proposes that Rosetta, while near perihelion, was typically not in electron-depleted regions of the inner coma of 67P. Our work suggests a typical level of electron depletion probably below 10% and possibly below 1%. In line with previous studies, we find, again with certain assumptions and other observables/parameters as input, that the observed negative spacecraft charging to a few tens of volts does not significantly impact the detection of charged dust grains, with a possible exception for grains with radii less than ∼10 nm.
Highlights
Regions of ionospheres where the number density of free electrons is significantly lower than the number density of positively charged ions are referred to as electron depleted
For a phase angle of 90° and near perihelion, the default gas expansion speed is, based on the work of Hansen et al (2016), set to ugas = 750 m s−1. This is in reasonable agreement with velocities inferred from measurements by the Microwave Instrument for the Rosetta Orbiter (MIRO; see Figure 5 in Biver et al 2019) and ∼20% higher than the terminal velocity of the smallest dust grains considered in this work
We have presented an analytical model for calculating the number density of electrons attached to dust particles, ne,dust, in the coma of 67P
Summary
Regions of ionospheres where the number density of free electrons is significantly lower than the number density of positively charged ions are referred to as electron depleted. Adding to our interest in the raised question is that grain charging has been raised as a plausible contributing reason as to why cometary ionospheric models assuming ions to move radially outwards at the same speed as the neutral gas perform worse in reproducing observed electron number densities near perihelion than at lower activity (e.g., Vigren et al 2019 and references therein). Adapting the grain charging formalism of Draine & Sutin (1987) and several simplifying assumptions, Vigren et al (2015) modeled (before the Rosetta arrival) grain charging in the coma of 67P From their results and sensitivity tests, they proposed that prominent electron depletion out to a few hundreds of kilometers from the nucleus requires a ubiquitous presence of grains in the nanoscale regime.
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