Building a Weakly Outgassing Comet from a Generalized Ohm’s Law

Abstract

When a weakly outgassing comet is sufficiently close to the Sun, the formation of an ionized coma results in solar wind mass loading and magnetic field draping around its nucleus. Using a 3D fully kinetic approach, we distill the components of a generalized Ohm's law and the effective electron equation of state directly from the self-consistently simulated electron dynamics and identify the driving physics in the various regions of the cometary plasma environment. Using the example of space plasmas, in particular multi-species cometary plasmas, we show how the description for the complex kinetic electron dynamics can be simplified through a simple effective closure, and identify where an isotropic single-electron fluid Ohm's law approximation can be used, and where it fails.Numerical models that seek to describe the evolution 1 of plasma without self-consistently including the electron 2 dynamics, such as (multi-)fluid and hybrid simulation 3 approaches [1], need to rely on a relation that prescribes 4 the behavior of the unresolved species. Typically a 5 generalized Ohm's law (GOL) is assumed [2], combined 6 with a closure relation such as a polytropic or a double 7 adiabatic evolution [3,4]. In this letter, we show how a 8 GOL can unravel the hidden mysteries of multi-species 9 plasma environments, such as the solar wind plasma 10 interaction with a weakly outgassing comet [5][6][7]. We 11 indicate where reduced plasma models can be applied, 12 e.g., to gain more direct access to the ongoing physics 13 and/or to decrease the needed amount of computational 14 resources, and show the consequences of this compromise. 15 * mailto: jandeca@gmail.com 16 The Rosetta spacecraft caught up with comet 17 67P/Churyumov-Gerasimenko (hereafter 67P) at a 18 heliocentric distance of 3.6 AU [8, 9]. At a few hundreds 19 of kilometers from the cometary nucleus, the Rosetta 20 plasma instruments, quite unexpectedly, picked up 21 the signatures of a plasma environment dominated 22 the plasma density with distance from the nucleus [19, 20] 34 or, in other words, there exists a continuously changing 35 ratio between the cometary and the upstream solar wind 36 plasma density throughout 67P's plasma environment, 37 both along the Sun-comet direction as well as in the 38 meridian plane [21-23]. To first order, for a weakly 39 outgassing comet, the dynamical interaction that de-40 termines the general structure of the cometary plasma 41 environment is representative of a four-fluid coupled 42 system (illustrated in Fig. 1), where the solar wind 43 electrons move to neutralize the cometary ions and the 44 cometary electrons organize themselves to neutralize the 45 solar wind ions [7]. 46 47 In addition to a detailed understanding of the kinetic 48 dynamics that governs the solar wind interaction with 49 a weakly outgassing comet, in this letter we provide 50 feedback to (multi-)fluid [24-29] and hybrid [16, 30-37] 51 models where the electrons dynamics is prescribed 52 through a GOL combined with an electron closure 53 relation. Using a f...

Observations of a mix of cold and warm electrons by RPC-MIP at 67P/Churyumov-Gerasimenko

“…The influence of the ambipolar electric potential, associated to the cometary plasma inhomogeneities and localized around the comet nucleus, in terms of particle trapping, might facilitate the electron cooling. This hypothesis shall be investigate in further details from global modeling (Deca et al 2017(Deca et al , 2019Sishtla et al 2019) to investigate the physical processes at the origin of the cold electron population far from perihelion reported in this study.…”

“…Second, from March 2016 to the end of the operations when the outgassing activity was low, no electron-neutral collisions were expected despite the fact that the spacecraft was getting closer to the nucleus. Therefore, the observation of cold electrons by RPC-MIP at the end of the Rosetta cometary operations is not consistent with the cooling of newborn electrons that are ballistically escaping the inner coma region, but is rather consistent with electron that would stay longer in the denser inner cometary atmosphere region, and therefore with electrons that got trapped in the inner coma by the ambipolar electric field as shown in recent kinetic simulations of cometary electron dynamics (Deca et al 2017(Deca et al , 2019Sishtla et al 2019). In this appendix, we explain how the main resonances are extracted from the acquired RPC-MIP mutual impedance spectra (i) when the spectrum is shaped by a single resonance (Fig.…”

“…Instead, the observations of cold electrons by RPC-MIP reported in this study are consistent with the existence of a trapped population of electrons in the inner come region, because of the effect of an ambipolar electric field. The presence of such an electric field is supported by full kinetic cometary plasma simulations that captures the electron kinetic dynamics (Deca et al 2017(Deca et al , 2019 and where evidence of trapped electrons is observed (Sishtla et al 2019). The cooling process is expected to be much more efficient for trapped electrons than for passing (or ballistic) electrons because the former travel much longer distances within the denser cometary neutral regions (Eriksson et al 2017).…”

Plasma characterization at comet 67P between 2 and 4 AU from the Sun with the RPC-MIP instrument

“…Second, in order to assess the influence of the electron acceleration processes reported in previous studies that were shown to be associated with the action of the ambipolar electric field that in turn is associated with the large-scale electron pressure inhomogeneity in the cometary ionosphere (Madanian et al 2016;Deca et al 2017Deca et al , 2019, we have used recent numerical simulations of the collisionless interaction of the solar wind with a comet to investigate the expected behavior of the electron temperature in the close cometary environment. The numerical simulation is described in Deca et al (2019) and was performed using a full kinetic particle-in-cell simulator that enabled the authors to model the kinetic behavior of electrons. This model is collisionless, so that electron cooling is not included in the model.…”

Vlasov simulation of electrons in the context of hybrid global models: A Vlasiator approach

“…An example of small-scale global, electromagnetic implicit PIC modelling for a weak comet has been performed by Deca et al (2017Deca et al ( , 2019 with a reduced proton-electron mass ratio of 100, and local simulations for a lunar minimagnetosphere (Deca et al, 2015) with a reduced proton-electron mass ratio of 256. GEM challenge, concluding that reconnection rates are well captured by smaller mass ratios of m p /m e = 180, although with modified electron kinetics.…”