RPC-MIP observations at comet 67P/Churyumov-Gerasimenko explained by a model including a sheath and two populations of electrons

Abstract

The response of the mutual impedance probe RPC-MIP on board Rosetta orbiter electrostatically modeled considering an unmagnetized and collisionless plasma with two Maxwellian electron populations. A vacuum sheath surrounding the probe was considered in our model in order to take the ion sheath into account that is located around the probe, which is immersed in the cometary plasma. For the first time, the simulated results are consistent with the data collected around comet 67P/Churyumov-Gerasimenko (67P), but strong discrepancies were identified with the previous simulations that neglected the plasma sheath around the probe. We studied the influence of the sheath thickness and of the electron populations. This work helps to better understand the initially unexpected responses of the mutual impedance probe that were acquired during the Rosetta mission. It suggests that two electron populations exist in the cometary plasma of 67P.

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

“…The existence of two different electron populations in the cometary plasma agrees with observations from the RPC-LAP dual Langmuir probe of Rosetta (Eriksson et al 2017) and with previous cometary measurements made during the flyby at comet 21P/Giacobini-Zinner of the International Cometary Explorer (ICE;Meyer-Vernet et al 1986). Wattieaux et al (2019) also underlined the influence of the positive ion sheath that surrounded the probe and the Rosetta spacecraft when they were exposed to the cometary plasma and were consequently negatively charged (Odelstad et al 2015).…”

“…Simulations have shown that the resonance occurs below the plasma frequency, while an antiresonance appears below the resonance frequency due to the occurrence of the sheath around the instrument ( Fig. 1 and more details in Wattieaux et al 2019). The antiresonance and the resonance frequencies and amplitudes depend on all the input parameters.…”

“…We here use the mutual impedance experiment model developed and validated on RPC-MIP data in Wattieaux et al (2019), which takes into account (i) the geometry of the Rosetta spacecraft as A&A 638, A124 (2020) well as the RPC-MIP quadrupolar antenna, (ii) the ion sheath surrounding the spacecraft and the experiment because of the negative spacecraft-charging reported at Rosetta (Odelstad et al 2015), and (iii) two different electron populations modeled by two Maxwellian distribution functions in the plasma dielectric function (Gilet et al 2017). The spectral power of the RPC-MIP experiment is referred to as the response of the probe (in dB units) P dB = 10 log 10 20 |∆V R | 2 , where ∆V R is the voltage drop between the receivers of the probe (in mV units) at a given frequency (Wattieaux et al 2019). The comparison of simulated with experimental responses provides four parameters that characterize the plasma eedf at comet 67P in terms of electron densities and temperatures.…”

“…In order to constrain the solutions, a double Maxwellian eedf involving a cold and a warm electron population is assumed at 67P. A recent electrostatic simulation of the probe operated in such a plasma (Wattieaux et al 2019) has shown that the instrumental response of the RPC-MIP experiment is not compatible with a single Maxwellian electron population, and it has provided simulated responses in accordance with experimental responses when two electron populations with different temperatures are considered. The existence of two different electron populations in the cometary plasma agrees with observations from the RPC-LAP dual Langmuir probe of Rosetta (Eriksson et al 2017) and with previous cometary measurements made during the flyby at comet 21P/Giacobini-Zinner of the International Cometary Explorer (ICE;Meyer-Vernet et al 1986).…”

Ionospheric total electron content of comet 67P/Churyumov-Gerasimenko

“…Comet 67P is reported to have a dynamic ionosphere (see Edberg et al 2015;Galand et al 2016;Vigren et al 2016;Hajra et al 2017Hajra et al , 2018aHenri et al 2017;Heritier et al 2017Heritier et al , 2018Engelhardt et al 2018). This ionosphere consists of newborn or freshly picked-up water group ions, such as H 2 O + and H 3 O + , and CO + and CO + 2 ions (Fuselier et al 2015(Fuselier et al , 2016Nilsson et al 2015;Goldstein et al 2017;Beth et al 2019), together with warm (∼5-10 eV) and cold (<0.1 eV) electrons (Eriksson et al 2017;Gilet et al 2017;Wattieaux et al 2019); there is also a lesser population of energetic (∼10-200 eV) electrons (Clark et al 2015;Broiles et al 2016;Myllys et al 2019). The photo-ionization or electron-impact ionization of cometary neutrals (e.g., H 2 O, CO 2 , and CO; Le Roy et al 2015;Fougere et al 2016), and, to a lesser extent, the charge-exchange of cometary neutrals with solar wind ions, have been shown to be the main sources of the cometary ions and electrons (Cravens et al 1987;Vigren & Galand 2013;Galand et al 2016;Vigren et al 2016;Heritier et al 2018;Simon Wedlund et al 2016).…”