Spatial distribution of low‐energy plasma around comet 67P/CG from Rosetta measurements

Abstract

We use measurements from the Rosetta plasma consortium Langmuir probe and mutual impedance probe to study the spatial distribution of low‐energy plasma in the near‐nucleus coma of comet 67P/Churyumov‐Gerasimenko. The spatial distribution is highly structured with the highest density in the summer hemisphere and above the region connecting the two main lobes of the comet, i.e., the neck region. There is a clear correlation with the neutral density and the plasma to neutral density ratio is found to be ∼1–2·10−6, at a cometocentric distance of 10 km and at 3.1 AU from the Sun. A clear 6.2 h modulation of the plasma is seen as the neck is exposed twice per rotation. The electron density of the collisionless plasma within 260 km from the nucleus falls off with radial distance as ∼1/r. The spatial structure indicates that local ionization of neutral gas is the dominant source of low‐energy plasma around the comet.

Mass-loading, pile-up, and mirror-mode waves at comet 67P/Churyumov-Gerasimenko

“…At that time, these newly created ions also indicated the "birth of a magnetosphere" for which the spatial distribution of the lowenergy plasma was discussed by Edberg et al (2015b). However, "conventional signatures" such as Alfvén waves or cyclotron waves were not observed.…”

“…There is a ∼ 6 h quasi-periodic variation in the neutral and plasma density (Hässig et al, 2015;Edberg et al, 2015b). First the effect of the mass loading on the induced magnetosphere is discussed, including magnetic field pile-up and draping, relating it to variations in the solar wind.…”

Evolution of the plasma environment of comet 67P from spacecraft potential measurements by the Rosetta Langmuir probe instrument

“…Since the charged spacecraft perturbs the potential of the surrounding plasma and the boom length is smaller than or comparable to the Debye length (∼1-10 m for the studied time period [Edberg et al, 2015]), the plasma potential at the probe will in general not be the same as that of the unperturbed plasma far away from the spacecraft. Simulations indicate that, for LAP1 in a tenuous solar wind environment, the ratio V ph ∕V S∕C is on the order of −1∕2 to −2∕3 [Sjogren et al, 2012].…”

“…The electron density and temperature cannot be disentangled by this method but for the purpose of this study we will generally attribute any variations in V S∕C to changes in n e , assuming a constant T e of 5 eV, which is supported by initial analysis of the LAP sweep measurements. Also, the consistency of the negative V S∕C shows that variations in T e must be much smaller than the variations of n e , which can be several orders of magnitude [Edberg et al, 2015]. Figure 1 shows an example of spacecraft potential measurements (upper panel), taken on 8 January 2015, when Rosetta was in near-terminator orbit at 28 km from the nucleus.…”

“…Edberg et al [2015] mapped the near-nucleus spatial distribution of this plasma using RPC-LAP during a 2 week period in October 2014 when Rosetta was at a cometocentric distance of ∼10 km (heliocentric distance ∼3.1 AU), finding a highly structured plasma environment with the highest density in the summer hemisphere and above the neck region of the comet. Altitude profiles from two flybys in February were consistent with a 1∕r dependence of plasma density on cometocentric distance, at least within 260 km of the nucleus.…”