Average cometary ion flow pattern in the vicinity of comet 67P from moment data

Hans Nilsson,Gabriella Stenberg Wieser,Ingo Richter,Hayley Williamson,Anders I Eriksson,Etienne Behar,Charlotte Goetz,Martin Wieser,Fredrik L Johansson,Sofia Bergman

doi:10.1093/mnras/staa2613

Abstract

ABSTRACT Average flow patterns of ions around comet 67P detected by the RPC-ICA instrument onboard Rosetta are presented both as a time series and as a spatial distribution of the average flow in the plane perpendicular to the comet – Sun direction (Y–Z plane in the coordinate systems used). Cometary ions in the energy range up to 60 eV flow radially away from the nucleus in the Y–Z plane, irrespective of the direction of the magnetic field, throughout the mission. These ions may however be strongly affected by the spacecraft potential, the uncertainty due to this is briefly discussed. Inside the solar wind ion cavity and in the periods just before and after, the cometary pick up ions moving antisunward are deflected against the inferred solar wind electric field direction. This is opposite to what is observed for lower levels of mass-loading. These pick up ions are behaving in a similar way to the solar wind ions and are deflected due to mass-loading. A spatial asymmetry can be seen in the observations of deflected pick up ions, with motion against the electric field primarily within a radius of 200 km of the nucleus and also in the negative electric field hemisphere. Cometary ions observed by RPC-ICA typically move in the antisunward direction throughout the mission. These are average patterns, full-resolution data show very much variability.

Highlights

Comets are Solar system bodies containing a significant amount of volatiles which are released into surrounding space when they are heated
We look at the average flow direction of cometary ions binned into different spatial regions in the Comet Sun Electric (CSE) field coordinate system Y–Z plane
The RPC-ICA moment data base which we have used reproduces the main features of many previous observations, notably the deflection of the solar wind, acceleration of pick-up ions along the solar wind electric field for the low-activity case, dominating antisunward flow of pick up ions and a radial expansion of low-energy cometary ions in the Y–Z plane (X towards the Sun)

Summary

Introduction

Comets are Solar system bodies containing a significant amount of volatiles which are released into surrounding space when they are heated. For typical comets in highly elliptical orbits the outgassing of volatiles, the comet activity, increases strongly as the comet approaches the Sun. The resulting gas and dust envelope, the coma, is gravitationally unbound, expanding into space with a velocity of typically 500–1000 m s−1 (Gulkis et al 2015). The expanding plasma of the coma, the comet ionosphere, is immersed in the solar wind. The two plasmas fill some common volume where they affect each other, though dense and large enough comas may expel the solar wind from their innermost regions. The solar wind is affected by the added mass of the newly formed cometary ions, in a process known as mass-loading Such a newborn ion accelerated by the solar wind electric field is said to be ‘picked up’ by the solar wind and becomes a ‘pick up’ ion

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