Abstract
Context. The cometary ionosphere is immersed in fast flowing solar wind. A polarisation electric field may arise for comets much smaller than the gyroradius of pickup ions because ions and electrons respond differently to the solar wind electric field.Aims. A situation similar to that found at a low activity comet has been modelled for barium releases in the Earth’s ionosphere. We aim to use such a model and apply it to the case of comet 67P Churyumov-Gerasimenko, the target of the Rosetta mission. We aim to explain the significant tailward acceleration of cometary ions through the modelled electric field.Methods. We obtained analytical solutions for the polarisation electric field of the comet ionosphere using a simplified geometry. This geometry is applicable to the comet in the inner part of the coma as the plasma density integrated along the magnetic field line remains rather constant. We studied the range of parameters for which a significant tailward electric field is obtained and compare this with the parameter range observed.Results. Observations of the local plasma density and magnetic field strength show that the parameter range of the observations agree very well with a significant polarisation electric field shielding the inner part of the coma from the solar wind electric field.Conclusions. The same process gives rise to a tailward directed electric field with a strength of the order of 10% of the solar wind electric field. Using a simple cloud model we have shown that the polarisation electric field, which arises because of the small size of the comet ionosphere as compared to the pick up ion gyroradius, can explain the observed significant tailward acceleration of cometary ions and is consistent with the observed lack of influence of the solar wind electric field in the inner coma.
Highlights
Models employed to understand solar wind interaction with a comet atmosphere were based on experience from highly active and large-scale comets, such as Halley (Neugebauer 1990), and employed methods mainly valid for large-scale phenomena
We obtained analytical solutions for the polarisation electric field of the comet ionosphere using a simplified geometry. This geometry is applicable to the comet in the inner part of the coma as the plasma density integrated along the magnetic field line remains rather constant
Observations of the local plasma density and magnetic field strength show that the parameter range of the observations agree very well with a significant polarisation electric field shielding the inner part of the coma from the solar wind electric field
Summary
Models employed to understand solar wind interaction with a comet atmosphere were based on experience from highly active and large-scale comets, such as Halley (Neugebauer 1990), and employed methods mainly valid for large-scale phenomena. The ESA mission Rosetta followed comet 67P over a large range of heliocentric distances, during which the comet ionosphere was small compared to a pick up ion gyroradius, but larger than the electron gyroradius This presents a special challenge and the situation at such a smallscale comet has more similarities with the plasma dynamics of barium release experiments than with large-scale comets (Coates et al 2015). (2018), showed how ions accelerated to several 10 eV energy and above were moving mainly antisunward, while the direction of the upstream solar wind electric field determined their motion in the plane perpendicular to the comet–sun line. Ions at lower energy were moving with a significant anti-sunward component, but in the plane perpendicular to the comet–sun line they were moving radially away from the comet nucleus irrespective of the direction of the solar wind electric field. We discuss the shortcomings and the strengths of the model and compare results obtained with the model to observations by instruments in the Rosetta Plasma Consortium (RPC) instrument suite (Carr et al 2007)
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