Size of a plasma cloud matters

The polarisation electric field of a small-scale comet ionosphere

¹ Swedish Institute of Space Physics, PO Box 812, 98128 Kiruna, Sweden
e-mail hans.nilsson@irf.se
² Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Rymdcampus 1, 98128 Kiruna, Sweden
³ Belgian Institute for Space Aeronomy, Avenue Circulaire 3, 1180 Brussels, Belgium
⁴ Department of Physics, Umeå University, 90187 Umeå, Sweden
⁵ Department of Space and Plasma Physics, School of Electrical Engineering and Computer Science, KTH Royal Institute of Technology, Stockholm, Sweden
⁶ Laboratoire de Physique et Chimie de l’Environnement et de l’Espace (LPC2E), UMR 7328 CNRS – Université d’Orléans, France
⁷ Technische Universität Braunschweig, Institute for Geophysics and Extraterrestrial Physics, Mendelssohnstraße 3, 38106 Braunschweig, Germany
⁸ Swedish Institute of Space Physics, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, Sweden

Received: 10 April 2018
Accepted: 9 May 2018

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.