The dynamics of low-beta plasma clouds as simulated by a 3-dimensional, electromagnetic particle code

Torsten Neubert, R.H. Miller, O. Buneman, K.-I. Nishikawa

Research output: Contribution to journalJournal articleResearchpeer-review

Abstract

The dynamics of low-beta-plasma clouds moving perpendicular to an ambient magnetic field in vacuum and in a background plasma is simulated by means of a three-dimensional, electromagnetic, and relativistic particle simulation code. The simulations show the formation of the space charge sheaths at the sides of the cloud with the associated polarization electric field which facilitate the cross-field propagation, as well as the sheaths at the front and rear end of the cloud caused by the larger ion Larmor radius, which allows ions to move ahead and lag behind the electrons as they gyrate. Results on the cloud dynamics and electromagnetic radiation include the following: (1) In a background plasma, electron and ion sheaths expand along the magnetic field at the same rate, whereas in vacuum the electron sheath expands much faster than the ion sheath. (2) Sheath electrons are accelerated up to relativistic energies. This result indicates that artificial plasma clouds released in the ionosphere or magnetosphere may generate optical emissions (aurora) as energetic sheath electrons scatter in the upper atmosphere. (3) The expansion of the electron sheaths is analogous to the ejection of high-intensity electron beams from spacecraft: a stagnation point is reached where the expansion is halted, allowing only a fraction of the electrons to escape. These electrons may have energies exceeding the initial bearn energy (sheath electrostatic potential energy). (4) Second-order and higher-order sheaths are formed which extend out into the ambient plasma. These sheaths are curved, the curvature increasing with the order of the sheath. (5) The formation of the sheaths and the polarization field reduces the forward momentum of the cloud. Furthermore, as the cloud moves across the field, sheath particles are peeled off and left behind, while fresh particles continue to drift into the sheaths in order to maintain these. This process requires continued energy supply which further decelerates the cloud. (6) The coherent component of the particle gyromotion (the particles are born with a coherent component at t=0) is damped in time as the particles establish a forward directed drift velocity. In addition, particles undergo orbital phase mixing associated with the establishment of the polarization electric field. For a dense cloud in vacuum the damping occurs very quickly, while for clouds in a background plasma the polarization fields are shorted, leaving the particles to gyrate as single particles. In this case, the coherent component is almost undamped. (7) The coherent particle gyrations generate electromagnetic radiation. The simulation indicate that artificial plasma clouds released in space will radiate dose to the cloud electron or ion cyclotron/upper hybrid frequencies.
Original languageEnglish
JournalJournal of Geophysical Research-space Physics
Volume97
Issue numberA8
Pages (from-to)12057-12072
ISSN2169-9380
DOIs
Publication statusPublished - 1 Aug 1992
Externally publishedYes

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