Rotating Spokes, Ionization Instability, and Electron Vortices in Partially Magnetized <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi mathvariant="bold-italic">E</mml:mi><mml:mo>×</mml:mo><mml:mi mathvariant="bold-italic">B</mml:mi></mml:math> Plasmas
Jean-Pierre Bœuf, M. Takahashi
Abstract
Regions of enhanced light emission rotating in the azimuthal direction are present in various $\mathbit{E}\ifmmode\times\else\texttimes\fi{}\mathbit{B}$ plasma devices. A kinetic model reveals that these ``rotating spokes'' are due to electron heating and enhanced ionization localized along a double layer at the interface between a region of large electric field and a quasi-equipotential region close to the anode potential. Electrons drifting along this interface are heated due to $\ensuremath{\nabla}B$ drift in the large electric field region. The formation of electron vortices due to the velocity shear in the double layer also contributes to electron heating. The possibility for spoke motion in the retrograde $\mathbit{E}\ifmmode\times\else\texttimes\fi{}\mathbit{B}$ direction as well as in the $\mathbit{E}\ifmmode\times\else\texttimes\fi{}\mathbit{B}$ direction, observed experimentally, can be reproduced and explained.