The Distribution of Vorticity in Auroral Arcs

  1. S.-I. Akasofu and
  2. J.R. Kan
  1. T. J. Hallinan

Published Online: 26 MAR 2013

DOI: 10.1029/GM025p0042

Physics of Auroral Arc Formation

Physics of Auroral Arc Formation

How to Cite

Hallinan, T. J. (1981) The Distribution of Vorticity in Auroral Arcs, in Physics of Auroral Arc Formation (eds S.-I. Akasofu and J.R. Kan), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM025p0042

Author Information

  1. Geophysical Institute, University of Alaska, Fairbanks, Alaska 99701

Publication History

  1. Published Online: 26 MAR 2013
  2. Published Print: 1 JAN 1981

ISBN Information

Print ISBN: 9780875900506

Online ISBN: 9781118664360



  • Auroras—Addresses, essays, lectures


In a magnetized plasma there is a direct proportionality between space charge and vorticity (shear). Because of this, a field-aligned sheet of negative space charge is subject to the Kelvin-Helmholtz instability and rapidly forms an array of vortices. These drift at the $$\overrightarrow {\rm E} \,{\rm X}\,\overrightarrow {\rm B}$$ velocity. In an auroral arc, electric fields parallel to the magnetic field may prevent the charge sheet from penetrating into the ionosphere. However, since energetic electrons reach the ionosphere after traversing this region, the vorticity maps to the ionosphere as an apparent vorticity in the visible auroral structures. The apparent velocity of auroral rays (vortices observed from the side) is the ionospheric manifestation of the $$\overrightarrow {\rm E} \,{\rm X}\,\overrightarrow {\rm B}$$ drift above the region of parallel electric fields. The perpendicular electric field (referred to the 100 km level) and the field-aligned potential are calculated from the observed vorticity in a typical rayed arc. The potential exceeds 2 kv. The field configuration and magnitude (1000 mv/m) are in excellent agreement with the “paired electrostatic shocks” observed from the S3-3 satellite. Despite this agreement, however, there is cause for concern. Msst auroral arcs, including some that are quite bright, exhibit active ray motions only sporadically. This suggests that large electric fields are not a necessary feature of auroral arcs. Hence the paired shocks, although highly significant, may not be the basic cause of discrete auroral arcs.